Bruce Fink Lynn Penn, U.S. Army Research Laboratory Univ of Kentucky
Richard Wool, Univ of Delaware
* Invited paper
SESSION M1: COMPOSITE SYNTHESIS AND PROCESSING
Chairs: Lynn S. Penn and Richard P. Wool
Monday Morning, December 1, 1997
Independence Center (S)
8:30 AM *M1.1
SYNTHESIS AND CHARACTERIZATION OF NEW THERMOSETTING POLYIMIDE COMPOSITE MATRIX RESIN SYSTEMS. James E. McGrath, Alfred Loos, Amba Ayambem, Hong Zhuang, Todd Bullions, Virginia Tech, Depts. of Chemistry and Engineering Science and Mechanics, NSF Science and Technology Center on High Performance Polymeric Adhesives and Composites, Blacksburg, VA; Biao Tan, Eastman Kodak Company, Rochester, NY.
The development of new materials as durable, long-term, solvent resistant adhesives for titanium 6-4 and carbon fiber composites has been investigated. Applications for these materials include the high speed civil transport plane, which requires adhesives to be durable for 60,000 hours at temperatures upwards of 200C. We have investigated crosslinkable arylene ether imide oligomers, which have been quantitatively fitted with phenylethynyl or phenylmaleic anhydride endcaps. Most of the effort has focused on applying this strategy toward a postulated cost-effective ìUltem-likeî material. Oligomers were prepared as a function of molecular weight from around 2,000ñ20,000 Mn. Initial screening studies suggested that the materials were quite ductile after the high temperature cure (45-90 minutes at 350-380C), even when the starting oligomer molecular weight was as low as 2,000 or 3,000. Since the higher crosslink density materials would demonstrate better solvent resistance, the 2,000 and 3,000 imide oligomers have been investigated in some detail. The subject materials produce a low viscosity material, which is soluble in fully imidized form, and allows for the generation of solvent resistant adhesives (>99% gel) with Tg values of 250C or higher. Scrim cloth adhesives have been prepared and evaluated both internally and by external laboratories. In all cases, the cured adhesives show an excellent combination of lap shear strength, thermal and solvent resistance that appears to be appropriate for the intended applications. This lecture will review the synthetic chemistry, characterization, processing, adhesive studies and initial investigations of continuous carbon fiber composites generated by a resin infusion or powder processes. It is also of interest that the crosslinkable polyimide oligomers have been found to be miscible with their linear thermoplastic counterparts, and cured thermoplastic/thermosetting hybrids have been prepared, which appear to show a good balance of processibility, toughness, and solvent resistance. Finally, phosphorus containing diamines were prepared and incorporated into polyimides that show good fire resistance.
9:00 AM M1.2
CARBON FIBER COMPOSITES FROM PHENYLETHYNYL TERMINATED THERMOSETTING POLYIMIDE OLIGOMER MATRIX SYSTEM: STRUCTURE-PROPERTY RELATIONSHIPS. T. A. Bullions, R. H. Mehta, B. Tan (a), J. E. McGrath, and A. C. Loos; Virginia Polytechnic Institute and State University; NSF Science and Technology Center: High Performance Polymeric Adhesives and Composites, Blacksburg, VA; A. Meyer, D. Hood, and D. Kranbuehl; Chemistry Department, College of William and Mary Williamsburg, VA; (a): Eastman Kodak Company, Rochester, NY.
Phenylethynylphthalic anhydride endcapped (4-PEPA) poly(etherimide) oligomer has been investigated as a matrix material in formation of carbon fiber reinforced composites from G30-500, 12k tow. Thermosetting poly(etherimide) exhibits excellent thermo-oxidative stability and solvent resistance upon curing at high temperatures. Therefore, this material is an attractive matrix system for carbon fiber reinforced composites, where the matrix is usually responsible for failure of the composite part in high temperature and aggressive solvent environments. In the course of the present investigation, a modified dry powder prepregging technique was employed to circumvent the processing difficulties associated with high viscosity and to perform the operation in organic solvent-free environments. Flat composite laminates were consolidated via manual lay-up from the powder coated towpreg. Resin curing was monitored using frequency dependent dielectric measurements (FDEMS). These composite panels have been characterized for their fiber volume fraction and density. Mechanical properties are related to the microstructure of the composites. The relationship between fiber/matrix interfacial bond strength and the mechanical and fracture behavior of composite panels has been elucidated by Mode I and Mode II fracture toughness as evaluated by the double cantilever beam (DCB) and end-notch flexure (ENF) test methods.
9:15 AM M1.3
SYNTHESIS AND CHARACTERIZATION OF CYANATE ESTER TERMINATED SILOXANES AS IMPACT MODIFERS FOR CYANATE ESTER RESINS. Steven K. Pollack and Zhidong Fu, Howard University Department of Chemistry and the Polymer Science & Engineering Program, Washington, DC.
Cyanate ester resins (CERs) possess many advantages for engineering applications. However, commercial CERs do not currently have the appropriate fracture toughness for applications such as airframe construction. The improvement of the performance of brittle polymer materials can be achieved by the addition of impact modifiers. Novel telechelic cyanate ester siloxanes for use as impact modifiers for CERs have been systhesized and characterized for the first time. These additives provide for improved mechanical properties along with an improved non-flammability of the base resins. NMR, GC/MS, and FTIR were used to characterize these reactive oligomers. Cross-linked homopolymers exibit a range of physical properties as a function of their molecular weight. Blends with commerical CERs show good microphase separation and dispersion.
9:30 AM M1.4
POLYMERS FROM RENEWABLE RESOURCES-LIQUID MOLDING RESINS BASED ON ACRYLATED EPOXIDIZED SOYBEAN OIL. Shrikant N. Khot, Richard P. Wool, Ralph Zhao, University of Delaware, Dept. of Chemical Engineering, Newark, DE; Selim Kusefoglu, Bogazici University, Dept. of Chemistry, Istanbul, TURKEY.
Using epoxidized soybean oil as a starting material, a wide range of thermoset polymers have been synthesized. These polymers are based on an acrylated form of epoxidized soybean oil. Acrylation makes the oil capable of polymerization in the presence of a reactive diluent. The resulting cross-linked polymer displays properties similar to the widely used epoxy and vinyl ester resins and are suitable for incorporation into a composite matrix by standard molding processes. In an effort to improve the glass transition temperature and mechanical properties of these polymers, several forms of acrylated epoxidized soybean oil have been synthesized. These synthetic routes involve pre-polymerization of the epoxidized soybean oil prior to acrylation as well as telomerization of previously acrylated soybean oil. Additionally, experiments with different reactive diluents have been conducted to determine their effect on the polymer properties.
9:45 AM M1.5
WATER DURABLE BIOABSORBABLE POLYPHOSPHATE/SOY PROTEIN PLASTIC COMPOSITES. Joshua U., Otaigbe, Dept. of Materials Science and Engineering, Iowa State University, Ames, IA.
The use of synthetic and natural bioabsorbable plastics has been severely limited due to their low stiffness and strength properties as well as their strong tendency to absorb moisture. This research focused on the development of bioabsorbable polyphosphate filler/soy protein plastic composites with enhanced stiffness, strength, and water resistance. Bioabsorbable polyphosphate fillers, biodegradable soy protein isolate, plasticizer, and adhesion promoter were homogenized and compression-molded. Physical, mechanical, and water absorption testing was performed on the molded specimens. Results showed improvements in stiffness, strength, and water resistance with increasing polyphosphate filler content up to 20 percent by weight. Application of a coupling agent produced further mechanical property enhancements and a dramatic improvement in water resistance, interpreted by an interfacial chemical bonding model. Examination of the fracture surfaces of the materials revealed that addition of the polyphosphate fillers changed the failure mode from brittle to pseudo-ductile. These results suggest that these materials are suitable for many load-bearing applications in both wet and dry environments where current soy protein plastics are not useable.
10:30 AM *M1.6
SYNTHESIS OF NON-FLAMMABLE, VERY HIGH CHAR YIELD, ACETYLENE-FUNCTIONAL POLYBENZOXAZINES AND THE EFFECT OF PROCESSING CONDITIONS ON THE CHAR FORMATION. Hatsuo Ishida, Hyun Jin Kim, Zdenka Brunovska, Case Western Reserve University, NSF Center for Molecular and Microstructure of Composites, (CMMC), Department of Macromolecular Science, Cleveland, OH.
Polybenzoxazines are attractive engineering materials because of their superb balance of mechanical and physical properties, as well as rich molecular design flexibility. Very recently, we have synthesized a class of polybenzoxazines that show char yield in the range of 71-82 polybenzoxazines are non-flammable and produce no polymerization by-products, resulting in composites with low void contents. Near-zero shrinkage behavior of the material, when the volumes before and after are compared, also helps produce high performance composites. When the processing conditions of the acetylene-functional benzoxazine resins are changed, the char formation is strongly influenced. This lecture summarizes general properties of polybenzoxazines and their composites and the new features that are obtained by the addition of acetylene functionality to benzoxazine chemistry.
11:00 AM M1.7
INVESTIGATIONS OF FIELD-STRUCTURED COMPOSITES. James E. Martin, Robert A. Anderson, Chris P. Tigges, Sandia National Laboratories, Albuquerque, NM.
When a colloidal suspension is subjected to a sufficiently large electric or magnetic field, the induced electrostatic or magnetic interactions cause particles to assemble into anisotropic structures, provided a sufficient permittivity or permeability mismatch exists, respectively. These structures are essentially one dimensional if the field is uniaxial, but two-dimensional structures can be created in a rotating field. If the continuous phase is polymerizable, e.g. a thermosetting resin, these anisotropic structures can be pinned by the gelling of the resin. The composite materials so produced have marked anisotropies in permittivity, electrical conductivity, dielectric breakdown strength, acoustical velocities and other properties, that are strongly dependent on the particle concentration, structuring time and quench path. In addition to synthesizing and characterizing such materials, we have written a large scale Langevin dynamics simulation (10,000 particles) of structure formation in an applied field, using the simple dipole interaction potential, Stokes friction for the spheres, and a correlated fluctuating thermal force. We have simulated particle ëcolumní formation in a uniaxial electric field and particle ësheetí formation in a rotating field and have characterized the evolution of structure via two-dimensional correlation functions, microcrystallinity, velocity correlations etc., and have computed anisotropies in the dipolar interaction energy, permittivity, dielectric breakdown strength, conductivity, and optical attenuation length. We will describe the concentration dependence of these properties, and will compare these to experimental data, including time-resolved light scattering scattering measurements.
11:15 AM M1.8
IN SITU REINFORCED THERMOPLASTIC FIBERS FOR FORMING COMPOSITE STRUCTURES. Donald G. Baird and Jianxian Xue, Virginia Tech, Department of Chemical Engineering, Blacksburg, VA.
A novel process has been used to generate thermoplastic fibers based on matrices reinforced with thermotropic liquid crystalline polymers(TLCP's) of higher melting point than the matrix. Because the TLCP reinforcing phase is of higher melting point than the matrix, the fibers can be heated in various processing steps to temperatures where the matrix is molten but the TLCP remains in a highly oriented state. The fibers have been woven into fabric prepregs suitable for compression molding and thermoforming. Materials containing about 30 wt% TLCP exhibit tensile and flexural properties similar to those of sheet molding compound but are only half as dense. Random fiber mats have been generated and compression molded. Properties and processing characteristics of these materials are similar to commercially available materials containing long glass fibers, but the density and surface appearance of the sample are more favorable. The fibers have been used in filament winding to form tube structures. Typically this is not readily done with thermoplastic based systems. A number of matrices have been used including polypropylene, nylon, and polyphenylenesulphide. Concentration levels of TLCP of up to 50 wt% have been used. For some applications the cost of the TLCP is a problem, but factors such as rate of processing, light-weight, and recyclability can overcome this limitation. Other factors under consideration at the present are durability and impact toughness.
11:30 AM M1.9
TAILORING THE RELATIVE MAGNETIC PERMEABILITY OF COMPOSITES BY FLOW INDUCED ORIENTATION OF FERROMAGNETIC FIBERS. Thomas Fiske, Rahmi Yazici, Halit S. Gokturk, Dilhan M. Kalyon, Highly Filled Materials Institute, Stevens Institute of Technology, Hoboken, NJ.
The magnetic permeability of composites filled with ferromagnetic particles is influenced by magnetic quality of the magnetic filler, its concentration and particle shape and size. These microstructural variables are complex and severely hinder the application of various available theoretical approaches to link magnetic permeability of a composite to the content and various characteristics of the magnetic powders. Currently only experimental studies can provide the necessary guidance to proper selection of gating and mold design to tailor composites with targeted magnetic properties. Composite samples consisting of ferromagnetic short fibers incorporated into a polyolefin binder were injection molded using custom designed molds, which produced preferential fiber orientations. The degree of fiber orientation was significantly affected by the size of the opening (gate) to the mold, and by the mold geometry going from an edge-gated cylindrical to a center-gated disk cavity. Fiber orientation distributions of the injection-molded samples were analyzed by applying x-ray microradiography and digitized image analysis. The fiber orientation function (J) was determined by numerical integration of the orientation data. The magnetic properties of the composites were characterized in terms of their relative permeability values and related to their microstructure. Relative permeability value of the composite is increased when the fiber orientation and the applied field were parallel to one another. For instance, highly aligned composite samples exhibited up to 30% greater relative permeability values compared to those samples that exhibit fiber orientation distributions approaching a random distribution. To our knowledge this is the first study that provides data linking the fiber orientation distribution functions of ferromagnetic asymmetric particles to the relative magnetic permeability values of injection molded composites.
11:45 AM M1.10
MORPHOLOGY AND MECHANICAL PERFORMANCE OF POLYSTYRENE/POLYETHYLENE COMPOSITES PREPARED WITH SUPERCRITICAL CARBON DIOXIDE. Edward Kung, Thomas J. McCarthy, and Alan J. Lesser, Univ of Massachusetts, Dept of Polymer Science and Engineering, Amherst, MA.
Supercritical carbon dioxide provides us with a means to polymerize a vinyl monomer within the bulk of a solid substrate polymer and produce a composite with unique morphological and mechanical characteristics. Polyethylene substrates were soaked in a solution of styrene, -butyl peroxybenzoate, and supercritical carbon dioxide at conditions of 80C and 240 atm. After sufficient time for the penetrants to reach equilibrium sorption, the temperature was raised to 100C to polymerize the styrene. We have found that we can control the final composition of the resulting polystyrene/polyethylene composites by controlling the time the system is allowed to react at 100C. Differential scanning calorimetry and wide angle X-ray diffraction indicate that the crystalline domains of the polyethylene are unaffected by this process and that the polyethylene and polystyrene are immiscible. Since the polyethylene remains solid during this process, the composites produced possess kinetically trapped morphologies. Both stained and etched samples observed under a scanning electron microscope (SEM) show that the polystyrene resides largely at the center and in between crystalline lamellae of the polyethylene spherulites. This results in composites with enhanced moduli and tensile strength over polystyrene/polyethylene composites produced by conventional melt blending processes. However, the composites show a marked reduction in fracture toughness as polystyrene content increases. Macroscopically, one can observe a concomitant decrease in the size of the damage zone at the crack tips. SEM images of these regions show how the damage micromechanism changes and provide some insight into how these changes affect overall toughness.
SESSION M2: COMPOSITE TAILORED INTERFACES
Chairs: Lynn S. Penn and Richard P. Wool
Monday Afternoon, December 1, 1997
Independence Center (S)
1:30 PM *M2.1
SELF-ASSEMBLY BETWEEN POLYMER-COATED SURFACES. S.S. Chern, G.T. Pickett, E.B. Zhulina, C. Singh, and A.C. Balazs, Chemical and Petroleum Engineering Department, University of Pittsburgh, Pittsburgh, PA.
Using self-consistent field calculations and scaling theory, we investigate the self-assembly and interaction between two planar surfaces that are coated with: (1) polyelectrolytes and (2) neutral, multi-block copolymers. The chains are tethered by one end and grafted at relatively low densities. The polyelectrolytes are immersed in a poor solvent, while the multi-block copolymers contain both solvophobic and solvophilic segments. For both systems, we calculate the energy of interaction versus distance profile and find that the curves show multiple minima as the surfaces are compressed. For colloidal particles, this profile indicates that the colloids can form two (or more) stable crystal structures. Such systems could play an important role in optical devices, such as bistable switches.
2:00 PM *M2.2
TAILORING INTERFACES WITH TELECHELIC POLYMERS. Erika Eiser, Jacob Klein, Weizmann Institute of Science, Rehovot, ISRAEL; Lewis J. Fetters, Exxon Research and Engineering, Clinton, NJ.
Interfacial interactions in polymer matrix composites often determine the ultimate shear strength of the interfacial bonds and thus the limiting properties of the composites. Using a surface force balance with uniquely high resolution and sensitivity for probing interfacial shear, we have examined the way in which surface attached telechelic polymer chains modify adhesion and friction (telechelic chains bear functional groups at each of their ends). Such chains form surface structures that are a combination of loops and tails, and dramatically modify the behaviour relative to analogous monofunctional brushes, in particular their adhesive and frictional properties. A simple molecular model based on association of the end groups together with a shear-rate independent dissipative mechanism can account well for our observations.
2:30 PM *M2.3
MEASUREMENTS OF SURFACE DEFORMATIONS AND OTHER TIME-DEPENDENT SHAPE CHANGES OF TWO INTERACTING SURFACES. Manfred Heuberger and Jacob Israelachvili, Materials Dept, University of California, Santa Barbara, CA.
The optical (multiple beam interference) technique that is commonly used in SFA experiments to measure surface separations has been further developed to allow for direct visualization of more complex interfacial phenomena at the molecular level in real time, such as surface deformations occurring during adhesional bonding or frictional sliding and lateral phase separation such as the formation of capillary bridges or cavities, in thin films between two interacting surfaces. These new experimental developments will be briefly described and some interesting new results presented of experiments on the non-equilibrium (time-dependent) elastic and plastic deformations of initially smooth polymer surfaces undergoing adhesive bonding and sliding.
3:30 PM M2.4
TAILORED INTERFACES IN COMPOSITES FOR DENTAL APPLICATIONS. Shrirang V. Ranade, Center for Composite Materials, University of Delaware, Newark, DE; Anthony T. DiBendetto, Xiang-Qun Xie; Institute of Materials Science, University of Connecticut, Storrs, CT.
Continuous S-Glass fiber reinforced polycarbonate composites have been evaluated clinically for orthodontic applications. It was found that the stability of the fiber/matrix interfaces in the oral environment was essential for satisfactory performance. To achieve maximum hydrolytic stability, polycarbonate oligomer and bisphenol A were chemically grafted onto the glass fiber surface through use of a silicon tetrachloride intermediary. The interfacial shear strength, fracture toughness and hydrolytic stability of the resulting interphase was measured and compared to those of two commercial sizings and ozone cleaned surfaces. The grafted interphases exhibited improved stress transmissibility and toughness, particularly after 24 hours in boiling water. The tenacity of the tightly bound oligomers was confirmed via DRIFT, TGA and GC/MS experiments on soxhlet extracted fibers. The grafting reaction was modeled on a high surface area silica and studied using solid state NMR to determine reasons for the greater stability of the treated surfaces. Measurements of spin-lattice relaxation times indicate that the oligomers are chemically attached to the surfaces, providing for a well bonded, water resistant interphase. Parallel experiments on a monomeric Bisphenol A-primed silica surface provided evidence that chemical bonding was primarily responsible for the greater hydrolytic stability. Temperature dependent measurement of relaxation times show that the molecules lose mobility upon grafting and the transitional response of the oligomer is lost.
3:45 PM M2.5
TAILORING THE INTERFACE IN POLYETHYLENE FIBER/MATRIX COMPOSITES: SURFACE-ENTANGLED INTERFACIAL LAYER. Yachin Cohen, Dmitry M. Rein, Lev E. Vaikhanski, Technion - Israel Institute of Technology, Dept of Chemical Engineering, Haifa, ISRAEL.
The material properties of ultra-high molecular weight polyethylene (UHMWPE) and of high-performance fibers fabricated from it are unique among polymeric materials which exhibit high modulus and strength. However, utilization of UHMWPE fibers and matrix in a single composite suffers from several drawbacks, in particular due to poor adhesion between PE fibers and matrix. We investigate a physical method for improvement of the interfacial adhesion between UHMWPE fibers and matrix, based on formation of an interfacial UHMWPE layer which is physically entangled with the surface of the fiber and crystallized on it together with the matrix material. As a result a strong interfacial adhesion is achieved, which allows utilization of all-UHMWPE composites having unique properties. For example: a unidirectional composite sample, containing 70% UHMWPE fibers in a matrix of UHMWPE ( 3,000,000 Da) exhibits (at 20 deg C): shear strength 25 MPa; longitudinal tensile strength 1.5 GPa; longitudinal modulus (ultrasonic) 140 GPa; transversal tensile strength 23 MPa; longitudinal elongation 2 %; transversal elongation 70 %.
4:00 PM M2.6
MOLECULAR ORIENTATION EFFECTS OF TRANSCRYSTALLINITY IN POLYETHYLENE AND POLYAMIDE MATRIX COMPOSITES. Gad Marom, Theodor Stern and Nava Klein, Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, Jerusalem, ISRAEL; E. Wachtel, Faculty of Chemistry, The Weizmann Institute of Science, Rehovot, ISRAEL.
Many semicrystalline thermoplastic composites exhibit transcrystalline layers due to the fact that the surfaces of the reinforcing fibers provide ample nucleation sites which enhance the rates of nucleation and crystallization of the matrix. As a result, a thick and uniform transcrystalline layer grows directly on the fibre surface in those composites. The existence of the transcrystalline layer influences the fibre dominated longitudinal properties of the composite material to an extent which cannot be accounted for by a simple ``rule-of-mixtures''. The suggested explanation, based on a series of studies aiming to link the mechanical performance to the microstructure, attributes the effect of the transcrystalline layer to a preferred crystallite orientation relative to the fibre, thereby conferring to the matrix in the fibre direction higher rigidity and reduced thermal expansion, which in turn lower the residual thermal stresses. The properties of the transcrystalline layer, when compared with those of the matrix, reflect a higher degree of order, which would result from a more compact crystal packing and possibly from a preferred crystalline alignment. The crystalline alignment may be characterized by a particular distribution in orientation of the c-axes of the crystallites. On the molecular level, since the c-axis is parallel to the chain axis of the polymer, the physical and mechanical properties in this direction reflect the covalent nature of the polymer chain, while the properties in the perpendicular directions, along the a- and b-axes, reflect weaker intermolecular interactions (van-der-Waals and hydrogen-bonding). For both reasons, it is expected that the orientation distribution of the polymer chains in the transcrystalline layer will determine the nature and extent of its effect on the properties of the composite material. X-ray diffraction analysis performed on three types of transcrystalline layers in microcomposites revealed preferential orientation of the polymer chain with respect to the fibre axis, so that their c-axes are inclined at specific angles relative to the fibre axis. The specific orientation in each case is a weighted average determined by the crystal growth mechanism, which results in an orientation distribution, and geometrical factors, such as the thickness. Consequently, the effect on the mechanical properties will depend on the thickness and thereby on the fibre volume fraction.
4:15 PM M2.7
INFLUENCES OF THE INTERPHASE ON THE MECHANICAL PROPERTIES OF NYLON-66 COMPOSITES. Ronald G. Kander and Richard L. Clark, Jr., Materials Science & Engineering Department, Virginia Polytechnic Institute & State University, Blacksburg, VA.
The mechanical properties of glass fiber and carbon fiber reinforced nylon 66 were investigated using both microscopic and macroscopic testing techniques. The objective was to determine how different interphase morphologies affect composite properties such as fiber-matrix adhesion, damping, ultimate stress, failure strain, and modulus. This was accomplished using a modified fiber debond test on single filament composites, and dynamic mechanical analysis, vibrational adhesion testing, and uni-axial tensile testing on bulk composite samples. Additional techniques such as scanning electron microscopy, DSC, profilometry, TGA, and water absorption measurements were performed to assist in data interpretation.
The specific interphase that forms in both glass reinforced and high modulus carbon fiber reinforced nylon 66 is termed transcrystallinity. Previous work has shown that this region can be altered by the addition of a specific diluent, poly(vinyl pyrrolidone), as either a blend to the matrix or as a fiber sizing. The diluent serves to dampen nucleation on the fiber surface, thus causing the interphase to change from transcrystalline to spherulitic in nature. The changes in composite properties that the different interphases produce were examined. Results from the modified fiber debond test showed that the interfacial shear strength decreases as the interphase becomes more spherulitic. Scanning electron microscopy revealed a more cohesive fracture surface for the samples having a transcrystalline interphase. Dynamic mechanical analysis showed that the damping behavior of E-glass/nylon 66 composites does not change with PVP sizing, while carbon fiber/nylon 66 composites showed a decrease in damping with the addition of sizing. Vibrational adhesion testing showed similar effects in the loss tangent of both composites versus fiber sizing. In addition, uni-axial tensile testing revealed an increase in the ultimate strength and toughness of both composites. On the other hand, neither the ultimate strain or modulus was a strong function of fiber sizing. These results are explained in terms of the total energy absorption capability of the interphase in both composite systems.
4:30 PM M2.8
THERMOMECHANICAL MODELING OF POLYMER INTERPHASES. Stephen A. Fossey, US Army Natick RD&E Center, Science & Technology Directorate, Natick, MA; Sukant Tripathy, Univ Mass at Lowell, Center for Advanced Materials, Dept Chemistry, Lowell, MA.
Motivated by recent work suggesting that interphases may be responsible for the remarkable mechanical toughness of spider silk we have developed a theoretical framework for calculating the properties of polypeptide interphases. In the framework the interphases are modeled as thin layers using molecular dynamics and molecular mechanics. Systems are created using a rotational isomeric states approach and subjected to simulated annealing to reach an equilibrium structure. Thin layers, typically two nm to four nm, have been simulated as well as isotropic amorphous systems. The calculated equilibrium free volume of the thin layers is less than that calculated for the isotropic amorphous material consistent with the increases in density measured for oriented polymer fibers. For example for poly(Ala-Gly) the increase in density of the thin layers results in an increased tensile modulus for a 4 nm layer of 30% over the isotropic amorphous morphology. These results suggest that two phase models (crystalline and amorphous) are inadequate to predict the thermomechanical behavior of fibers and composites and comprehensive treatment of the interphase material must be considered.
4:45 PM M2.9
INTERCALATION OF POLYMER CHAINS BETWEEN NARROW INORGANIC LAYERS. Yulia Lyatskaya and Anna C. Balazs, Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA.
Intercalation of polymer chains was recently reported to be the main mechanism of the formation of a new class of nanocomposite materials comprising flat silicate layers and long polymer chains(1). The combination of large flat inorganic particles and flexible polymers lead to unique properties of a composite material, such as large increases in tensile strength, heat distortion temperature, and large decrease in gas permeability(2). We use an analytical self-consistent-field theory to analyze the morphology of the polymer/inorganic mixture and isolate the conditions under which polymer intercalates into inorganic galleries. In particular, we consider the role of the following factors on the morphology of the composite: 1)the molecular weight of the polymer, 2)the acidity of the polymer, 3)the affinity of the polymer to inorganic layers, 4)the solvent quality. We then analyze the dynamics of a polymer chains from the melt into narrow inorganic galleries. Dependencies of the diffusion coefficient on the molecular weight of the polymer are discussed. The findings are important for tailoring the properties of polymer/inorganic mixtures for manufacturing nanocomposite materials.
SESSION M3: NANOCOMPOSITES
Chairs: T. S. Chow and Bruce K. Fink
Tuesday Morning, December 2, 1997
Independence Center (S)
8:30 AM *M3.1
NANO-STRUCTURED INORGANIC-ORGANIC POLYMER MATRIX COMPOSITES. R.E. Taylor-Smith, F.C. Schilling, P. Wiltzius, Bell Laboratories, Lucent Technologies, Murray Hill, NJ.
Multicomponent polymeric systems enable the engineering of materials with novel physico-chemical characteristics. We are investigating organic polymer matrix systems with internal architectures designed to act as molecular reinforcements; such systems incorporate high surface/volume inorganic particulate inclusions dispersed within an organic superstructure. The generic synthetic methodology utilized involves generation of the inorganic phase from metal alkoxides or related organometallic compounds via sol-gel chemistry, grafting of a difunctional coupling agent bearing pendant metal alkoxide reactive functional groups in addition to defined polymerizable organic reactive functionalities, solvation in organic monomer with subsequent polymerization to generate a surrounding polymer matrix. In various studies of this synthetic approach, hydroxy-terminated monomers (-hydroxystyrene, 2-hydroxyethylmethacrylate etc.) proved most system-compatible as the matrix component, minimizing propensity for phase-segregation. Various analytical techniques provided insight into the inorganic particulate morphology, verifying formation of an inorganic nano-phase of sparse, open fibrous microstructure, with high surface/volume. Interfacial linking of the coupling agent was confined via nuclear magnetic resonance spectroscopy. The system inorganic:organic ratio was applied as the degree of freedom in this work and homogeneous, optically-clear materials were produced over a wide range of compositions. Controlled tuning of material composition permitted generation of composite materials with enhanced thermal, mechanical and dielectric properties, yielding credence to the premise that such multicomponent systems might successfully be designed for a range of active and passive applications.
9:00 AM M3.2
CONTROLLED MINERALIZATION IN DIBLOCK COPOLYMER MICELLES AND THE REGULAR ASSAMBLY OF METAL NANOPARTICLES IN THIN FILMS. Joachim P. Spatz, Stefan Mössmer, Martin Möller, University of Ulm, Organische Chemie III-Makromolekulare Chemie, Ulm, GERMANY.
Stabilization of nanoparticles by block copolymers does not only allow accurate control of particle size and inter particle distance, but also offers the possibility of producing thin optical transparent, catalytic active films and a new approach for lithography in the nanometer range. Diblock copolymers can form micelles in a solvent which is selective for one of the constituent blocks and organize in a microdomain structure in the bulk state. If the blocks which form the core of the micelle or the dispersed domains are polar and are able to interact with a transition metal compound, the latter can be selectively incorporated within the micelles or the domains. The thus formed nano compartments can serve as a locus for the mineralization of sterically stabilized inorganic crystallites or clusters. The particle size and interparticle distance of these crystallites or clusters can be controlled exclusively by the block copolymer. Problems which are encountered with this concept are the formation of several particles per compartment and the exchange of the inorganic species between the micelles causing loss of control of the particle formation. The mineralization can be subdivided into three stages: (i) reduction of the incorporated metal precursor, (ii) mineralization and (iii) exchange. Accurate control and separation of each of these stages was found to be essential to obtain particles identical in size and shape in each single micelle. Changing the thermodynamic environment and/or the macromolecular properties result in a coagulation of spherical block copolymer micelles to cylindrical micelles, i.e., the formation of micellar wires with an aspect ratio up to 10.000. This process of micelle coagulation is used for studying the percolation behavior of well defined metal particles in an organized polymer matrix.
9:15 AM M3.3
MOLECULAR COMPOSITES AND NANOCOMPOSITES OF RIGID-ROD AND FLEXIBLE-COIL POLYMERS. Samson A. Jenekhe, Departments of Chemical Engineering and Chemistry, University of Rochester, Rochester, NY.
Polymer molecular composites, in which a stiff-chain rodlike polymer is molecularly dispersed in the matrix of a flexible-coil polymer, have been of wide scientific and technological interest in the past 15 years. If achieved, molecular level reinforcement of a flexible-coil polymer matrix with a rigid-rod polymer would produce single-phase composite materials with vastly superior mechanical and thermal properties. Phase separation at the nano/micro/macro scale due to the thermodynamic incompatibility of rodlike and coillike polymers has been a major challenge to developing molecular composites. Many rod/coil homopolymer and block copolymer systems and processing approaches have been explored as routes to overcome phase separation. Recent work in our laboratory has developed novel and general methods for the preparation and characterization of diverse molecular composites and nanocomposites of rodlike and coillike polymers (1,2). Complexation mediated processing and coagulation of mixtures of rigid-rod polymers such as poly (p-phenylene benzobisthiazole), poly (p-phenylene benzobisoxazole), polyquinolines, and poly (benzimidazobenzophenantroline ladder) and coillike polymers such as aliphatic polyamides (nylons), poly (methyl methacrylate), and poly (benzimidazobenzophenantroline) from ternary solutions in organic solvents yield polymer composites with either molecular level or nanoscale dispersion. Steady state and time-resolved fluorescence techniques have been shown to be capable of differentiating between molecular level dispersion and nanophase separation where electron microscopy and x-ray diffraction could not resolve aggregation of rodllike polymers for sizes below 5-20 nm. In addition to structural applications, these rod/coil polymer composites are also of interest in optoelectronics and photonics.
9:30 AM M3.4
NANOPOROUS PARTICLES AS REINFORCEMENTS FOR THERMOSETTING POLYMERS. Jiazhong Luo 1, Robert R. Seghi2 and John J. Lannutti1, 1 Department of Materials Science & Engineering, 2 College of Dentistry, The Ohio State University, Columbus, OH.
Particulate reinforcement of polymer matrices is pervasive at all levels of technology. Given this, our recent discoveries concerning the control available through engineered levels of particle porosity are surprising. These composites are processed by simple infiltration by the desired monomers followed by either light- or heat-catalyzed polymerization. The type of polymerization had little effect on macroscopic properties. Tailored interfaces between porous ceramic particles and the polymer matrix caused the inherent toughness of the matrix to triple following only limited particle additions (<10 wt%). This is attributed to the influence of the interface on the fracture process. The corresponding fracture surfaces show that good interpenetration leads to clean fracture through the reinforcing particles. Poor interpenetration results in crack path selection around the reinforcement. This has substantial implications for fiber-reinforced and single-phase thermosets across a broad range of applications. The well-accepted practice of using coupling agents in this setting actually produces severe negative consequences. This is apparently due to a competition for the available porosity between the coupling agent and the polymer matrix.
10:15 AM M3.5
PROPERTY COMPARSION OF CLAY-REINFORCED AND NANO-INORGANIC CLUSTERS REINFORCED POLYMER MATRIX COMPOSITES. Andre Lee, Michigan State University, Dept. of Materials Science and Mechanics, E. Lansing, MI.
The Properties of hybrid organic-inorganic composite materials are greatly influenced by the length scale of the component phases. Nanoscale dispersion of the inorganic component typically optimizes the desired properties. In this study, we are interested in comparing properties of exfoliated clay reinforced polymers and polyhedral oligosilsesquioxane inorganic clusters reinforced polymers. In particular, the thermal properties such as the glass transition temperature and thermal expansion coefficient are investigated as function of weight fraction of inorganic phase. Mechanical properties such as tensile and shear modulus, strength are also compared. Furthermore, we are interested in the viscoelastic behavior of these hybrid organic-organic polymers, where dimensional stability and long-term structural performance of composite material can be examined using time-temperature, time-aging time superposition principles.
10:30 AM M3.6
MICROSPECTROSCOPY OF NANOSTRUCTURES. N. Afanasyeva, Department of Physics/220, University of Nevada Reno, Reno, NV.
Recently, new form of graphite-carbon needles, so-called 'nanotubules' have been studied by means of microspectroscopy methods in the near (NIR) to far (FIR) infrared (IR) region. Numerous phonon bands in the NIR, MIR and FIR regions have been observed. Three types of different nanotubes (powder, thin films and tubes from graphite matrix) with diameter of the order of 10 to 20 A and length of 10 A have been studied. The nanotubes on substrates were oriented. The observed MID-IR spectra of these samples have several distinct bands in the region 500-1200 1/cm. These bands are located at 1085-1100, 930-790,680 and 513 1/cm. The main difference between the spectra of large and small nanotubes is related to the presence of bands at 930 or 790 1/cm associated with nonplanar deformations of double bonds in cis-forms. The difference between large and small nanotubes is due to different confinement induced zone-folding graphite-like elements. Some of the series of overtones and combination modes have been observed in NIR (7000-4000 1/cm). Distinct bands have been registered also in the FAR-IR and even in the submillimeter regions (240-10 1/cm). The presence of these additional peaks can be associated with quantum size effects. Some of them can be assigned to surface phonons along these nanostructures. Furthermore micro-Raman spectra of the carbon-based materials and their new forms have been recorded. The materials were prepared under high pressure and temperatures leading to several superhard structures. From the spectra the level of disorder in such polymer forms of carbon and graphite was determined.
10:45 AM M3.7
STRUCTURE-PROPERTY ANALYSIS OF PLASMAPOLYMER METAL COMPOSITE FILMS. Andreas Heilmann, Jens Werner, Anne-Dorothea Mueller, Mike Gruner,Technical University Chemnitz-Zwickau, Institute of Physics, Chemnitz, GERMANY.
If metal nanoparticles of silver, gold or indium are embedded, e.g., in a plasmapolymer thin film matrix, their size and shape can be modified by thermally induced processes like reshapening, coalescence, as well as Ostwald ripening. These thermal induced processes can be caused by laser or electron beam irradiation. Especially by electron irradiation, it is possible to limit microstructural changes only at the irradiated sample part and to get artificially structured nanomaterials. The electron beam irradiation was done with a nanofocus electron source (10 keV) as well as with the electron source of the electron microscope (200 keV). Before and after electron irradiation the microstructure was observed by transmission electron microscopy (TEM) and analysed by optical image processing. The thermally induced microstructural changes result in changes of the electrical and optical properties. After annealing of silver nanoparticles, a blue shift of the spectral position of the optical plasma resonance absorption was found. At samples which have a microsstructure close to the percolation threshold, reshapening of silver particles was enforced by electromigration.
11:00 AM M3.8
BIOPOLYMER/METAL COMPOSITE FOR NON-LINEAR OPTICAL APPLICATIONS. Joseph D. Gresser, Charles M. Lyons, Edgardo J. Mantilla, Debra J. Trantolo, Cambridge Scientific, Inc., Boston, MA; , Donald L. Wise, Gemao Wong, Northeastern University, Department of Chemical Engineering, Boston, MA.
Oriented biopolymers possess larger nonlinear optical (i.e. ^(2)and(3)) susceptibilities. Development of the new NLO-active materials with optimized optical nonlinearity is under investigation. For this new NLO material a ^(3)-active TCVA(tricyanovinyl aniline)/silver sol is incorporated into a(2)-active polymeric host to develop a NLO-active polymer/metal composite (''polymet''). Aligning the composite structure in an electric field under microgravity conditions further optimizes the non-linear properties of the polymet. The homogeneity and distribution of the metal nanoparticles in the thin polymer films affect the NLO activity. This presentation will focus on the preparation of the polymer films and the sols; incorporation of the two to produce a polymer-metal adduct; and, preliminary studies into the non-linear optical behavior of the system.
11:15 AM M3.9
GROWTH OF POLY(HEXYL ISOCYANATE)CHAINS FROM GOLD NANOPARTICLES: A MODEL SYSTEM TO INVESTIGATE SURFACE INITIATED POLYMERIZATIONS. Dale L. Huber, and Thomas A. P. Seery, University of Connecticut Polymer Program, Storrs, CT.
Gold nanoparticles have been prepared with a titanium trichloro alkoxide functionality capable of inserting isocyanate monomers. Polymerization of hexyl isocyanate was conducted at ambient temperature under an inert atmoshpere. The resultant structure consists of a central core of gold surrounded by a covalently bound monolayer of hexyl isocyanate polymer. The reaction chemistry has been characterized by NMR and IR spectroscopies. The kinetics of the surface reaction have been probed by systematically varying reaction times and concentrations, and compared to the kinetics of the homogeneous polymerization. This system represents a model to investigate the broader issue of polymerizations initiated by the surface of a substrate. The application of this synthetic approach to planar and cylindrical surfaces will be considered.
11:30 AM M3.10
POLYPYRROLE COLLOIDS-ASSISTED PROTON TRANSPORT IN WATER SWOLLEN POLYACRYLIC ACID MATRIX. Liang Hong, Yujie Zhou, En-Tang Kang, National Univ of Singapore, Dept of Chemical Engineering, SINGAPORE.
Polypyrrole(PPy) colloidal particles were synthesized in an emulsion system using water-soluble poly(vinyl alcohol)(PVA) as a stabilizer. Initially, the dispersed phase of the emulsion consisted of either pyrrole or a solution of pyrrole in an organic solvent. An aqueous solution of the oxidant, ferric chloride, was then added to the emulsion to trigger off the oxidative polymerization of the pyrrole in tiny dispersed droplets. When pyrrole was dissolved in an organic solvent that possesses some solubility to low molecular weight PPy, the finally obtained PPy particles have lower densities than the rest. Most of PVA were removed from PPy particles after washed several times by water according to FT-IR analysis. Scanning electronic microscopic observation showed that the resulting PPy blacks were comprised of basically with sub-micrometer particles (colloidal particles). A remarkable feature of the PPy blacks is thus their feasibility to be dispersed in water, particularly with the assistance of ultrasonic treatment. Interactions between PPy colloidal particles and polyacrylic acid (PAA) were observed in aqueous medium via the study of its rheological behaviors: (i) Viscosity of a PAA (M = 0.75 million) solution reduced monotonically with increase of the content of the dispersed PPy colloidal particles in it. (ii) The shear-thinning effect of the PAA solution became appreciable in the presence of a small amount of PPy colloidal particles moving around in it. On the basis of this investigation, a new type of composite of PPy colloids and hydrophilic polymers was prepared, where PPy colloidal particles were distributed in a semi-interpenetracting network (semi-IPN) consisting of linear PAA (M = 2,000) in crosslinked poly(acrylamide - methylenebisacrylamide). The weight ratio of PPy in it was controlled in the range from 0.2% to 2.3%, and as a result of such low PPy content the composites are electrically non-conductive in dry state. However, they become conductive when swollen by water. There is a percolation threshold of water-content against the resistivity, the semi-IPNs containing PPy display a lower resitivity than does the blank semi-IPN. For example, as the water-content was increased from 14 wt% to 18 wt%, the resistivity of the semi-IPN embracing 0.8 wt% PPy reduced 10 times, while that of the blank one only 1.3 times. In conclusion, the phenomenon that PPy colloids enhance proton transport in water-swollen polymer matrixes could be utilized to improve the humility-sensing materilas as well as proton-conducting electrolytes.
11:45 AM M3.11
INTERACIAL EFFECTS OF THE Li+-ION TRANSPORT AND MECHANICAL PROPERTIES OF POLYMER/LAYERED INORGANIC SOLID NANOCOMPOSITES. Kecheng Gong, Younghua Zhang, Wen Zhang, Chengya Huang, Lan Liu, Polymer Structure & Modification Res. Lab. South China Univ. of Technology, Guangzhou, CHINA.
In this paper two types of novel polymer nanocomposites are: conductive polyaniline intercalated between the -MnO2 lattice layers, and the kaolin, montmorillonite layers are exfoliated and dispersed in a continuous polymer (PVC, PP, PE, SBR, NBR, BR, NR) matrix by melt mixing. Interfacial effects play a key role in the formation. Li-4-ion transport and mechanical properties of those materials. Polyaniline intercalated -MnO2 lattice composite cathode are prepared. Conductive polyaniline in the interlamellar space (distance between -MnO2 layers from 7.1 exfoliated to 15.4) increase the specific capacity twice as large as -MnO2 and improved Li-4-ion transport-rate-as-well as the Thermal Analysis, Impedance and Properties of battery. The few contents (0.25 1 wt PVC, 2 5 wt montmorillomite dispersed in polymers increase toughness, tear and tensile strength of NR-modified montmorillonite (5 wt twice or three times as large as that of NR-silica (5 wt composite, respectively. The impact strength of PP-organic kaolin or montmorillonite intercalated nanocomposites are greatly enhanced by several times. The structure and mechanism were studied by XRD, FTIR, SEM, TEM, DSC, and DMA, etc.
SESSION M4: FRACTALS AND PERCOLATION IN COMPOSITES
Chairs: Bruce K. Fink and Richard P. Wool
Tuesday Afternoon, December 2, 1997
Independence Center (S)
1:30 PM *M4.1
TIME PATTERNS IN ADVANCED COMPOSITE FABRICATION STEPS. T. Gutowski, Massachusetts Institute of Technology, Cambridge, MA.
Time estimation is an important part of manufacturing system design and cost estimating. although empirical data provides the basis for most time estimation methods, new processes, such as those under development for advanced composites, have only limited available data. Therefore the observation of repeating patterns with physical and/or probabilistic explanations could provide useful ``scaling laws'' for making estimates with limited data. Three topics will be addressed; 1) variation in existing process times, 2) Pareto charts and Zipf distributions for time steps, and 3) scaling laws for design and production variables. Three specific cases of scaling laws will be discussed for the effects of size, complexity, and learning. Finally, an example of time estimate ``without data'' will be given and compared to time estimates with data.
2:00 PM M4.2
FRACTAL PERMEATION CHARACTERISTICS OF PREFORMS USED IN LIQUID COMPOSITE MOLDING. Ranga Pitchumani and Bhaskar Ramakrishnan, Composites Processing Laboratory, University of Connecticut, Department of Mechanical Engineering, Storrs, CT.
Determination of the permeabilities of preforms is critical for an accurate analysis of mold filling during liquid composite molding processes. The complex labyrinth of the preform pore structures, however, presents a major challenge to a quantitative description of the microstructures, and conseqently, the evaluation of their permeabilities. Towards addressing this problem, a fundamental description of the disordered preform pore structures using fractal techniques is presented. A fractal permeation model is developed which relates the preform permeabilities to the actual microstructures in terms of two fractal dimensions--one relating the size of the capillary flow pathways to their population, and the other describing the tortuosity of the capillary pathways. Excellent agreement is demonstrated between the analytical model predictions and the experimental measurements on the permeabilities, for a wide range of process parameters. The model development, experimental studies and model validation are presented and discussed.
2:15 PM *M4.3
DYNAMIC PROBES OF MECHANICAL PROCESSES AT POLYMER INTERFACES. J. Thomas Dickinson, Washington State University, Department of Physics, Pullman, WA.
When ceramic-, glass-, or metal-polymer interfaces are under stress, deformation and physical separation (debonding) of the surfaces are possible events. In addition, interfaces under stress are also more vulnerable to attack by fluids such as water or corrosive chemicals. In this talk we present recently developed instrumentation and measurements designed to dynamically probe detachment of polymers from both conductors and inorganic insulators to examine in a time resolved fashion the microscopic events and kinetics of detachment and failure for polymer interfaces (a) that are mechanically strained, (b) under exposure to environmental agents, and (c) under tribological loading (sliding interfaces in shear). In many cases, the dynamics of these events can be detected even for buried interfaces. We also show experiments involving an AFM fitted with a conducting tip to probe interfacial debonding with spatial resolution of dimensions << 100 nm. This work supported in part by the National Science Foundation, Grant CMS-9414405.
3:00 PM M4.4
A PERCOLATION EXPLANATION OF THE TEMPERATURE DEPENDENT RESISTIVITY IN DISORDERED CARBON-BLACK/POLYMER COMPOSITES. Michael B. Heaney and Heidi Pan, Research and Development Division, Raychem Corporation, Menlo Park, CA.
We have measured the temperature dependence of the resistivity of a series of disordered carbon-blackñpolymer composites. The data can be explained by percolation scaling laws with universal exponents. The temperature dependence of the percolation parameters suggests the changes in resistivity are caused by breaking of some conductive pathways, and not increases in the resistivity of every pathway.
3:15 PM M4.5
MORPHOLOGICAL AND DIFFUSIVE PROBE ANALYSIS OF POLY(VINYLIDENEFLUORIDE)/GRAPHITE COMPOSITES. Deanna Busick, C. Maurice Balik, Richard Spontak, North Carolina State Univ, Dept of Materials Science & Engineering, Raleigh, NC.
Polymer matrix composites containing graphite are of tremendous interest for a wide variety of commercial applications. In this work, we examine the morphological characteristics and interfacial properties of poly(vinylidenefluoride) (PVDF) films containing up to 70 wt% graphite. The percolation threshold in these materials is identified using electrical resistivity measurements, as well as environmental scanning electron microscopy. Electron micrographs of cryomicrotomed composite surfaces are analyzed by a procedure that extends established stereological principles. Isothermal gravimetric sorption of carbon dioxide into thin composite sheets yields the diffusion coefficient and solubility of carbon dioxide in the composites, as well as the fraction of voids within each composite. Correlation of these quantities with graphite concentration reveals that the voids are not percolated (even though the graphite particles are), most likely due to incomplete particulate wetting. These results are compared with heats of PVDF fusion obtained from differential scanning calorimetry to ascertain the extent of transport-controlling heterogeneous nucleation inherent in these composites.
3:30 PM M4.6
MICROSCOPIC INTERACTIONS IN NANOCOMPOSITES, T.S. Chow, Xerox Corporation, Wilson Center for Research and Technology, Webster, NY.
A nanocomposite model is presented to describe the effect of microscopic interactions on the macroscopic flow and deformation of composite materials. The materials are filled with particles ranging from nanometers to submicrons. We shall demonstrate the breakdown of the traditional composite theories, which are based on the continuum mechanics, and point out the importance of microscopic interactions. The short range molecular interactions within the microstructure are analyzed. Our analysis reveals that the macroscopic behavior of nanocomposites undergoes a percolation transition as the particle volume fraction approaches a critical value. The percolation threshold is determined as a function of the repulsive interparticle potential, particle size, shape, and orientation. This critical phenomenon cannot be explained by the traditional composite theories, which underestimate the concentration dependence. The theoretical calculations based on our nanocomposite model are in good agreement with experimental data of composite materials at the microscopic scale.
SESSION M5: POSTER SESSION
Tuesday Evening, December 2, 1997
Grand Ballroom (S)
ON THE PERCOLATION ESTIMATION OF THE CONDUCTIVITY AND ELASTIC MODULUS OF A FILLED POLYMER. Yu.N. Kryuchkov, Institute for Problems in Materials Science, NASU, Kiev, UKRAINE.
The study of the dependence of the conductivity and the elastic modulus of filled compositions on the concentration, ratio of particle's size and packing density of fillers has been made. The data obtained is compared with the theoretical values of the conductivity and the moduli of filled systems. Taking into account the maximum packing density of filler allows to predict correctly the dependence of the compression modulus of compositions versus content and size of filler particles.
A MICROSCOPIC TO MACROSCOPIC VECTOR PERCOLATION MODEL OF FRACTURE IN POLYMER COMPOSITE MATRICES. Richard P. Wool, Center for Composite Materials and Department of Chemical Engineering, University of Delaware, Newark DE.
Fracture in polymer composite matrices is analyzed in terms of vector percolation concepts, which connects microscopic events to macroscopic fracture energies J1c. The entanglement and crosslinked networks in semicrytsalline, amorphous and thermosetting polymers are sloppy and non uniform such that applied macroscopic stresses produce highly stressed Hot Bonds which we observe by infrared and Raman spectroscopy. The hot bonds break at molecular stesses ca 10 GPa while the macroscopic stresses are 50-100 MPa. The initial stored energy U [P - Pc]v, is dissipated by the randomly distributed hot bonds, each consuming a known energy Uhb. At the vector percolation threshold Pc, a precice number of hot bonds are broken in the network, connectivity between the broken percolation cluster occurs and using the Embedded Zone Process fracture mechanics model (EPZ), the deformation zone at the crack tip fractures. Knowing how much strain energy needs to be stored to fracture the Pc number of hotbonds of a specific energy, we can solve for the fracture stress and fracture energy exactly in terms of known material molecular parameters. The Vector Percolation model has application to durability, mechanochemically controlled lifetimes of polymer composites, molecular weight effects, use of coupling agents at tailored polymer-polymer and polymer-solid interfaces, and fatigue.
ELECTROMAGNETIC FUSION BONDING OF THERMOPLASTIC COMPOSITES USING OPTIMIZED MESH SUSCEPTORS. S. Yarlagadda, Center for Composite Materials, J.W. Gillespie Jr., Materials Science Program, University of Delaware, Newark, DE; B.K. Fink, Army Research Laboratory, Aberdeen Proving Ground, MD.
A numerical model has been developed to estimate bond strengths of induction heated thermoplastics, using optimized metal mesh susceptors. By designing cut patterns, the susceptors were optimized for uniform temperature distributions in the bondline. Temperature distributions in the bondline and through the thickness were calculated, using coil, heat generation and finite element models, for various mesh geometries and cut patterns. Using temperature and pressure profiles, healing and intimate contact models were used to predict bond strengths. Lap-shear specimens were fabricated and tested to compare with predicted bond strengths.
NEW POLYMER CERAMIC COMPOSITE FOR HIGH ENERGY DENSITY CAPACITORS. Kristen Law, Yvonne Spooner, Kirk Slenes, TPL, Inc., Albuquerque, NM.
A newly synthesized high energy density polysulfone-barium titanate composite dielectric material is described. The new composite material achieves increases in performance through two processing methods: the in situ hydrolysis of metal-organic precursors to form monodisperse high dielectric constant ceramic particles of sub-micron scale; and the synthesis of high polarity pendant groups onto the polymer backbone. In situ ceramic formation allows for tailoring of the ceramic particle size while eliminating difficulties involved in dispersing high surface area powders. The uniform dispersion of ceramic has allowed for an 80% increase in the dielectric constant of the composite without the traditional loss in breakdown strength associated with the incorporation of inorganic into polymer. Unique coupling of the organic precursor to the polymer backbone has actually increased the DC breakdown strength of the composite. Synthesis of 25 % of the polysulfone monomer sites with high polarity pendant groups has also allowed for increases in both the dielectric constant and breakdown strength to yield energy density increases of more than 2-fold over base polysulfone polymer.
OF SOLVENT EFFECTS ON ELECTRODEPOSITION OF POLYTHIOPHENE THIN FILMS BY ATOMIC FORCE MICROSCOPY. Terence Kin Shun Wong, Shubo Gao, Photonics laboratory, School of EEE, Nanyang Technological University, Singapore, SINGAPORE; Xiao Hu, Hongmei Liu, Polymer laboratory, School of Applied Science, Nanyang Technological University, Singapore, SINGAPORE.
Polythiophene(PT) is a novel type of conjugated polymers which can combine the optical and electronic properties of inorganic semiconductors with the processing and mechanical properties of conventional plastics. In this paper, solvent effects on the electrodeposition of PT thin films based on indium tin oxide(ITO) glasses have been studied by atomic force microscope(AFM). The relationship between morphology and mechanical property will be addressed. The electrodeposition of PT was carried out in single compartment cell with three electrodes connected to a potentiostat. The working electrode is a ITO glass. A high pure platinum plate and a silver wire(99.99 electrical resistance of a 32 composite during 0 tension and compression. The number of fiber-fiber contacts was found to decrease by 0.7 (resistance increasing) and increase by 1.1 -0.5% strain (resistance decreasing), such that this number deceased with increasing strain and the resistance varied linearly with strain (fractional resistance change per unit strain=2), until damage (probably matrix cracking) occurred and caused the resistance to increase with compressive strain beyond 1 increase abruptly with tensile strain beyond 0.5 Prior to damage, the resistance varied with strain reversibly; slight reversibility was due to plastic deformation of the matrix.
POLYMER-SOLID COMPOSITE INTERFACES: INFLUENCE OF STICKER GROUPS ON INTERFACIAL BONDING. Liezhong Gong and Richard P. Wool, Univ. of Delaware, Dept. of Chemical Engineering, Newark, DE.
Adhesion between polymers and solid substrates is an issue of fundamental importance since it controls the performance of polymer-solid composites. The addition of sticker groups to polymer chains is an efficient strategy and has been widely used to improve adhesion. Generally, introduction of sticker groups could (1) improve interfacial bonding, (2) perturb chain connectivity, and (3) modify chain dynamics. However, the details of the influence of sticker groups is not yet well understood. The key parameters to control the adhesion properties of polymer-solid interfaces are the areal chain density, chain length, concentration of the sticker groups and the type of sticker groups, as well as the chain architectures. In this study, the influence of these molecular details were explored. Computer modeling was used to predict how chain architectures, the type of sticker groups and the concentration of the sticker groups modify chain structure at interfaces. Peeling tests on a model polymer-solid interface, carboxylated polybutadiene - aluminum, were conducted to examine how the fracture energy is related to interfacial structure.
DIELECTRIC AND PYROELECTRIC PROPERTIES OF P(VDF-TrFE) AND PCLT/P(VDF-TrFE) 0-3 NANOCOMPOSITE FILMS. Q.Q. Zhang, H.L.W. Chan, and C.L. Choy, Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hung Hom, Kowloon, HONG KONG.
Calcium and lanthanum modified lead titanate (PCLT) nanocrystalline powder has been derived from the sol-gel process. The PCLT powder was incorporated in a vinylidene fluoride-trifluoroethylene [P(VDF-TrFE)] 70/30 mol % to form 0-3 nanocomposites. PCLT/P(VDF-TrFE) 0-3 nanocomposite films for IR sensor application have been fabricated by solution spin-casting. Pyroelectric and dielectric properties of the P(VDF-TrFE) copolymer and nanocomposite have been measured and the results are discussed.
SESSION M6: COMPOSITE FRACTURE & DURABILITY
Chairs: Jan-Anders E. Manson and Richard P. Wool
Wednesday Morning, December 3, 1997
Independence Center (S)
8:30 AM *M6.1
INTERFACE STRENGTH, WORK OF ADHESION AND PLASTICITY IN THE PEEL TEST. Yueguang Wei and John W. Hutchinson, Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA.
A cohesive zone model is proposed and analysed for steady-state peeling of a thin rate-independent, elastic-plastic film bonded to an elastic substrate. A traction-separation description of the interface is embedded within continuum characterizations of the film and substrate. The primary parameters characterizing the traction-separation relation are the work of adhesion and the peak separation stress, termed the interface strength. The objective of the study is the determination of the relationship of the peel force to the work of adhesion of the interface and its strength, with due regard to plastic deformation in the film.
9:00 AM M6.2
MOISTURE-ASSISTED CRACK GROWTH AT EPOXY/GLASS INTERFACES. J.E. Ritter, J.R. Fox, D.I. Hutko, and T.J. Lardner, University of Massachusetts, Department of Mechanical and Industrial Engineering, Amherst, MA.
The adhesion between polymer and glass is of critical importance in determining the toughness and moisture resistance of fiber glass reinforced plastic composites. The Double Cleavage Drilled Compression (DCDC) test was used to measure the critical energy release rate, moisture-assisted crack growth, and fatigue threshold for epoxy/glass interfaces treated with and without a silane coupling agent. The DCDC specimen consists of two glass beams (either soda-lime or fused silica) bonded together with an epoxy adhesive. A through-the-width hole is drilled in the center of the specimen. In the DCDC test, compressive loading causes tensile stresses to develop at the poles of the drilled hole. Cracks then nucleate in the epoxy/glass interface, extend from the poles and propagate axially in primarily mode I loading. The resistance to moisture-assisted crack growth at untreated epoxy/glass interfaces is significantly less than in monolithic glass specimens. However, the resistance to moisture-assisted crack growth at silane coupled epoxy/glass interfaces can be comparable to or greater than that in monolithic glass. Silane coupling of epoxy to glass is more effective with fused silica than soda-lime glass with the fatigue limit of silane coupled epoxy/fused silica interfaces being about 2.5 times greater than that for silane coupled epoxy/soda-lime glass. These results are discussed in terms of possible interfacial crack growth mechanisms.
9:15 AM M6.3
CRACK WAKE EFFECTS ON RIGID PHASE TOUGHENING OF POLYMERS. Shanti V. Nair, Dept of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA; and Lloyd A. Goettler, Nylon Tech Center, Monsanto Company, Cantonnment, FL.
Nylon 6,6 and SAN were reinforced with both rigid and non-rigid phases, individually and in combination. The non-rigid phase was a rubbery phase, while the rigid phases were both a rigid polymer phase as well as chopped glass fiber reinforcements. The fracture behavior was characterized not only by standard fracture initiation toughness measurements by also in terms of the fracture propagation toughness when stable crack extension occurs. The latter provides a clear quantitative measure of toughness contributions from the wake of the propagating crack. A significant portion of polymer toughening in toughened polymers was found to arise from the wake of the crack as a result of deviatoric plasticity associated with rigid and non-rigid phases. In particular, crack wake toughening due to rigid phases depended on rigid phase morphology, rigid phase/matrix interface and the presence or absence of a non-rigid phase.
9:30 AM M6.4
A HIERARCHICAL INVESTIGATION INTO THE YIELD RESPONSE OF GLASSY THERMOSETS SUBJECTED TO MULTIAXIAL STRESS STATES. Robert S. Kody, Alan J. Lesser, Univ of Massachusetts, Polymer Science and Engineering Dept, Amherst, MA.
Many applications, in which glassy thermosets are used today, demand that the resin survive multi-axial stresses and severe environments. Also, due to the wide array of resins, curing agents, and modifiers available today, it is important to understand how molecular architecture and morphology affect the yield behavior of glassy thermosets. Along these lines, this paper reports the results of a hierarchical investigation into the origins of yield on a macroscopic, microscopic, and molecular level. We consider the effect of hydrostatic stress, on both a macroscopic and microscopic level, and relate macroscopic behavior to molecular interactions. Macroscopic yield studies are conducted on pressurized thin walled hollow cylinders which are tested in stress states ranging from uniaxial compression to biaxial tension as a function of strain rate, temperature and molecular architecture. The molecular architecture of an epoxy based thermoset is controlled by altering the molecular weight between crosslinks and changing the curing agents stiffness (e.g. aliphatic to aromatic). We propose a generalized yield model, based on a thermally activated process applied to a modified von Mises yield criterion. The model can predict the effects of both testing conditions and molecular parameters. On a microscopic level, rubber toughened epoxies are tested biaxially in combination with dilatometry measurements and SEM to investigate inelastic void growth after particle cavitation. Finally we study the molecular motions associated with sample dilation and distortion, and discuss yield on the basis of intermolecular interactions and conformational changes.
9:45 AM M6.5
TOUGHENING GLASS FIBER REINFORCED EPOXY LAMINATES WITH HYDROXYL-TERMINATED POLY(BUTADIENE-CO-ACRYLONITRILE). S. Samajdar, Center for Materials Science and Department of Technology Systems, Bowling Green State University, Bowling Green, OH.
Composite laminates used in high performance applications, such as aerospace structural components, are often fabricated with toughened plastics matrix for improved damage resistance. Such matrices are usually multiphase polymers. Incorporating an elastomeric phase in the microstructure elevates the interlaminar fracture energy of thermosets such as epoxy. Carboxyl-terminated poly(butadiene-co-acrylonitrile), commonly abbreviated as CTBN, has so far been the dominant modifier used for this purpose. However, another closely related elastomer, namely hydroxyl-terminated poly(butadiene-co-acrylonitrile), in short HTBN, has remained conspicuously unexplored as a toughening agent for plastics and composites. This work examines the fracture behavior of glass fiber reinforced epoxy laminates with and without HTBN-modification of the matrix. End notched flexure (ENF) specimens were employed to determine toughness (GII) in interlaminar mode II fracture, while single edge notched (SEN) specimens were tested in tension to measure toughness (JI) in multilaminar mode I fracture. Results show that only 35 to 3.61 kJ/m2, and the associated nonlinearity factor by 33 kJ/m^2. The interlaminar toughness results exhibit marked strain rate sensitivity. In case of specimens with4 three-point bending tests carried out at a crosshead speed of 0.5mm/s yields a toughness value of 4.69 kJ/m2, compared with 3.61 kJ/m2 at a speed of 0.05mm/s. Experimental data also indicate that although HTBN-modification aggravates the reduction in interlaminar fracture toughness under hygrothermal attack, it virtually arrests such deterioration in multilaminar mode I fracture. Scanning electron microscopic (SEM) observations reveal corroborative evidence of micromechanisms that provide plausible explanations for the macroscopic behavior of the specimens. In conclusion, the efficacy of HTBN, vis-à-vis CTBN, as a toughening agent for reinforced epoxy laminates is critically reviewed.
10:30 AM *M6.6
MACROFRAGMENTATION AND MICROFRAGMENTATION PHENOMENA IN POLYMER-BASED COMPOSITES. H. Daniel Wagner, Oleg Lourie, The Weizmann Institute of Science, Dept of Materials and Interfaces, Rehovot, ISRAEL.
The single-fiber fragmentation phenomenon has been used for many years, mainly to study the mechanical stress transfer efficiency of fiber-matrix interfaces. In the present paper we review the fragmentation phenomenon and demonstrate that it may be used in a variety of ways, using tensile and compressive stresses, macroscopic and microscopic material phases, mechanical and thermal stresses. In particular we focus on two novel and interesting aspects, as follows. First, to demonstrate the role of thermal residual stresses and of fiber pre-loading, an experimental exercise is performed using a microcomposite specimen that comprises two identical single fibers within a thin polymeric film. The fibers are embedded parallel to each other, at a distance sufficiently large for fiber-fiber interactions or perturbations to be negligible. One of the fibers is pre-loaded at its ends prior to (and during) matrix polymerization, whereas the other fiber experiences only typical thermal residual stresses due to specimen preparation. A continuously monitored tensile fragmentation test is performed, and the results are discussed using a new energy balance approach. The fiber fragmentation patterns observed in both fibers, as well as the calculated interface energies from both fibers, are found to differ significantly. These observations and conclusions give rise to fundamental questions and possible solutions regarding the meaning, interpretation, power, and accuracy of the fragmentation test. Second, we report here the observation of the fragmentation (and other fracture modes) of single carbon nanotubes embedded within a polymeric film. Experimental data of this type are presented for the first time, and we discuss the mechanical properties of single and multi-wall carbon nanotubes that may be inferred from such experiments.
11:00 AM M6.7
FUNDAMENTAL PARAMETERS THAT CONTROL THE DURABILITY OF HIGH TEMPERATURE POLYMER MATRIX COMPOSITES*. Roger J. Morgan, E. Eugene Shin, Jason E. Lincoln, Jiming Zhou, Michigan State University, Advanced Materials Engineering Experiment Station, Midland, MI; Jin Choi, Andre Lee, Michigan State University, Composite Materials & Structures Center, E. Lansing, MI; David Curliss, Air Force Materials Laboratory, Dayton, OH.
Present and future polymer-matrix fibrous composite aerospace applications, for bismaleimide(BMI) and polyimide (PI)-carbon fiber composites, involve combined complex synergistic stress, thermal, moisture, time, radiation and oxygen service environment-induced degradation mechanisms of composite performance. In this paper we report and characterize the critical aging mechanisms that control damage initiation in BMI and PI carbon fiber composites in aerospace environments.
For BMI-carbon fiber composites systematic Fourier Transform infrared spectroscopy, FTIR, and differential scanning calorimetry, DSC studies reveal that these resins are not fully cured and can continue to react in service environment as a result of formation of dehydration induced ether crosslinks. This further crosslinking causes Tg increases, mechanical property decreases and further resin shrinkage with associated microcrack development. The relations between network structure and BMI thermal and mechanical properties are discussed.
For PI-C fiber composites moisture-time-temperature-stress phase diagrams of the physical and chemical structural states of PI matrices and their composites are discussed in terms of unacceptable damage thresholds for specific future aerospace service environments. The PI physical structural state is described in terms of elastic and inelastic cavitation, glassy-state free volume, and the structural molecular environment of water molecules within the polyimide. The PI chemical structure is described in terms of depolymerization of imide rings via hydrolysis to amide groups and ultimately hydrolytic chain scission of the amide groups.
11:15 AM M6.8
STUDIES OF THE FAILURE MECHANISMS OF POLYMER-BONDED EXPLOSIVES BY HIGH RESOLUTION MOIRE INTERFEROMETRY AND ENVIRONMENTAL SCANNING ELECTRON MICROSCOPY. H.T. Goldrein, S.J.P. Palmer, P.J. Rae, Cambridge University, Cavendish Laboratory, Cambridge, UNITED KINGDOM.
Polymer bonded explosives (PBXs) are highly filled polymer composites, with a typical solids loading of 95^-4 s^-1m, and a displacement sensitivity approaching 10 nm. The optical arrangement in our experiment allows direct correlation of the measured displacement fields with features in the composite microstructure. This is made possible by the acquisition of white-light microstructural images together with laser interferograms from which the displacements are calculated. The second technique uses Environmental Scanning Electron Microscopy (ESEM) which is ideally suited to the study of slow crack growth in insulating PBX samples. By allowing a small amount of gas into the specimen chamber, charging of the sample from the electron beam is neutralized. This allows freshly exposed uncoated fracture surfaces to be viewed directly. It is thus possible to watch in real time, with a spatial resolution of approximately 0.1 hydroxy-terminated polybutadiene binder (HTPB). The techniques described, however, are widely applicable to many other composite systems.
11:30 AM M6.9
FATIGUE DAMAGE AND DEGRADATION IN GLASS FIBRE REINFORCED COMPOSITES. David H.Isaac, Kirsten Dyer, Department of Materials Engineering, University of Wales Swansea, Swansea, UK.
The fatigue performance of a range of continuous glass fibre reinforced layups in two different resins has been determined. In particular, S-N curves have been generated to compare a standard polyester resin with a polyurethane-vinyl-ester resin designed to have an increased toughness. The study included a woven roving cloth with [90,0]2s layup and stitch-bonded cloths with three different layups: [90,0]2s, [90,0,+45,-45,+45,-45,0,90] and [+45,-45]4. All the fatigue tests were carried out in tension at 1Hz and with an R-value of 0.1. Also, damage accumulation during fatigue testing was measured using a slightly waisted sample geometry, with an extensometer across the central region, so that reduction in modulus as fatigue progressed could be determined. Light microscopy and scanning electron microscopy of samples at various fractions of their predicted lifetimes and of fracture surfaces were used to assess the development of damage and the failure mechanisms. As expected, S-N graphs indicated that layups containing angle ply fibres had lower fatigue strength, and showed up the differences between the matrix materials more clearly. The higher toughness polyurethane-vinyl-ester exhibited superior fatigue properties. On the other hand, in the [90,0] layups, the dominating effect of fibre performance in determining fatigue behaviour was evident, particularly at higher stress levels (lower lifetimes). Comparing matrix materials, there were larger differences in fatigue strength at lower stress levels. This is consistent with changing damage micromechanisms, so that at high stresses fibre failure was more significant, whereas at high lifetimes matrix cracking had more time to develop and affect the performance. Graphs of damage accumulation showed that the modulus decreased in either two or three stages. Samples containing 0 and 90 fibres exhibited three stages of modulus decay, whereas the modulus of specimens with only 45 fibres decreased in just two stages, although the shorter lifetimes of these latter samples made the correlation inconclusive. Similar fatigue damage mechanisms, including matrix cracking, longitudinal splitting, delamination and fibre fracture appeared across the range of materials, although their relative contributions varied.
11:45 AM M6.10
EVALUATION OF THE FIBER-MATRIX INTERFACE AT HIGH RATES. L. S. Penn, D. S. Kalika, M. J. Greenfield, A. Pedicini, Department of Chemical and Materials Engineering, Lexington, KY.
We report a high-rate test for the interface in filamentary composites. Such a test is needed because of the increasing interest in crashworthy composites, where energy absorption during the failure process is a key factor. The test is a modified single fiber fragmentation test, with specific requirements for success at high rates. Results on the ability of the test to provide interfacial information for several different fiber-matrix systems will be presented. High rate test data will be compared with low rate test data.
SESSION M7: COMPOSITE INTERFACE ADHESION
Chairs: Andre Y. Lee and Lynn S. Penn
Wednesday Afternoon, December 3, 1997
Independence Center (S)
1:30 PM *M7.1
INTERPHASE ANALYSIS. C. Jones.
Abstract not available
2:00 PM M7.2
EVALUATION OF THE CUMULATIVE STRESS TRANSFER FUNCTION AS AN ADHESION PARAMETER, USING FIBRES WITH PLASMA POLYMERISED COATINGS OF SPECIFIC CHEMICAL FUNCTIONALITY. F.R. Jones, A.P. Kettle, N. Lopattananon, A.J. Beck and R.D. Sort, University of Sheffield, Department of Engineering Materials, Sheffield, UNITED KINGDOM.
The mechanism of adhesion of carbon fibers to epoxy and related resins results from a complex interaction of chemical functionality, microposity and active sites at the fibre/matrix interface. This paper reports the controlled functionalisation of untreated Type A carbon fibers by plasma polymerisation for the evaluation of the Cumulative Stress Transfer Function (or CSTF) as a measure of adhesion. The degrees of adhesion has been estimated using the plasticity effect model of Tripanthi and Jones (J. Comp. Materials 1996,30 1514) to calculate the CSFT from individual values of debond and fragment lengths in a continuously monitored fragmentation test. The methodology incorporates the yield strength of the matrix and therefore has the potential for being generally valid. Since the microporosity and chemical functionality of the untreated fibre surface can be concealed, any functionality incorporated into the film can be considered to provide the principal adhesion mechanism. The radio frequence-induced plasma copolymerization of acrylic acid/hexane, allyl alcohol/hexane and allylamine/octadiene gas mixtures is used to obtain a rang of functionalised coatings on Type A carbon fibre surfaces. Analysis and quantification of the surface groups was determined by means of XPS. Coatings prepared by the homopolymerisation of hexane and that of octadiene are strongly hydrocarbon in nature and inhibit chemical interaction between the fibre and matrix resulting in poorer adhesion than the parent untreated fibers. The introduction of commoners; acrylic acid, allyl alcohol and allyl amine individually to the plasma feed resulted in increased levels of specific fibre surface functionalities and improvements in the level of adhesion. The CSTF value which is calculated from the tensile stress profile for each fragment, duly modified to account for debonding and matrix yielding, has been shown to be much more effective in differentiating between the efficacy of differing functional groups in forming a strong adhesive bond to epoxy resins. Thus carboxylic acid and amine groups are more effective than hydroxyl groups, which in agreement with their recognized reactivity towards epoxides, and the probable formation of interfacial covalent bonds.
2:15 PM M7.3
STRESS-INDUCED DEGRADATION OF INTERFACE STRUCTURE IN EPOXY FILMS ON SILICON SUBSTRATES: AN IN-SITU STUDY BY NEUTRON REFLECTION. Michael Kent, Frere McNamara, Sandia National Labs., Albuquerque, NM; Jaroslaw Majewski, Greg S. Smith, Los Alamos National Labs, Los Alamos, NM.
Neutron reflection is used to detect and characterize (in-situ) the degradation of interface structure in model epoxy/silicon samples subjected to a cyclic stress. A cyclic in-plane stress (maximum of roughly 30 MPa) is achieved in the 2 micron epoxy films (Tg 70 C) by thermal cycling from 40 C to -65 C. Samples are examined both in the dry state, and after swelling with a good solvent. The experiment yields the interfacial density profile for the dry samples, and the interfacial composition profile of the swelling solvent for the swelled samples. The latter is sensitive to the local crosslink density of the network. Results have indicated a very strong dependence upon i) the strength of the interaction between the epoxy and the substrate surface, and ii) the temperature at which the sample is cured. The as-prepared interface strength of the epoxy/silicon wafer samples (with native oxide) is very strong. A weak epoxy/substrate interaction strength is achieved by coating the silicon wafer with octadecyltrichlorosilane (ODTS) prior to application of the epoxy. The structure of the epoxy in the interface region is reported for both weak and strong interface samples at various stages of thermal cycling.
2:30 PM M7.4
SURFACE-DRIVEN CHEMICAL SEGREGATION AT SIMULATED GLASS/EPOXY INTERFACES. N. C. Beck Tan, S. H. McKnight, Materials Division, US Army Research Laboratory, Aberdeen Proving Grounds, MD; G. R. Palmese, Center for Composite Materials, University of Delaware, Newark, DE; W. L. Wu, Polymers Division, S. Satija, Reactor Radiation Division, National Institute of Standards and Technology, Gaithersburg, MD.
Chemical and physical interactions between a solid surface and a polymer often lead to the formation of an interfacial layer of material adjacent to the surface possessesing properties which are significantly different from those of the solid and those of the bulk polymer. The chemical and structural characteristics of such an interfacial region at the fiber/matrix boundary in advanced polymer composites are often crucial in determining the macroscopic properties and performance of these materials. Previous computational studies have suggested that chemical segregation in epoxy precursor mixtures may be induced by the interaction between the components of the mixture (amine crosslinker plus epoxide) and the surface of a reinforcing fiber; however, direct experimental validation of this segregation phenomena has not been presented. In this study, we have used neutron reflectivity techniques to investigate the chemical composition of isotopically labeled epoxy networks in contact with a simulated glass fiber surface. Our results demonstrate the first direct evidence of surface-induced segregation phenomena in these systems, which we find to occur within a few nanometers of the polymer/solid interface. The chemical profiles and surface effects in several systems based on commercial epoxides and amine crosslinking agents are presented. Implications of the results on the mechanical properties of advanced polymeric composite materials are discussed.
2:45 PM M7.5
STRUCTURAL AND CALORIMETRIC CHARACTERIZATION OF THE INTERPHASE IN Al-FILLED DGEBA-BASED EPOXIES. D. Arayasantiparb and M. Libera, Stevens Institute of Technology, Hoboken, NJ.
There are now many studies indicating that there is a region at the epoxy/adherend interface known as the interphase with chemical and structural properties different from the bulk epoxy. There remains, however, inadequate understanding of the interphase microstructure. This paper describes differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) studies of the interphase between DGEBA-based epoxies and aluminum metal surfaces. DSC results show differences in resin-cure behavior and glass transition temperature between filled and unfilled specimens. High-angle annular-dark-field STEM imaging of the interphase in DGEBA/mPDA epoxy embedded with anodized aluminum wire show a RuO4 stain depletion within 200nm of the wire surface. The variation in electron scattering due to the stain can be attributed to a curing-agent depletion such that the interphase has an enhanced degree of homopolymerization. First studies by electron energy-loss spectroscopy of DGEBA/PACM20 epoxies indicate that the distribution of resin and curing agent can be measured with approximately nanometer resolution by following a characteristic pi-pi* transition associated with the aromatic ring in the DGEBA resin.
3:30 PM *M7.6
THE EFFECT OF INTERFACE STRENGTH AND DUCTILITY ON THE MICROMECHANICAL BEHAVIOR OF GRAPHITE/POLYMER COMPOSITES. Linda S. Schadler, S. Narayanan, P. Gopalakrishnan, Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, NY; M.S. Amer, Center for Composite Materials, University of Delaware, Newark, DE; S. L. Phoenix, Dept. of Theoretical and Applied Mechanics, Cornell University, Ithaca, NY; I.J. Beyerlein, Los Alamos National Laboratory, Los Alamos, NM.
In order to predict failure of composite materials, damage initiation and propagation must be understood and quantified at the micromechanical level. For example, a fiber break causes a strain concentration in the intact neighboring fibers and the magnitude and spatial extent of overloads generated together with the fiber flaw statistics will determine if and where the intact fiber fails. In addition, the role of the interface must be quantified both in terms of the strength and ductility of the interface region and in terms of its effect on the strain concentration. Micro Raman spectroscopy (MRS) can measure the strain in embedded graphite fibers with 2µm spatial resolution and is thus an excellent tool for studying micromechanical deformation. Our group has been using MRS to study failure initiation in both tension and compression for both long and short fiber composites. This talk will present the details of the MRS technique and our latest results on damage initiation. A computational mechanics techniques called quadratic influence superposition (QIS) has been developed to calculate the fiber and matrix stresses and displacements generated by fiber breaks and accompanying forms interfacial deformation. QIS is used to interpret the MRS strain profiles in a single broken fiber embedded in a microcomposite. From this analysis we can predict the in situ yield and frictional sliding stresses as well as the extent of the yield and debond zones with no a priori assumptions on their location or length. Armed with this information, micromechanics modeling can be used to predict the statistical aspects of damage initiation, evolution and tensile strength involving many distributed fiber breaks in bulk composites. This talk will also present results from such comparisons and predictions for graphite fiber/epoxy composites.
4:00 PM M7.7
STRUCTURE OF POLYMER COMPOSITE INTERFACES. Keith A. Welp and Richard P. Wool, Univ. of Delaware, Dept. of Chemical Engineering, Newark, DE.
Predicting strength in polymer composite materials requires understanding of the structure at polymer-polymer interfaces. This structure is in turn controlled by the polymer dynamics. Several dynamics models have been presented, but none have been proven correct, for describing the polymer-polymer interface. Experiments involving specially labeled poly(styrene) chains have been designed which are capable of directly probing the dynamics at an interface. Two materials are prepared, each with approximately 50% deuterium along the length. The deuterons are located in the center 50% of the chain (HDH) or 25% in from each end (DHD). Concentration depth profiles of deuterium with welding time are obtained by Dynamic Secondary Ion Mass Spectroscopy (DSIMS) and Specular Neutron Reflectivity (SNR). These profiles give detailed information about the polymer dynamics at the symmetric polymer-polymer interface. Results for a 400k molecular weight, fully entangled, HDH/DHD pair will be presented. Complementary experiments involve selective tracking of center or end portions of the chain. This is accomplished by forming interfaces between HDH materials and fully deuterated or protonated polystyrene. The results to date are consistent with the predictions of the reptation model. Work on polydispersity, by studying bidisperse blends of triblocks, incompatible interfaces, HDH poly(styrene)/PMMA and HDH poly(styrene)/dPMMA, and interdiffusion into crosslinked polystyrene are also being investigated. These studies lead to exact predictions for strength of interfaces and required welding time as a function of temperature and pressure to obtain optimal strength and fatigue resistance.
4:15 PM M7.8
BONDING OF THERMOPLASTIC COMPOSITES TO THERMOSETTING COMPOSITES. Mary B. Chan-Park, Huang-Shin Ngew, Singapore Productivity and Standards Board, SINGAPORE; En-Tang Kang, Tong-Earn Tay, Wee-Sing Chok, National University Of Singapore, SINGAPORE.
Thermosetting epoxy/graphites are used widely in aircraft structures. Repair of these materials are typically done using composite patches of the same grade as the parent rnaterial. However, many different types of epoxy/graphite composites are used on a single aircraft and the epoxy/graphite prepregs do have limited shelf life of typically six months when properly stored. Hence, it was thought that thermoplastic composites such as polyetheretherketone (PEEK) / graphite composites which do not have any shelf life may be a better alternative for repair patches However, due to the low surface energies of thermoplastics, they are not easily bonded using thermosetting structural adhesives such as epoxies. However, when the PEEK/graphite composite was argon plasma treated and then graft copolymerized with glycidal methacrylate in the presence of a UV light, the joint between PEEK/graphite and epoxy/graphite changed from interfacial failure to cohesive failure within the adhesive or substrate failure. Both Dim and 2-part epoxies were investigated. X-ray photoelectron spectroscopy was used to investigated the chemical groups present on the surface.
4:30 PM M7.9
MICRO RAMAN SPECTROSCOPIC STUDY OF THE ORIGIN OF KINK BANDS FORMED UNDER COMPRESSIVE LOADING IN GRAPHITE/EPOXY COMPOSITES. S. Narayanan, Materials Engineering Department, Drexel University, Philadelphia, PA; L.S. Schadler, Materials Science and Engineering Department, Troy, NY.
Graphite/Epoxy composites are gaining prominence as structural materials, and as advanced materials for light weight, high strength applications. During service these composites are subject to compressive stresses. In order to improve the design criteria for these composites in compressive loading it is essential to understand the different failure mechanisms under compressive loading. To date, the failure mechanisms in intermediate/high modulus graphite fiber reinforced epoxy have been broadly classified as due to fiber delamination and delamination buckling, fiber microbuckling, and fiber compressive failure in several planes. The matrix and fiber properties were considered the most important factors in determining the failure mechanism. Recent work in this area has brought forward the importance of the interface and the interaction between fibers in determining the failure mechanism. The interface limits the load transfer ability and thus determines the strain distribution in the composite. Fiber microbuckling is one of the mechanisms of failure in these composites and there has been a need to understand the sequence of events that lead to fiber microbucking (also referred to as kink band formation) as well as the role of the fiber, matrix, and interface in the formation of kink bands. Micro Raman Spectroscopy (MRS), a technique capable of obtaining fiber strains with a spatial resolution of less than 2 microns, is being used in this work to monitor the strain in the fibers and the load transfer from the matrix to the fibers, before and after the kind band formation in order to understand this failure mechanism. Unsized, sized and coated fibers in the same matrix material are compared in this study to see the effect of different interface conditions on the failure mechanism.
SESSION M8: TOUGHNESS, CHARACTERIZATION, SENSING
Chair: Bruce K. Fink
Thursday Morning, December 4, 1997
Independence Center (S)
8:30 AM *M8.1
DENDRITIC TOUGHENERS FOR EPOXY COMPOSITES. Louis C.N. Boogh, Bo Pettersson*, Jan-Anders E. Manson, Laboratoire de Technologie des Composites et Polymères, Ecole Polytechnique Fédérale de Lausanne, SWITZERLAND. *Perstorp Polyols, Perstop AB, Perstorp, SWEDEN.
Hyperbranched dendritic polymers (HBP) have shown outstanding performances as tougheners in many thermally cured epoxy matrix resins. Toughening is induced without loss of thermo-mechanical properties as often otherwise the case in conventional rubber toughening. The HBP does not alter the viscosity of the resin, since it has an intrinsically low viscosity and already low volume fractions are sufficient for a substantial increase of the KIc toughness properties. Low viscosity is valuable for processing methods such as RTM where long flow distances and high shear rates are present. The HBP is functionalised with epoxy groups ensuring an efficient stress transfer between the toughener and the matrix for optimum properties. The HBP / matrix mixture is initially homogeneous and is tailored for a controlled phase separation upon curing, yielding a finely dispersed particle toughened material. Filtering of the toughener while impregnating the fiber bed is avoided by completing the impregnation before the commencement of phase separation. A processing window limited by the time to phase separation is defined for the epoxy-toughener system. By modifying the chemical structure of the HBP, the processing window can be modified to suit a wide range of composite processing methods, such as wet lay-up, prepregging and RTM. The fiber reinforcements influence the phase separation process and the fiber surface treatment must be taken into account when using dendritic tougheners in epoxy composites.
9:00 AM M8.2
DESIGNED POLYMERIC INTERPHASES FOR CARBON FIBER REINFORCED VINYL ESTER MATRIX COMPOSITES. M. A. F. Robertson, S. R. McCartney, M. B. Bump, K. A. Wilson, N. S. Broyles, M. C. Flynn, K. E. Verghese, J. J. Lesko, and J. S. Riffle, The Designed Interface Group (DIG) of the NSF Science and Technology Center for High Performance Polymeric Adhesives and Composites, Virginia Polytechnic Institute and State University, Blacksburg, VA.
As part of our ongoing research on designed interphases for carbon fiber composites, we have begun to study selected thermoplastic polyhydroxyethers and polyurethanes as sizing materials with vinyl ester matrices. Bilayer films were prepared by casting thin films of the candidate sizing materials, then by curing the vinyl ester in contact with the films. Cross-sections of these bilayers were characterized using atomic force microscopy (AFM). We have also made use of the nano-indentation AFM technique in an attempt to quantify gradients in mechanical properties across these interfaces. Cross-ply AS-4 reinforced composites have been prepared with several of these sizings and fatigue tests have been conducted. The durability to fatigue cycling with a bisphenol-A/epichlorohydrin polyhydroxyether sizing material is significantly enhanced with 0.5 weight percent of the coating material incorporated into the composite. The nano-mechanical properties across the interphase regions are being compared and correlated with composite properties.
9:15 AM M8.3
AFM CHARACTERIZATION OF FIBER-MATRIX INTERFACE USING A CARBON NANOTUBE MODIFIED PROBE. Seung Bin Park, Michael Buss, and Ronald Andres, School of Chemical Engineering, Purdue University, West Lafayette, IN.
Microscopic characterization of the elastic properties of the fiber-matrix interface is critical to obtaining a fundamental understanding of the macroscopic behavior of fiber reinforced composites. Such characterization has proven difficult. What is needed is a characterization tool that is capable of nanometer-scale spatial resolution and that measures directly the elastic properties of the interface. Dynamic force microscopy using an Atomic Force Microscope (AFM) seems ideally suited to this problem, however, it has proven difficult to obtain nanometer-scale resolution with this technique because of the blunt tips present on conventional AFM cantilevers. In order to surmount this problem we have modified a conventional AFM cantilever by attaching a carbon nanotube onto the tip of the probe. We demonstrate the power of this tool by exploring the interfacial structure of a steel reinforced rubber.
9:30 AM M8.4
EVALUATION OF STRUCTURAL AND ADHESIVE PROPERTIES OF NYLON AND TEFLON ALIGNMENT FILMS BY MEANS OF ATOMIC FORCE MICROSCOPY. G. Padeletti, CNR-Instituo di Chimica dei Materiali, Roma, ITALY; S. Pergolini, LOT Oriel Italia, Roma, ITALY; G. Montesperelli, Univ TorVergata, Dip. Scienze Chimiche e Tecnologiche, Roma, ITALY; A. D'Alessandro, F. Campoli, P. Maltese, Univ La Sapienza, Dip. Ingegneria Electronica, Roma, ITALY.
Atomic Force Microscopy (AFM) has become a powerful technique for submicron investigation of surface properties. The capability of this technique to observe conductive and non-conductive samples allows the investigation of dielectric films as the alignment layers used in the fabrication of surface stabilized ferroelectric liquid crystal (SSFLC) displays, which are the object of the present work. In fact the device final performance strongly depends on the alignment layer quality and our study is focussed on the comparison between the more conventional polymer as nylon 6, and polytetrafluoroethilene (PTFE or Teflon), generally used as alignment films. A micromorphological characterization of the sample surface has been carried out, in order to correlate structure with alignment properties of both polymer films. The results show that reduced roughness and periodicity wavelength are related to smaller anchoring forces and therefore to a faster response. In addition to the topographic characterization, AFM non-conventional measurements have been performed in order to obtain local information on the adhesive properties of the studied materials. These observations confirm the smaller anchoring force of PTFE with respect to nylon.
9:45 AM M8.5
MICROMECHANICAL PHENOMENA CONTROLLING COMPOSITE TOUGHNESS; EXPERIMENTAL AND MODELING INVESTIGATION OF THE INTERPHASE EFFECT. Maher S. Amer, Center for Composite Materials, University of Delaware, Newark, DE; L.S. Schadler, Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, NY; Bassel Iskandarani, MERCK Research Laboratory, Westpoint, PA.
Macromechanical behavior of fiber composites is influenced by a number of micromechanical phenomena such as fiber fragmentation, interfacial and matrix cracking, and fiber/fiber interactions. The stress concentration phenomenon due to fiber/fiber interactions controls, to a large extent, the composite toughness and failure behavior. In this study Micro-Raman spectroscopy, with the ability to measure strains in graphite fibers with a spatial resolution of 2m, was used to measure fiber strain and monitor fiber interactions in model multi-fiber graphite/epoxy composites. Model multi-fiber composites with interface properties that varied in adhesion level and ductility were used to investigate the effect of interfacial properties on the stress concentration phenomenon. It was found that a brittle interphase that promotes interfacial cracking significantly reduces the fiber/fiber interaction in the composite while low interfacial adhesion increase such interaction. Experimental results were found to be in agreement with the predictions of an energy based approach that is used to model the phenomenon. The energy based model was also used to calculate interfacial strain energy release rates (Gi) for the different interphases based on experimental results. It was found that commercial sized fibers yields a value of 282 J/m2 for Gi when embedded in epoxy matrix. Details of the energy approach and its prediction capabilities are fully discussed.
10:30 AM *M8.6
OPTICAL COHERENCE TOMOGRAPHY OF POLYMER COMPOSITES. Joy P. Dunkers, Richard S. Parnas, Richard C. Peterson, Carl G. Zimba, and Kathleen M. Flynn, Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD; Brett E. Bouma and James G. Fujimoto, Department of Electrical Engineering and Computer Sciences and the Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge MA.
Optical Coherence Tomography (OCT) is a technique initially developed to non-destructively image the microstructure of biological samples. In this work, OCT is applied to even more challenging samples: polymer composites. Briefly, OCT is a technology that uses a low coherence source such as a diode laser to perform interferometric reflectometry. Reflectance information is gathered as a function of sample depth for any position on the sample, and cross-sectional information is assembled by moving the sample with a motorized micro-positioning stage. Specific micro-structural information obtained from OCT is reinforcement placement and shape and also void formation. Samples of epoxy and vinyl ester E-glass reinforced composites were imaged, and features from those images are compared to optical micrographs. The advantages and limitations of this technique are discussed.
11:00 AM *M8.7
OPTICAL AND MICROWAVE CHARACTERIZATION OF POLYMERS. Kevin Webb and Mark Webster, Sch Elect and Comp Engineering, Purdue University,West Lafayette, IN; Neal Gallagher, Dept Elect Engineering, University of Delaware, Newark, DE.
Polymers and polymer composites have been characterized using both optical and microwave measurement techniques. In the case of highly scattering, white plastics, laser speckle statistics have been used to evaluate optical scattering coefficients through the application of a diffusion equation model, and preliminary imaging experiments have been conducted. In addition, modulated incoherent light measurements have been used to locate inhomogeneities in a diffuse liquid, an approach which could also be applied to polymers with low absorption at optical wavelengths. These optical techniques should prove useful in the evaluation of silica fiber composites. In the case of carbon fiber composites, microwave measurement results show the potential for imaging thin samples. Possible approaches to interrogate composites with microwave signals will be suggested.
11:30 AM M8.8
SENSING DELAMINATION IN A CARBON FIBER POLYMER-MATRIX COMPOSITE DURING FATIGUE BY ELECTRICAL RESISTANCE MEASUREMENT. Xiaojun Wang and D.D.L. Chung, Composite Materials Research Laboratory, State University of New York at Buffalo, Buffalo, NY.
Delamination in a crossply [0/90] continuous carbon fiber polymner-matrix composite was sensed in real time during fatigue by measuring the electrical resistance of the composite in the through-thickness direetion. Upon 0 tension-tension fatigue at a maximum stress of 57% of the fracture stress, the resistance irreversibly increased both in spurts and continuously, due to delamination, which started at 33% of the fatigue life. The resistance increased upon loading and decreased upon subsequent unloading in every cycle, thereby allowing strain sensing. The minimum resistance at the end of a cycle irreversibly increased during the first 0.1% of the fatigue life. The resistance became noisy starting at 62% of the fatigue life, at which delamination occurred rapidly and the fraction of laminate area delaminated reached 4.3%.
11:45 AM M8.9
EFFECTS OF PARTICLE SIZE AND DISTRIBUTION ON ELECTROMAGNETIC HEATING OF FERROMAGNETIC PARTICLE FILLED POLYMERS. S. Yarlagadda, Center for Composite Materials, J.M. Riley, J.Q. Xiao, Dept. of Physics, J.W. Gillespie Jr., Materials Science Program, University of Delaware, Newark, DE; B.K. Fink, Army Research Laboratory, Aberdeen Proving Ground, MD.
Induction heating properties of polymer films with Ni and Fe-Ni particles have been studied as a function of particle size and distribution. Ni and Fe-Ni particles of various compositions, between 20-50 nm, were fabricated using the ball milling technique. Magnetic properties (hysteresis) and size distributions were measured. Polymer films, upto 60% concentration, were fabricated using solvent casting techniques. Film heating rates were measured by subjecting them to alternating magnetic fields at various frequencies and measuring the temperature-time profile.
SESSION M9: COMPOSITE PROPERTIES vs. STRUCTURE
Chairs: Lynn S. Penn and Richard P. Wool
Thursday Afternoon, December 4, 1997
Independence Center (S)
1:30 PM *M9.1
MECHANICAL PERFORMANCE OF MOLECULAR COMPOSITES: CREEP AND STRESS RELAXATION OF BLENDS OF POLYPROPYLENE(PP) WITH A POLYMER LIQUID CRYSTAL(PLC). Witold Brostow, Nandika Anne D'Souza, Department of Materials Science, University of North Texas, Denton, TX; Josef Kubat, Department of Polymeric Materials, Chalmers University of Technology, Gothenburg, SWEDEN; Robert D. Maksimov, Institute of Polymer Mechanics of the Latvian Academy of Sciences, Riga, LATVIA.
The PLC is longitudinal, with LC sequences in the main chain along the backbone. We have determined short-term (hours) as well as long-term (2 months, 1 year) creep for PP and for its blends containing 5, 10, 15, 20 wt. between 20 and 100C. Stress-dependent shift factor a equation was obtained; used in conjunction with the Hartmann equation of state it provides good predictions of the creep compliance master curves. Stress relaxation (sr) experiments confirm the cooperative theory of one of us (JK) and agree with molecular dynamics simulations. Temperature shift factors a from sr agree with those from creep and with predictions. Effects of PLC addition on PP properties are explained.
2:00 PM M9.2
MORPHOLOGY IN DIGLYCIDYL ETHER OF 4,4'-DIHYDROXY--METHYLSTILBENE LIQUID CRYSTALLINE EPOXY COMPOSITES. H.-J. Sue1, J.D. Earls2, and R.E. Hefner, Jr.2, 1Department of Mechanical Engineering, Texas A&M University, College Station, TX; 2Organic Product Department, Dow Chemical USA, Freeport, TX.
The morphologies and mechanical behaviors of various diglycidyl ether of 4,4-dihydroxy--methylstilbene based liquid crystalline epoxy (LCE) composites are studied. It is found that, depending on how the resins are cured and the level of mesogenic monomers are incorporated, the morphology in LCE composites may vary from being amorphous, to containing micro-domain of various size, and to the formation of preferred molecular orientation. Evidence of mesogenic molecular alignment along the fiber direction is detected in LCE composites. The preliminary results indicate that the oriented LCE composites exhibit high compression after impact strength, high environmental stress cracking resistance, and superior hot-wet performance. Methodology for aligning the mesogenic LCE molecules in composites are discussed.
2:15 PM M9.3
EFFECT OF ISOMER ON THE MISCIBILITY OF POLYMER BLENDS: A SYNCHROTRON SAXS STUDY. M. Rabeony, Henry H. Shao, K. S. Liang, Corporate Research Laboratory, Exxon Research & Engineering Co., Annandale, NJ.
We use synchrotron small angle x-ray scattering, calorimetric technique and cloud point measurements to investigate the miscibility of poly(cyclohexylacrylate) (PCHA) with the ortho, meta, and para isomer of poly(bromostyrene). The results show that PCHA is immiscible with poly(para-bromostyrene) (P4BrS) whereas blends with poly(ortho-bromostyrene) (P2BrS) and poly(meta-bromostyrene) (P3BrS) exhibit lower critical solution temperature (LCST). X-ray measurements of the absolute scattered intensity enable the determination of the interaction parameter () within the framework of the Random Phase Approximation. We observe the following order: (PCHA-P2BrS) < (PCHA-P3BrS) < (PCHA-P4BrS). These results emphasize the importance of polymer microstructures on their phase behavior.
2:30 PM M9.4
MICROSTRUCTURAL EVOLUTION IN TATB COMPOSITE MATERIALS. Richard H. Howell, George E. Overturf III and Philip A. Sterne, Lawrence Livermore National Laboratory, Livermore, CA.
The explosive molecular solid TATB is commonly formed in small crystallites from precipitation. Shaped parts are made by including a small amount of a thermoplastic binder ( 5 wt%) and forming the composite systems under heat and pressure. Changes in these composite systems due to environmental stress can effect the mechanical and chemical properties of the composite. We have measured detailed changes in the microscopic open volume of TATB/KEL-F 800 (a fluorochloropolymer) composites by the use of positron annihilation lifetime spectroscopy performed with the high energy beam at the LLNL Positron Facility. Separate measurements of KEL-F 800 aged at elevated temperature, of TATB as received from production and of TATB/KEL-F 800 composites, aged and unaged, were performed. By separate measurements of the composite components we have separated distinct annihilation rates that are correlated with changes in the KEL-F 800 binder and to the TATB material. Open volume size was seen to shrink in KEL-F 800 aged at elevated temperature. The open volume size in the TATB crystallites was found to be significantly smaller than the binder and appears to grow as the material is stressed or aged. However TATB showed a new correlation between the open volume size and signal strength . The source of this correlation has not yet been identified. It may indicate that consideration of effects beyond structural changes ,such as a dependence on chemical changes, are required for a full interpretation of the positron data in this case. This work was performed under the auspices of the US Department of Energy by LLNL under contract No. W-7405-ENG-48.
2:45 PM M9.5
MORPHOLOGIES OF POLY(ETHER SULFONE)-MODIFIED POLYCYANURATES. Jin-Long Hong, Inst. of Materials Science and Engineering, National Sun Yat-Sen Univ., Kaohsiung, TAIWAN; Jer-Yen Chang, Dept. of Chemical Engineering, Yung-Ta Junior Inst. of Technology and Commerce Ping-Tung, TAIWAN.
Polycyanurates prepared from cure of bisphenol A dicyanate (BPADCy) were modified with either hydroxyl-terminated or cyanated poly(ether sulfone)s (as HPESs or CPESs, respectively) of different molecular weights (MWs). With HPES (or CPES) of high MW, the resulting resin showed a two-phase morphology in contrast to the single-phase morphology generated from the curing reaciton of the low MW HPES (or CPES). This result can be thermodynamically explained. Observations from scanning electron microscopy suggest a discrete-continuous fracture surface for the HPES-modified polycyanurate but for CPES-modified resin, a blur interface between the dispersed particles and matrix was observed. In addition, the blur interface can be observed if catalyst was added. Results from infrared spectroscopy suggest that the hydroxyl terminals in HPES can react with cyante groups in BPADCy to form iminocarbonate bonds, resulting in the blur interfacial zone.
3:30 PM M9.6
PROPERTIES AND APPLICATIONS OF EXTERNALLY IMPREGNATED SHAPED FIBERS. Ronald P. Rohrbach, Peter Unger, Alex Lobovsky, Lixin Xue, Daniel Bause, Russel Dondero, AlliedSignal Inc., Corporate Research, Morristown, NJ; Gordon Jones, AlliedSignal Inc. Filters and Spark Plugs, Perrysburg, OH.
Polymeric fibers have been produced in an array of geometric cross sections, all of which possess deep channels along the length of the fiber. These shaped fibers have been made in many different formats including wovens, nonwovens, and parallel arrays. A number of polymeric materials are suitable to retain these nonround cross sections during spinning, they include polyolefins, polyesters, and polyamides. Specfic cross sections have been observed to capture and to tenaciously retain high levels of both liquids and finely divided solids within the channels of the fibers. The liquids are held through capillary forces while the solids are mechanically entrapped within the channels and do not require adhesives to be bound. Their retention is sufficient to allow these impregnated fibers to be used in high flow applications without experiencing loss of the reagents. Exploiting this property, one can use this type of fiber to support a host of reagents in a practical format for various applications. This highly dispersed form of reagents naturally lends itself to filtration applications where efficient mass transfer and low pressure drop are critical. However, the range of agents which can be entrapped with the fiber suggests far wider applications, including ferromagnetic fibers, electrical conducting fibers, mini-chromatography and micromolding. Comparative performance analysis of an impregnated fiber versus a traditional granular carbon adsorbent for gas phase purification will be presented. Additionally, a continuous gas phase removal system will also be described using this shaped fiber in a parallel array.
4:00 PM M9.8
METASTABILITY IN POLYMERS: INFLUENCE OF CRYSTAL SIZE. S. Rastogi, L. Kurelec, P.J. Lemstra, Eindhoven Polymer Laboratories, Eindhoven University of Technology, Eindhoven, NETHERLANDS.
Polymorphism is a well established phenomenon in crystalline materials and is important for pharmaceutical and polymeric materials. In our study concerning the processability of polymers, we came across an unusual observation related to polymorphism induced by pressure. The experimental observation is that polyethylene crystals transform from the stable orthorhombic crystal structure into a transient hexagonal phase, which on annealing isothermally and isobarically transforms back into the stable orthorhombic phase. In-situ X-ray, optical microscopy and transmission electron microscopy lead us to conclusion that the occurence of a transient hexagonal phase is dependent on the polymer crystal size; smaller crystals which are more metastable transform into the transient hexagonal phase at temperatures and pressures much below the thermodynamic critical point. Since chain mobility is rather high in the hexagonal phase we used our findings in the fusion of ultar high molecular weight polyethylene particles, which are nearly impossible to fuse via the conventional methods. The polymer possesses excellent abrasion resistance and for this reason it is used in demanding applications like in artificial hip-joints as the interface between the bone of the hip and the metal bar which is inserted in the leg.
4:15 PM M9.9
EXTRUSION FREEFORM FABRICATION OF FIBER REINFORCED COMPOSITES. Paul Calvert and Tung Liang Lin, Department of Materials Science and Engineering, University of Arizona, Tucson, AZ.
Extrusion freeform fabrication is a 3-D layerwise writing technique for forming objects directly under the control of a CAD program. This method is one of a family of rapid prototyping methods which include stereolithography, selective laser sintering and fused deposition modeling. Short-fiber reinforced epoxy resins can be formed using this method, with up to 40 wt modulus from about 3 GPa for the pure matrix to about 9 GPa with fibers. The extrusion process causes alignment of the fibers parallel to the writing direction, which can be across or parallel to the axis of a test bar, resulting in a lower or higher elastic modulus. The freeform process also allows tougher unreinforced layers or regions to be introduced between the stiff layers. This should allow optimization of local stiffness and toughness. In principle, the properties of short fiber composites should approach those of continuous fiber composites, if high levels of fiber alignment could be obtained. It is possible to extrude fibers with a sufficient aspect ratio to give high levels of reinforcement, the problem is to achieve sufficient orientation. The interaction between fiber alignment and the writing process will be discussed.
4:45 PM M9.10
ON INFLUENCE OF ADHESION IN MONOFIBRE-THERMOPLAST MICROCONTACT ON STRENGTH OF FIBRE-REINFORCED COMPOSITES. Vladimir Meshkov and Sergey Chizhik, MPRI, Dept of Tribomaterials, Gomel, BELARUS.
One of the promising methods of fibre-reinforced polymer composites study is modelling an adhesional contact between a fibre and matrix. The given work presents the results of modelling an adhesional contact of a polymer monofibre with a thermoplastic binder and influence of adhesion on strength of fibre-reinforced composites. To estimate adhesional strength in the interphase zone of components two contact models have been compared: a contact - adhesional model and a model of ``viscous flow''. In the first case the formation of the contact is due to the external pressure and molecular attraction of surfaces. The second model assumes the flowing of the melt into the cavities of the monofibre surface. The calculation of the area and adhesion depending on the pressure and flow depth of the thermo-plastic binder has been made on the basis of the AFM-image analysis of the initial and biochemically treated aramide fibre. It is shown that the treated surface compared with the initial one possesses some advantage. The advantage is expressed in the formation of a large area of the adhesional contact, mainly on the subroughness level. Calculated and experimental estimation of adhesion of the mono-fibre to the thermoplast has shown that the biochemical treatment allows to increase considerably a number of adhesionally and cohesionally destructing contacts, and also to increase the adhesions strength. Rising strength properties of the thermoplast reinforced with treated fibres is accompanied by changing character of their fracture. The appearance of the second peak on the curve of the acoustic emission spectrum is associated with increasing energy of the components interphase interaction. Such materials have high strength at static and dynamic loading.