Meetings & Events

Spring 1999 logo1999 MRS Spring Meeting & Exhibit

April 5-9, 1999 | San Francisco
Meeting Chairs: Katayun Barmak, James S. Speck, Raymond T. Tung, Paul D. Calvert



Symposium HH—Soft Condensed Matter-Fundamentals and Applications

-MRS-

Chairs

Bill Carty 
Dept of Ceramic Engr 
Alfred Univ 
Alfred, NY 14802 
607-871-2451

Seth Fraden
Physics Dept
Brandeis Univ
MS 057
Waltham, MA 02254
781-736-2888

Terry Garino 
Materials Processing Dept 
Sandia National Labs 
MS 1411 
Albuquerque, NM 87185-1411 
505-845-8762

Robert Prud'homme
Dept of Chem Engr
Princeton Univ
A301 Equad
Princeton, NJ 08544
609-258-4577

  * Invited paper

SESSION HH1: GELS
Chair: Robert K. Prud'homme
Monday Morning, April 5, 1999
Salon 13 (M)

8:30 AM *HH1.1
RHEOLOGY OF PHYSICAL GELATION. H. Henning Winter , Dept. Chemical Engineering and Dept. Polymer Science and Engr., Univ. of Massachusetts, Amherst, MA.

Polymers at their transition from liquid to solid (critical gels) exhibit a distinct relaxation pattern which allows detection of the gel point and direct observation of gelation dynamics. The slow molecular dynamics of critical gels is governed by a power law distribution of relaxation modes. The time dependent relaxation modulus, G(t)=St-n, decays with a relaxation exponent n and front factor S (stiffness of critical gel). This can be measured, for instance, by mechanical spectroscopy at low frequencies which shows distinct power laws of dynamic moduli, G' G wn, and a frequency-independent loss tangent, G/G'. Not only is rheology a sensitive measure of molecular mobility at a wide range of length scales. Rheology has also become more quantitative due to the recent discovery of the rheological laws which govern the gel point. Universality of the transition behavior has been found for chemical and physical gelation (Winter HH, Mours M, Advances in Polymer Science 134:165-234, 1997). The finite life time of physical junctions imposes a problem if the ratio of life time and experimental time (gel number) is within a decade below or above order unity. Two physical gels will be discussed: crystallizing popyolefins and micro-phase separating block copolymers.

9:00 AM HH1.2
COLLOIDAL GELS OF TETHERED SILICA PARTICLES IN POLAR MEDIA: DIRECT CORRELATION BETWEEN DYNAMIC RHEOLOGY AND INTERACTION PARAMETERS. Srinivasa R. Raghavan, Saad A. Khan , North Carolina State University, Department of Chemical Engineering, Raleigh, NC; Jun Hou, Gregory L. Baker, Michigan State University, Department of Chemistry, East Lansing, MI.

Colloidal interactions between particles dispersed in a liquid can be suitably tailored by modifying the surface chemistry of the particles. In the case of fumed silica particles, the surface can be systematically altered from hydrophilic to hydrophobic by replacing a portion of the original silanol (Si-OH) groups by non-polar alkyl chains. In this study, we probe the effect of surface modification of fumed silica on their rheology and microstructure in polar media. Variables of interest include the length of tethered alkyl chain and the extent of surface coverage. For the continuous phase, we examine a range of polyether liquids comprising different architectures and molecular weights. We find that when the alkyl chains are C8 or longer, and are attached at saturation levels, a dense non-polar surface layer is formed on each silica unit. Such particles experience strong interactions in polar media, leading to the formation of a volume-filling gel network. We show that these interactions arise as a result of a negative free energy of mixing between the tethered chains, owing to the mismatch in chemical nature between chains and solvent. In this flocculation process van der Waals interactions between the particles play a negligible role. We also find that the greater the mismatch between particle surface and liquid, the greater the ``stickiness'' of the surface chains and correspondingly, the higher the elastic modulus (G$\prime$) of the fumed silica network. This leads to a unique correlation between G$\prime$ and a term comprising the $\chi$ parameter for the chain-solvent pair. An approximate but useful form of this correlation can be written as G$\prime$   ($\delta$s$\delta$m)4 where the latter expression characterizes the mismatch in solubility parameters between the surface chains ($\delta$s) and the liquid medium ($\delta$m).

9:15 AM HH1.3
MORPHOLOGICAL AND PROPERTY CHARACTERISTICS OF BLOCK COPOLYMER GELS AND MESOGELS. Jonathan Laurer, Nicole Jackson, Megan King, Saad Khan, Richard Spontak , NC State Univ, Depts of Materials Science & Engineering and Chemical Engineering, Raleigh, NC; Scott White, Becton Dickinson Technologies, Polymer Science & Technology Dept, Research Triangle Park, NC; Steven Smith, Procter & Gamble Company, Corporate Research Div, Cincinnati, OH.

Triblock copolymers in the presence of a midblock-selective solvent are capable of forming thermo-reversible (physical) gels if the endblocks are sufficiently rigid to serve as physical crosslink sites. Such networked materials are of fundamental importance in the study of macromolecular self-assembly, as well as commercial importance in the production of shock-absorbing media and sealants. In this work, the morphological features and mechanical properties of several copolymer-solvent systems in which gelation occurs due to glassy styrenic endblocks are described. The copolymer midblocks investigated here include isoprene, ethylene-co-propylene and ethylene-co-butylene. Material and process variables to be discussed include midblock-solvent compatibility, temperature stability, shear orientation, blend composition and endblock reinforcement (with polyphenylene oxide). Images of the microstructural elements (typically micelles) in these blends have been obtained by energy-filtered and cryofracture-replication transmission electron microscopy, and accompanying mechanical properties have been measured by dynamic rheology. In addition to copolymer-solvent blends allowed to reach near-equilibrium, the preparation and characteristics of solvent-rich nonequilibrium mesogels, i.e., anisotropic physical gels composed of highly swollen glassy bilayers, will be presented.

9:30 AM HH1.4
GEL-LIKE MODES IN POLY(ETHYLENE OXIDE) MELTS. R. Walkenhorst, J.C. Selser , G. Piet, University of Nevada, Las Vegas, Physics Department, Las Vegas, NV.

Two slow, long-ranged relaxations in melts of poly(ethylene oxide) were studied using the noninvasive dynamic light scattering technique of photon correlation spectroscopy. Measurements were made for two PEO samples, one with a molecular weight distinctly above the entanglement value and a second sample witha molecular weight distinctly below the entanglement value. Based on several lines of evidence, it is concluded that the faster of these relaxations, which can be identified with the cluster or Fischer mode observed earlier in other homopolymer melts, results from the gel-like behavior of a physical network formed by the polymer in the melt while larger scale rearrangements in the polymer network structure are responsible for the slower relaxation.

9:45 AM HH1.5
HIGHLY SWOLLEN HYDROGELS: RHEOLOGY AND PHASE BEHAVIOR. H. Jiang, Anteon Inc., Dayton, OH; W. Su, TMCI, Dayton, OH; P.T. Mather , T.J. Bunning, AFRL/MLBP and MLPJ, Materials and Manufacturing Directorate, WPAFB, OH.

We have investigated the phase transition of water within polyacrylate/chitosan blend hydrogels, with focus on the effect of crosslinking extent, by means of oscillatory shear rheology. Interest in these materials stems from our recent report of excellent laser damage resistance where the measured laser damage threshold (LDT) is better than BK7 glass and quartz, and 20 to 35 times higher than commercial PMMA. A strong correlation between the hydrogel viscoelastic properties and water phase behavior is found, with the presence of the hydrophilic moieties on the macromolecular chains and the restriction of the polymer network influencing this correlation. Trends among the measured shear storage modulus (Gí), shear loss modulus (Gî), and shear loss tangent (tan d) are shown to be related to the mobility of water within the gels. While an increase in the extent of crosslinking is found to enhance the gel storage modulus at room temperature, the storage modulus below the freezing point of water is found to decrease. Our results are discussed with respect to the influence of polymer microstructure on water mobility, confinement, and crystallization.

10:30 AM HH1.6
INTERACTION POTENTIAL BETWEEN POLYAMPHOLYTE STABILIZED COLLOIDS. Christos N. Likos, Hartmut Loewen , Institut fuer Theoretische Physik II, Heinrich-Heine-Universitaet Duesseldorf, Universitaetsstrasse, Duesseldorf, GERMANY; Abe Vaynberg, Norman J. Wagner, Center for Molecular and Engineering Thermodynamics, Dept. of Chemical Engineering, University of Delaware, Newark, DE.

To understand stabilizing effects of gelatin we explore how the adsorbed gelatin affects the interparticle effective pair potential. Experimental measurements of the structure factor of colloidal dispersions (60nm and 100nm diameter) stabilized with gelatin are analyzed to obtain effective interparticle potentials. We use liquid integral equation theories to fit the experimental structure factors to a simple model proposed by Kamiyama and Israelachvili [Macromolecules 25: 5081 (1995)]. To this end, we start with SANS profiles obtained with solutions of bare, charged latex particles at different concentrations and fit them using a DLVO potential. Having determined the various parameters characterizing the bare latex particles, we proceed with the gelatin-coated solutions, where an additional ``compressional'' energy arising from the brushes is considered. In this manner, the stabilizing potential of an adsorbed polyampholyte is quantified These results are used to understand measurments of the stability of gelatin stabilized dispersions under flow and their rheology.

10:45 AM HH1.7
THE STRUCTURE OF ADSORBED GELATIN ON FLAT SURFACES AND COLLOIDS. K. Abraham Vaynberg , Norman J. Wagner, Heiko Ahrens, Dirk Eck, Christiane A. Helm, Christos N. Likos, Hartmut Lowen.

Polyampholytes, such as gelatin, are well known stabilizing colloids used extensively in the chemical and food industries. In this work we explore gelatin interfacial layer properties by a number of methods in order to facilitate the understanding of gelatin stabilization. Measurements of the interfacial gelatin layer were done at the air-water and air-arachidic acid interface. With X-ray reflectivity, the electron density profiles of the adsorbed layers were measured. For the air/water interface, the electron density profile consisted of two parts, a 1.5 nm thick concentrated gelatin layer, and then a long trail (7 - 9nm). Only a diffuse tail was found in the case of adsorption to the air-arachidic acid interface. These layer structures were compared to previous results of the gelatin structure on polymer colloids with various surface properties by small angle neutron scattering (SANS) and dynamic light scattering (DLS) (Vaynberg et al. JCIS, 205: 131-140 (1998)). SANS measurements of the structure factor of concentrated colloidal dispersions with adsorbed gelatin layers provide a further method to characterize the adsorbed layer. Monte Carlo modeling of the structure factors yields interparticle forces that could be understood in terms of the adsorbed layer structure. These results help explain experimental measurements of the strong stabilizing properties of gelatin in shear flows and at rest.

11:00 AM HH1.8
MICROSCOPIC AND BULK PROPERTIES OF CYANOGEL SYSTEMS: A NOVEL AQUEOUS SOL-GEL SYSTEM BASED ON THE CHEMISTRY OF TRANSITION METAL CYANOMETALATE COMPLEXES. Andrew Bocarsly , Ileana Flintoff, Jochim Gross, George Scherer, Stefanie Sharp, Jennifer Wilson, Princeton University, Dept. of Chemistry, Princeton, NJ.

Recently, we reported that a new broad class of hydrogels can be synthesized based on the reaction of tetrachlorometalates with a wide variety of transition metal cyanometalate complexes in an aqueous environment. Such gels are composed of a coordination polymer having a star morphology. We term these materials `cyanogels', based on the use of a bridging cyanide ligand as a structural motif within the gel. Cyanogel formation follows a classic sol-gel synthesis pathway with gel products typically containing $\sim$95% water. Due to its unusual chemical stability, the gel synthesized from tetrachloropalladate and hexacyanocobaltate has been the focus of our characterization efforts. Palladium cobalt cyanogels can be dehydrated to form xerogels, or solvent exchanged using supercirtical carbon dioxide to form aerogels. These materials are microporous offering interesting gas adsorption properties. The bulk and microscopic properties of the solvent free gel materials have been examined using a variety of spectroscopic techniques in conjunction with electron microscopy and mechanial perturbation experiments. Based on these experiments the gels appear collodial in nature with two different length scales for porosity; one at the molecular limit the other a nanometer scale. The bulk properties of the palladium.cobalt cyanogel have been examined using beam bending experiments. In addition to permiability information, these experiments have produced the mechanical moduli for this material. Available results allow a comparision of this material to the better studies silica gel system.

11:15 AM HH1.9
LAMELLAR HYDROGELS FROM CATANIONIC SURFACTANT MIXTURES AND SIMPLE AMPHIPHILIC POLYMERS. Bret A. Coldren , Hee-Tae Jung, Joseph A. Zasadzinski, Materials Research Laboratory and Department of Chemical Engineering, University of California, Santa Barbara, CA; Hidetaka Iwai, Kao Corporation, JAPAN.

Highly swollen bilayers of dimyristoylphosphatidylcholine, pentanol and polyethylene glycol polymer-lipid (PEG-lipid) form gel-like phases with up to 90% water. Freeze-fracture electron microscopy shows that these gels are induced by the proliferation of interconnected, high curvature liquid crystalline defects which are stabilized by the steric requirements of the wedge-shaped PEG-lipid. This novel behavior can be generalized to create similar hydrogels using dilute lamellar phases from cationic and anionic surfactant mixtures. These catanionic mixtures form lamellar phases of up to 80% water, in addition to spontaneous vesicle phases at lower surfactant concentrations. Electron microscopy shows addition of PEG-lipid to these dilute catanionic lamellar phases induces similar interconnected defects, resulting in gelation. This finding supports theoretical predictions and experimental observations that catanionic bilayers have a characteristically low bending modulus. Additionally, we find that it is possible to replace the PEG-lipid molecules with simple block copolymers and nonionic surfactants of comparable molecular weight. The lower costs and enhanced chemical stability associated with catanionic surfactants and block copolymers makes these novel gel materials potentially useful as thickening agents, personal care products, detergents, etc., where viscosity control at high water fractions is desirable.

11:30 AM HH1.10
LYOTROPIC LIQUID CRYSTALLINE POLYMER GELS-A NEW THERMOTROPIC MATERIAL. Arno Seeboth , Hans-Rainer Holzbauer, Jörg Kriwanek, Th. Fischer, WITEGA, Dept. Adv. Mater., Berlin, GERMANY.

In recent years thermotropic materials have met with growing interest. Such materials (for example polymer blends or gels can be used as thermotropic materials for intelligent shade-giver (light valve) or Large Area Displays. The aim of this work was to examine the possibility of preparing gels with new optical properties. Of particular interest were materials where the optical transparency of the system varies with temperature without the gelation behaviour being perturbed. The optical behaviour of lyotropic liquid crystalline polymer gel networks have been described by us. These gels change their optical properties extremely when temperature increases or decreases by few degrees. A phase transition between the lyotropic liquid crystalline and isoptropic phase in the system is the cause for the optical effect. The contrast ratio of the aqueous gel (1 wt $\%$ polyethylene glycol and 12 wt $\%$ polyvinyl alcohol) in the temperature range between 27$^{\circ}$C and 33$^{\circ}$C is nearly 85: 1. This is a similar contrast ratio to that found for a phase transition from a thermotropic nematic phase to an isotropic phase or in electro-optically addressed LCD's. It is higher than that of photo-addressed LCD's. However, for all known thermotropic materials only one mode transition (clear-cloudy, or cloudy-clear) takes place with increasing or decreasing temperature. Therefore additional studies were done to examine the possibility of preparing gel networks that exhibit multiple phase transitions in connection with changing the transparency of the system depending on temperature variation. Furthermore, preliminary investigations show, that the gels also suitable as hybrid light- and heat valves.

11:45 AM HH1.11
REVERSIBLE COLOR CHANGES IN AQUEOUS POLYMER GEL NETWORKS. Jörg Kriwanek , Arno Seeboth, WITEGA, Department Advance Materials Ltd, Berlin, GERMANY.

The aim of the work is to examine the possibility of preparing gels with markedly reversible color changes with temperature (thermochromism) in the range of 5$^{\circ}$ to 80$^{\circ}$. Such gels are interesting for the production of new color filters, as temperature sensors, and especially for uses in intelligent windows. We have found, that some pyridinium N-phenolate betaines (so called Reichardt dyes) exhibit thermochromic effects in aqueous polyvinylalcohol (PVA)- borax gel networks: For instance the Reichardt dye ET(30), by addition of N-tetradecylbetaine as surfactant solubilized in a PVA-borax gel, change its color gradually from near colorless at 10$^{\circ}$ to deep violett at 80$^{\circ}$ at pH = 8.5. Additionally to the changes in the absorption intensities the absorption maxima are shifted bathochrom from 537 nm (10$^{\circ}$) to 555 nm (80$^{\circ}$). The color changes in dependence with temperature can be explained primarily by reversible protonation of the phenolate structure and forming of the corresponding phenole structure of the dye molecules, since the phenole structure is preferred in the microenviroment of the gel network at lower tempertures. By heating the gel network is gradually destroyed and the phenolate structures is forming which exhibit in the UV/Vis-absorption spectra the CT-transition from the phenolate to the pyridinium part of the molecule.
 

SESSION HH2: GELS AND SPHERICAL COLLOIDS
Chair: Bill Carty
Monday Afternoon, April 5, 1999
Salon 13 (M)
1:30 PM HH2.1
GELS FORMED FROM ASSOCIATING POLYMERS IN SOLUTION. Subramanian Kesevan, Robert K. Prud'homme , Princeton University, Princeton, NJ.

Guar and hydroxypropyl guar (HPG) polymer solutions can be crosslinked with borate ions to form gels. The gels are not true gels, but association fluids with tunable relaxation times. Chemical equilibria involving boric acid, borate ions, and borate ions associated with cis-diol sites on the polysaccharide chains determine the number of crosslinks and, thus, the rheology of the borate gels. Measurements were performed over the temperature range from 15 C to 65 C, and the pH range from 6.35 to 9.5. The storage and loss moduli obey time-temperature superposition so that master curves can be constructed over 6 decades in frequency. The shift factors along the time axis follow a single exponential Arrhenius form.. The activation energies for guar and HPG are identical, which is consistent with the cis-diols on each polymer being the active site for crosslinking. Also, the data at different pH values can be superimposed by time-pH shifting to produce master curves. The reduced moduli data follow a single time constant Maxwell model. The results are interpretted using the model of Leibler, et al. for the rheology of associating polymer systems.

1:45 PM HH2.2
SIMULATION OF HARD PARTICLES IN A PHASE SEPARATING BINARY FLUID. Valerly V. Ginzburg, Feng Qiu, Marco Paniconi, David Jasnow and Anna C. Balazs , University of Pittsburgh, Depts of Physics and Chemical Engineering, Pittsburgh, PA.

We simulate the motion of spherical particles in a mixture of two immiscible fluids. As the particles are diffusing through the medium, the fluids are undergoing phase separation. In addition, one of the fluids wets the surface of the mesoscopic particles. Thus, the system exhibits a competition between spinodal decomposition and wetting. By combining a cell dynamical systems model with the Langevin dynamics for particles, we show that the addition of hard particles significantly changes both the speed and the morphology of the phase separation. The domain growth rate shows a nontrivial dependence on the particle density, in qualitative agreement with earlier experimental studies. We also consider the effect of shear on the morphology and rate of domain growth in this system. We find that applying an externally imposed shear provides a distinct means of tailoring the characteristics of the mixture.

2:00 PM HH2.3
DYNAMIC LIGHT SCATTERING OF COMPLEX FLUIDS. Thomas A.P. Seery , Amit Sehgal, R.A. Weiss and Kaushik Chakrabarty University of Connecticut, Polymer Program, Storrs, CT.

Recent efforts to apply dynamic light scattering to the study of complex systems has led to the development of novel experimental approaches as well as advances in data analysis. Studies of associating systems that exhibit multiple relaxations have been examined using a combination of dynamic light scattering and total intensity measurements to place contributions of each mode of scattering on an absolute intensity scale. Polyelectrolytes in high dielectric constant solvents and ionomers in non-polar solvents are two systems where recent advances have been made using this approach. In addition, studies of conducting polymers as well as heme containing proteins in solution have led to efforts to address the inherent problems in scattering from light absorbing solutions. Absorption of the incident light produces a cascade of related physical phenomena that greatly complicate the acquisition and interpretation of data. The primary effect is local heating of the solution that generates a temperature gradient. The temperature gradient in turn creates a gradient in refractive index, density, and viscosity that are accompanied by thermal lensing and convective flow. The observation of these effects as a function of input power allows for interpretation of data. The unique oscillations seen in the correlation functions may also provide a diagnostic of the presence or absence of bimodal size distributions.

2:15 PM HH2.4
STRUCTURAL STUDIES OF COLLOIDAL GOLD-SILICA AEROGELS. R.M. Stroud , M.L. Anderson, C.A. Morris, C.I. Merzbacher and D.R. Rolison, Naval Research Laboratory, Washington, DC.

Colloidal metal aerogels are composite materials that combine the porosity of the >85% free-space aerogel structure with the optical and chemical properties of metal nanoparticles. This combination of properties is desirable for sensing, catalytic and electrocatalytic applications. The properties of a specific colloidal metal aerogel depend on the domain size and pore structure of the aerogel matrix, the size and composition of the metal nanoparticles, and also the matrix environment surrounding the metal. We have prepared a series of base-catalyzed silica aerogels with gold colloids ranging in size from 5 nm to 100 nm. The aerogels have been characterized using scanning and transmission electron microscopy in order to determine the aerogel domain size, the metal colloid size, and the spatial distribution of the colloids. Distinct morphologies, in which the colloid size is smaller, equal to, or larger than the silica domain size have been identified. The accessibility of the gold surface to species introduced from solution, which is a critical feature for many applications, will be discussed in terms of the morphology of the metal colloid-aerogel matrix.

2:30 PM *HH2.5
VISCOELASTIC PROPERTIES OF PARTICLE GELS. Lennart Bergström , Joseph A. Yanez and Eric Laarz, Institute for Surface Chemistry, Stockholm, SWEDEN.

The effect of strength of attraction and volume fraction on the mechanical properties of alumina particle networks were investigated. Alumina particle gels were formed reversibly and in situ in the rheometer by cooling alumina particle suspensions with adsorbed poly(12-hydroxy stearic acid) suspended in a marginal solvent, hexanol. The collapse of the polymer layer with decreasing solvency (temperature) induces flocculation when the long-range van der Waals force overcomes the remaining steric repulsion. The gelation temperature depends on volume fraction. At the gel temperature, Tgel, the gel becomes predominantly elastic; at temperatures below Tgel, the elasticity increases with decreasing temperature. We will discuss the functional form of the volume fraction scaling of the elastic modulus, measured over a wide range of volume fraction (0.2 < f< 0.425) and temperature (10-14$^{\circ}$C), based on current theories.

3:30 PM *HH2.6
HARD SPHERE SUSPENSION CRYSTALLIZATION KINETICS. Zhengdong Cheng , Paul M. Chaikin, Princeton Univ, Dept of Physics, Princeton, NJ; William B. Russel, Princeton Univ, Dept of Chemical Engineering, Princeton, NJ.

Kinetics of crystallization of colloidal hard sphere suspension are investigated using time resolved Bragg scattering. Three samples of PMMA/PHSA, 600 nm in diameter, in decalin/tetralin with volume fractions $\phi=0.528, 0.552, 0.575$, were studied in m g. The effect of gravity was clarified by comparison with studies on ground with the same samples. The Bragg scattering from the close packing planes and the diffusive scattering from the stacking disorder were captured by a CCD camera. As FCC 200 peak shows up in all three samples during the coarsening process, indicating FCC might be the equilibrium structure of the hard sphere crystal. The crystallites initially compressed by the surrounding liquid, expanded as the crystallization progressed, as measured by position of the main peak. The integrated intensity of the main peak measures the crystalline fraction of the sample, while the width determines the crystallite size. The ratio of these two quantities indicates the number of crystallites, which reflects the nucleation or coarsening processes.

4:00 PM HH2.7
MONODISPERSE SILICA AND ZnS PARTICLES WITH CONTINUOUSLY VARIABLE SHAPE MADE BY ION IRRADIATION OF MICRO-SPHERES. E. Snoeks ,a A. van Blaaderen,a,b M.L. Brongersma,a T. van Dillen,a C.M. van Kats,b K. Velikov,b and A. Polmana; a FOM-Institute for Atomic and Molecular Physics, Amsterdam, THE NETHERLANDS; b Van 't Hoff Laboratorium, Debye Institute, Utrecht, THE NETHERLANDS.

An ion-irradiation induced anisotropic plastic deformation effect is employed to continuously and controlably change the shape spherically shaped micro-particles. Oblate and prolate spheroids are obtained. Silica and ZnS spheres were obtained with diameters of $300\,$nm and $1030\,$nm with a polydispersity in size of $2\,\%$, and placed on the surface of a Si substrate. The spheres on the substrate were then irradiated with Xe ions at an energy of $4.0\,$MeV, to fluences ranging from $3\times 10^{13}$ to $1\times 10^{15}\,$ions/cm2; and analysed by scanning electron microscopy (SEM) under various tilt angles. A spectacular change in shape is observed for both small and large particles of ZnS and silica: oblate particles (smarty shape) are obtained with an aspect ratio of upto 3.2 for the highest fluence. When viewed along the ion beam direction, the particles are circular with a diameter of upto 1.6 times larger than the original, while in the plane perpendicular to the ion beam they are elliptically shaped with the shortest cross section 0.5 times the original diameter. Prolate spheroids (cigar-shape) are made by a second irradiation of oblate particles under a direction perpendicular to the original ion beam. At $4\,$MeV all Xe ions pass through the spheres, each ion depositing about $2\,$MeV into a local cylindrically shaped spike in the micro-spheres. In the spike region, the temperature rises by more than $2500\,$K within picoseconds after ion impact. The hot region expands, and causes a small anisotropic plastic deformation for each incoming ion. The macroscopically oberved deformation is a cummulative result of over a million of ion impacts, and therefore does not lead to additional polydispersity. This method to produce non-spherical particles with continuously variable shape will open paths to experimental studies on the properties of crystallization and optical scattering from photonic crystals of non-spherical particles. Optical scattering measurements are underway.

4:15 PM HH2.8
X-RAY PHOTON CORRELATION SPECTROSCOPY STUDY OF THE MICROSCOPIC DYNAMICS OF POLYSTYRENE LATEX SUSPENDED IN GLYCEROL: DIFFUSIVE DYNAMICS AND THE ROLE OF CONFINEMENT. D. Lumma * 1, L.B. Lurio* 1, A.R. Sandy* 1, P. Falus1, M.A. Borthwick1, S.G.J. Mochrie1, M. Sutton2, J.F. Pelletier2, A. Malik3, G.B. Stephenson3, L.J. Regan4-1Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA; 2Physics Department, McGill University, Montréal, CANADA; 3Materials Science Division, Argonne National Laboratory, Argonne, IL; 4Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT. *These authors contributed equally to this work.

The dynamics of polystyrene spheres [$\O\:\:142\:\mbox{nm}$] suspended in glycerol have been studied, using the emerging technique of X-ray photon correlation spectroscopy [XPCS]. Our experiments investigate suspensions of various concentrations from the dilute limit to the onset of crystallization , extend over one order of magnitude in length scales [$0.0012\:\mbox{\AA}^{-1} \leq Q \leq 0.0140\:\mbox{\AA}^{-1}$], and span four orders of magnitude in the time domain [$0.03\:\mbox{s} < \Delta t < 400\:\mbox{s}$]. In dilute suspensions, the dynamic behavior is well-described by simple Brownian motion. By contrast, in highly-concentrated suspensions, the inverse diffusion coefficient, 1/D, displays a strong wavevector dependence and mimics the static structure factor, S(Q). These results demonstrate how suspended particles adopt different microscopic dynamics in response to the confinement imposed by their neighbors.
Our time-correlation functions exhibit high signal-to-noise levels, almost comparable to those of dynamic light scattering [DLS]. Yet unlike DLS, XPCS is capable of characterizing opaque systems, and moreover of supplying S(Q) over a wide range in reciprocal space. This work marks a crucial step in the progression of XPCS towards becoming a routine characterization technique for low-energy dynamics. As such, it will have major impact on future investigations of the microscopic dynamics of dense colloidal and micellar systems, block- and copolymer blends, and entangled homopolymer melts.

4:30 PM HH2.9
DETERMINATION OF PARTICLE SIZE DISTRIBUTIONS IN CONCENTRATED SLURRIES FROM ACOUSTIC AND OPTICAL ATTENUATION DATA. Christopher A. White, Melanie L. Carasso , Sanjay S. Patel, Jorge L. Valdes, Bell Laboratories Lucent Technologies, Murray Hill, NJ.

Concentrated suspensions containing particles of radius 10 nm to 10 microns are important for many industrial processes including chemical mechanical polishing and sol-gel chemistry. Acoustic attenuation and optical turbidity provide sensitive measures of the particle size distribution in these suspensions at process concentrations. We have developed a robust, stable, and fast inversion method for the determination of particle size distributions from measured attenuation spectra. The inversion method can be applied to both optical and acoustic attenuation measurements. Theoretical predictions using experimental data demonstrate that this new algorithm outperforms commercially available methods for the determination of particle size distributions from attenuation data. The level of characterization afforded by this new algorithm provides a unique opportunity for greater understanding and control of manufacturing processes.

4:45 PM HH2.10
MECHANICAL PROPERTIES OF SILICA AEROGELS BY DEPTH-SENSING MICRO-INDENTATION. Magda Moner-Girona, Anna Roig , Elies Molins, Carles Miravitlles, Institut de Ciència de Materials (CSIC), Barcelona, SPAIN; Elena Martínez, Joan Esteve, Univ of Barcelona, Dept of Applied Physics and Electronics, Barcelona, SPAIN; Joan Llibre, Carburos Metálicos SA, R&D, Barcelona, SPAIN.

Silica aerogels are highly porous materials with nanometer scale pore diameters. Samples were synthesized by supercritical solvent evacuation from a wet gel. Because very small loads are sufficient to crack these fragile materials, standard Vickers and Knoop tests are not sensitive enough to measure their mechanical properties. Therefore, we used a new dynamical method called Depth-Sensing Micro-Indentation (DSMI). The DSMI method allows one to simultaneously measure the applied load and the penetration depth of a Berkovich diamond tip, hysteresis load displacement curves are obtained. Very small indentation loads ($\sim$1mN) com be applied and the load and depth resolutions are better than 1$\mu$N and 0.1nm respectively. Young's modulus (E), hardness and the elastic recovery parameter (ERP) were measured in samples that were synthesized by a variety of the sol-gel processes. Results were analyzed as functions of density ($\rho$), morphology and pore size distribution. A relation of the type, E $\propto\rho^{\alpha}$ with $\alpha\sim$2.9 was found for the acetone-synthesized series. As a function of aerogel density, two different regimes of mechanical behavior are observed. The lowest density aerogels are elastic but the denser aerogels are elasto-plastic materials.
 

SESSION HH3: MAGNETORHEOLOGICAL FLUIDS/
ELECTRORHEOLOGICAL FLUIDS
Chair: Terry J. Garino
Tuesday Morning, April 6, 1999
Salon 13 (M)
8:30 AM *HH3.1
MR FLUID SPONGES AND DEVICES. J. David Carlson , Lord Corporation, Materials Division, Cary, NC.

Stable, high-strength, magnetorheological (MR) fluids that enable the benefits of controllable fluid technology are now commercially available. Rotary MR fluid brakes have been manufactured and sold in modest numbers into the exercise equipment market since 1995 as easily controlled, variable resistance elements in stair-climber and cycling machines. Early in 1998 a real-time, commercial vibration control system based on linear MR fluid shock absorbers became available for use in the seats of large Class 8, i.e., eighteen wheeler, trucks. Recently, a new way of using MR fluids in which the fluid is contained in an absorbent matrix has been developed. Such MR fluid sponge devices enable low-cost, controllable fluid applications by eliminating the need for most of the high-cost components normally associated with a fluid filled device. MR fluid sponge devices compliment existing ways of using MR fluids. They are particularly appropriate for less demanding, low-force applications where a high degree of control is desired. MR fluid sponge devices contain MR fluid that is constrained by capillary action in an absorbent matrix such as a sponge, open-celled foam, felt or fabric. The sponge serves to keep the MR fluid located in the active region of the device where the magnetic field is applied. The sponge allows a minimum volume of MR fluid to be operated in a direct shear mode without seals, bearings or precision mechanical tolerances. They are not susceptible to gravitational settling or sedimentation of the MR fluid suspension. Examples of a variety of recently developed MR fluid sponge devices will be described.

9:00 AM HH3.2
AGGREGATION KINETICS AND STABILITY OF STRUCTURES FORMED BY MAGNETIC MICROSPHERE. Weijia Wen , F. Kun, K.F. Pal, D.W. Zheng and K.N. Tu.

Rings, chains, and clusters formed by magnetization-controllable microspheres suspended in the liquid at zero field are reported in this paper. The magnetic microspheres were made by plating glass beads with Ni film of a critical thickness. We found that the ring leading to magnetic flux closure is the most stable configuration. At high concentrations all individual rings, chains and clusters join together to form a net-like structure. A computer simulation based on the dipole-dipole interaction, without thermal noise has been carried out and the results are in good agreement with the experimental observations. Based on an analytic approach we give a simple explanation of the formation and stability of rings. In addition, the chain-column transition under an external magnetic field has also been investigated and it is found that the ground state of magnetorheological fluids is BCT structure

9:15 AM HH3.3
STRUCTURES OF A MAGNETORHEOLOGICAL FLUID. Gerald Gulley , Sandia National Labs, Albuquerque, NM; Rongjia Tao, Southern Illinois University, Dept of Physics, Carbondale, IL.

Extensive computer simulations were carried out to find the underlying structures of a magnetorheological fluid. The simulations took into account dipolar forces, the viscous drag force, and Brownian motion. Three structures were found: the bct lattice, the liquid state, and a columnar structure that bears resemblance to the worm-like structure often found in magnetorheological fluids.

9:30 AM HH3.4
EFFECTS OF STRUCTURES ON RHEOLOGY IN A MODEL MAGNETORHEOLOGICAL FLUID. Mustapha Chaker , Neal Breslin and Jing Liu, Department of Physics and Astronomy, California State University, Long Beach, CA.

An externally applied magnetic field can induce different types of structures in a magnetorheological (MR) fluid ranging from separated columns to a worm pattern of bent walls. The type of structure obtained depends upon not only the ramping rate of the magnetic field, but also on the particle volume fraction. In this work, we study the influence of these structures on rheology using magnetic polystyrene colloids. We found that these changes of structure in the fluid lead to variations in its apparent viscosity and the yield stress. Using a controlled-strain-rate rheometer, the apparent viscosity and yield stress as a function of the magnetic field strength and the particle volume fraction were obtained. As suspected, experimental results show that for a given shear rate, the apparent viscosity increases with increasing magnetic field. However, after a certain time, a restructuring occurs within the fluid, which in turn causes the viscosity to drop abruptly before leveling off. This variation profile of the viscosity as a function of time will be reported as our on-going comprehensive studies of the effects of structures on rheology in our model magnetorheological fluid.

9:45 AM HH3.5
ORIGIN OF CORROSION IN MAGNETORHEOLOGICAL FLUIDS USED FOR OPTICAL FINISHING. Irina A. Kozhinova a, Steven R. Arrasmitha, Leslie L. Gregga, Stephen D. Jacobsa,b, Univ. of Rochester, Center for Optics Manufacturinga and Laboratory for Laser Energeticsb, Rochester, NY.

Finish polishing of optical glasses, crystals, and ceramics is one of the most promising uses of magnetorheological fluids. Commercial, computer numerically controlled (CNC) magnetorheological finishing (MRF) machines have recently come on-line in U.S. optics companies to produce precision flat, spherical and aspheric optical components. MRF eliminates subsurface damage, smoothes rms microroughness to less than 10$\AA$, and corrects p-v surface figure errors to 0.03$\mu$ in minutes. Fundamental to the success of this technology is a stable magnetorheological (MR) fluid composed of 36 vol.$\%$ carbonyl iron (CI) and 6 vol.$\%$ cerium oxide in water. Invariance in the viscosity, and hence material removal efficiency, of this high solids concentration fluid throughout a work day is critical to stability and repeatability of the process. This is achieved with a fluid containment system that provides constant agitation and circulation to prevent sedimentation, constant temperature control with a heat exchanger, and use of an automated dripper system to make up for water lost through evaporation.Current MRF machine practice calls for periodic replacement of the MR fluid because it develops an unpleasant odor. Analysis of the used MR fluid shows evidence of corrosion. Here we present results from a study of corrosion in aqueous based MR fluids. Details of a spectrophotometric technique for quantifying iron ion corrosion product concentration in the supernatant are given. We find that the corrosion resistance of the CI powder itself is good. Experiments show that accelerated corrosion of the CI in the MR fluid is caused by additives incorporated into the cerium oxide abrasive.

10:30 AM HH3.6
DESIGN AND TESTING OF A NEW MAGNETORHEOMETER. Aric B. Shorey a,b, Stephen D. Jacobsa,b,c, William I. Kordonskid, Sergei R. Gorodkine, Kevin M. Kwongb, Univ of Rochester Materials Science Programa, Center for Optics Manufacturingb and Laboratory for Laser Energeticsc, Rochester, NY; QED Technologies, LLCd, Rochester, NY; Belarus Academy of Sciences, A.V. Luikov, Heat and Mass Transfer Institute, Minsk, BELARUSe.

Magnetorheological Finishing (MRF) is a newly developed and recently commercialized method for finishing optical components. The MR fluid is a water based suspension consisting of magnetic carbonyl iron (CI), nonmagnetic abrasives and various stabilizers. This fluid stiffens under the action of a magnetic field to provide a compliant, abrasive lap that is used in the computer-controlled MRF process. The increase in stiffness can be attributed to magnetic particle alignment along field lines which tends to create a chain structure. This means the bulk properties of the fluid will be anisotropic, and so it is important that the material be tested in the same orientation that it is to be used. There were several issues that were addressed in the manufacture of this device. The design of the pole pieces for the electromagnet proved difficult. The fluid must be subjected to a field that is symmetric in one direction, provides a gradient in a second direction while it is sheared in a third direction. Commercial software was used to model the field and aid in the design of the pole pieces. Slight variations in the pole design gave significant variations in field distribution as well as measured fluid properties. This will be demonstrated through experimental results. A second major issue that was addressed is compliance with the no slip boundary condition at the moving boundaries confining the fluid. The novel solution to this problem will also be discussed.

10:45 AM HH3.7
DYNAMICS OF A CONCENTRATED FERROFLUID EMULSION STUDIED BY DIFFUSING-WAVE SPECTROSCOPY. Serge Cutillas and Jing Liu, Department of Physics and Astronomy, California State University, Long Beach, CA.

A ferrofluid emulsion is used as a model magnetorheological fluid to study the effect of a dipole-dipole interaction on particle dynamics in concentrated particle region. When a magnetic field is applied, an induced dipolar moment appears on each droplet of ferrofluid dispersed in water. Dipolar forces between particles lead to an important structure change inside the emulsion. The emulsion goes from a simple colloidal system to complex aggregates like chains, columns or walls oriented to the magnetic field direction. In most current MR fluids applications, particle concentration is normally high. Thus, Diffusing Wave Spectroscopy is used to probe particle dynamics in these concentrated dipolar fluids. We show that the electric field autocorrelation function from multiple scattering light is not a simple decaying exponential but has a long tail following the decay at short times. This behavior is interpreted by the binding nature of particle interactions. Particles are trapped by the presence of the others all around them. This situation is very similar to that of colloidal crystals. However, essential differences exist since particle interactions are not controlled by typical isotropic interaction resulted from ionic or particle concentration, but rather by magnetic dipole-dipole energy that is anisotropic. From correlation function, the mean square displacement of particles in aggregates will be measured as a function of time. As the magnetic field strength and the particle volume fraction increases, we expect that the mean square displacement decreases reflecting the liquid to solid phase transition. Real time video microscopy is also used to correlate the structure with the dynamic measurements. Since a link is possible between dynamic properties measured optically and rheological properties, we will also compare our results with mechanical measurements of rheological properties in the same frequency range.

11:00 AM HH3.8
MAGNETORHEOLOGICAL MEDIUM BASED ON POLYMERIC COMPOSITIONS WITH FERROUS OXIDES. T.G. Lazareva , Institute of General and Inorganic Chemistry of the National Academy of Sciences of Belarus, Minsk, BELARUS.

The basic moments of synthesis of fillers of magnetorheological media based on iron oxides are discussed. An influence of the synthesis conditions, namely, a composition of the compounds, pH and a temperature of the solution, the presence of surfactants, precipitators on the structure and properties (magnetic, sorption properties) of the fillers are evaluated. The sorption capacity of various fillers with respect to a series of such water-soluble polymer dopants as cellulose esters, polyvinyl alcohol is determined. The conditions of synthesis of carbon-mineral fillers and their properties depending on the type of polymer dopants in synthesis, their concentration and heat treatment temperature are considered. A comparative study is made of magnetorheological properties of the compositions in oils depending on the type, structure, and properties of the fillers, the presence of surfactants, moisture content of the fillers. Recommendations are given on the synthesis of optimum magnetorhelological fillers of the investigated compounds. The main regularities of the formation of the compositions based on magnetic fillers (Fe3O4$\gamma$-Fe2O3) and binders - water-soluble carboxymethyl cellulose, methyl cellulose, polyvinyl alcohol phenol-formaldehyde, epoxyacrylic and pentaphthalic resins have been considered using the rheological method. The flow curves have been estimated depending on composition structure, type of binder and filler, composition temperature, size of filler's particles, it's porosity, the introduction of joining agents. The influence of the filler on the composition's viscosity and type of solution's flow has been estimated. The concentrational intervals of gelatinization depending on weight correlations of binder and filler have been found out. The phenomenological Akay-Leslie reological equation of state for anisotropic viscoelastic liquids (A-L model) was chosen for describing the flow resistance of the compositions. The data available as to the initial composition viscosity have been classified based on types of coherence characterized by the indication of degree of concentrational dependence of viscosity. The magnetorheological effect of compositions depending on the type of composition, weight correlation of the filler and binder, temperature and the type of coherence has been estimated. The conclusion has been made as to the influence of type of coherence (which depends on intermolecular interaction at the phases boundary line: polymeric solution - fillers and on initial solution structure) on magnetorheological effect.

11:15 AM HH3.9
SOUND PROPAGATION IN A MAGNETORHEOLOGICAL FLUID. Yuri Nahmad, Camilo Arancibia, Carlos Ruiz , Dept of Applied Physics, CINVESTAV-IPN, Merida, Yucatan, MEXICO.

The purpose of this report is to discuss experimental evidence of two long wavelength longitudinal acoustic modes propagating through a fluid made of hydrogen-reduced iron particles suspended in glycerine. The first mode travels through glycerine channels within the suspension whereas the second one propagates through the fibrillated structure formed by the iron particles. This second mode is very strong in amplitude and travels at very low speeds, depending on the applied magnetic field. The behavior of the system resembles the propagation of sound through a fluid-saturated porous material. However, some distinct physical effects are observed, like the structural saturation and the rheological hysteresis of the velocity of sound in the suspension. A time-of-flight acoustic technique, which consists in producing and detecting a pulse of pressure within the suspension, is used. The pulse is created by a plane piezo-electric transducer and detected by a similar piezo crystal.

11:30 AM *HH3.10
ASPECTS OF NONLINEAR CONDUCTION IN ELECTRORHEOLOGICAL SUSPENSIONS. P.J. Rankin, Y.M. Shkel, D.J. Klingenberg , Dept. of Chemical Engineering, University of Wisconsin, Madison, WI; J.L. Shohet, Dept. of Electrical and Computer Engineering, University of Wisconsin, Madison, WI.

It is well known that the field-controlled stress transfer properties of electrorheological suspensions arise from the electrostatic interactions between the polarized particles. Idealized models of particle polarization have successfully reproduced, at least qualitatively, a wide variety of experimental observations. However, nonideal features of particle polarization are apparent, especially at large electric field strengths where applications are likely to operate. Nonlinear conduction within the suspension is believed to dominate such nonidealities, giving rise to increased current and reduced stress transfer. We will present results from various experiments aimed at understanding the origin of nonlinear conduction in electrorheological suspensions. Varying the particles' surface chemistry can have a pronounced effect on the nonlinear character of the electrorheological response, suggesting that charge transport at the particle surface is of paramount importance. Transient electrical measurements reveal information about the polarization kinetics, providing additional insight into the physical chemistry of nonlinear conduction.
 

SESSION HH4: ELECTRORHEOLOGICAL FLUIDS/
SUSPENSIONS OF RODS
Chair: J. David Carlson
Tuesday Afternoon, April 6, 1999
Salon 13 (M)
1:30 PM *HH4.1
THE NONLINEAR RHEOLOGY OF ELECTRORHEOLOGICAL FLUIDS. James E. Martin , Sandia National Laboratories, Albuquerque, NM.

When a colloidal suspension is exposed to a uniaxial electric field the polarized particles chain along field lines causing a macroscopic solidification of the fluid, the basis of the so-called electrorheological (ER) effect. Likewise, in a rotating electric field particles form sheets in the plane of the field, which we call the rotary ER effect. Both of these fluids exhibit a nonlinear, shear thinning rheology, due to shear-induced structural relaxations. Because the fluid stress can be controlled by the applied field, a number of applications are possible, including electromechanical actuators, clutches, and active vibration dampers. To design these devices, and to develop effective control loop algorithms, it is necessary to understand the strongly nonlinear rheology of these fluids. We have used time-resolved, two-dimensional light scattering on a concentrated colloidal silica fluid in steady and oscillatory shear to demonstrate that the fragmentation and aggregation of chain-like particle microstructures is the cause of flow nonlinearities. We show that the light scattering is an indirect measure of the fluid stress. These observations form the basis of a kinetic chain model we developed to describe the nonlinear dynamics of the microstructures in ER fluids in nonstationary shear flows. Understanding the microstructural dynamics then leads us to a theory of the macroscopic rheology of these fluids in nonstationary, low Reynolds number flows. Finally, we have conducted extensive large-scale (1000-10000 particles) simulations of these fluids in steady and oscillatory shear, and will compare these results to theory and experiment.

2:00 PM HH4.2
STRUCTURE EVOLUTION AND CORRESPONDING ELECTRORHEOLOGICAL EFFECT. Weijia Wen , D.W. Zheng and K.N. Tu, Department of Materials Science and Engineering University of California at Los Angeles, CA.

We present an investigation about chain/column evolution and the corresponding electrorheological (ER) effect performed with glass/oil ER fluid. Our results demonstrate that once the field applied to the ER fluids surpasses a certain time period, the particles begin aggregating to form chains. It is found that both the initial response time and the chain's formation time depend on the conductivity of the microspheres. The overall trend is that the higher the micro-sphere's conductivity, the shorter the response time and the stronger the particles' interaction under the same external electric field. Moreover, These chains then coarsen and eventually form columns in the direction of the external field. We found that different column structures can be obtained depending on how the electric field is applied to the ER fluid. Only a loose column structure can be achieved if a square pulse field is applied to the ER fluid, yet a compact column is formed when the field strength is increased slowly. We have measured the ER effect with a sensitive yield-stress testing device as the structure varies. The results indicate that there exist three increasing tendencies of interaction among particles corresponding to three processes of sequential transition between states; they are (1) random spatial configuration to chain, (2) chain to metastable column, and (3) metastable column to stable column.

2:15 PM HH4.3
CONFOCAL MICROSCOPY STUDY OF ELECTRORHEOLOGICAL FLUIDS. Ujitha Dassanayake, Seth Fraden , Complex Fluids Group, Brandeis University, Waltham, MA; Alfons van Blaaderen, FOM Inst. for Atomic and Molecular Physics, Amsterdam, NETHERLANDS.

Specially synthesized fluorescently labelled core-shell silica spheres were used as a model electrorheological fluid to experimentally explore structure formation and evolution under conditions of no shear. Using Laser Scanning Confocal Microscopy we measured the location of each colloid in three dimensions. We observed the equilibrium body-centered tetragonal phase and several non-equilibrium structures such as sheet-like labrynths or isolated chains of colloids. The formation of non-equilibrium structures was studied as a function of the volume fraction, electric field strength and starting configuration of the colloid. We compare our observations to previous experiments, simulations and calculations.

2:30 PM HH4.4
A THERMODYNAMIC APPROACH TO FIELD-INDUCED STRESSES IN ELECTRO- AND MAGNETOACTIVE COMPOSITES. Yuri M. Shkel and Daniel J. Klingenberg , Dept. of Chemical Engineering, Univ. of Wisconsin, Madison, WI.

Field-induced stresses give rise to electro- and magnetoactive behavior in a variety of soft materials, such as electro- and magnetostrictive elastomers, and electro- and magnetorheological suspensions and gels. The study of electro- and magnetoactivity of stiff materials has traditionally focused on their apparent responses to applied fields. Such an approach is not appropriated for soft materials, where the deformation arising from field-induced forces is strongly affected by the conditions at the boundaries. We employ an alternative approach where field-induced stresses are determined directly. These stresses are related to the system free energy through thermodynamic variables that describe how the permittivitty (permeability) depends on material deformation. This reduces such problems as determining field-induced deformations or rheological properties to determining well-defined material parameters that are independent of boundary conditions. In this presentation, we will describe this thermodynamic formalism and illustrate examples where electroactive responses are described in terms of these thermodynamic parameters. We will show how these parameters can be determined via dielectric measurements, and present results for several different systems. We will also describe how these parameters can be calculated from microscopic model of the materials. Our results illustrate that electroactivity is very sensitive to the material microstructure, particularly the degree of anisotropy. For example, electrostrictive deformations of anisotropic structures are predicted to be more than order of magnitude larger than the response of an isotropic medium with the same chemical composition. The approach is quite general, capable of describing a variety of systems, such as polymer films, liquid crystals, as well as electro- and magnetorheological composites.

2:45 PM HH4.5
THE MOTION EQUATION OF ELECTRORHEOLOGICAL FLUID. Albert A. Mokeev , Vitebsk Univ, Vitebsk, Blearuss, Andrew A. Mokeev, Moscow Univ, Moscow, RUSSIA.

The current of the electrorheological suspensions in an electrical field is by sequence of breaks of a virtual lattice of a chains of particles, connected among themselves and with electrodes - walls of the channel - by the electrostatic interaction and them reconstructionÕs for small intervals of time about relacsation time is lower on current from others fragments of chains, that creates a pressure of resistance to current, not dependent on current velocity, similarly to a pressure of a dry friction.
Units of the particles which have been not included in a virtual lattice, as a result of collisions between by itself create resistance of a viscous friction, which pressure, proportional velocity of shift, exponential depends on a difference of bond energy of units and kinetic energy of their relative movement referred to their average energy, similar thermal. The particles not included in a virtual lattice and in units, electrically interact with collisions also will form a suspension component, rendering resistance to current proportional to its velocity, and linearly growing with growth of an intensity E of an electrical field. Total pressure of resistance to current linearly growing with growth E, that as against results of known theoretical works will be coordinated with the experienced data, with small velocity of current grows almost linearly with grow of a current velocity and after a failing site aspires again to whether - linearly to grow appropriate much to smaller viscousity.
The equation of a movement of electrorheological suspension with such rheological law has the kind similar to Nouet-Stoks equation, in which the effective structural viscosity whether the sum of the exponential function from a difference of value g1, proportional to the bond energy of units both square of a gradient of velocity and their function, so with small gradients - near to an axis of the channel, where the velocity is constant in process of shift from an axis, it transforms in the Nouet-Stoks equation with a large viscosity, and with large gradients of velocity - near to walls channel again coincides with the Nouet-Stoks equation with the small viscosity. The profile of velocity is a conjugate of the flat and abrupt parabolas, in comaxis - cylindrical slot-hole channel, in cylindrical coordinates this equation has a kind

\rm \rho \cdot \frac{dV_z}{dt}= -P + T \cdot \frac{Uo^2}{Eo^2}\cdot \frac{1}{r... ...ht)^2}{go(E)^2}{{ \right] \right] \cdot \left( \frac{d}{dr}V_z \right) \right]

P- the difference of pressure, Uo- electrical voltage between walls of the channel, Eo- intensity of an electrical field with which the saturation of polarization of particles initiates, $\eta$o - viscosity in absence of an electrical field, B(E) - linear component of viscosity, g1(E) - gradient of speeds appropriate to bond energy of unite, go- to average their energy.

3:30 PM *HH4.6
THE RANDOM DENSE ROD PACKING. Albert P. Philipse , Van 't Hoff Laboratory for Physical and Colloid Chemistry, Debye Institute, Utrecht University, Utrecht, THE NETHTERLANDS.

The experimentally observed asymptotic scaling $\phi$(L/D) = constant, for the random packing volume fraction ($\phi$) of rods with high aspect ratio (L/D) is shown to be simply related to (the distribution of) particle contacts in the packing. Consequences of this geometrical analysis are discussed for anisotropic colloids and powders. It is argued, for example, that many random rod packings are actually meta-stable rod glasses with respect to an ordered (nematic) structure.

4:00 PM *HH4.7
MATERIALS FROM CHIRAL ROD-LIKE CELLULOSE SPECIES. Derek G. Gray , Pulp and Paper Research Centre, McGill University, Montreal, CANADA.

Cellulose is the most abundant material in the biosphere, and cellulose fibres are constituents of wood, wood composites, paper and textiles. Natural cellulose fibres often show evidence of chiral structure. For example, the thickest layer in wood cell walls consists of cellulose microfibrils twisted into a shallow right-handed helix. This chiral structure results in a chiral response to changes in relative humidity. Quite different chiral structures and properties are displayed when natural cellulose fibres are broken down chemically to give rod-like colloidal particles. An aqueous suspension of these cellulose crystallites forms a chiral nematic liquid crystalline phase in dilute aqueous suspension. Removal of water gives a solid chiral nematic film composed of pure cellulose with the optical properties of cholesteric liquid crystals. Some other properties of cellulosic materials that result from of the chiral rod-like nature of their constituents will be discussed.

4:30 PM HH4.8
COLLOIDAL DYNAMICS IN A LIQUID CRYSTAL. Roan Lavery , Wilson Poon, Jason Crain, Univ of Edinburgh, Dept of Physics and Astronomy, Edinburgh, UNITED KINGDOM.

We present the results of a study of the translational diffusion of colloidal PMMA spheres in the isotropic phase of a liquid crystal. A significant increase in the hydrodynamic radius is seen as the nematic phase is approached. We attribute these observations to an increasingly ordered layer of liquid crystal molecules forming at the surface of the particles, an effect known as orientational pre-wetting. In addition, we present evidence that a small coating of another solvent can be formed around the particles during preparation and this coating will remain stable while the particles are dispersed in solution. This extra coating has the effect of altering the boundary conditions at the surface of the particles from the stick boundary condition, implicit in the Stokes-Einstein expression, to partial slip boundary conditions and consequently changing the dynamics dramatically. This is seen in both mesogenic and non-mesogenic solvents. For the case of a liquid crystal solvent the coating also affects the ability of the liquid crystal molecules to order at the surface of the particles, and therefore their ability to act as defect sites. This means that the isotropic-nematic phase transition temperature will be relativley unaltered by the presence of the particles. It is shown that larger coatings lead to a smaller reduction in the phase transition temperature and a decrease in orientational pre-wetting as the nematic phase is approached.

4:45 PM HH4.9
ENGINEERING ENTROPY: BUILDING ORDER WITH DISORDER. Zvonimir Dogic, Marie Adams, Seth Fraden , Complex Fluids Group, Department of Physics, Brandeis University, Waltham, MA.

We study mixtures of colloidal suspensions of rodlike and spherical colloids under conditions approximating hard particle fluids, meaning the colloids behave as impenetrable objects with no other interaction potential. These simple, hard particle fluids exhibit unexpectedly rich phase behavior ranging from liquid crystalline phases, to bulk separation of the rods and spheres, and to microphase separation of the rods and spheres in alternating layers. Through experiment, simulation, and theory we explore how phase behavior is controlled solely by the shape and concentration of the particles. Our studies suggest that the observed phase behavior is entropically driven by a competition between the entropy of mixing and packing of the particles. Because all molecules have a hard particle core, the phenomena are likely to be quite general, applying also for example to low-molecular mass liquid crystals, amphiphiles and block copolymers, to bioseparation methods and DNA partitioning in prokaryotes, and to protein crystallization, and the manufacture of composite materials.
 

SESSION HH5: PAPER, FIBER, POLYMERS
Chair: Seth Fraden
Wednesday Morning, April 7, 1999
Salon 13 (M)
8:30 AM *HH5.1
Abstract Withdrawn.

9:00 AM *HH5.2
THE ROLE OF FIBRE FLOCCULATION IN PAPER STRUCTURE. R.J. Kerekes , Pulp and Paper Institute of Canada, Pulp and Paper Centre, University of British Columbia, Vancouver, CANADA.

Pulp fibres flocculate into aggregates which impart a characteristic non-uniformity to paper on a scale of several millimetres. This non-uniformity, visible in transmitted light, diminishes the physical and optical properties of paper. Consequently, minimization of fibre flocculation has long been an objective of papermaking.
It was established many years ago that mechanical rather than colloidal forces governed fibre flocculation in the shear flows used in papermaking. However, the process by which individual flocs form and the conditions required for their creation have only recently been investigated in detail. This paper will review recent research on this topic at the University of British Columbia.
The paper will focus on the formation of fibre flocs, their propreties, and the role they play in the micro-uniformity of paper. Specific attention will be paid to the effect of fibre length, coarseness (weight per unit length), and refining on fibre flocculation in paper structure.

9:30 AM *HH5.3
TENSILE STRENGTH OF WET PAPER . Andre Schroder , Djamel Bensarsa, Ludwik Leibler, CNRS/ELF Atochem, Levallois-Perret, FRANCE.

Fiber bonds play a major role in the in-plane tensile strength of paper. It is well known that the Young's modulus (E) of a well-bonded dry paper sheet is one third of the Young's modulus (Ef) of its individual fibers. However, this relation is not valid for wet paper. We have measured in-plane Young's (E) and shear (G') moduli of wet paper sheets. Various wetting liquids have been studied. We have observed that the shear modulus (G') varyied over two decades from 0.1 MPa to 10 Mpa. We have also measured the Young's modulus (Ef) of wet individual fibers, which was found to be much higher than (G'). We find that the Young's modulus (E) of these various wet papers is proportional to their shear modulus (G'). Hence, we conclude that the Young's modulus (E) of wet paper is governed by the shear modulus of fibre-fibre crossings, rather than by the friction between fibers in the bonding region, as has been hitherto assumed. We can interprete quantitatively our results by considering a simple model for the fiber network. Moreover, the present study enables us to rationalize the role of wet strength additives, which in particular increase the shear modulus of fiber-fiber crossings.

10:30 AM *HH5.4
SIMULATIONS OF FLEXIBLE FIBER SUSPENSIONS. C.F. Schmid, L. Switzer, D.J. Klingenberg , Dept. of Chemical Engineering, University of Wisconsin, Madison, WI.

A simulation method is employed to investigate particle aggregation phenomena in fiber suspensions undergoing low Reynolds number flow. In contrast to aggregation in colloidal dispersions that arises from van der Waals attractive forces between particles, aggregation in elongated particle suspensions is attributed primarily to mechanical effects. Microscopic fiber features such as aspect ratio, intrinsic shape, flexibility, and surface roughness are thought to play a role in mechanical aggregation. Experiments aimed at elucidating the relationships between microscopic fiber features and macroscopic flocculation behavior are difficult as fibers are small, opaque, and rapidly moving in most applications. Thus, simulations that provide detailed information about the structure (fiber orientations, positions) and mechanics (interparticle forces, velocities) in the suspension can provide insight into the aggregation process. In this simulation technique, a fiber is modeled as a chain of rigid rods connected by elastic hinges. Fibers interact via excluded volume and frictional contact forces. The effects of such fiber features as elastic fiber deformation, irregular fiber equilibrium shape, and interparticle friction on the flocculation process are addressed. Interesting similarities between the rheology of aggregating fiber suspensions and micelle solutions that form shear-induced structures will be discussed.

11:00 AM HH5.5
ATTRACTIVE INTERACTIONS BETWEEN FILAMENTOUS BIOPOLYMERS OF LIKE CHARGES. Jay X. Tang , Paul A. Janmey, Brigham and Women's Hospital, Harvard Medical School, Boston, MA; Alexander Lyubartsev, Lars Nordenskiold, Stockholm University, Physical Chemistry, Stockholm, SWEDEN.

Interactions between charged macromolecules in solution are essential for the material properties of soft condensed matters including colloidal suspensions, polyelectrolytes, and other complex fluids. Our recent studies focus on the polyelectrolyte nature of a class of filamentous biopolymers, with the goal of best defining attractive interactions between them. Our approach is to compare results from experimental studies that define many features of counterion mediated lateral aggregation with Monte-Carlo simulations considering realistic ion size and molecular structures. Filamentous phages fd and M13, which are identical filaments with exception of different surface charge, are selected as model biopolymers for a quantitative comparison between simulation results and the experimental data. In such a context, we will discuss a number of intriguing features including sharpness of bundling transition, variation of the threshold condition, re-solubilazation, limited bundle size, and ion specificity. The knowledge obtained using model biopolymers is aimed at enhancing our overall understanding of both biological and industrial materials, and to provide valuable guidance for rational drug design, paper and food processing, etc.

11:15 AM HH5.6
AVOIDANCE MODEL FOR CHARGED SPHERES AND RODS BEYOND THE DILUTE LIMIT. Judith Herzfeld , Eric M. Kramer, Jining Han, Brandeis University, Dept of Chemistry, Waltham, MA.

The effects of soft repulsions are calculated using a relatively simple and intuitive model that takes account of local order. Comparison with exact results in the dilute limit indicates that the avoidance model is remarkably accurate for charged spheres. For charged spherocylinders, where further approximations are needed, the accuracy is less satisfactory, but still useful. Two advantages of the avoidance model are the ease of application to high concentrations and the concomitant estimation of local order. The local order is shown to decrease with increasing concentration, except for jumps at the long-range ordering transitions. The predicted shift of the isotropic-nematic transition to lower concentrations mimics the observed effect of electrostatic repulsions in suspensions of virus particles. Positional ordering transitions are also predicted to be shifted to lower concentrations, consistent with the observed behavior of charged lyotropics. This work was supported by National Institutes of Health grant HL36546 and National Research Service Award GM18932.

11:30 AM *HH5.7 SHAPE-DEPENDENT COLLOIDAL SEPARATIONS. Thomas G. Mason , Exxon Research and Engineering Co., Corporate Research, Annandale, NJ.

Dispersions consisting of both micron-sized disks and spheres in a micellar solution exhibit novel aggregation phenomena arising from entropic excluded volume interactions. By adjusting the micelle concentration, these attractive interactions can be controlled so that they induce a preferential aggregation of the disks into striking columnar structures, leaving the spheres unaggregated. These columnar aggregates of disks are spatially segregated from the spheres by creaming, providing a novel mechanism for shape-dependent colloidal separations.
 
 
 

SESSION HH6: POSTER SESSION
Wednesday Evening, April 7, 1999
8:00 P.M.
Salon 7 (M)
HH6.1
KINETICS OF AGGREGATION IN ASSOCIATING SULFONATED POLYSTYRENE IONOMER SOLUTIONS. Laura Harris , Robert A. Weiss and Thomas A.P. Seery, University of Connecticut, Chemical Engineering Department, Chemistry Department and Polymer Program, Storrs, CT.

There have been remarkably few studies of associating ionomers given their industrial significance. Solvent studies are extremely useful in developing an understanding of how isolated ionomer chains interact, which may then help explain bulk or solid phase interactions. Previous investigation of the aggregation behavior of solutions of the acid form of sulfonated polystyrene in decalin have characterized the phase behavior as a function of molecular weight and percent sulfonation. These studies have shown the existence of large aggregates that pre-associate above the macrophase separation temperature. Dynamic light scattering has shown a temperature and concentration dependence on the nature of the aggregates formed in solution. Parallel measurements on matched pairs of the ionomer and the parent homopolymer provide the means to isolate the effect of ionic moieties and compensate for variations in solvent quality or viscosity. The system under consideration has a macrophase separation line near the theta condition for the parent homopolymer so that chain associations are being probe under conditions where the backbone sections between ionic groups should be ideal gaussian subchains. Current efforts are focused on the rate of aggregate formation. By conducting dynamic temperature jump studies, we will gain an understanding of the kinetics of the microphase aggregate formation which appears to provide the driving force for phase separation.

HH6.2
ASSOCIATIVE POLYMERS BEARING n-ALKYL HYDROPHOBES: RHEOLOGICAL EVIDENCE FOR MICROGEL-LIKE BEHAVIOR IN ALKALINE MEDIA. Srinivasa R. Raghavan, Saad A. Khan , North Carolina State University, Dept of Chemical Engineering, Raleigh, NC; Robert J. English, The University of Leeds, Dept of Color Chemistry, Leeds, UNITED KINGDOM; Richard D. Jenkins, Union Carbide Asia Pacific Inc, SINGAPORE.

Rheological techniques are used to probe the behavior of hydrophobic alkali-swellable emulsion (HASE) polymers, bearing n-alkyl hydrophobes, in aqueous alkaline media. The polymers possess a comb-like architecture with a polyelectrolyte backbone (ethyl acrylate-co-methacrylic acid) and hydrophobes (ca. 10 per polymer chain) tethered to the backbone via polyether side chains. The size of the hydrophobes is varied from n-C8 to n-C20 in this study. We demonstrate that, at such a level of hydrophobic modification, and at relatively high polymer concentrations, the microstructure in these polymer systems is akin to that existing in concentrated microgels. Thus, the original polymer latex particles swell extensively in alkaline media and disintegrate to form a system of close-packed, compressible (``soft'') aggregates. This is reflected in the rheological response of the system, where we observe a ``pseudo-yield stress'' coupled with considerable shear-thinning under steady shear, and a characteristic power-law behavior of the dynamic elastic (G') and viscous (G'') moduli with frequency ()under oscillatory shear (G', G''   0.4) persisting to very long time scales. Concentration-independent master curves are obtained for the elastic modulus G', with the level of G' increasing with hydrophobe size. The similarity in the dynamic response suggests that there exists a qualitative equivalence in microstructure over the range of systems, the only difference being the ``softness'' or compressibility of the particles. Data from this study are also contrasted with those for a similar HASE polymer bearing a smaller number of alkylaryl hydrophobes [English et al. (1997)]. In the latter case, the rheology can be interpreted in terms of hydrophobic associations and chain entanglements occurring in solution. Thus, subtle variations in molecular architecture are shown to cause significant differences in morphology and microstructure for these polymer systems.

HH6.3
POLYURETHANE GELS IN THE CONTROLLED ADMINISTRATION OF ANTICANCER DRUGS. Erkesh O. Batyrbekov , Rinat Iskakov, Bulat A. Zhubanov, Institute for Chemical Sciences, Almaty, KAZAKSTAN.

Polymeric gels on the base of segmented polyurethanes are widely used as biomedical materials because of their excellent physico-mechanical properties. The most predominant factor is a phase separation being typical for these polymers. One allows mainly to regulate the release profile of incorporated drug. In the order to clarify the release of anticancer drugs, such as cyclophosphamide and blemicine, from segmented polyurethanes the supermolecular structure of microdomain polymer and the interaction between the drug and polyurethane were preliminary studied. Supermolecular structure of polyurethane gels was described with x-ray scattering ir-spectroscopy and proton NMR study. It was established the phase separation in polyurethane is intensified by means of increase of molecular weight of soft segments. The drug-concentrated domains of hard segments are microheterogeneously dispersed in the amorphous soft segments controlled a diffusion of drug from gels. These results indicate that supermolecular structure design of segmented polyurethane allows to control the drug release from the polymer matrix.


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3/8/1999