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 D—Liquid Crystal Materials and Devices

-MRS-

Chairs

Timothy Bunning
MLPJ
Air Force Wright Lab
Bldg 651
Wright Patterson AFB, OH 45433-7702
937-255-3808 x3177

Shaw Chen
Center for Optoelectronics
Univ of Rochester
COI Room 1213
Rochester, NY 14623-1212
716-275-0909

Liang-Chy Chien
Liquid Crystal Institute
Kent State Univ
Kent, OH 44242
330-672-3827

Tisato Kajiyama
Dept of Matls Physics & Chem
Kyushu Univ
Fukuoka, 812-81 JAPAN
81-92-6411101 x5601

Naoyuki Koide
Dept of Chemistry
Science Univ of Tokyo
Tokyo, 162-8601 JAPAN
81-3-3260-4272 x2262

Shui-Chih Lien
IBM T.J. Watson Research Lab
Yorktown Heights, NY 10598
914-945-3401

Symposium Support

  • Air Force Research Laboratory
  • Everlight Chemical Industrial Corp., Taiwan
 
Proceedings published as Volume 559
of the Materials Research Society
Symposium Proceedings Series.
* Invited paper
SESSION D1: NEW MATERIALS AND EFFECTS
Chairs: Liang-Chy Chien and E. Samulski
Monday Morning, April 5, 1999
Concordia (A)
8:30 AM *D1.1
JAVELINS, HOCKEY-STICKS, AND BOMERANGS; THE ROLE OF MESOGEN SHAPE IN PHASE STABILITY AND PHASE TYPE. E.T. Samulski , University of North Carolina, Department of Chemistry, Chapel Hill, NC.

In order to obtain more insight into the relation between molecular shape and phase symmetry and stability we have synthesized thiophene analogues of p-quinquephenyl (PPPPP). We have systematically ``moved'' 2.5-thiophene from a terminal position (PPPPT) to the center of the molecule (PPTPP, a symmetric bent molecule), and by doing so, changed the overall molecular shape. Liquid crystal transition temperatures drop considerably when the non-linear thiophene segment moves to the center of the molecule, and the asymmetric molecule PPPTP exhibits a broad LC phase (289-356C) and displays both a smectic A and nematic phase. Such systems, are particularly suited for realistic Monte Carlo simulations and it is anticipated that a comparison of simulation results with experiment will provides an assessment of the influence of molecular shape and dipolar interactions on phase structure.

9:00 AM *D1.2
DIRECT STRUCTURAL CHARACTERIZATION OF FERRI- AND ANTIFERRO-ELECTRIC CHIRAL SMECTIC-C LIQUID-CRYSTAL PHASES. Peter Mach , Dept of Physics, University of Minnesota, Minneapolis, MN.

A great deal of effort has been directed in recent years at determining the detailed structures of various chiral smectic-C phases exhibiting antiferroelectric (SmCA*) or ferrielectric (SmCFI1*, SmCFI2*, SmC$_{\alpha}$*) electrooptic responses. Along with the ferroelectric SmC*, these phases hold great promise for future generations of high-speed optical switching devices. By applying polarized resonant x-ray scattering at the sulfur K edge to freely-suspended films of one thiobenzoate liquid-crystal enantiomer, we made the first direct structural observations of superlattice periodicities associated with the antiferro- and ferri-electric phases [1]. Respectively, we observed helical superlattices of four-, three-, and two-layer periodicity in the SmCFI2*, SmCFI1*, and SmCA* phases, and periodicity incommensurate with layer spacing in SmC$_{\alpha}$*. For x-ray energy differing from the sulfur K edge by more then 25 eV, the diffraction peaks of superlattice origin disappear. We have also measured the polarization states of these resonant diffraction peaks. The observed polarizations agree with the clock model of chiral smectic-C variants, and rule out other proposals made to date for these structures. Results on the thiobenzoate racemic compound and a thiophene-containing liquid-crystal will be presented as well. 1. P. Mach et al., Phys. Rev. Lett. 81, 1015 (1998). *Collaborators: R. Pindak, A.-M. Levelut, P. Barois, H. T. Nguyen, J. Goodby, C. C. Huang, and L. Furenlid.

9:30 AM *D1.3
FERROELECTRIC AND ANTIFERROELECTRIC LIQUID CRYSTALS FROM ACHIRAL MOLECULES. David M. Walba , Eva Korblova, Univ of Colorado, Dept of Chemistry, Boulder, CO; Renfan Shao, Joseph E. Maclennan, Darren R. Link, Noel A. Clark, Univ of Colorado, Dept of Physics, Boulder, CO.

In bent-core mesogenic structures and in tilted bi-layer smectics, a class of ferro- and antiferroelectric phases composed of achiral or racemic molecules has recently been discovered. A discussion of the requirements for antiferroelectricity in such systems will be given, and the directed design of new antiferroelectric and ferroelectric phases composed of racemic molecules will be described. Achiral symmetry breaking in the ferroelectric phase provides a unique liquid crystalline conglomerate.

10:30 AM *D1.4
ROLE OF THE CHOLESTERIC PHASE IN THE FORMATION OF TWISTED SMECTIC STRUCTURES. S.W. Suh, Licom Technologies, Inc., State College, PA; Angela L. Campbell, Materials and Manufacturing Directorate, AFRL/MLPJ, Wright-Patterson AFB, OH; J.S. Patel , Department of Physics and Electrical Engineering, The Pennsylvania State University, University Park, PA.

We have recently demonstrated a new ferroelectric liquid crystal structure that combines the fast speed of ferroelectric and the optical behavior of the twisted nematic structures. This ferroelectric twisted smectic structure (FETS) allows fast switching and at the same time allows variable transmission controlled by an externally applied field. In the talk, we will present our recent results, which show the importance of structure in the cholesteric phase in the formation of twisted smectic structure. We also show that a presence of a helical inversion close to the cholesteric to smectic C* phase transition temperature in our material is responsible for the different structures that are obtained by using different cooling rates.

11:00 AM *D1.5
PHOTOCONDUCTIVITY OF DISCOTIC LIQUID CRYSTALS DERIVED FROM A LONG-CHAIN TETRAPHENYLPORPHYRIN. Yo Shimizu , Hirosato Monobe, Shoji Mima, Taizo Higashiyama, Tomonori Fuchita, Takushi Sugino, Osaka National Research Institute (ONRI) AIST-MITI, Dept. of Organic Materials, Ikeda, JAPAN.

Some tetraphenylporphyrin-based mesophase materials (C15TPP) were synthesized and investigated on their photoconductive properties. These all exhibit one or two lamellar mesophases with high order of molecular alignments. All measurements of photocurrent were carried out for ITO/C15TPP/ITO type cells. The metal-free compound showed strong dependence of photocurrent behavior on the mesomorphic phase transitions. A drastic change of the applied voltage characteristics was observed on the phase transition between two lamellar mesophases and this phenomenon was interpreted in terms of the variant mechanism of charged carrier generation depending on the phase transition by the results of the action spectra and illumination light intensity dependence of photocurrent.

11:30 AM *D1.6 THE EFFECT OF SIDE CHAIN SUBSTITUTION ON THE FIELD-DEPENDENT BIREFRINGENCE IN A SERIES OF CHIRAL SMECTIC A LIQUID CRYSTALS. J.R. Lindle , F.J. Bartoli, S.R. Flom, J.V. Selinger, R. Shashidhar and B.R. Ratna, Naval Research Lab, Washington, DC.

The field-dependent birefringence and the electroclinic tilt angle are measured for a homologous series of four chiral liquid crystals in the smectic A phase. These are designated as KN125, SiKN105, DSiKN65 and TSiKN105. The aromatic core and chiral center of these molecules are identical, and they differ only in the side-chain substituent at the end opposite to the chiral center. The zero-field birefringence decreased significantly and monotonically with the length of the siloxane side chain. Measurements of the wavelength dispersion of the birefringence show that changes in the resonant wavelength of the molecules account for only a small fraction of the observed results. This work examines the origin of the change in birefringence and possible roles played by changes in the polarizability anisotropy and the molecular order parameter for this series. In all four samples, both the electroclinic tilt angle and the birefringence increased substantially with the application of a field. At 10 V/um, the electroclinic tilt angle in DSiKN65 and TSiKN105 exceeded 22.5 degrees and the phase retardation due to the change in the birefringence exceeded a quarter wave. In all four liquid crystals, the electroclinic tilt angle and the field-induced change in birefringence were found to follow a quadratic relationship, and surprisingly the same quadratic coefficient could be used to fit the data for the four samples. Moreover, this quadratic relationship persists even at high fields where both processes undergo saturation. A model based on a field-dependent order parameter is presented to describe the strong coupling observed between the field-dependent birefringence and the electroclinic effect. The mechanisms responsible for the large induced change in the birefringence and the implications for device performance will be discussed.
 

SESSION D2: POLYMER/LIQUID CRYSTAL COMPOSITES
Chairs: Timothy J. Bunning and Toru Fujisawa
Monday Afternoon, April 5, 1999
Concordia (A)
1:30 PM *D2.1
AN ANALYSIS OF THE PHOTO-POLYMERIZATION INDUCED PHASE SEPARATION PROCESS IN LIQUID CRYSTAL/POLYMER COMPOSITE FILMS. Toru Fujisawa , Masao Hayashi, Hideo Nakada, Siger Matumoto, Yuitiro, Tani, Masao Aizawa, Dainippon Ink & Chemicals, Inc., Central Research Laboratories, Sakura, JAPAN.

In the optical device that is prepared by the photo-polymerization induced phase separation process, the molecular structures, molecular interactions of the materials at a polymer surface, and the morphology play an important role to determine their electro-optical properties. The morphology in the films is strongly affected by several parameters such as UV curing conditions, LC concentration, and a type of materials. Therefore, understanding the relation between the phase separation process during photo-polymerization and the parameters based on the materials is extremely important to control of microstructure in the films. In this paper, we present the results of analysis in the micro-phase separations during photo-polymerization based on the system that employs mixtures of liquid crystal materials containing alkyl cyanobiphenyl compounds and various photo-curable diacrylates having the different alkyl main chain in order to investigate the effects of the molecular structure, because the microstructure in the films are dependent on what kinds of materials are used. In the phase diagram during polymerization, the change in the phase diagram during curing depends on the type of monomers. In the measurement of time resolved light-scattering profiles during curing, it is confirmed that the process of phase separation during curing is affected by the number of methylene groups in the main chain of diacrylates. Moreover, from the scanning electron micrographs, we can observed the tendency that the shape of surface in polymer layer in the films changes a smooth into a interconnected spherical particles as the number of methylene groups decreases. Consequently, it turned out that the molecular structure in diacrylate reflects the process of phase separation during curing.

2:00 PM *D2.2
FACTORS AFFECTING NANO-SCALE MORPHOLOGY IN PDLC BRAGG GRATINGS. Pogue, R.T. , S.A. Siwecki, L.V. Natarajan, V.P. Tondiglia, R.L. Sutherland, Science Applications International Corp., Beavercreek, OH and T.J. Bunning, AFRL, WPAFB, OH.

Polymer-dispersed liquid crystal (PDLC) volume gratings have all of the advantages and applications of traditional holographic gratings with the additional benefit that the optical properties can be electrically switched. The optical and electro-optical properties of these gratings, however, are determined by the nano-scale morphologies of the PDLC films. In order to optimize these properties it is necessary to understand the factors which contribute to the final film morphologies and how these factors can be controlled. In this study we have evaluated formulation and processing changes which effect these morphologies. Such factors as average monomer functionality, photoinitiator concentration and cure temperature have been studied and each is shown to have a significant effect on the resulting film morphologies. Polymerization kinetics have been investigated using differential photo-calorimetry (DPC), and the film morphologies were evaluated using low-voltage SEM. The results indicate that increased functionality, altered either by a change in the primary monomer or the co-monomer concentration leads to larger LC domains, higher diffraction efficiencies and lower switching voltages. Increased initiator concentration also lead to larger LC droplets. Domain size decreased, however, with increasing cure temperature. This result was initially unexpected. Discussion in this presentation will focus on the effects of each of these parameters on the film properties and will briefly include some mention of the mechanisms that may drive the phase separation process. Some relationships will be postulated based on comparison of the polymerization kinetics with respect to real-time diffraction efficiency measurements obtained during writing of the holographic gratings.

2:30 PM D2.3
EFFECT OF PHASE ORDERING ON PHASE SEPARATION KINETICS IN LIQUID-CRYSTAL/POLYMER MIXTURES. A.M. Lapeñ a, Univ of California-Los Angeles, Dept of Chemistry, Los Angeles, CA; S.A. Langer, NIST, Information Technology Laboratory, Gaithersburg, MD; S.C. Glotzer, NIST, Polymers Division and CTCMS, Gaithersburg, MD; A.J. Liu, Univ of California-Los Angeles, Dept of Chemistry, Los Angeles, CA.

Materials based on mixtures of liquid crystals with a small amount of polymer are used for a variety of electro-optical devices, because the polymer tends to form a network that aligns the liquid crystals. Often, making such mixtures involves the interplay of three kinetic processes: phase separation, phase ordering, and polymerization. We focus on the interplay of the first two processes, using a simple model.1,2 This model allows for composition and orientation fields to evolve with time in a coupled fashion. We conduct numerical analyses and present results for a system quenched from a mixed isotropic phase into coexisting polymer-rich isotropic and liquid-crystal-rich nematic phases. We find that orientational ordering not only affects resulting domain morphologies, but also affects the time evolution of domain growth.
1A.J. Liu and G.H. Fredrickson, Macromolecules 29, 8000, (1996)
2A.M. Lapeña, S.C. Glotzer, S.A. Langer, and A.J. Liu. Preprint.

2:45 PM D2.4
THE EFFECT OF FLUORINE-SUBSTITUTED MONOFUNCTIONAL MONOMERS ON POLYMER DISPERSED LIQUID CRYSTAL ELECTRO-OPTICAL AND MORPHOLOGICAL PROPERTIES. Michael D. Schulte and Stephen J. Clarson, University of Cincinnati, Dept MS&E, Cincinnati, OH; Lalgudi V. Natarajan and Vincent P. Tondiglia, SAIC Inc., Dayton, OH; Timothy J. Bunning, AFRL/MLPJ WPAFB, OH.

Floodlit and holographic polymer dispersed liquid crystals (PDLCís) with semi-fluorinated host matrices have been investigated. Films were formed by photopolymerization-induced phase separation from an initially isotropic solution of monomer and liquid crystal. We report the observed electro-optical and morphological properties that result from the systematic addition of fluorine-containing acrylates and methacrylates to a pentaacrylate-based prepolymer solution. General increases in polymer network phase separation were observed using low voltage scanning electron microscopy (SEM) in floodlit specimens with additions of fluorinated monomers up to 10% by weight. Electro-optical properties such as contrast ratio, switching voltage and relaxation time are reported. Contrast ratio and relaxation time increased with fluorinated monomer concentration in floodlit films. The less reactive methacrylate monomers revealed greater relaxation times and tended to form large polymer beads while the faster reacting acrylate monomers resulted in smaller polymer aggregates and shorter relaxation times. Photo differential scanning calorimetry (PDSC) was utilized to study the polymerization reaction kinetics of prepolymer syrups. In holographic PDLC films, the inclusion of fluorine-substituted monomers produced initially promising morphological features.

3:30 PM *D2.5
LIGHT TRANSMISSION-LIGHT SCATTERING REVERSE MODE SWITCHING OF (LIQUID CRYSTALLINE POLYMER/LIQUID CRYSTALS) COMPOSITE SYSTEM. Hirotsugu Kikuchi , Shigeru Kibe, Tisato Kajiyama, Kyushu University, Graduate School of Engineering, Fukuoka, JAPAN.

Conventional (polymer/liquid crystal) composite films exhibit the normal mode electro-optical switching, that is, the light scattering-light transmission switching upon electric field OFF-ON states, respectively. In this study, the ``reverse mode'' electro-optical switching, that is, the light transmission-light scattering switching upon electric field OFF-ON states, respectively, has been investigated for a novel type of composite system consisting of the side chain type liquid crystalline polymer (LCP) and the low molecular weight liquid crystals (LCs). The polarizing optical microscopic observation and X-ray diffraction study revealed that the two types of smectic-like short range orderings were present in the (LCP/LCs) composite system in a nematic state. It was ascertained from the electric capacitance measurements of the homogeneous alignment cell that these phases with different smectic-like short range orderings exhibited each different value of threshold voltage. Although the homogeneous alignment cell of the (LCP/LCs) composite system was the light transmission state in the absence of an electric field, the cell turned into the light scattering state upon the application of electric field of 1.0-1.5 MV/m. The transmission-scattering states were reversible in the electric field OFF-ON states, respectively. The light scattering state of the (LCP/LCs) composite system upon the application of electric field might be due to the appearance of optically heterogeneous structure induced by heterogeneous reorientation behavior of the mesogenic groups of LCP and LC molecules in an electric field ON state. A possible origin for the field-induced heterogeneous reorientation is the different values of the threshold voltage between the two types of smectic-like phases with different short range orderings. These phases might be in a phase-separated state with an optical dimension. In conclusion, the reversible ``reverse mode'' electro-optical switching was realized for the (LCP/LCs) composite system in a nematic state in which the two types of smectic-like short range orderings were separately formed in optical size level.

4:00 PM *D2.6
POLYMER DISPERSED LIQUID CRYSTALS AS OPTICAL STORAGE MATERIALS. Francesco Simoni , Sonia Di Bella, Liana Lucchetti, Dept. Scienze dei Materiali e della Terra, Univ. Ancona, Ancona, ITALY; Gabriella Cipparrone, Alfredo Mazzulla, Dept. Fisica, Univ. della Calabria, Cosenza, ITALY and Istituto Nazionale per la Fisica della Materia.

In the last years several experiments have been carried out in order to investigate memory effects in Polymer Dispersed Liquid Crystals (PDLC). Two different approaches have been used: in the first one the optical information is stored during the polymerization process by fixing a specific droplets' morphology in the material, while in a second approach the information is recorded on a PDLC sample after the curing process, by exploiting different phenomena. We present our more recent results obtained by the mentioned methods. In the first case UV curable polymers have been used and a detailed investigation of the phase separation process has been performed using both conventional UV lamp and UV laser lines. Very different morphologies have been obtained leading to droplets or channels formation. Under different experimental conditions either opaque images on transparent background or transparent images on opaque background have been obtained. Moreover all the recorded images could be switched off by a moderate applied voltage. Using the second method of recording optical patterns on already cured samples, it was possible to write and erase many times holographic gratings on the same spot. Two beams interference was used to record the grating, while single beam illumination lead to erasure of the optical pattern. A detailed investigation of this phenomenon has demonstrated that photorefractive effect plays a major role in this process.

4:30 PM D2.7
OPTICALLY SWITCHABLE GRATINGS BASED ON POLYMER-DISPERSED LIQUID CRYSTAL FILMS DOPED WITH A GUEST-HOST DYE. Andy Ying-Guey Fuh , Ming-Shan Tsai, Li-Jue Huang and Tsung-Chi Liu, Department of Physics, National Cheng Kung University, Tainan, TAIWAN.

In this work, we add a small amount of a guest-host dye G-206 in the polymerdispersed liquid crystal (PDLC) mixtures to fabricate holographic gratings. Following formation, the grating is irradiated by a single Ar' laser beam while probed by a He-Ne laser. Experimental results indicate that the first-order diffracted intensity of the probe beam significantly increases. Such an optically switchable diffraction effect is attributed to thermal expansion. The G-206 dye absorbs Ar+ laser subsequently resulting in thermal expansion. The liquid crystal (LC) droplets are then squeezed so that the LC molecules within the droplets are reoriented collectively. Thus, the refractive index difference between the LC-rich and polymer-rich stripes, $\Delta$ n, increases.

4:45 PM D2.8
OPTICAL POWER LIMITING IN POLYMER-DISPERSED LIQUID CRYSTAL FILMS DOPED WITH FULLERENES. N.V. Kamanina , L.N. Kaporskii, Vavilov State Optical Inst, St. Petersburg, RUSSIA.

Effect of reverse saturable absorption has been investigated in the liquid crystal composites based on polymers (polyimide and polyvinyl alcohol) and fullerenes. With stirring, the initial mixture of polymer and nematic liquid crystal (NLC) was prepared in the ratio of 3 polymer to 2 NLC as long as an uniform emulsion was obtained. ZhK999, ZhK1289, and E7 were used as NLC. A finely divided powder of fullerenes was introduced either in NLC or in a polyimide solution in 1,1,2,2-tetrachloroethane. The emulsion was poured over a substrate with calibrated spacers followed by drying in order to remove the solvent.
A thickness of the samples was 10$\mu$m. A size of NLC drops in the polymer matrix was 2$\mu$m. The second harmonic of a pulsed Nd-laser at wavelength of 532 nm was used to investigate optical limiting.
Under laser radiation the samples with fullerenes in millijoule range of powers, the attenuation of laser power was found at least by a factor of 10-15. The effect was caused by an increase in absorption cross-section of the fullerene molecule in excited state in comparison with that in ground state. By contrast, transmission augmentation was observed in the samples without fullerenes. The transmission augmentation was caused by laser-heating NLC, resulting in the transition of NLC to isotropic phase with refractive index close to the one of the polymer. In this case transmission was varied by the factor 1.7-2. Moreover, a memory effect was observed after the transmission augmentation of the samples when the laser power decreased.
 

SESSION D3: NEW MATERIALS AND EFFECTS
Chair: Liang-Chy Chien
Monday Evening, April 5, 1999
Concordia (A)
8:00 PM D3.1
COMPLEX FORMATION IN MIXTURES OF CYANOBIPHENYLS AND 4, 4$^{\prime}$-DISUBSTITUTED DIPHENYLDIACETYLENES. Mary E. Neubert , Sandra S. Keast, Erica E. Humeniuk, Margaret E. Walsh and Kyle J. Miller, Glenn H. Brown, Liquid Crystal Institute, Kent State Univ., Kent, OH; Rolfe G. Petschek, Physics Dept, Case Western Reserve Univ., Cleveland, OH.

A variety of 1:1 mixtures of 4,4'-disubstituted-diphenyidiacetylenes (PTTP series) with cyano containing mesogens were investigated for possible complex formation. Complexes were formed when alkyl/alkoxycyanobiphenyls (CB series) were mixed with alkyl/alkoxy PTTP homologs. Two types of complexes were observed, depending on chain length and the presence of an alkoxy group: those with a melting temperature higher than that for either component and those with lower melting temperatures. A fluro substituent on the PTTP gave an enhanced nematic phase and a cyclohexane ring in place of a benzene ring yielded an enhanced smectic A phase but a cyano group produced a poor mixture. Phase diagrams were constructed for PTTP-24/5-CB and PTTP-24/80-CB. Both complexes formed had melting temperatures higher than those for either component. These complexes were recrystallized and shown by X-ray studies to be 1:1 complexes. The PTTP-24/5-CB complex showed only a monotropic nematic phase but the PTTp-24/80-CB complex showed induced smectic A and hexatic B phases in addition to an enantiotropic nematic phase.
*This work was supported in part by the NSF Center for Advanced Liquid Crystalline Optical Materials (ALCOM), DMR89-20147.

8:15 PM D3.2
RELATION BETWEEN MOLECULAR AND MACROSCOPIC PROPERTIES OF NEMATIC LIQUID CRYSTALS. Kim F. Ferris and Gregory J. Exarhos, Pacific Northwest National Laboratories, Richland, WA; Steven M. Risser, Texas A&M University Commerce, TX.

While many studies have focused on static structural and electronic properties, dynamic factors such as molecular anisotropy, partial ordering and structural fluctuations have hindered descriptions linking liquid crystalline properties from the molecular level with the macroscopic level (as is measured experimentally). In this paper, we examine the transition from molecular to bulk properties for the nematogen 5-alkyl-cyanobiphenyl (5CB), using both molecular dynamics and quantum mechanical calculations. We perform molecular dynamics simulations of a collection of molecules in the nematic phase, and sample the trajectory to create a representative collection of structures of individual molecules. We also create a collection of structures for clusters of 5CB molecules. We then perform electronic structure calculations on the structures sampled from the trajectory. By varying the number of molecules included in the cluster, we explore the evolution of the electronic properties from the molecular to the macroscopic level. We will also compare these all-molecule results to calculations using point-charge representations of adjacent molecules to examine how inter-molecular interactions alter the collective response. This work was supported in part by the US Department of Energy, Office of Basic Energy Sciences, Materials Sciences Directorate under contract DE-AC06 76 RLO 1830.

8:30 PM D3.3
THE DYNAMIC PROPERTIES OF 5CB FILLED WITH AEROSIL PARTICLES INVESTIGATED BY PCS. F.M. Aliev a, M. Kreuzerb and Yu.P. Panarina, aDept. of Physics, University of Puerto Rico, San Juan, PUERTO RICO; bIAP, Darmstadt University of Technology, Darmstadt, GERMANY.

Nematic liquid crystal filled with aerosil particles, the prospective composite material for optoelectronic application, has been investigated by static light scattering and Photon Correlation Spectroscopy (PCS). The aerosil particles in filled nematics (FN) liquid crystals form the network structure with linear size of LC domains about 250 nm with random distribution of the director orientation of each domain. From this point of view, the dynamical properties of filled nematics are expected to be similar the properties of LC confined in random porous matrices. However the flexibility of the framework of filled nematics may cause new properties and phenomena which absent in rigid matrices. We found that the properties of 5CB are considerably affected by the network. The N-I phase transition in filled 5CB was found to be depressed while the nematic phase retains even 20 K below crystallization point of bulk 5CB. PCS experiment shows that two new relaxation processes appear in filled 5CB in addition to the director fluctuation process in bulk. The slow relaxation process, with a broad spectrum of relaxation times, is somewhat similar to the slow decay, which observed in confined nematic liquid crystal. The middle frequency process, which doesn't exist neither in bulk nor in pores, was assigned to the director fluctuations in the surface layer formed at particle-LC interface. The decay function describing this relaxation process is stretched exponential $(\beta \approx 0.7)$. The temperature dependence of the relaxation times of the middle frequency process was found to follow to Vogel-Fulcher low, while the temperature dependence of the relaxation times of the bulk-like process was not. Such a temperature dependence, accompanied by broad spectrum of relaxation times suggests the glass-like dynamics of the director fluctuations near the aerosil particle-LC interface.
 

SESSION D4: LIQUID CRYSTALLINE POLYMERS AND APPLICATIONS
Chairs: Shaw H. Chen and Julie A. Kornfield
Tuesday Morning, April 6, 1999
Concordia (A)
8:30 AM *D4.1
DYNAMICS OF SIDE-GROUP LIQUID CRYSTALLINE POLYMERS. J.A. Kornfield , M. Kempe, W.J. Zhou, Caltech, Pasadena, CA; C. Ober, Y.U. Kim, Y.C.Chao, Cornell University, Ithaca, NY.

Chain dynamics of side-group liquid crystalline polymers provide a powerful tool to manipulate alignment of the liquid crystalline phase formed from their side groups. As the length of the chains increases, their relaxation dynamics slow down. This can be used to advantage, since deformation of the material can then readily alter chain conformation. Therefore we are interested in synthetic strategies to produce well-defined SG-LCPs that are long relative to their entanglement molecular weight ($M_e \sim$ 300,000 g/mol). Extending polymer analogous chemistries to produce SG-LCPs of a few million g/mol, we can prepare polymers with narrow distributions of chain length starting from anionically synthesized prepolymers (e.g. 1,2-polybutadiene). The dynamics of these materials in the melt and when solvated by small-molecule liquid crystals will be described.

9:00 AM D4.2
EFFECT OF COMPOSITION ON THE MORPHOLOGY AND ELECTRO-OPTICAL PROPERTIES OF PHYSICALLY CROSSLINKED LIQUID CRYSTALS. Steffen Geppert, Armin Schneider, Richard Spontak , Wolfram Gronski, Heino Finkelmann, Albert Ludwigs Universitat Freiburg, Inst fur Makromolekulare Chemie, Freiburg, GERMANY.

In a conventional polymer-dispersed liquid crystal (PDLC) composite material, a small-molecule liquid crystal is uniformly dispersed within a matrix of glassy polymer typically through either polymerization-induced phase separation or emulsification. The liquid crystals residing in the resultant dispersions inherently possess a random molecular director, but can be oriented through the application of an electric field to yield a highly transparent medium. An alternative to PDLCs, in which the liquid crystal constitutes the minor component confined within discrete micron-size dispersions, is the physically crosslinked liquid crystal (PCLC). A PCLC consists of liquid crystal molecules physically bound within a linear multiblock copolymer network. In this case, the copolymer comprises the minor component, whereas the liquid crystal is the major component. In this work, we report on the phase stability, morphological characteristics and electro-optical properties of blends of E7, a commercial liquid crystal mixture exhibiting a nematic mesophase, and novel triblock copolymers with styrenic endblocks and side-chain liquid crystalline midblocks. Since the copolymer endblocks are glassy at ambient temperature, they serve as physical crosslink sites, thereby forming a thermally reversible copolymer network in which the E7 molecules reside.

9:15 AM D4.3
DESIGN AND SYNTHESIS OF THERMOTROPIC CHIRAL-NEMATIC CONJUGATED POLYMERS. John C. Mastrangelo , Brooke M. Conger, Philip H. Chen, Shaw H. Chen, Andrew S. Kende, Materials Science Program, Departments of Chemical Engineering and Chemistry, Center for Photoinduced Charge Transfer and Laboratory for Laser Energetics, University of Rochester, Rochester, NY.

With various nematogenic and chiral pendants, thermotropic polymers with p-phenylene, thiophene, and p-phenylenevinylene conjugated backbones were synthesized and characterized. Spontaneous assembly of the conjugated systems into helically stacked planes inherent to cholesteric mesomorphism was characterized with circularly polarized fluorescence. Unique features include: (1) ability to form macroscopically ordered solid films; (2) both right- and left-handded circular polarization of incident light; (3) circularly polarized photoluminescence with unpolarized, UV-excitation. Potential applications of this novel class of materials are being explored.

9:30 AM D4.4
SYNTHESIS AND OPTO-ELECTRIC PROPERTIES OF LIQUID CRYSTALLINE POLYMERS CONTAINING CARBAZOLYL GROUP. Naoyuki Koide , Koji Arai, Science University of Tokyo, Department of Chemistry, Tokyo, JAPAN; Tetsuya Wada and Hiroyuki Sasabe, The Institute of Physical and Chemical Research, Bio-polymer Physics Laboratory, Saitama, JAPAN.

The paper reports synthesis of side-chain type liquid crystalline polymers containing carbazolyl group and their thermal and opto-electric properties. The polymers containing carbazolyl group at the side of methacrylate polymer backbone via spacer length exhibited smectic phases. One of the methacrylate type polymer( PMICZ) was turned out to have photoconductivity, and a hole mobility of PMICZ was 4.24 x 10-5 cm2/Vs at an electric field of 3.00 x 105V/cm at room temperature by the time-of-flight measurements. This value of PMICz was superior to that of PVK. In general amorphous polymer, such as PVK, the mobility of PVK becomes larger with increasing electric field. However, the mobility of PMICZ became smaller in proportion to the applied electric field. This tendency would be attributed by characteristic of the mesogenic state .

9:45 AM D4.5
DETERMINATION OF LOCAL FIBER TEXTURE IN LIQUID CRYSTAL POLYMER BY ELECTRON DIFFRACTION. Jennifer Taylor , Matthew Libera, Stevens Institute of Technology, Dept of Materials Science, Hoboken, NJ.

Liquid crystal polymers (LCP) are being increasingly used for applications requiring light weight material with high yield strength and high elastic modulus. High yield strength originates from crystalline and semicrystalline LCP domains separated by poorly oriented or amorphous LCP matrix. A ``skin-core effect'' is observed in certain LCP fibers and moldings where an unoriented core is covered by a surface layer of variable thickness oriented along the flow direction. By controlling the shear dynamics during melt-forming operations, the morphology may be manipulated to maximize crystallinity thereby improving mechanical properties. Conventional selected-area electron diffraction is being used here to study the morphological variations resulting from the ``skin-core effect.'' Successive thin sections from an LCP fiber exhibit features indicating a varying degree of orientation from the skin to the center of the fiber. A simulation of chain morphology is also presented to understand the real-space orientations represented by the diffraction data. The effects of electron radiation damage are qualified by the decay of diffraction features as a function of electron dose.

10:30 AM *D4.6
NEW FERROELECTRIC LIQUID CRYSTALS FOR NON-DISPLAY APPLICATIONS. W. Haase, D. Ganzke, T. Weyrauch, Institute of Physical Chemistry, Darmstadt University of Technology, Darmstadt, GERMANY; E. Pozhidaev, P.N. Lebedev Physical Institute, Moscow, RUSSIA, Institute of Physical Chemistry, Darmstadt University of Technology, Darmstadt, GERMANY.

New Ferroelectric Liquid Crystals (FLC's) considering mainly Deformed Helix Ferroelectric (DHF) or Electroclinic (E) effect will be demonstrated. The feasibility of the electrooptical performance of broad temperature range DHF FLC's from below -20$^\circ$C to 90$^\circ$C will be shown. Such mixtures are favorable for both low voltage light shutters (contrast ration more than 100:1) and electrically controlled phase retarders. The birefringence can be changed more than twice with a typical response time 0.2-10 ms under action of 2-7 A.C. voltage in an analogous mode. Single Electroclinic light shutters providing an electrically induced inclination more than 20$^\circ$ at R.T. under A.C. electric field strength of about 15V/$\mu$m Those cells provide practically the same level of light transmission as nematic based cells but response time is $\sim$1$\mu$s at R.T. Main goal of the report is to present the state of the art overview on the various fields of such FLC's of non-display application for shutters, switchers, diffraction gratings, adaptive optics etc. in electrically driven mode by special consideration of the requirements for each of the FLC-mixtures applied. Moreover, some recent results on optically driven spatial light modulators will be presented.

11:00 AM D4.7
HOLOGRAPHIC GRATING FORMED BY PHOTOCHEMICAL PHASE TRANSITION OF POLYMER AZOBENZENE LIQUID CRYSTAL. Takahiro Yamamoto , Satoshi Yoneyama, Makoto Hasegawa, Akihiko Kanazawa, Takeshi Shiono, Tomiki Ikeda, Tokyo Inst. of Technology, Research Laboratory of Resources Utilization, Yokohama, JAPAN.

To establish the liquid-crystalline materials for photonics in which properties of light are controlled by a stimulating light, we have performed so far study on photochemical nematic (N) to isotropic (I) phase transition behavior of polymer azobenzene liquid crystals (PALCs). We already reported optical switching and two-dimensional image storage by transmission-, reflection- and scattering-mode analyses. We present here the formation of the holographic grating by diffraction-mode analysis. When two writing beams (argon-ion laser, 488 nm, unpolarized) were interfered on the surface of PALC film (thickness, $\sim$500 nm), multiple diffraction signals were observed immediately, resulting from formation of the grating. Such generation of the diffraction signals was assumed to be due to a periodic induction of photochemical phase transition in the bright regions of the interference pattern. The grating is namely made up of a periodic arrangement of N (dark region) and I phases. In fact, the intensity of the diffraction signals was affected by the polarization direction of the readout beam (He-Ne laser, 633 nm, linearly polarized). In Raman-Nath diffraction, diffraction efficiency (D.E.) depends on the difference in the refractive indices between the bright and the dark regions ($\Delta$n). When the plane of polarization of the beam was parallel to the director of liquid crystal in the dark region, D.E. was highest ($\sim$20$\%$). On the other hand, D.E. was lowest when the plane of the polarization was perpendicular to the director because the $\Delta$n was smaller in this case. Observation of recorded interference pattern under a cross-polarized optical microscope also supported our speculation for the structure of the grating. On the basis of these results, we succeeded in recording image hologram of photomask as an object.

11:15 AM D4.8
FORMATION OF POLYMER STABILIZED FERROELECTRIC LIQUID CRYSTALS USING A FLUORINATED DIACRYLATE. C. Allan Guymon , University of Southern Mississippi, Department of Polymer Science, Hattiesburg, MS; Christopher N. Bowman, Department of Chemical Engineering, University of Colorado, Boulder, CO.

Ferroelectric liquid crystals (FLCs) have shown great potential for use in electro-optic and display technology due to their inherently fast switching speeds and bistability. Recently, considerable research has been devoted to FLCs mechanically stabilized by a polymer network. The network is formed typically by in situ polymerization of a monomer dissolved in the FLC. Because of the inherent order in the FLC, the polymerization behavior may be significantly different than what might be expected in solution polymerizations. These deviations result largely from the segregation properties of the monomer in the liquid crystal. One class of monomers, namely fluorinated acrylates, is a likely candidate for inducing novel segregation, polymerization and electro-optic behavior in polymer stabilized ferroelectric liquid crystals (PSFLCs). The use of fluorinated moieties has a significant impact on the phase and polymerization behavior of liquid crystal systems. This study focuses on the polymerization of a fluorinated diacrylate, octafluoro 1,6-hexanediol diacrylate (FHDDA), to form PSFLCs and the consequent impact of the polymerization on the ultimate performance. Interestingly, as the temperature is increased and the order of the system decreases, a dramatic increase in the polymerization rate is observed. This increase is especially prominent for polymerizations in the smectic C* phase for which the rate is more than five times that exhibited at much higher temperatures in the isotropic phase. As with other monomer/FLC systems, the segregation of the monomer plays a role in this polymerization behavior as the monomer segregates between the smectic layers of the liquid crystal. The segregation properties also significantly impact the ultimate electro-optic properties. Both ferroelectric polarization and response time of the PSFLC change markedly with different polymerization temperatures, and approach values very close to those of the neat FLC under appropriate polymerization conditions. This behavior not only provides a unique mechanism for rate acceleration in PSFLCs, but also paves the way for new methods to optimize performance in these materials.

11:30 AM D4.9
NEW WHOLLY-AROMATIC THERMOTROPIC POLYESTERS WITH CONTROLLED FLEXIBILITY. Devdatt S. Nagvekar, Univ. Dayton Research Inst., AFRL/MLBP, Wright Patterson AFB, OH; Patrick T. Mather , L.-Seng Tan, AFRL/MLBP, Wright Patterson AFB, OH; Hong G. Jeon, Systran Corp., AFRL/MLBP, Wright Patterson AFB, OH.

Aromatic copolyesters which are para-linked are known to feature thermotropic behavior in which heating the semicrystalline solid results in melting to a mesomorphic phase, often nematic, giving rise to desirable molding characteristics, mechanical properties, and solvent resistance. Often, their high modulus and strength come at the expense of low toughness, manifested as failure strains less than 5%. In addition, such polymers feature nematic-isotropic transition temperatures in excess of thermal decomposition, eliminating the potential benefit of traversing the isotropic-nematic phase transition during processing. In this work, we present the synthesis and characterization of a new series of wholly-aromatic copolyesters derived from the condensation of various weight fractions of 4,4´ -(o-phenylenedioxy) dibenzoyl chloride (OPDB) and substituted terephthaloyl chloride (BTA) with 2-phenylhydroquinone (PHQ). The Hagashi method[1], involving tosyl chloride and pyridine as solvent, was employed to yield polymer with significant molecular weight. These polymers are intended to enable accessible clearing transition and to control the balance of stiffness and toughness in melt-spun fibers systematically. We report the synthetic details along with characterization of quiescent phase behavior and morphology. Fiber orientation, microstructure, and mechanical properties for selected samples are also presented. [1] Higashi, F.; Akiyama, N.; Takahashi, I.; Koyama, T. J. Polym. Sci. A: Polym. Chem. Ed. 1984, 22, 1653.

11:45 AM D4.10
DYNAMICS AND ALIGNMENT BEHAVIOR OF A THERMOTROPIC MAIN-CHAIN LIQUID CRYSTALLINE POLYMER. Weijun Zhou, Julia A. Kornfield , Caltech, Pasadena, CA; Victor M. Ugaz, Wesley R. Burghardt, Northwestern University, Evanston, IL.

The material properties of thermotropic liquid crystalline polymers (TLCPs) are strongly influenced by the flow-history they experience during processing because it can dramatically alter the orientation distribution of the polymer and the texture of the liquid crystalline domains. However, the flow behavior of TLCPs have received relatively little attention due to experimental difficulities. Here we describe the rheology and flow-induced orientation behavior of a model TLCP, which has good thermal stability and an accessible isotropization temperature: DHMS-7,9, $T_m=90^\circ$C, $T_{xn}=120^\circ$C, $T_{ni}=197^\circ$C, where Txn is the transition temperature from an unidentified mesophase (X) to the nematic phase. Systematic rheological studies were carried out on DHMS-7,9 as a function of temperature and Mw (Mw=11K to 65K g/mol). The storage modulius ($G^\prime$) showed a plateau at an intermediate frequency in Phase X, but no plateau for $G^\prime$ was observed over the whole nematic range. This TLCP exhibits qualitatively different flow-aligning behavior in the X mesophase than in the nematic phase. Wide-angle X-ray diffraction shows that all steady shear conditions probed in the nematic phase induce a preferential orientation along the flow direction (``parallel), whereas a log-rolling alignment results under steady shear conditions in Phase X for low enough shear rates. The orientation can be flipped back and forth by raising or lowering the shear rate or temperature across the boundary between the parallel and log-rolling regimes.
 

SESSION D5: DISPLAY TECHNOLOGIES AND MODELING
Chairs: Shui-Chih Alan Lien and Shunsuke Kobayashi
Tuesday Afternoon, April 6, 1999
Concordia (A)
1:30 PM *D5.1
POLYMER STABILIZED FLCD. H. Furue, T. Takahashi, T. Miyama, M. Shikada, R. Kurihara and S. Kobayashi , Liquid Crystal Institute, Science University of Tokyo in Yamaguchi, Onoda, Yamaguchi, JAPAN.

We present electrooptical performance of polymer-stabilized (PS) FLCDs and their applications to active matrix LCD and field sequential full color LCD. The PS-FLCD, which is fabricated by UV photocuring doped mesogenic sidechain polymer, exhibits high contrast ratio (230:1) with thresholdless monostability and fast response speed (40micro seconds). We confirmed that our PS-FLCD is addressable using switching FETs with the gate pulse condition for SXGA specifications having 1024 strobing lines. We also demonstrate a field sequential fullcolor LCD using PS-FLCD.

2:00 PM *D5.2
COLORLESS HIGH DIELECTRIC COMPOUNDS FOR LOW VOLTAGE LIQUID CRYSTAL APPLICATION. Shin-Tson Wu , Robert N. Schwartz, HRL Laboratories, Malibu, CA; Qing T. Zhang, Seth Marder, Beckman Institute, California Institute of Technology, Pasadena, CA.

Several colorless compounds with high dielectric anisotropy were designed and synthesized. Their physical properties, such as phase transition temperature, heat fusion enthalpy, absorption spectra, dielectric constants and visco-elastic coefficient were measured. Confirming molecular modelings on the structures and dipole moments of these compounds are developed. These compounds are useful for lowering the operation voltage of liquid crystal devices.

2:30 PM *D5.3
ELECTRIC PROPERTIES OF LIQUID-CRYSTAL MATERIALS FOR DISPLAY APPLICATIONS. Shohei Naemura , Merck Japan Ltd., Atsugi Technical Center, Kanagawa, JAPAN.

High image quality is an essential requirement for displays, and, especially for mobile applications, low power consumption is another key issue. In order to lower the power consumption of liquid crystal displays (LCDs), an anisotropy of the dielectric property is required to be sufficiently large for LC materials and, at the same time, the dielectric constant value is required to be kept low enough. Electrical conduction is also important not only for the dielectric loss but also for the image quality. In practical LCD panels driven by AC voltage, the electrical conduction is governed by a long-range movement of ions in a LC slab. Mobile ions also act as a space charge and affect the dielectric property in a very low frequency range. Moreover, when localized at an interface of dielectric layers, they can cause an internal electric-potential and affect the effective field across the LC slab. The electric properties of LC materials such as dielectric and conductive properties are essentially important for LCDs, as far as they are addressed electrically. In order to understand these electric properties of LC materials, behavior of ions in LC materials are indispensable, and will be focused in the presentation.

3:30 PM *D5.4
COMPUTER SIMULATION OF THE OPTICAL PERFORMANCE OF LIQUID CRYSTAL DEVICES. Jack R. Kelly and Weimin Liu, Liquid Crystal Institute, Kent State University, Kent, OH.

We present various approaches for simulating the optical performance of liquid crystal devices in more than one dimension. Two ray optics approaches based on the extended Jones matrix method and geometrical optics are appealing because of their simplicity. These approaches are expected to break down when the wavefronts develop significant curvature, as occurs in the neighborhood of a defect or a shadow edge. Near defects where the wavefronts distort smoothly, a modified Jones matrix approach can yield useful results, while for shadows, the geometrical theory of diffraction can be generalized to birefringent media. We compare the simulations with experimental transmission measurements of a liquid crystal device: an in-plane switching device with homeotropic boundary conditions.

4:00 PM *D5.5
REFLECTIVE FULL COLOUR LIQUIED-CRYSTAL DISPLAY: ACTIVITIES IN ASET. Yoshiharu Nakajima , Association of Super-Advanced Electronics Technologies (ASET), Tokyo, JAPAN.

In February 1996, ASET was established and made the contract with NEDO (New Energy Development Organization) to promote this Government research project ``Super-Advanced Electronic Technology Development Promotion Project'' formed by Ministry of International Trade and Industry (MITI). Main research subjects conducting in ASET are next generation silicon semiconductors, Ultrahigh density magnetic storage and low power liquid crystal display. Reflective full colour liquid crystal display project is one of major subject in this government research project. Five materials and four devices companies are participating. The research aim is to make technological breakthrough with the objective of creating full colour displays that can provide high resolution and high quality images with very low power consumption. To meet this aim, we are now doing the research and development in the following sub-fields: 1) forming the fine composite structures creating new display mode such as stacked holographic one, 2) to create high gain effective light reflecting characteristics, 3) retentive memories mode by using ferroelectric properties, in addition to these devise related research, 4) development of new liquid crystal, optical and alignment film materials which have large anisotoropy, high scattering efficiency, pure and high reliable and to show new alignment mode, respectively. We have just passed intermediate stage of our first five years project term, the results obtained so far will be discussed as well as project outline. This work was performed under the management of ASET in the MITIÕs R & A program supported by NEDO.

4:30 PM D5.6
12.1 SVGA WIDE VIEWING ANGLE TFT-LCDS BASED ON RIDGE AND FRINGE-FIELD STRUCTURES. A. Lien , C. Caiü, R.A. John, E. Galligan, R. Nunes, J. Wilson, H. Ifill, IBM T. J. Watson Research Center, Yorktown Heights, NY; Department of Chemical Engineering, Chemistry and Material Science, Potytechnic University, Brooklyn, NY.

Compared with cathode ray tube (CRT) displays, it is well known that liquid crystal displays (LCDs) have a much narrower viewing angle [1]. To solve this problem, the in-plane switching (IPS) mode [2,3,4] is a popular approach. However, because of the low optical transmission compared to the conventional twisted nematic (TN) mode, the IPS mode is restricted to desktop applications and not suitable for notebook applications. We have recently proposed a ridge and fringe-field multi-domain homeotropic (RFFMH) mode [5] as another approach to achieve wide viewing angle for LCDs. The operation of this new structure is based on the combined effect of the ridge structure and the pixel fringe electric field to control the tilt direction of LC molecules inside each pixel. We have also demonstrated that RFFMD structure is suitable for the desktop application [6]. In this paper we investigated whether the RFFMH mode is suitable for the notebook application. To do that, several RFFMH structures were built on the 12.1 SVGA panels. The result shows that RFFMH method is suitable for the high-end large-area notebook application since it provides a wide viewing angle with a reasonably high transmission. The ridge structure is constructed only on top of the indium tin oxide (ITO) layer of the color filter (CF) substrate, and can be fabricated during color filter manufacture. This process is thus compatible with the conventional TFT-LCD process, and quires only one extra photolithographic step, but without rubbing treatment. References: [1] A. Lien, H. Takano, S. Suzuki and H. Uchida, The Symmetry Property of a 90 degree Nematic Liquid Crystal Cell, Mol. Cryst. Liq. Cryst. 198, 37 (1991). [2] R. A. Soref, Field Effects in Nematic Liquid Crystal Obtained with Interdigital Electrodes, Jour. of Appl. Phys, 45, 5466 (1974). [3] R. Kiefer, B. Weber, F. Windscheid and G. Baur, In-Plane Switching of Nematic Liquid Crystals, Japan Display'92, 547 (1992). [4] M. Ohta, M. Oh-e and K. Kondo, Development of Super-TFT-LCDs with In-Plane Switching Display Mode, Asia Display'95, 707 (1995). [5] A. Lien, C. Cai, R. Nunes, R. A. John, A. E. Colgan, J. Wilson, Ridge and Fringe Field Multi-Domain Homeotropic Liquid Crystal Display, submitted to SID'98. [6] A. Lien, C. Cai, R. Nunes, S. L. Wright, R. A. John, E. A. Galligan, W. S. Wilson,Wide Viewing Angle TFT-LCD Based on Ridge and Fringe Field Multi-Domain Homeotropic Structure, Asia Display 98, p 375 (1998).
 
 

4:45 PM D5.7
SIMULATION OF REORIENTATION DYNAMICS IN BIPOLAR NEMATIC DROPLETS. Philip K. Chan , Ryerson Polytechnic Univ, School of Chemical Engineering, Toronto, Ontario, CANADA; Alejandro D. Rey, McGill Univ, Dept of Chemical Engineering, Montreal, Quebec, CANADA.

A two-dimensional model composed of a synthesis of the Leslie-Erickson continuum theory of nematics and the Euler-Lagrange equation for surface director motion is used to study the magnetic-induced director reorientation dynamics confined in spherical bipolar droplets with viscoelastic surfaces. The magnetic field is restricted to the droplet axis of symmetry direction. The numerical results indicate that the surface viscosity and anchoring strength must be taken into account to describe accurately director reorientation dynamics in droplets. In addition, the numerical results replicate frequently reported experimental observations on the performance of polymer dispersed liquid crystal films. These observations include the familiar exponential increase followed by saturation in light transmittance as the external applied field increases, and the exponential increase (decrease) followed by saturation as time increases in the on (off) state. Furthermore, this model is able to predict precisely the relationships between the rise and decay times and the external applied field strength, and the fact that the switching field strength is inversely proportional to droplet size.
 

SESSION D6: POSTER SESSION:
LIQUID CRYSTAL MATERIALS AND DEVICES
Chair: Naoyuki Koide
Tuesday Evening, April 6, 1999
8:00 P.M.
Metropolitan Ballroom (A)
D6.1
ELECTRO-OPTICAL RESPONSE OF UNIAXIALLY ORIENTED PDCHLC FILMS WITH VARIOUS DROPLETS ANISOMETRY AND CONCENTRATION OF THE CHIRAL ADDITIVE. Vladimir V. Presnyakov , Sergei A. Vetoshkin, Victor Ya. Zyryanov, Vasily F. Shabanov, L.V. Kirensky Institute of Physics, Krasnoyarsk, RUSSIA.

Uniaxially oriented films of polymer dispersed cholesteric liquid crystals (PDChLC) is a new perspective material for multifunctional optoelectronic elements. The material presents a polymer film with an ensemble of elongated cholesteric droplets dispersed therein. Long axes of the droplets are oriented preferably uniaxially in the film plane. These films reveal a great anisotropy of the polarized light transparency, which depends strongly on the applied electric field, concentration of the chiral additive and shape of the droplets. We have found the optimal composition and droplets anisometry to produce the films, which can be switched by applied voltage between three different optical states: scattering light of any polarization, transmitting the one linear-polarized component only and transparency for light any polarization. A device employing these films may be used as a modulator of the whole of the light, as a modulator of one linearly polarized component only and as a light polarizer [1].
[1] V.V. Presnyakov, S.L. Smorgon, V.Ya. Zyryanov, V.F. Shabanov, Polyfunctional optoelectronic elements based on oriented PDCLC films, SPIE Proceedings, 1998, V.3348, pp.98-102.

D6.2
VOLTAGE CREEP IN HOLOGRAPHIC PDLC GRATINGS. A.M. Klostermann , S. Lakhia and T.J. Bunning, AFRL/MLPJ, Materials and Manufacturing Directorate, WPAFB, OH; R.T. Pogue, L.V. Natarajan, V.P. Tondiglia, SAIC, Dayton, OH.

It is well known that with conventional polymer dispersed liquid crystals (PDLCs) a minimum voltage is necessary in order to switch the cell between a scattering and a transparent state. In holographic gratings formed using anisotropic photopolymerization of free-radical monomers, a particular voltage is also necessary to switch the grating between a diffracting and a transmitting state. In this work we report on the gradual increase of this switching voltage with time after initial fabrication. We explore the magnitude of the effect in a pentaacrylate/E7 material systems and compare to the effect observed for a diacrylate/E7 material system. This latter material combination crept considerably more. There was however no change in the measured contrast ratios which suggest little increase in the microphase separation as a function of time. We also report on processing issues examined in an attempt to mitigate the increase in voltage. These include the application of heat after the initial polymerization and extended laser curing exposure time.
 

D6.3
EFFECTS OF MOLECULAR ARCHITECTURE ON PHYSICAL AND LIQUID CRYSTALLINE TRANSFORMATIONS IN LIQUID CRYSTALLINE THERMOSETS (LCT'S). Arthur J. Gavrin , Elliot P. Douglas, University of Florida, Dept of Materials Science and Engineering, Gainesville, FL.

Two homologous series of bis-acetylene LCT's have been synthesized. Our goal is to investigate how changes in molecular architecture affect liquid crystalline transitions and physical transformations. These thermosets are thermotropic liquid crystalline monomers with flexible alkyl ``wings''. The flexible segments range in size from three to eight carbons. Differential scanning calorimetry and optical microscopy were used to determine the liquid crystalline transitions for the various monomers. The monomers with longer flexible units had both lower melting temperatures and more stable smectic phases. Preliminary studies on the liquid crystalline phase transformations show that these materials progress from more ordered to less ordered phases during isothermal cure. These results are contrary to the trends typically seen in other LCT's.

D6.4
ELECTRICALLY SWITCHABLE HOLOGRAPHIC REFLECTION GRATINGS IN POLYMER DISPERSED LIQUID CRYSTALS. L.V. Natarajan , R.L. Sutherland, V.P. Tondiglia, S. Siweeki, R. Pogue, M. Schmitt, D. Brandelik, B. Epling, G. Berman, C. Wendel, M. Ritter and M. Stallings, Science Applications International Corporation, Dayton, OH; and T.J. Bunning, Materials Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, OH.

Electrically switchable reflection holograms were recorded in polymer dispersed liquid crystals using a prepolymer syrup containing multifunctional monomers, a photosensitizing dye, a coinitiator and a nematic liquid crystal. Using Argon ion laser wavelengths 476 and 514nm, blue, green and red reflection holograms were made with high efficiency. The holograms were switched electrically with contrast ratios >100. Real time study of the holograms indicated that the hologram formation was completed in $\sim$20 seconds. Shrinkage of the polymer during writing blue shifted the reflection notch. Low voltage high resolution SEM studies showed the presence of discrete phase separated nematic droplets of 30-70 nm size.

D6.5
DIELECTRIC RELAXATION IN FILLED NEMATIC LIQUID CRYSTALS. G.P. Sinha a, M. Kreuzerb and F.M. Alieva, aDept. of Physics, University of Puerto Rico, San Juan, PUERTO RICO; bIAP, Darmstadt University of Technology, Darmstadt, GERMANY.

Nematic liquid crystals (LCs) filled with aerosil particles are new heterogenous materials important for different optoelectronic applications. These materials are suspensions of small silica particles, about 10-17 nm in diameter, filling liquid crystals. The particles are known to form into network structure dividing liquid crystal into domains with linear size of about 250 nm. This material has very developed liquid crystals-solid particle interface that makes the role of the surface layers of LC extremely important in the determination of the properties of the material. We used particles, both with hydrophilic and hydrophobic surfaces, filling nematic liquid crystal-5CB. Broad band dielectric spectroscopy (0.1 mHz - 1.5 GHz) was applied for the investigations of the dielectric properties of these materials. Two bulk-like modes due to the rotation of molecules around short axes and the tumbling motion were observed in filled 5CB. Additionally, a low frequency relaxation process and the dispersion of dielectric permittivity due to conductivity were also observed. The modification of the surface of filling particles has strongest influence on the properties of the slow process and it is less important for molecular modes. The characteristic frequencies of the slow process are lower for hydrophilic particles and in this sample the contribution of this process to the total polarization was greater than in the case of hydrophobic particles. These facts suggest that low frequency relaxation is aerosil particle-liquid crystal interface related phenomena and the origin of this process maybe explained on the basis of the surface induced polarization. Both materials show up nematic-like behavior up to 20 degrees below bulk crystallization temperature. Deviations from the Debye type behavior were observed and were more pronounced at low temperatures.

D6.6
THE DYNAMIC PROPERTIES OF CONFINED ANTIFERROELECTRIC LIQUID CRYSTAL INVESTIGATED BY PCS. Yu.P. Panarin a, C. Rosenblattb and F.M. Alieva, aDept. of Physics, University of Puerto Rico, San Juan, PUERTO RICO; bDepartment of Physics, Case Western Reserve University, Cleveland, OH.

Since the discovery Antiferroelectric Liquid Crystals (AFLCs) - materials important in display technology -various novel liquid crystalline phases were found in the temperature range between the antiferroelectric SmCA and ferroelectric SmC* phases. These phases were found to be ferrielectric-like. The appearance of these phases seems to be due to the competition between the antiferro- and ferroelectric interactions in adjacent smectic layers that stabilize SmCA and SmC* phases. In the present work we report on the influence of confinement on the dynamic properties and the appearance of ferrielectric phases as determined by photon correlation spectroscopy. The bulk AFLC under investigation (AS573) possesses various ferrielectric and antiferroelectric phases between SmCA and SmC* phases. Dynamic light scattering of the sample was measured in the bulk and in cylindrical pores of two different diameters: 200 Å and 2000 Å. The intensity/intensity autocorrelation function of the bulk sample consists of three relaxation processes, while in the confined liquid crystal the fastest relaxation process (50-250 msec) does not exist. The analysis of the autocorrelation functions and the temperature dependencies of the relaxation times show that the ferrielectric phases (SmCg, AF, FiLC) are not formed in the pores. It was shown that in the smallest pores the electrostatic interactions between the spontaneous polarization of neighboring smectic layers are weaker than in the larger pores and in bulk. This fact stresses the importance of long-range electrostatic interactions for the existence of ferrielectricity. The SmC* - SmA phase transition temperature in the pores is about 4 $^\circ$C lower than in the bulk sample, while the SmCA - SmC* transition in pores and the SmCA- SmCg transition in bulk are of the same value of temperature. These facts have been explained by the structural aspects of AFLCs in a confined geometry.

D6.7
IN-SITU OBSERVATION OF THERMAL AMPLIFICATION OF PHOTOGENERATED OPTICAL ANISOTROPY OF FILMS OF AZOBENZENE POLYMERS. Masatoshi Kidowaki , Takenori Fujiwara, Shin'ya Morino, Kunihiro Ichimura, Tokyo Institute of Technology, Research Laboratory and Resources Utilization, Yokohama, JAPAN; Joachim Stumpe, Institute for Thin Film Technology and Microsensorics, Erieseering, Berlin, GERMANY.

Photo-induced optical anisotropy in polymeric films has been extensively studied for optical data storage and processing. Polymers containing azobenzene derivatives are one of the promising materials for this purpose since irradiation with linearly polarized light (LPL) to polymer films generates dichroism and birefringence due to the axis-selective photoisomerization and the reorientation of azobenzene moieties. We demonstrated that azimuthal alignment of nematic liquid crystals (LCs) is regulated by LPL-irradiation of thin films of polymers substituted with azobenzenes in their side-chains, leading to the fabrication of rubbing-free LC-aligning films. This work aims at revealing photochemical and thermal characteristics of photoaligned film of polymethacrylates with azobenzene moieties to be applied to LC aligning films. We followed the generation of optical anisotropy of LPL-irradiated films of three types of polymethacrylates with azobenzene side-chains exhibiting amorphous, crystalline and liquid-crystalline natures during heat treatment by mean of elipsometry and polarized UV-visible spectroscopy. Whereas photogenerated optical anisotropy of amorphous polymer films was disappeared above glass transition temperature, the photo-induced anisotropy of films of liquid crystalline and crystalline polymer films was significantly amplified by heat treatment. In case of crystalline film it was maintained even at 573 K. Homogeneous LC alignment induced by this polymer film showed extraordinary stability because of the generally stable anisotropy.

D6.8
LIGHT ABSORPTION PROPERTIES OF ANTHRAQUINONE DYES FOR GUEST-HOST LIQUID CRYSTAL DISPLAYS. Shuangxi Wang , Sugat Abeygunaratne and Liang-Chy Chien, Department of Chemical Physics, Liquid Crystal Institute and NSF ALCOM Center, Kent State University, Kent, OH.

Substitution of the anthraquinone systems has provided dyes suitable for guest-host liquid crystal display applications. In general, we seek to elongate the anthraquinonic system through the synthesis of 1,5-bis(butylphenylamino)-2-(4-heptoxyphenyl)-4,8-dihydroxy-9,10-anthracedione. The light absorption properties of anthraquinone dye in different hosts are compared in order to demonstrate the influence of the dye-liquid crystal interactions on dye order parameter. Surprisingly, the anthraquinone dye have a higher order parameter in liquid crystalline mixtures of positive dielectric anisotropy as well as in a liquid crystalline mixture of negative dielectric anisotropy (S = 0.71-0.82). As we mentioned, many types of liquid crystal displays have taken advantages of dichroic dye additives. Such displays include Heilmeier displays may be made to operate transmissively, reflectively, or transflectively. The most common use is in the transmissive mode with a backlight. We studied the Heilmeier displays use homogeneous alignment. The display is operated at the reverse mode, transparent at zero field. The switching voltage between the bright and dark states for these displays is about 2-3 volts.

D6.9
THREE-DIMENSIONAL PHOTOREORIENTATION OF SELF-ORGANIZED AZOBENZENE CHROMOPHORES IN LIQUID-CRYSTALLINE POLYMER FILMS. Mina Han , Shin'ya Morino, and Kunihiro Ichimura, Tokyo Institute of Technology, Research Laboratory of Resources Utilization, Yokohama, JAPAN.

Recently many efforts have been focused on developing photoresponsive systems which demonstrate optical anisotropy changes upon irradiation of azobenzene containing photochromic materials with linearly polarized light as a result of the photoreorientation of azobenzene moieties via reversible trans-cis-trans isomerization. Our recent work has revealed that three-dimensional control of the orientational direction of azobenzene moieties is in fact attained by slantwise photoirradiation of polymer films containing azobenzene moieties with non-polarized light. Upon slantwise irradiation of polymer thin films with non-polarized visible light, photodichroism was generated in a thin film of an amorphous polymer even though the level of the optical anisotropy was small compared to that induced by linearly polarized light irradiation. In other words, the orientational direction of azobenzene chromophores can be controlled by the propagation direction of actinic light as a result of physical reorientation of the chromophores. The main purpose of this report is to investigate the photocontrol of three-dimensional orientation of a liquid-crystalline polymer with azobenzene side chains. The extent of the three-dimensionally photoaligned orientational order of azobenzene residues was considerably enhanced and stabilized by the formation of aggregates owing to the liquid crystallinity, which was revealed by measurement of polarized absorption spectra. The optical anisotropy generated by non-polarized light irradiation was much increased by annealing at temperature close to glass transition temperature.

D6.10
MICROSTRUCTURE AND MORPHOLOGY OF SELF-ASSEMBLED LIQUID CRYSTALLINE MATERIALS. Hee-Tae Jung , Dept. of Chemical Engineering, University of California, Santa Barbara, CA; Steven D. Hudson, Virgil Percec, Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH.

We present new examples of hexagonal columnar and cubic thermotropic liquid crystalline phases by transmission electron microscopy. The phase symmety, dimension and local structure of the amphiphilic molecules have been determined by controlling the orientation of the mesophase by surface anchoring and thermal treatment. Low surface energy planes were revealed by the faceting of discontinuous films. For polymeric materials, in which the backbone is contained within the cylinder core, homeotropic alignment was possible only when the film thickness was significantly larger than the mininum size of the polymeric cylinders. The stiffness and microstructure of the mesophase are also discussed.

D6.11
LIQUID CRYSTALLINE ZINC CHLORIDE. James D. Martin , Todd A. Thornton, Department of Chemistry, North Carolina State University, Raleigh, NC.

The templated syntheses of our zeolite-type copper zinc chloride frameworks frequently result in the formation of glassy phases or viscous liquids that might be described as ambient temperature molten salts. Having discovered that certain of these materials exhibit remarkable birefringence, we have investigated the structure of both the glass and liquid forms of alkylammonium templated zinc chloride materials by neutron and X-ray diffraction, as well as by DSC and polarizing microscopy techniques. Further template control using alkylammonium surfactants as directors has yielded an entire family of liquid crystalline materials with up to 90% inorganic content. For certain phases, liquid crystallinity has been observed from between 275 C to below room temperature. The phase characteristics of this unique family of anionic, inorganic liquid crystals willl be described in terms of the relation between the structure of glassy and liquid zinc chloride and a lamellar structure commonly found for surfactant based metallomesogens.

D6.12
STUDY ON THE BULGE OF TN ELECTRO-OPTICAL PERFORMANCE CURVE. Xinyu Zhu , Xibin Shao, Tao Yu, Zhihua Ling, Kai Ma, Ximin Huang, Changchun Institute of Physics, Chinese Academy of Sciences, Changchun, P.R.CHINA; North LCDs Research and Development Center, Changchun, P.R.CHINA.

There exists a bulge in TN Electro-Optical performance Curve. Here we measure the TN EO curve for visible Light and some monochromatic light: 450nm, 500nm, 550nm, 600nm, 650nm and 700nm. In the experiment, we keep the polarizer and analyzer vertical to each other and change the angle between polarizer and rubbing direction. We make comparison among them. Based on the comparison, we get the following three important results. First, with increase of wavelength of monochromatic light, the voltage where occurs the bulge will decrease. This is because the phase retardation of light is inversely proportional to the wavelength of light and the light distance of two waves will decrease with the increase of operating voltage near the threshold voltage. While for visible light the place where occurs the bulge is close to that of 550nm, this is because the measurement of electro-optical curve for visible light is corrected by the visible spectrum sensitive function. Second, with increase of the angle between polarizer and rubbing direction from 0 degree to 45 degrees, the transmission of light in the place of bulge will decrease. While the angle increase from 45 degrees to 90 degrees, the transmission will increase evidently. This is because that if the angle between polarizer and rubbing direction is not equal to 0 degree of 90 degrees, the light in the liquid crystal layers is far from linear polarization. This leads to the polarized state of output light also in the nonlinear polarized state, hence reduce the transmission. Three, when the angle between polarizer and rubbing direction is 90 degrees, the contrast ratio is higher than in the condition of 0 degree. This is because the effect of pretilt angle. This explains the reason that for conventional commercial TN LCDs, the polarizer is often vertical to the rubbing direction.

D6.13
STUDY OF LC MATERIAL'S ELASTIC CONSTANTS IN MAGNETIC FIELD. Yan Shi , Zhu Xinyu, Yu Tao, Ma Kai, Huang Ximin, Changchun Inst. of Physics, Chinese Academy of Science, CHINA; North LC R&D Center, Changchun, CHINA.

By using the magnetic field measurement of LC elastic constants, we got the ratio of splay elastic constant and bend elastic constant (k3/k1), and the ratio of twist elastic constant and bend elastic constant (k2/k1), then got the value of k1, k2, k3 through calculation. The devlation brought while measuring elastic constant by using electric field measurement is greatly diminished by using magnetic field measurement. The magnetic field measurement of LC elastic constants is both in favor of studying the aligning mechanism and the anchoring energy of the LC molecule surfaces, and in favor of optimizing the apparatus parameter of LCD implement.
 

SESSION D7: NEW EMERGING TECHNOLOGIES
Chairs: Tisato Kajiyama and Patrick J. Hood
Wednesday Morning, April 7, 1999
Concordia (A)
8:30 AM *D7.1
TWO-DIMENSIONAL SUPERLATTICE SELF-FORMED BY NOVEL IONIC LIQUID CRYSTALS AND ITS PHOTOFUNCTIONAL PROPERTY. Akihiko Kanazawa , Tomiki Ikeda, Tokyo Inst. of Technology, Research Laboratory of Resources Utilization, Yokohama, JAPAN.

The ability to control structure of organizations at the molecular or atomic levels is very important in development of new functionalized materials. In crystalline materials, it is usually difficult to control the crystalline structure and the molecular arrangement in the bulk. Practical applications in magnetic, optical, or electrical materials are therefore limited by the ease with which single crystals of a given material can be grown. Liquid crystals have self-assembly characteristics and provide additional features such as an ability to respond to applied external fields. Such self-assembled materials may be one of the most potential groups for applications in high-performance devices. In this study, we present a new concept ``in-plane ferroelectrics'' as a two-dimensional acentric superlattice. Its existence was revealed through the evaluation of second-order nonlinear optical property of the multilamellar assembly organized by the phosphonium liquid crystals which are novel thermotropic liquid crystal composed of non-metal ions and long alkyl segments. In a two-dimensional ionic layer within the phosphonium assembly, it was found that spontaneous polarization (Ps) occurs because of the spontaneous displacement of ions. The Ps directions in each in-plane ferroelectrics could be controlled by application of an electric field. Through the theoretical calculations, such unusual ferroelectric feature was found to originate from the variety of bonding manner (i.e., the characteristic electronic structure) of phosphorus atom. Furthermore, it was found that introduction of bivalent metal ions into the amphiphiles leads to enhancement of the thermal properties. The metal-containing ammonium complexes showed a stable liquid-crystalline phase in the expanded temperature range in comparison with the parent compounds. These ionic metallomesogens formed organizations with layered structure based on two-dimensional arrangement of the metal ions, i.e., ``sheet metal complex'' which was separated by the insulating alkyl segments.

9:00 AM D7.2
ORIENTED PEARL-NECKLACE ARRAYS OF METALLIC NANOPARTICLES IN POLYMERS: A NEW ROUTE TOWARDS POLARIZATION DEPENDENT COLOR FILTERS. Yvo Dirix , Cees Bastiaansen, Walter Caseri, Paul Smith, ETH Zurich, Dept. of Materials, Zurich, SWITZERLAND.

Nanocomposites of high-density polyethylene containing surface-coated silver or gold nanoparticles were prepared by melt-extrusion or solution-casting techniques. The nanocomposites were subsequently oriented by drawing in the solid state at temperatures close to, but below the melting temperature of the polyethylene. It was found that the absorption spectrum in the visible wavelength range of the drawn nanocomposites strongly depends on the polarization direction of the incident light. For instance, the polyethylene/silver nanocomposites appear bright yellow and bright red in linearly polarized light with its vibration direction, respectively, perpendicular or parallel to the drawing axis. The optical anisotropy of the drawn nanocomposites originates from uniaxially oriented, pearl-necklace type of arrays of nanoparticles possessing high aspect ratios. The color of the composites in polarized light can be tuned either by using different metals (silver versus gold), or by varying the size of the nanoparticles. For example, annealing of the polyethylene/silver nanocomposite prior to drawing results in an increase of the particle size, due to Ostwald ripening, and consequently a range of polarization dependent colors can be generated in the drawn films. These polarization dependent color filters are potentially useful in LCD applications. A conventional color LCD switches between a colored (off) state and a dark (on) state, were no light is transmitted. In a new display set-up, a single polymer/metal nanocomposite film with a polarization dependent color replaces both the polarizer (analyzer) and color filter. The latter transmits colored light in both the on and off state, which in principle raises the brightness and light efficiency of the device.

9:15 AM D7.3
POLYMER ULTRATHIN FILMS VIA ALTERNATE SELF-ASSEMBLY ADSORPTION OF POLYELECTROLYTE AND AZO-DYE MOLECULES: PHOTO-INDUCED ALIGNMENT AND LC DISPLAY PROPERTIES. Rigoberto C. Advincula , Department of Chemistry, University of Alabama at Birmingham, AL; Akira Baba and Futao Kaneko, Department of Electrical Engineering, Niigata University, Niigata, JAPAN.

The fabrication and characterization of ultrathin polymer films containing photoisomerizable azo dyes is described. These films are fabricated using the alternate-polyelectrolyte self-assembly adsorption technique of polyions originally described by Decher and co-workers. While most materials reported involves oppositely charged polyelectrolytes, we report the use of a small molecule dye/ polymer combination. Surface sensitive spectroscopic and microscopic investigations were made to elucidate the formation of the thin film on a layer-by-layer basis. We used these dyes to influence the packing arrangement and ordering in these ultrathin films. We investigated their photoisomerization characteristics in polarized light and compared them with similarly investigated spin-coated films. The substrates were subsequently used to investigate LC alignment properties for the so-called command layer effect. A rearrangement of the dye long-axis perpendicular to the direction of polarization was observed. This dramatically affected the layer ordering and orientational properties of the substrate.

9:30 AM *D7.4 OPTICAL PROPERTIES AND APPLICATIONS OF NEMATIC GLASSY LIQUID CRYSTALS. Patrick J. Hood , Cornerstone Research Group, Inc., Dayton, OH; John C. Mastrangelo and Shaw H. Chen, Center for Optoelectronics and Imaging, University of Rochester, Rochester, NY.

Glassy liquid crystal (GLC) materials are a relatively new class of organic material which possess the properties of conventional nematic low-molar-mass liquid crystals at temperature above the glass transition temperature (Tg) and the properties of a hard plastic at temperatures below Tg. Originally, this class of material was developed for passive optical devices, however, recent research offers the promise of novel active devices. In this paper, we review the structural and physical properties of GLC materials, the temperature- and wavelength-dependence of the GLC films, as well as device applications for this new class of materials. The performance of passive devices as well as novel active devices will be discussed in relation to the unique properties of the materials. Finally, a discussion of the future directions being pursued and improvements necessary in materials performance required to commercialize devices based on this technology are presented.
 
 

10:30 AM *D7.5
ADVANCES IN REFLECTIVE HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL TECHNOLOGY. Chris C. Bowley , Gregory P. Crawford, Department of Physics and Division of Engineering, Brown University, Providence, RI.

Holographically-formed polymer dispersed liquid crystals (H-PDLCs) have been demonstrated in both reflection and transmission mode. These materials are a natural evolutionary step from regular polymer dispersed liquid crystals (PDLCs), discovered in the mid-1980's. Their formation in an interference fringe pattern, instead of a blanket-exposure, results in a spatial modulation in the density of liquid crystal droplets through the polymer binder. In regular PDLCs, LC droplets are randomly scattered through the binder. In the off-state, the random orientation of the droplet directors throughout the sample results in a net refractive index mismatch between the polymer-rich and LC droplet-rich planes. In this state the H-PDLC behaves as a regular holographic grating. The application of an electric field causes the directors to align and the refractive index modulation is cancelled out. The material no longer behaves as a grating and transmits all incident radiation. H-PDLCs show great promise for various applications, including reflective displays, diffractive optics, fiber optics and as filters for remote sensing. For display applications, the reflection efficiency of volume H-PDLCs cannot, for practical purposes, be increased by making the cell arbitrarily thick because of the implications on driving voltage. It is important, therefore, to maximise the separation of LC and polymer into Bragg planes thereby maximising the intrinsic effciciency of these holograms. We report on improved reflection efficiency of volume H-PDLCs formed using commercially available, high-functionality acrylate monomers. We compare the performance of these H-PDLCs to those formed from similar monomers of lower functionality. We also report on novel H-PDLCís exhibiting two distinct reflection peaks. A simple phenomenological model is presented to describe this effect.

11:00 AM D7.6
PHOTOLUMINESCENCE FROM THE RESONANCE REGION OF VITRIFIED CHIRAL-NEMATIC FILMS. Dimitris Katsis , Richard Jin, Ansgar W. Schmid, Shaw H. Chen, Materials Science Program and Chemical Engineering Department, NSF Center for Photoinduced Charge Transfer and Laboratory for Laser Energetics, University of Rochester, Rochester, NY; Tetsuo Tsutsui, Department of Materials Science and Technology, Kyushu University, JAPAN; Thomas N. Blanton, Analytical Technology Division, Eastman Kodak Company, Rochester, NY.

Luminophores such as Exalite 428 and Eu-complexes were placed in vitrifiable chiral-nematic liquid crystalline films via doping and functionalization. The polarization state of photoluminescence (PL) with UV-excitation from the selective reflection region of the host was analyzed. Novel features were observed: (1) nearly pure circularly polarized emission; and (2) handedness reversal of emitted light in a single-handed host. New insights into PL from periodically structured films will be presented.

11:15 AM D7.7
MID-WAVELENGTH IR (MWIR) POLARIZERS FROM GLASSY CHOLESTERIC LIQUID CRYSTALS. P.T. Mather , W. Barnes, T.J. Bunning, AFRL/MLBP and MLPJ, Materials and Manufacturing Directorate, WPAFB, OH; P.J. Hood, Cornerstone Research Co., Dayton, OH.

The investigation of glass forming liquid crystalline materials for mid-wave infrared polarization applications is driven by their low melt viscosity and ability to vitrify order and thus functionality into films with a wide range of thicknesses. Commercially available polarizers which function in the mid-wave infrared region suffer from poor polarization contrast, high cost, and limited size. In this work, we explore the feasibility of using cyclic siloxane-based LCs with chiral mesogens to form circular polarizers in the mid-wave infrared spectrum (3-5 mm). Specifically, we have designed a cholesteric molecular blend to possess the proper helical twisting power to exhibit a selective reflection notch in the 4.0-4.5 micron region. We have fabricated circular polarizers using shear under a number of processing conditions and explored their performance as measured by polarization contrast. Processed films with reflection notches at the proper wavelength and near theoretical reflection performance have been prepared. The use of simple alignment layers was demonstrated to yield consistent formation of Grandjean monodomains by reducing the tendency of the large pitch blends to spontaneously form a fingerprint molecular orientation observed in cells with untreated surfaces. The measured polarization contrast of >70:1 exceeds the values obtained from state-of-the art commercial polarizers in this wavelength regime.
 
 

11:30 AM D7.8
EFFECT OF NETWORK ELASTICITY ON NEMATIC LIQUID CRYSTAL/CROSSLINKED POLYMER PHASE DIAGRAM. Domasius Nwabunma, Thein Kyu , Institute of Polymer Engineering, The University of Akron, OH; Robert T. Pogue, SAIC, OH; Timothy J. Bunning, AFRL/MLPJ, Materials and Manufacturing Directorate, WPAFB, OH.

The influence of elasticity on the phase behavior of a mixture of nematic liquid crystal (LC) and crosslinked polymer has been investigated. The LC used is the low molar mass eutectic mixture E7, while the polymer network was made in-situ from crosslinking photopolymerization of a mixture of two monomers: dipentaerythrol hydroxylpentaacrylate (DPHPA) and N-vinylpyrrollidone (NVP) using a mixture of two photoinitiators: Rose Bengal (RB) and N-phenylglycine (NPG). Of particular significance is that the experimental phase diagram supported by theoretical calculation showed no critical point.Instead, the binodal curve exhibits an upward asymptotic behavior as E7 concentration approaches unity due to domination arising from network elasticity. An examination of the effect of average repeat units between cross-links and network functionality on the theoretical phase diagram of E7/crosslinked polymer showed that, although both parameters exert similar effect, the former parameter exerts a greater influence.

11:45 AM D7.9
EFFECT OF PROCESSING VARIABLES ON THE MAGNETIC FIELD ORIENTATION OF LIQUID CRYSTALLINE THERMOSETS. Derek M. Lincoln and Elliot P. Douglas , Department of Materials Science and Engineering, University of Florida, Gainesville, FL.

We have determined the effect of magnetic field processing on the orientation and properties of the 4,4$^{\prime}$-bis(2,3-epoxypropoxy)- $\alpha$-methylstilbene (EPAMS)/sulfanilamide (SAA) epoxy system. A statistical experimental design was generated and analyzed to determine the effects of the time in the magnetic field, field strength, and the amount of B-staging. A regression model with a coefficient of determination of 0.8577 was calculated to predict the orientation parameter for the input variables. An optimization study was also conducted to check the prediction of the orientation parameters by the model. The model can predict the experimentally measured orientation in regions where the model predicts physically realistic values for the orientation. Physical properties of the oriented material have also been measured. The tensile modulus increases with increasing field strength, and fits a simple model for the modulus of anisotropic materials.
 


System Administrator

3/8/1999