Session XC12 - Liquid Crystals.
ORAL session, Friday morning, March 26
Room 268W, GWCC

[XC12.01] Twisted Single Crystals and Phase Behaviors in a Chiral Main Chain Liquid Crystalline Polyester

Helical structures and morphologies have been found in low ordered chiral liquid crystalline (LC) phases. However, when the chiral liquid crystals form highly ordered smectic crystal phases, the helical morphology is suppressed due to the crystallization process. This study describes a synthetic chrial main-chain LC polyester which is able to form both flat and helical lamellar crystals from the anisotropic melt. The helical geometry can be identified as a double-twisted helix. Both the flat and helical crystals possess the same structure, although macroscopic translational symmetry is broken in the twisted helical geometry. The helical morphologies are also dependent on the liquid crystal phases from which the crystals are formed.

[XC12.02] Spatio-Temporal Growth of Nematic Domains in a Liquid Crystal/ Polymer Mixture

Dynamics of phase separation and morphology development in mixtures of a low molar mass liquid crystal and a polymer have been investigated theoretically and experimentally. The combined free energy densities of Flory-Huggins theory for isotropic mixing and Maier-Saupe theory for nematic ordering have been incorporated into the time-dependent Ginzburg-Landau equation (type C). The temporal evolution of the structure factor and the emergence of phase separated liquid crystal domains have been simulated based on a square lattice with a periodic boundary condition. Of particular interest is the observed plateau (or inflection) region in the growth dynamic curve, which may be attributed to the breakdown of the interconnected domains caused by the nematic ordering. This unique behavior has been verified experimentally in terms of the growth of structure factor following several temperature quenches into a nematic + liquid region of the phase diagram of an E7/poly(methyl methacrylate) mixture. Further the emergence of LC domains in the metastable and unstable nematic-liquid spinodal regions has been investigated.

[XC12.03] The Study of the Crystallization Kinetics for Main-chain Liquid-crystal Polymers by Using Temperature-modulated Calorimetry

Temperature modulated differential scanning calorimetry (TMDSC) is a new technique that applies a sinusoidal temperature modulation to a conventional DSC run, followed by analysis of the response in heat flow as a function of frequency. This technique has been well established for the determination of the crystal-growth rate of polyethylene and PET by Toda et al. In this paper, the same TMDSC technique is applied to the study of the crystallization kinetics of a main-chain liquid crystal polymer (an aromatic polyester-imide). The main goal is to explore the influence of the prior existing molecular order formed in the mesophase on its crystallization rate. An attempt will be made to link the observed crystallization kinetics to an earlier proposed molecular nucleation mechanism.

[XC12.04] Ordering and Disordering of Thermotropic Polymers in Shear Flow

The strong coupling of flow and order in thermotropic polymers leads to outstanding mechanical properties in fiber spinning but not necessarily in shear-dominated processes such as injection molding. In order to begin narrowing the gap between fiber and molding properties we have investigated the role of various architectural features of model thermotropic polymers in determining the evolution of macroscopic birefringence and micron-scale morphology in shearing flow. Specifically, we will report the impact of spacer length (n) and mesogen pendant group (m) in PmHQn homopolyesters on the steady state birefringence-shear rate flow curves as well as transient response to shear start-up and reversal. Band formation following a reversal in shearing direction is examined in detail for PSHQ10 using in-situ polarizing optical microscopy (POM) and conoscopic small-angle light scattering (cSALS) to reveal a surprising transient tilting of the bands (oriented along the shear-plane normal) toward the flow direction. Finally, a new stress-optical correlation is reported for various PmHQn polymers which relates the ratio of first normal stress difference over shear stress to the projection of birefringence on the flow-vorticity plane.

[XC12.05] Visualization of Director Evolution under Weak Shear Flow for a Model Thermotropic Liquid Crystalline Polymer

Continuum theory predicts that liquid crystalline materials respond to shear flow in two distinct ways in the linear limit (De\ll1), flow-aligning or director tumbling. Although director tumbling has been experimently confirmed in lyotropic systems, it is not clear whether thermotropes exhibit flow-aligning or director tumbling behavior. Optical conocopy was used to monitor the director orientation of a model thermotropic liquid crystalline polymer under shear. A thermally stable polyether (DHMS-7,9) was chosen as a model system in our studies. It has an accessible isotropization point (T_ni=181^oC), a relatively wide nematic range and good optical transparency. Our results showed that DHMS-7,9 (M_w=11,000 g/mol) is flow-aligning in the nematic phase. The effect of temperature and shear rate on the director dynamics will also be discussed.

[XC12.06] In Situ X-Ray Scattering Measurements of Molecular Orientation in Channel Flows of Thermotropic Liquid Crystalline Polymers.

We quantify the orientation developed in LCPs subjected to flow fields such as those found in extrusion and injection molding. To accomplish this, we have constructed a channel flow extrusion die which allows collection of x-ray scattering patterns as a function of position in the die. The scattering patterns are then used to characterize the orientation, in terms of both the direction of orientation and the degree of alignment. Interchangeable spacers allow the channel flow geometry to be altered. We compare behavior of two thermotropic liquid crystalline polymers (Xydar resin supplied by Amoco and hydroxypropylcellulose from Aqualon), emphasizing results from expansion flow experiments. A sharp decrease in orientation is observed at the expansion followed by a recovery in the straight downstream channel. Scattering patterns reveal orientation transverse to the flow direction induced by unfavorable extensional gradients.

[XC12.07] Ionic Main-Chain Thermotropic Liquid Crystalline Polymers(TLCPs)

To prepare new materials, we have synthesized main-chain thermotropic liquid crystalline polymers (TLCPs) that contain ionic groups. Base polymers are wholly aromatic copolyesters containing naphthalene units (VectraŽ type), and a small amount of ionic monomeric units is incorporated into the polymer chains with various counterions (Na, K, Mg, Ca, Ba, and Zn). A fiber-forming molecular weight is achieved for all the ionic TLCPs, and they exhibit nematic mesophase textures over a wide temperature range, without showing a transition to an isotropic phase, at least up to 380 oC. We have found that both tensile and compressive properties of these ionic TLCPs are enhanced. Such an increase in mechanical properties is attributed to enhanced lateral support, via ionic bonds (cross-links), between highly aligned TLCP chains.

[XC12.08] A TGB-like Twisted Smectic Phase in Monodisperse Poly(\gamma-be nzyl \alpha,L-glutamate) Produced by Recombinant DNA Techniques

Smectic ordering has been observed in perfectly monodisperse PBLG which was synthesized using recombinant DNA technology. Transmission electron microscopy (TEM) and electron diffraction reveals a banded morphology with an approximately 120 nm period which provides strong evidence for helical rotation of the director field as in a cholesteric or twisted smectic. Detailed examination of the relative orientation of the banding in the morphology images and the reflections in the electron diffraction patterns leads to the conclusion that the structure observed is a twisted smectic phase. The relationship between the twist and the layering is found to be that of the twist grain boundary (TGB) phase. Thus, we will refer to the phase as TGB-like. Conventional, polydisperse PBLG is well known to form cholesteric phases as a result of the chirality of the helical rod. The formation of a TGB-like phase in monodisperse PBLG is consistent with the superposition of a smectic-A layering resulting from the uniform rod length on the twisted texture present in the cholesteric.

[XC12.09] The Geometry of Hexatic Order

Several arguments indicate that the low temperature phase of flexible fixed-connectivity (crystalline) membranes is most likely an orientationally-ordered (hexatic) phase. Motivated by this we study the physical meaning of hexatic order on a general two-dimensional manifold. We find a very rich underlying geometrical structure characterizing hexatic phases. This allows us to analyze hexatic order in the plane from a new perspective. Results in more general geometries will also be discusssed.

[XC12.10] Syntheses of photoluminescenct spiro molecules

Novel spiro compounds, which have two perpendicular, well defined, rigid oligomeric chains fused together through a spiro center, were synthesized. Amorphous films of the spiro compounds formed by spin coating method have good optical qualities. The absorption and emission spectra can be tuned by differring the chain lengthes and the substituents. The spiro compounds were found have very high photoluminescence.

[XC12.11] Novel Liquid Crystallinity in Flexible Polymers

We construct a liquid-state, integral equation theory describing a liquid crystalline transition of a dense liquid of fully flexible Gaussian polymer chains. The polymers interact only through athermal, repulsive hard-core forces. We demonstrate a novel lyotropic transition from an isotropic to coexisting nematic and discotic phases in the Onsager-like limit of small physical chain thickness compared to the chain statistical segment length. The transition is second order in this limit, but becomes first-order for finite hard-core chain thickness. The structural correlations are anisotropic in the ordered phase. Also, density fluctuations are correlated in an anisotropic sense, indicating that the underlying semi-dilute mesh is anisotropic as well. This structural anisotropy has a natural interpretation in terms of lessened monomer-monomer collisions along the director axis, and thus is a starting point for understanding the enhanced flow properties of liquid crystalline polymers. We demonstrate that our formalism reproduces the qualitative features of Onsager's theory of the isotropic-nematic transition of hard, thin rods.

[XC12.12] Use of FTIR Spectroscopy to monitor Formation of Polymer Dispersed Liquid Crystals

Polymer Dispersed Liquid Crystals (PDLCs) are dispersions of liquid crystal droplets in a polymeric matrix. The optical properties of these materials can be controlled by the application of an electric field. PDLCs are formed by inducing phase separation in an initially homogeneous mixture of pre-polymer and liquid crystal. The preferred method of formation is by UV-induced curing of the matrix material. The phenomena of interest in the formation process are curing reactions of the matrix, point of gelation, conversion at which phase separation takes place and nematic ordering of the liquid crystal in the formed droplets. We present a novel application of FTIR spectroscopy where the whole formation process and multiple phenomena during the process are simultaneously monitored in real time. This allows us to understand the phase separation process and optimize material usage for PDLC formation.

[XC12.13] Dynamics of Side-Group Liquid Crystalline Polymers

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 (Me approx. 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.

[XC12.14] Surface Characterization of Rubbed Polyimide Films: A New Mathod to Determine Liquid Crystal Anchoring Properties

The polymer chain distribution and the direction of liquid crystal (LC) alignment induced by rubbing in a polyimide (PI) film can be altered by exposure to linearly polarized UV light. We studied the rotation of the direction of LC alignment on rubbed PI films as a function of UV exposure time and determined the changes in polymer chain distribution function using a simple model. The width of the distribution depended inversely on the rubbing strength and, thus the azimuthal anchoring energy. The time rate of change of the alignment direction was found to be inversely proportional to the anchoring energy. The results show that measurement of the width of the polymer chain distribution is an alternative LC independent and experimentally simpler way to determine the anchoring properties.

[XC12.15] Investigation of a Polymer/Small Molecule Liquid Crystal Interface Using Reflectivity

Important areas of liquid crystal display technology include the use of polymer stabilized liquid crystals and polymer dispersed liquid crystals (PDLC). The optical properties of PDLC systems are controlled by the alignment of the confined liquid crystal by an applied electric field. The nature of the polymer/liquid crystal interface also controls the surface induced orientation of the liquid crystal, a parameter that affects the ultimate optical properties of the display device. Even though the nature of the polymer/liquid crystal interface strongly influences the optical properties of the resulting structure, there is very little experimental characterization of these interfaces. Previous X-ray reflectivity results have shown that the as cast interface between thin layers of poly(methyl methacrylate) and 8CB is fairly broad. Also, annealing above the glass transition temperature of the polymer in this system results in diffusion of the liquid crystal into the polymer layer, and thus a broadening of the interface. The effect of annealing at more moderate temperatures is currently under examination. Thus, neutron reflectivity results will be presented which broadly describe the effect of thermal treatment on the evolution of the interface between a polymer and small molecule liquid crystal.

Part X of program listing