Session I30 - Liquid Crystalline Polymers.
MIXED session, Wednesday morning, March 19
Room 4204A, Conv. Center

[I'30.01] X-ray Scattering Study of Molecular Orientation in a Thermotropic LCP Under Shear

While much is known about the rheology and structure of lyotropic liquid crystalline polymers (LCPs) subjected to shear flow, thermotropic LCPs are relatively less understood. To a significant degree, this reflects experimental difficulties associated with thermal degradation, recrystallization, or transesterification processes in the commercial thermotropes most widely available. Here we describe results of in situ x-ray scattering studies of molecular orientation in a model thermotropic copolyester subjected to shear flow. The polymer, synthesized by Chang and Han, consists of rigid mesogens randomly copolymerized with flexible spacer chains of two different lengths. This effectively suppresses any complications due to crystallization. In addition, the sample has an accessible clearing temperature, that allows establishment of a well-defined thermal history prior to application of shear flow. We will present studies of steady state orientation as a function of shear rate and temperature, as well as measurements of orientation during relaxation upon cessation of shear flow.

[I'30.02] Organo-Soluble Aromatic Polyimide Films with Optical Anisotropy and Complicated Relaxation Behaviors

This abstract was not submitted electronically.

[I'30.03] Electrorheological Behavior of Liquid Crystal Polymers dissolved in Nematic Solvents

When an electric field is applied orthogonal to the shear direction, low molar mass nematics with positive dielectric anisotropy show a small electrorheological (ER) effect, the viscosity with the field ON being 2-3 times larger than with the field OFF. The magnitude of the ER effect is greatly enhanced when a liquid crystal polymer (LCP) is dissolved in the nematic, and is much larger for main-chain LCPs versus side-chain LCPs. In dilute solution, the viscosities are a linear function of LCP concentration, and we determine an intrinsic viscosity with the field ON and OFF, viz. [ON] and [OFF], respectively. For main-chain LCPs, [ON] is a strong function of molecular weight, [ON] ~ M, and is an order of magnitude larger than [OFF], which is weakly dependent on molecular weight. For side-chain LCPs, the reverse is found, i.e. [ON] is weakly dependent on molecular weight, and 3-5 times smaller than [OFF], which varies ~ sqrtM. These results are consistent with hydrodynamic theory, if it is assumed that the conformation of the main-chain LCP in the nematic medium is prolate, whereas that of the side-chain LCP is oblate.

[I'30.04] Origins of Defect Texture in Liquid Crystal Polymers

We have studied the nucleation and growth process of point defects in nematic liquid droplets of the polyether DHMS-7,9 as it is cooled below its isotropic to nematic transition temperature. The director field distribution was revealed using a solidification induced banding method on quenched samples. At a low percent conversion of isotropic to nematic, the defect density is dominated by domain density and by boundary conditions between the nematic domains and the isotropic matrix. At higher percent conversions the defect density is controlled by domain interaction events. The defect density rises as the domains nucleate and impinge but decreases after the generation of defects of negative strengths and an annihilation process begins. Larger domains created in the coalescence process can then grow and impinge and continue defect generation via domain interaction. The absence of negative strength defects during the preimpingement phase indicates these defects arise solely due to the impingement of domains during the growth process.

[I'30.05] Vitrified Liquid Crystalline Order in LC Thermoset films under Electric Fields.

The curing of liquid crystalline (LC) dicyanates and epoxies containing phenylbenzoate mesogens were studied under applied A.C. electric fields. Characteristics of the two families and the difference between dicyanates and epoxies will be explained by x-ray diffraction and POM measurements. Especially, curing reactions of epoxies under low frequencies resulted in a parallel molecular orientation, initially, but gradually changed the alignment until the final network showed molecular orientation perpendicular to the E-field, independent of the applied frequency. This effect is different for dicyanates. The crossover frequency of molecular orientation shifted to lower frequencies as the curing reaction proceeded in a much shallower transition and the initial frequency decides whether the material is aligned parallel or perpendicular to the E-field for dicyanates. The gelation point of both systems vitrifies the orientation of the LC director and retains this alignment until decomposition. Curing reactions under low frequencies took place faster than those at high frequencies, due to electrohydrodynamic effects. The electric field also affected the formation of the smectic epoxy networks.

[I'30.06] Microstructure of Hairy-Rod Polymers at the Air-Water Interface

To better understand the role of long flexible side groups on the microstructure of "hairy-rod" polymer thin films, a series of poly(\gamma-methyl-L-glutamate-co-\gamma-n-octadecyl-L- glutamate) of varying degree copolymerization has been investigated at the air-water interface by external reflectance infrared spectroscopy. Coupled with a Langmuir trough, the microstructure of the monolayer films was charaterized directly at the air-water interface as a function of varying surface packing density. The conformational order of the n-octadecyl side groups was subsequently shown to vary strongly as a function of n-octadecyl side group content as well as surface packing density. When conformationally disordered, the n-octadecyl side groups could be likened to that of a Rsolvent-likeS matrix for the rigid- rod main chains in the plane of the interface. By controlling the conformational order and therefore the Rsolvent-likeS character of the n-octadecyl side groups, it was possible to control an in-plane orientation of the rigid-rod main chains relative to that of the compression axis of the Langmuir trough. The orientation of the n- octadecyl side groups out of the plane of the air-water interface was also determined.

[I'30.07] Solvent Effects on the Flow of Lyotropic Polymers as Determined by Neutron Scattering

Small angle neutron scattering is utilized to examine the orientation of liquid crystalline polymer solutions under a flow field. It is found that the alignment behavior of poly(benzyl L-glutamate) (PBLG) in deuterated benzyl alcohol (DBA) differs significantly from the alignment of PBLG in deuterated m-cresol (DMC) while a liquid crystalline solution of hydroxypropylcellulose in deuterated water exhibits similar alignment behavior to the PBLG/DBA solution. This is unexpected as DMC is an isomer of DBA while PBLG is a rigid molecule in solution and HPC behaves as a semiflexible or wormlike molecule. To better understand this behavior the relaxation of shear induced orientation in PBLG/DBA and PBLG/DMC after the cessation of shear is currently under investigation. Preliminary results show that an aligned state is stable up to 6 hours after the cessation of shear for the PBLG/DMC solution while the alignment of PBLG in DBA readily relaxes.

[I'30.08] Toughening Thermoplastics with Thermotropic Liquid Crystal Polymers

Blends of thermotropic liquid crystalline and thermoplastic polymers have improvements in mechanical properties when high elongational flow provides LC needle shaped domains. This means the test sample must be a filament. Without elongational flow the LC domains are globular in shape and provide little or no mechanical property enhancement. One solution to this problem is to make the LC domains small enough so that their size approaches that of microvoids in the thermoplastic polymer system. Such an approach can potentially provide a pseudo-compatible blend. Thus if the thermoplastic matrix polymer is transparent, the nano size LC domains can provide reinforcement, modified optical properties, and still leave the bulk material transparent. This investigation focusses on the process and improvement in mechanical and fracture resistant properties.

[I'30.09] Deutron NMR Study of Spacer Flexibility Effects on Molecular Order in Nematic Melts of Semiflexible Polymers

We have investigated the conformational and orientational order in nematic melts of semiflexible polymers which comprise rigid and flexible sequences in alternating succession. The average flexibility is varied continuously by changing the molar ratio of two flexible spacer groups, O(CH_2)_10O and O(CH_2CH_2O)_3, which are distributed randomly along the chain. The enthalpy of nematic-isotropic transition does not increase linearly with the decamethylene fraction, but rather exhibits a sharp jump above ca. 30 mole-%. Such a phase behavior indicates an abrupt, drastic change in the nature of nematic order as the average flexibility of the spacer sequences decreases beyond a critical value, as predicted by theory. Deuteron NMR patterns from deuterated spacers, measured for polymers of varying comonomer ratios, provide a direct proof that, depending on the average degree of flexibility of spacer sequences, polymer chains in nematic melts undergo an abrupt change from random conformations with low orientational order to extended conformations with very high orientational order.

[I'30.10] Phase Diagrams of a Binary Smectic Liquid Crystal Mixture

A theoretical model for predicting phase diagrams of binary smectic liquid crystal mixtures has been developed based on combination of Flory-Huggins (FH) theory for isotropic mixing and Maier-Saupe-McMillan (MSM) theory for anisotropic ordering of liquid crystals (LC). Two clearing transitions along with two nematic order parameters and two smectic order parameters have been incorporated in the calculation. The FH/MSM model is capable of predicting a variety of phase diagrams of a binary smectic mixture in which both LC components have either a smectic-A-nematic-isotropic transition or a smectic-A-isotropic transition. Phase diagrams involving induced nematic and induced smectic phases are also calculated and tested critically with the reported experimental results.

[I'30.11] Translational and Rotational Diffusion of Tobacco Mosaic Virus in Semidilute Dextran Solutions

Depolarized dynamic light scattering was used to measure translational and rotational diffusion of tobacco mosaic virus, TMV, in aqueous solutions of dextran. TMV is a charged, rigid rod (300 x 18 nm). Dextran is an uncharged, flexible polymer. TMV was held at a fixed, dilute concentration; dextran concentrations spanned both dilute and semidilute regimes. TMV did not phase separate or strongly aggregate, and it did dominate the depolarized scattering at all dextran concentrations. Beyond 6.5 percent dextran, large failures of the Stokes-Einstein relation between diffusion and viscosity occurred. Translational diffusion continued more rapidly than expected based on the macroscopic viscosity, while rotational diffusion fell sharply below expectation. The quotient Dr/Dt of rotational and translational diffusion also dropped suddenly above the transition. These findings are consistent with sudden onset of topological constraints to rotational motion of the TMV before translation is affected. Temperature studies showed that either the solution or the solvent viscosity can describe translation and rotation at concentrations below the transition.

[I'30.12] Deformation of an Amorphous and Liquid Crystalline Epoxy-Based Thermoset

Liquid crystalline thermosets (LCT's) are a new class of materials which have unusual mechanical properties due to their unique ``polydomain'' network structure which consists of microscopic regions of uniform molecular orientation. The LCT's used in this study were prepared by curing a difunctional LC epoxy monomer, the diglycidyl ether of 4,4'-dihydroxy-\alpha-methylstilbene (DGDHMS), with the tetrafunctional crosslinker, methylene dianiline (MDA) at different temperatures. The type of local molecular order (e.g. nematic, smectic, or isotropic) of the networks depends on which phase the epoxy/amine mixture is crosslinked from and is controlled through the initial cure temperature. A commercial, amorphous epoxy of similar starting molecular weight was also cured with MDA to produce a non-LC thermoset for comparison. Dynamic mechanical analysis (DMA) revealed reduced glassy moduli, increased rubbery moduli, and broadened glass transitions for the LCT's compared to the amorphous thermoset. At room temperature, the true stress/true strain curves of the LCT thermosets under uniaxial compression show no strain softening region, substantial plastic deformation (\epsilon_f=0.5) and increased strain hardening compared to the amorphous thermoset. Fracture toughness values show an increasing trend with increasing local order for the LCT's ranging from a GIc (kJ/m^2) of 0.68 (isotropic) to 0.75 (nematic) to 1.62 (smectic).

[I'30.13] Studies of Molecular Orientation and Order in Self-Assembled Semifluorinated n-Alkylthiols: Single and Dual Component Mixtures

The structure, orientation and morphology of self-assembled monolayers of a semifluorinated n-alkythiol, F(CF_2)_8 (CH_2)_11 SH (F8H11SH), have been investigated by polarized IR, angular dependent XPS, time-of-flight SIMS, contact angle and ellipsometric measurements. The orientation of the all trans hydrocarbon segment was found to be tilted much less from the surface normal than the 30 degree tilt found for octadecylthiol. This has been attributed to the steric constraints imposed by the larger cross section fluorocarbon helices which subsequently are tilted from the surface normal. In addition, studies of dual component mixtures of F8H11SH/F8SH and F8SH/F8H2SH have revealed that competitive adsorption occurs in the former producing monolayers which are deficient in the shorter F8SH molecules while in the latter equal representation of both F8SH and F8H2SH molecules are found on the surface due to their similar molecular lengths. These well-defined surfaces were investigated as alignment media for liquid crystals and a number of these templates have shown homeotropic and degenerate planar alignment of adjacent liquid crystal layers. The morphology of these surface layer is found to be very important in controlling the liquid crystal alignment.

Part I of program listing