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A simple model is proposed for the molecular weight dependence of the relaxation time spectrum of linear flexible polymers with molecules of nearly uniform length. A systematic approach for determining the parameters of the relaxation time spectrum from dynamic mechanical data, G'(w) and G"(o), shows the possibilities and the limits of this model. The necessary parameters are ne, G N O , and A , for the entanglement and flow behavior and A, and n, for the crossover to the high-frequency glass behavior. Generic values are proposed for polystyrene and polybutadiene of high molecular weight.

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In this paper, we report the mechanical relaxation behavior of low molecular weight glassforming side-chain liquid crystalline compounds with cyclic cores and their linear polymeric analogues near the glass transition temperature. We examined two systems: one based on cyclic and linear siloxane backbones and one based on cyclic (cyclohexane) and linear aliphatic backbones. Dynamic mechanical spectroscopy is used to measure the dynamic shear moduli and the complex viscosity near but above Tg. The temperature dependence of the zero-shear viscosity of the cyclic compounds is well described by the Vogel-Tammann-Fulcher (VTF) equation. The strong temperature dependence of the viscosity along with the values of the fitted parameters of the VTF equation shows that the cyclic LC compounds are "fragile" liquids. All cyclic LC compounds, regardless of chemical structure, show identical relaxation behavior when viscosity is plotted versus normalized temperature (Tg/T), where Tg is the temperature obtained calorimetrically at a heating rate of 10 K/min. All cyclic compounds show lower viscosity than their linear analogues when plotted versus Tg/T. The difference in viscosity between the cyclic and linear siloxane compounds is much more pronounced than the difference observed in the aliphatic compounds. For the cyclic compounds, master curves of G′ and G′′ can be described by a single Maxwell mode. The linear compounds exhibit much broader mechanical spectra, suggesting a more complex relaxation phenomenon is taking place. Our results show that, while there is little difference in relaxation behavior among low molecular weight cyclic liquid crystalline compounds, the behavior of the linear polymeric systems is quite different.

Two types of chitosan hydrogel systems have been prepared that have a laser damage threshold (LDT) up to 35 times higher than commercial PMMA bulk materials. For these samples, the LDT increases with increasing water content. The mechanism of laser damage and the contribution of water to their high laser damage resistance have been examined. DSC measurements indicate water within the hydrogels exist in various states, each with different laser damage resistance properties. These various states play a key role in determining the LDT by controlling the dissipation of laser energy and providing a mechanism for self‐healing. This preliminary research shows that polymer hydrogels have potential for high power laser applications because they combine good mechanical integrity due to the polymer frame and good energy dissipation and healing characteristics due to the molecular mobility of water. These two traits allow for bulk shape‐retaining films with high laser damage thresholds and potential reversibility in damage processes. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 769–778, 1999

ABSTRACT:We demonstrate the tuning of the diffraction efficiency in a polymer-dispersed liquid crystal (PDLC) diffraction grating by mechanical shear deformation. We chose a photocurable thermoset with elastic mechanical properties as the host material to demonstrate the tuning effect. Raman-Nath gratings were prepared by anisotropically curing a mixture of commercial photopolymer doped with liquid crystal. The mixture was contained between glass slides which were mounted in a simple shearing device. During the photopolymerization, the liquid crystal phase separated into 2-mm droplets organized into channels with 3.8-mm spacing. Gratings were deformed by shearing the samples in a direction perpendicular to the channel orientation. The diffraction intensity and morphology were monitored as a function of shear strain. Reversible tuning was observed due to the elastic nature of the matrix.

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