Viscoelasticity and dynamics of alignment in side-group liquid crystalline polymers

Citation

Rubin, Susan Felicia (1996) Viscoelasticity and dynamics of alignment in side-group liquid crystalline polymers. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/bdjk-n270. https://resolver.caltech.edu/CaltechETD:etd-01032008-082416

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Side-group liquid-crystalline polymers (SG-LCPs) consist of a flexible polymeric backbone and rigid mesogens (molecules forming LC phases), attached laterally to the backbone via flexible spacers. Since the dynamics of the mesogen field and polymeric backbone are partially decoupled, uniformly-aligned SG-LCP materials show promise in a variety of applications such as non-linear optical materials, optical data storage media, and stress sensors, which require switching of the mesogen orientation. Recent work has shown that uniform alignment of SG-LCP materials can be accomplished using flow-fields. However, the mechanisms of alignment, which are essential for the development of effective and rational processing strategies for the SG-LCP materials, remain poorly understood. To address this need, we focus on the viscoelastic properties of nematic and smectic SG-LCPs and the dynamics of field-induced alignment of SG-LCPs. We have investigated the dynamic mechanical response of SG-LCPs having methacrylate backbone, hexamethylene spacer, and phenyl benzoate mesogens as a function of molecular weight in the isotropic, nematic, and smectic phases [...], and have discovered a unique molecular-weight dependence of the sensitivity of the dynamic modulus to nematic order of the melt. Nematic order produced a profound change in the dynamics of the entangled SG-LPCs relative to the isotropic phase; however, this effect was absent in the unentangled SG-LCPs. In SGLCPs with smectic order, there was increase in the elastic character of the fluid with smectic ordering, but the incremental effect in a system that was entangled was relatively small. Oscillatory shear with large amplitude [...] induced macroscopic alignment in the nematic phase for all the SG-LCPs studied and could be used to alter the microstructure in the smectic liquid. Shearing the smectic phase produced a decrease in modulus, whereas shearing in the nematic phase followed by cooling into the smectic phase produced an increase in modulus. To assess the effect of the coupling between the mesogen and the backbone on field- induced orientation of nematic SG-LCPs, we have compared magnetically aligned and flow-aligned nematic SG-LCPs. Magnetic forces act primarily on the mesogens, and the backbone conformation changes to accommodate the torques on its pendant mesogens. In contrast, oscillatory shear can couple to the relaxation modes of both the director field and backbone. We have discovered that while the flow-aligned material exhibits distinct low-frequency relaxation dynamics, the relaxation of magnetically- aligned monodomains is indistinguishable from that of polydomain nematic melts. This suggests that flow alignment and magnetic alignment produce qualitatively distinct changes in the fluid miscostructure. We have further compared the processes of shear- and magnetic-alignment by monitoring the evolution of director orientation in nematic SG-LCP melts, in-situ, through visible transmittance (related to the liquid-crystalline domain size) and visible birefringence (related to the molecular orientation).

Thesis Files