Stress relaxation modes in liquid crystalline polymers

Rheological constitutive equations, developed for traditional isotropic polymers, inadequately portray viscoelasticity in anisotropic liquid crystalline polymers (LCPs). Even the equations explicitly developed for rodlike polymers do not adequately depict prominent transient features of the LCP mesophase. Therefore, the objective of this research is to improve our understanding of the viscoelastic behavior for this expanding class of polymers, so that better constitutive models may be developed.;In this work, the phenomenological emulsion based "droplet" model was developed to characterize the relaxation behavior for real LCP systems. This model incorporates the influences of the molecular and supramolecular morphologies on the viscoelasticity. The resulting equation was used to describe the viscoelastic stress relaxation, which has its origin in large scale supramolecular morphological changes.;The primary experimental technique used to study the LCP viscoelasticity was the flow cessation experiment. Two liquid crystalline polymers, each at several polymer concentrations, were used as model systems. The major contributions to the viscoelasticity of the mesophase were correlated with supramolecular changes observed optically. Excellent quantitative agreement was observed between the emulsion based droplet model and the experimental stress measurements for the supramolecular-scale stress relaxation. The resulting model parameters were found to have realistic physical interpretations which further support the supramolecular nature of the relaxation process.;The success of the droplet deformation model, in characterizing the mesophasic stress relaxation, strongly supports this description of the supramolecular structures in terms of a two-phase emulsion. As a consequence of this dominance of the supramolecular structure in the stress relaxation, the principal mode for storing free energy changes in LCPs is best characterized by interfacial energies, or equivalently, scalar Frank distortional energies.