Rheological studies on nematic thermotropic liquid crystalline polymers

Liquid crystalline polymers (LCPs) are a new class of high strength materials. Their rheological behavior is different from that of ordinary isotropic polymers because they are inherently anisotropic in the melt state. Crystallization in rigid-rod mesogenic systems in the nematic melt and super-cooled states has been studied by small amplitude oscillatory shear, which has been found to be in some ways more sensitive than conventional techniques like DSC or X-ray scattering (in that changes are measured by orders of magnitude instead of merely on a linear scale). A scheme of preheating is suggested for unsubstituted, rigid-rod polymers whereby a metastable nematic melt can be achieved, effectively suppressing crystallization and enabling a thorough rheological characterization. For example, an excellent agreement has been obtained for the stress relaxation modulus between experimental values and those calculated from dynamic oscillatory measurements, thereby confirming the existence of a linear viscoelastic range for the LCP. Non-linear creep studies illustrate how rheological properties are strongly affected by structural changes upon deformation. Structure in unsubstituted rigid-rod nematic systems is hypothesized to exist at two levels--Non-Periodic Layer (NPL) Crystallites and Domains/Disclination Network.; Another aspect of research concerns a comparative study of the phase behavior of liquid crystalline components in closely related blends and copolymers.