Influence of Molecular Weight on the Thermotropic Mesophases of Poly(6-(4-methoxy-(4'-oxy)-alpha-methylstilbene)hexyl Methacrylate)

摘要

Poly(6-(4-methoxy-(4'-oxy)-alpha-methylstilbene)hexyl methacrylate) (4-6-PMA) with different molecular weights and molecular weight distributions was synthesized by group transfer and radical polymerization of the corresponding monomer. The phase behavior of liquid crystalline 4-6-PMA with varying molecular weights was compared to that of its monomeric and dimeric model compounds. The number and nature (i.e., virtual, monotropic or enantiotropic) of phase transitions was determined by a combination of thermodynamic and kinetic factors. Since the rate of formation of highly ordered phases (crystalline and smectic) decreases with increasing polymer molecular weight, the number and nature of the liquid crystalline phases exhibited by 4-6-PMA varies with increasing molecular weight according to the following trend: monotropic nematic, enantiotropic nematic and enantiotropic smectic, enantiotropic nematic and monotropic smectic, enantiotropic nematic. Phase transition temperatures increase up to a degree of polymerization of about ten to twelve, and are thereafter essentially molecular weight independent. The rate by which the number of transitions decreases also decreases with increasing polymer weight. However, the enthalpy change associated with the nematic-isotropic transition is molecular weight independent. (AW)