MELT VISCOSITY OF MAIN-CHAIN THERMOTROPIC LIQUID-CRYSTALLINE POLYMERSWITHOUT FLEXIBLE SPACERS

Viscoelastic properties for a series of all-aromatic main-chain thermo tropic copolymers based on random units of 75% 1-hydroxy-4-benzoic aci d and 25% 2-hydroxy-6-naphthoic acid (B-N, 3:1) were measured in the n ematic state. Zero-shear rate viscosity eta(0) obtained from small-str ain oscillatory and steady-state measurements reveals that eta(0) scal es with the molecular weight with a power of 4.1, i.e., eta(0) proport ional to M(4.1). Measurements of viscosity for the small-molecule liqu id crystal (SMLC) bis[4-(phenoxycarbonyl)-phenyl] 1,4-benzenedicarboxy lic acid ester and data for other SMLC compounds taken from the litera ture establish that this power dependence holds over a range covering 3 decades in molecular weight and 8 decades in viscosity. This exponen t 4.1 is distinctly different from that of conventional polymer melts, 3.4, but lies within the range of values reported for nematic polymer s with flexible spacers.