KINETICS OF ISOTROPIC SMECTIC PHASE-TRANSITION IN LIQUID-CRYSTALLINE POLYETHERS

The isotropic-smectic mesophase (ism) transition in two thermotropic main-chain polyethers based on bis(4-hydroxyphenoxy)-p-xylene and two different methylene spacers with 9 and 11 carbons, respectively, have been studied under isothermal conditions by polarized-light microscopy and differential scanning calorimetry. The ism transition, which displays all the characteristics of a nucleation controlled process, follows the general Avrami equation with an exponent typically between 2.5 and 3.0. The growth rate (dL/dt) of the mesomorphic mosaic domains, which is greater along than perpendicular to the chain axis, is, at isothermal conditions, constant with time and follows dL/dt = c1e-Kg/T(T1-T) where C1 and Kg are constants and Ti is the equilibrium clearing temperature. DSC data revealing the overall rate are consistent with this view. A second process which occurred at rates almost comparable with the ism transition at high temperatures displayed an Arrhenius type temperature dependence with an activation energy of about 400-850 kJ mol-1. This is indicative of a rearrangement, e.g., a perfection of the smectic mesophase occurring without nucleation. This process followed the Avrami equation with an exponent of about 2.0-2.2. © 1990, American Chemical Society. All rights reserved.