Glass transition of thermotropic polymers based upon vanillic acid, p-hydroxybenzoic acid, and poly(ethylene terephthalate)

Differential scanning calorimetry (DSC) and dynamic mechanical and thermomechanical analyses are used to study the glass transition temperature (T-g) of the thermotropic liquid crystalline polyesters prepared by molten polycondensation of 4-acetoxyvanillic acid (V), 4-acetoxybenzoic acid (B), and poly(ethylene terephthalate) (E). The polyesters exhibit two glass transition ranges at 66-83 as well as 136-140 degrees C, which are attributed to E segmental motion in the E-rich/B-rich phases of the polyesters, as well as to the local mobility of the B component in the B-rich phase. The lower T-g of the polyesters increases with an increase in the B and V unit contents or with increasing heating rate in the DSC measurements, but the higher T-g decreases slightly. The experimental T-g values are compared with the T-g values predicted from three equations on the basis of the sequence distribution and the monomer reactivity ratio of the polyesters. Uematsu's, Johnston's, and Couchman's equations all give a better fit with the experimental results. (C) 1996 John Wiley & Sons, Inc.