Effect on TSDC relaxation spectra of substitutions in the mesogenic unit and in the flexible spacer of poly(tetramethylene terephtaloyl-bis-4-oxybenzoate)

A family of four thermotropic polyesters starting with the poly(tetramethyl ene terephthaloyl bis-4-oxybenzoate), substituting either asymmetrically in the tetramethylene spacer or incorporating polar substituents onto the aro matic units of the mesogen, is studied by the Thermally Stimulated Depolari zation Currents (TSDC) technique. The results are compared to the Dynamic-M echanical Analysis at frequencies ranging from 0.1 to 1 Hz from 123 to 423 K. The unsubstituted polymer has a complex low-temperature TSDC spectrum co rresponding to local reorientation modes due to the motion of the COO group s with different locations along the main chain. By comparing the effect of the Cl and CH3 substituents on the relative intensity and on the mean ener gies of the distribution of relaxation times determined by the DSA procedur e, the lowest temperature mode is attributed to the COO peripheral groups a nd the modes located at higher temperatures, to the internal COO groups, wh ich may be accompanied by adjacent segments. The mechanic and dielectric al pha-transitions are also very sensitive to the substituents, the addition o f CH3 in the 1,4 flexible spacer shifting the glass transition temperature above room temperature. The existence of the three-dimensional order that i s present in the materials with a linear spacer significantly broadens the alpha-relaxation. (C) 1999 John Wiley & Sons, Inc.