Effect on TSDC relaxation spectra of substitutions in the mesogenic unit and in the flexible spacer of poly(tetramethylene terephtaloyl-bis-4-oxybenzoate)
✍ Scribed by M. C. Hernández; E. Laredo; A. Bello; M. A. Gómez; C. Marco
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 279 KB
- Volume
- 37
- Category
- Article
- ISSN
- 0887-6266
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✦ Synopsis
A family of four thermotropic polyesters starting with the poly(tetramethylene terephthaloyl bis-4-oxybenzoate), substituting either asymmetrically in the tetramethylene spacer or incorporating polar substituents onto the aromatic units of the mesogen, is studied by the Thermally Stimulated Depolarization Currents (TSDC) technique. The results are compared to the Dynamic-Mechanical 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 corresponding to local reorientation modes due to the motion of the COO groups with different locations along the main chain. By comparing the effect of the Cl and CH 3 substituents on the relative intensity and on the mean energies of the distribution of relaxation times determined by the DSA procedure, the lowest temperature mode is attributed to the COO peripheral groups and the modes located at higher temperatures, to the internal COO groups, which may be accompanied by adjacent segments. The mechanic and dielectric ␣-transitions are also very sensitive to the substituents, the addition of CH 3 in the 1,4 flexible spacer shifting the glass transition temperature above room temperature. The existence of the three-dimensional order that is present in the materials with a linear spacer significantly broadens the ␣-relaxation.