Internal friction in two longitudinal polymer liquid crystal esters

Internal friction (IF) was measured for two polymer liquid crystal (PLC) esters, PET/xPHB, where PET is poly(ethylene terephthalate), PHB is p-hydroxybenzoic acid, with the mole fractions x of the PHB in the copolymers 0.5 and 0.6. Pure PET was also studied for comparison. The internal friction parameter Q to the -1 and the square of the output vibration frequency f squared were determined as a function of temperature between -150 degrees C and +400 degrees C for a fixed input frequency. Values of the latter were varied between 0.5 and 30 Hz; all spectra were recorded under vacuum of 0.10 Torr. The IF results are virtually independent of the substrate used, namely silica glass or platinum. Multiple transitions found in each sample agree fairly well with those determined by other methods, including thermally stimulated depolarization. In contrast to other dynamic mechanical methods, a single internal friction vs. temperature run can be used to locate transitions in solid as well as in melt phases.