Molecular mobility in para-substituted polyaryls .3. Low-temperature dynamics

The low-temperature mechanical relaxations in poly(oxy-1,4-phenyleneoxy-1,4-phenylene-carbonyl-1,4-phenylene [poly(aryl ether ether ketone, PEEK], poly(thio-1,4-phenylene) [poly(phenylene sulfide), PPS], and poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene), [poly(aryl ether sulfone), PES] are investigated. All three polymers exhibit a low-temperature gamma relaxation, in the temperature range from 40 to 100 K at 1 Hz. The apparent activation energies for the gamma relaxations (15-25 kJ/mol) are close to the values derived from empirical force field molecular mechanics calculations of the ''crankshatt motion'' in an isolated chain portion. Only polymers with a polar interaromatic bridge (PEEK and PES) exhibit another secondary beta relaxation, in the temperature range from 100 to 250 K at 1 Hz. Moreover, the apparent activation energies of the beta processes are significantly higher than the activation energies of the gamma relaxations. Therefore, it is concluded that polar intermolecular interactions play an important role in the molecular events leading to the beta relaxation, whereas the gamma relaxation is due to conformational changes mostly controlled by an intramolecular energy.