DETECTION OF GLASS-TRANSITION IN POLY(ETHYLENE-TEREPHTHALATE) AND NYLON-6 BY POSITRON-ANNIHILATION

Positron annihilation lifetime spectroscopy (PALS) was used to investi gate the phase transitions, mainly the glass transition, of poly(ethyl ene terephthalate) (PET) and nylon-6 during the thermal treatment of t hese polymers. The longest-lived component lifetime and intensity, ind icative of ortho-positronium pick-off, exhibit thermal dependencies th at can be attributed to the free-volume changes associated with struct ural transitions. Glass transition temperatures and the volume of inte rmolecular-space holes among polymer chains were obtained from the lif etime, tau(3), and intensity of formation, I-3, Of the long-lived comp onent of ortho-positronium. For PET, the free-volume fraction and ther mal expansion coefficients related to the free-volume fraction were al so obtained. Double glass transition behavior was noted in the analyze d polymers, which was consistent with their semicrystalline nature as revealed by differential scanning calorimetry. Increases in the slope of the lifetime-temperature plots for nylon-6 and PET were interpreted to suggest that glass transitions are followed by an increased free-v olume cavity expansion as temperature is increased. The intensity resp onse for PET was consistent with the association of glass transition w ith the reduction of crystalline constraint on segmental mobility in t he amorphous phase. In contrast, the intensity behavior during the the rmal treatment of nylon-6 seems to be governed more by the electronic effects occurring when the polymer chains acquire mobility than by fre e-volume changes. Since the sensitivity of PALS is in the order of nan ometers, it is expected to give an alternative novel technique to esti mate phase transitions and relaxations in polymers from the point of v iew of the free volume. (C) 1996 John Wiley & Sons, Inc.