STUDIES ON THE FREE-VOLUME CHANGE IN ANNEALED ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE BY THE POSITRON-ANNIHILATION TECHNIQUE

Ultra-high molecular weight polyethylene (UHMWPE) samples are annealed at 100-degrees-C and the positron annihilation technique is used to s tudy the variances of crystallinity, mean free-volume hole size, and f ree volume fraction. At the initial annealing stage, the crystallinity of UHMWPE increases. Then the increasing rate slows down along with t he annealing time and the maximum crystallinity is reached after about 7 min. The mean free-volume size and free-volume fraction also increa se at first, but decrease after reaching their maximum values. Anneali ng of UHMWPE using a high heating rate is a complicated process in whi ch the thickening of lamellae, recrystallization, changing of void con centration in the crystalline region and free-volume formation, diffus ion and relaxation can occur at the same time. The increase of crystal linity can be explained as the thickening of lamellae and perfecting o f crystal. The change of lamellae could be described through Wunderlic h's equation a2-dl/dt = -k dG(s)/dl. The relation between the O-Ps int ensity I3 and free-volume fraction is eluciated by the model put forwa rd by Jean et al. The experimental results are in good agreement with the theories.