材料期刊网

Ultra-high molecular weight polyethylene (UHMWPE) samples are annealed at 100-degrees-C and the positron annihilation technique is used to study the variances of crystallinity, mean free-volume hole size, and free volume fraction. At the initial annealing stage, the crystallinity of UHMWPE increases. Then the increasing rate slows down along with the annealing time and the maximum crystallinity is reached after about 7 min. The mean free-volume size and free-volume fraction also increase at first, but decrease after reaching their maximum values. Annealing of UHMWPE using a high heating rate is a complicated process in which the thickening of lamellae, recrystallization, changing of void concentration in the crystalline region and free-volume formation, diffusion and relaxation can occur at the same time. The increase of crystallinity can be explained as the thickening of lamellae and perfecting of crystal. The change of lamellae could be described through Wunderlich's equation a2-dl/dt = -k dG(s)/dl. The relation between the O-Ps intensity I3 and free-volume fraction is eluciated by the model put forward by Jean et al. The experimental results are in good agreement with the theories.