Characteristics of Ultradrawn Polyethylene Films as a Function of Temperature Estimated by the Positron Annihilation Lifetime Method

Abstract

Three kinds of mechanical relaxations, the α, β, and λ transitions, of ultradrawn polyethylene films were obtained by dynamic mechanical measurements, and the results were analyzed by positron annihilation. The gels were prepared by crystallization from dilute solutions and were then dried. The dried gel films were stretched by up to a draw ratio of 300 times (λ = 300). The temperature dependence of the intensity (I3) of the long-lived component of the orthopositronium (o-Ps) as well as that of the lifetime (λ3) was observed for undrawn and drawn polyethylene films. For the undrawn film, the peak positions of the γ and β transitions corresponded to the first and second transitions of τ3. The β dispersion was not observed for the films with λ = 50 and 300. The second transition of τ3, corresponding to the β dispersion, was observed for drawn films with λ = 50 but was not observed for the film with λ = 300. A histogram of τ3 for three kinds of specimens showed that the size distribution for the undrawn film (λ = 1) became broader, and the peak position shifted to a larger value of the lifetime (τ3) with increasing temperature. This indicates that the free volume holes within the specimen became bigger and had a wider size distribution with increasing temperature and, at the same time, the molecular motion became more active. For the ultradrawn film with λ = 300, the lifetime distribution at 100 °C showed a much wider and asymmetry profile, and the peak position shifted to a shorter value than those for the other two films with drawing ratios of λ = 1 and λ = 50. This indicated poor activity of a large movement (macro-Brownian motion) of amorphous chains. These experimental results justify that the thermal expansion of the film due to an increase in the molecular mobility was almost zero in a given temperature range from 0 to 120 °C and that the storage modulus at 100 °C was higher than 130 GPa.