Dynamic mechanical studies of α and β relaxations of polyethylenes

Abstract

The α and β relaxations of a variety of polyethylenes have been extensively studied using lowfrequency dynamic mechanical methods. The main focus of this work has been both the control and the quantitative measurement of the key structural factors that describe semicrystalline polymer systems. The structural factors that have been examined in detail include the level of crystallinity, the crystallite thickness, the interfacial content, and the supermolecular structure. Consequently a variety of other types of supplementary measurements were made to accomplish the necessary characterization. The location of the α transition is found to depend primarily on the crystallite thickness. There also is the distinct possibility that the interfacial structure exerts an important influence. The level of crystallinity and the supermolecular structure do not play a significant role in the location of Tα. A strong correlation is found with the carbon‐13 NMR crystalline T1, which is reported in a separate paper. From analysis of the influence of the different structural factors on the β transition, it is concluded that this transition results from the relaxation of chain units which are located in the interfacial region. The elusiveness of this transition and the contradictory reports that have existed in the literature are given a ready explanation. The enhancement of this transition by branching and copolymerization follows naturally as does its invariance with counit content.