Polyethylene specific volume, crystallinity, and glass transition

Abstract

Abstract In all polymer crystallinity determinations, the basic assumptions are made that there exist only two well‐characterized phases and that the volumes of those phases are additive. For the case of polyethylene, the specific volume of the amorphous phase cannot be measured directly and is usually obtained as a linear extrapolation of the melt specific volume. This is shown to be invalid for the particular case of a high density polyethylene studied here. A new approach is proposed which utilizes temperature dependent determinations of total specifiac volume and of crystalline specific volume. The results indicate that the specific volume of the amorphous phase is less than the extrapolated liquid specific volume, and this is attributed to the restraining effect of the crystalline regions. The crystallinity of the particular high density polyethylene studied is found to have a constant value of about 70% up to 80°C., where melting commences. During this work, the glass transition of polyethylene has been detected as a 15% increase in the slope of the total specific volume‐temperature curve at −130°C.