Mechanical γ and β relaxations in polyethylene—I. Glass transitions of polyethylene

Abstract

The mechanical γ relaxation of a linear polyethylene has been extensively analysed by means of high resolution mechanical spectrometry. This analysis was completed by quantitative measurements of the morphological parameters governing the characteristics of the γ relaxation, such as the crystallinity index and the crystalline lamellar thickness distribution determined from differential scanning calorimetry. The γ relaxation was found to have the characteristics of mechanical relaxation due to glass transition: there is a high apparent activation energy; linear polyethylene exhibits structural recovery induced by annealing at a temperature below the γ peak, whereas the γ relaxation has a WLF temperature dependence. As for the γ relaxation, it was found that the β relaxation displayed by a branched polyethylene has the characteristics of a glass transition (high value of apparent activation energy; structural recovery during ageing at T < T β ). Recalling definitions of the glass temperature, it was proposed that the lower glass transition (γ region) is characteristic of Van der Waals interactions between CH 2 groups while the upper glass transition (β region) is specific of Van der Waals interactions between CH 2 and CH 3 groups.