MECHANICAL-GAMMA AND BETA-RELAXATIONS IN POLYETHYLENE .1. GLASS TRANSITIONS OF POLYETHYLENE

The mechanical gamma-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 gamma-relaxation, such as the crystallinity index and the crystalline lamellar thickness distribution determined from differential scanning calorimetry. The gamma-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 gamma-peak, whereas the gamma-relaxation has a WLF temperature dependence. As for the gamma-relaxation, it was found that the beta-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(beta)). Recalling definitions of the glass temperature, it was proposed that the lower glass transition (gamma-region) is characteristic of Van der Waals interactions between CH2 groups while the upper glass transition (beta-region) is specific of Van der Waals interactions between CH2 and CH3 groups.