The temperature- and frequency-dependent dynamic-mechanical properties in the temperature regime of the γ-transition were determined for a number of polyethylenes and ethylene/α-olefin-copolymers differing in their crystallinity and crystal morphology. The temperature dependence of the γ-transition was found to obey the Arrhenius law for thermally activated processes. The γ-transition temperature determined at a frequency of 1 Hz and the corresponding activation energy were analyzed as a function of the crystallinity. As an overall trend, both quantities are found
to decrease with decreasing crystallinity, which is explained by the increase in free volume due to the incorporation of short-chain branches or the thermal pretreatment (e.g., quenching). Taking into account that the crystal morphology of polyethylenes can be classified into four different groups, a more detailed picture appears. Within one type of morphology both quantities, namely the transition temperature and the activation energy, increase with decreasing crystallinity
independent of the α-olefin used as the comonomer. These findings can be explained by partial orientations of the molecule segments in the interlamellar amorphous space in the case of HDPE or by the increased steric hindrance of the crankshaft motion by the short-chain branches. From the findings of this series of studies, it was concluded that the glass transition in polyethylene and polyethylene/α-olefin-copolymers is the β- and not the γ-transition.
Glowinkowski S, Makrocka-Rydzyk M, Wanke S, Jurga S, European Polym. J., 38(5), 961, 2002
Most of the activation energies determined were significantly larger than the value of 38 kJ/mol reported by Gl.owinkowski et al. [33] and Hartwig [2]. The reason for these differences might lie in the measurement method, as both groups determined the activation energy by NMR, which seems not to produce comparable results with dynamic- mechanical measurements due to the different length scales being analyzed (see also part II of this series [19]).
Popli R, Glotin M, Mandelkern L, J. Polym. Sci. Part B: Polym.Phys., 407, 1983