Dynamic-mechanical behavior of polyethylenes and ethene-/α-olefin- copolymers. Part I. α′-relaxation

Polyethylenes and polyethylene/α-olefin-copolymers covering a range in crystallinity between 12 and 85% were investigated by means of dynamic-mechanical measurements between -145 °C and their melting point. From the temperature and frequency dependence of the complex modulus α′-, α-, β- and γ-relaxations were analyzed. The α′-relaxation was discovered in all HDPE-, LDPE- and LLDPE-samples but not in plastomer- and elastomer-samples. The activation energies (30-140 kJ/mol) of this relaxation were found to decrease with increasing crystallinity. The α′-transition temperature at a fixed frequency rises with increasing degree of crystallinity and tends to reach the melting point when approaching the fully crystalline state. Thus, it is concluded that the α′-relaxation originates from the interface between crystal lamellae and amorphous interlamellar regions. By extrapolation of the storage modulus to the amorphous state the entanglement molar mass was calculated as 2300 g/mol for a completely amorphous polyethylene/α-olefin-copolymer.