The polydispersity of ethylene sequence in metallocene ethylene/α-olefin copolymers III: crystallization and melting behavior of short ethylene sequence

Crystallization and melting behavior of short ethylene sequence of metallocene ethylene/α-olefin copolymer with high comonomer content have been studied by standard DSC and modulated-temperature differential scanning calorimetry (M-TDSC) technique. In addition to high temperature endotherm around 120°C, a low temperature endotherm is observed at lower temperatures (40–80°C), depending on time and temperature of isothermal crystallization. The peak position of the low temperature endotherm Tmlow varies linearly with the logarithm of crystallization time and the slope, D, decreases with increasing crystallization temperature Tc. The Tmlow also depends on the thermal history before the crystallization at Tc, and an extrapolation of Tmlow (30.6°C) to a few seconds has been obtained after two step isothermal crystallization before the crystallization at 30°C. The Tmlow is nearly equal to Tc, and it indicates that the initial crystallization at low temperature is nearly reversible. Direct evidence of conformational entropy change of secondary crystallization has been obtained by using M-TDSC technique. Both the M-TDSC result and the activation energy analysis of temperature dependence suggest that crystal perfection process and conformational entropy decreasing in residual amorphous co-exist during secondary crystallization.