Temperature-modulated differential scanning calorimetric measurements on pre-melting behavior of nascent ultrahigh molecular mass polyethylene

Temperature-modulated differential scanning calorimetric (TMDSC) measurements on nascent ultrahigh molecular mass polyethylene (UHMMPE) in scanning as well as quasi-isothermal mode are presented. From these measurements different processes, which contribute to the modulated heat flow in the pre-melting region, both exothermic and endothermic in character, can be separated. One of them only occur on the first heating of the nascent material. Analysis of quasi-isothermal measurements, performed between 90 and 130degreesC, show that there are two slow (exponential) relaxation processes with time constants of 2-5 and 10-100 min, respectively. One, exothermic in character, seems to be connected with irreversible structural changes (crystal thickening and ordering). The low activation energy (ca 40 kJ mol(-1)) points to a chain diffusion process rather than melting and crystallization. The other process (activation energy 60 kJ mol(-1)) seems to be endothermic. In the melting region, a slow (>100min) 3rd relaxation process with high activation energy (300 kJ mol(-1)) can be separated. (C) 2002 Elsevier Science B.V. All rights reserved.