Dynamics of surface crystallization and melting in polyethylene and poly(ethylene oxide) studied by temperature-modulated DSC and heat wave spectroscopy

Polymers with a high longitudinal diffuse mobility within the crystallites are known to show a continuous, reversible surface melting and crystallization; temperature changes are accompanied by shifts of the crystalline-amorphous interface, resulting in a crystal thickening on cooling and a thickening of the amorphous layers on heating. In measurements of the dynamic heat capacity c*(omega), the process shows up as a strong excess contribution which increases up to the temperature of the final irreversible crystal melting. Experiments were carried out for linear polyethylene (LPE) and poly(ethylene oxide) (PEO). Employing both a temperature-modulated differential scanning calorimeter (TMDSC) and a heat wave spectrometer (HWS), thereby covering the frequency range from 10(-3) to 10(2) Hz, we could analyze the process dynamics. The times required for the surface melting or crystallization were deduced from the change at the signal amplitude with frequency. They are remarkably long. For temperatures near to the respective final melting points we found about 12 s for the LPE sample and 120 s for PEO. The dynamic heat capacity of PE measured by TMDSC at low frequencies corresponds to the temperature dependence of the crystallinity. A comparison with the results of a SAXS structure analysis showed perfect agreement. For PEO the quasi-stationary conditions were not reached. Even at the lowest frequencies probed by TMDSC, the dynamic heat capacity was still below the value expected on a basis of the temperature dependence of the crystallinity determined by SAYS. In determinations of the dynamic heat capacity by TMDSC and HWS, it is in general necessary to correct the raw data to account for the inner heat flow resistance, additional heat capacities, and delay times introduced by the electronics. The corrections can be accomplished by an appropriate modeling of the measuring devices.