Reversible melting probed by temperature modulated dynamic mechanical and calorimetric measurements

Temperature-modulated DSC (TMDSC) and dynamic-mechanical analysis (DMA) allows the study of degree of crystallinity changes of polymers. From the comparison of the two methods using the same temperature-time program, one expects additional information about the processes occurring in the melting region. A first description of temperature-modulated DMA is given. The effect of reversible melting during every period of temperature modulation has been observed in the melting region of PEEK. The number of molecules which undergo reversible melting for a given quasi-isotherm decreases with time. At 600 K the fraction of the material involved in this process during one modulation cycle is between 0.35% and 0.25%, but has a non-zero value at infinity. The kinetics of this decrease indicates that the process of the structural changes is most likely related to the melt. It can be explained considering an entanglement of the disentangled melt surrounding the just molten crystals. The results are in agreement with the four-state scheme for polymer crystallization and melting proposed by Strobl.