Molecular dynamics of the α relaxation during crystallization of a glassy polymer: A real-time dielectric spectroscopy study

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Abstract
The change of the α relaxation process occurring during isothermal crystallization of a glassy polymer, poly(aryl ether ketone ketone), has been followed in real time through measurement of the dielectric complex permittivity. The dielectric strength has been correlated with the degree of crystallinity derived from real time wide angle x-ray patterns using synchrotron radiation. The phenomenological Havriliak-Negami description has been used to analyze the changes of the dielectric strength, central relaxation time and shape parameters describing the relaxation, as crystallization proceeds. The evolution of the dielectric magnitudes with crystallization time has been interpreted in the light of the Schönhals-Schlosser model. According to this model, a restriction of the long scale motions of the polymeric chains as the material is filled in with crystals is observed. The derived dipole moment time correlation functions have been calculated for different crystallization times and fitted by the Kohlrausch-Williams-Watts function. The variation of the stretching parameters with crystallization time can be interpreted in terms of the coupling model assuming an increase of the intermolecular cooperativity of the α relaxation as the polymeric chains are constrained to move between crystalline regions. These results offer a contribution to the understanding of molecular dynamics of a glassy polymers during crystallization.

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