The influence of crystal formation on segmental mobility in polymers: hints from a statistical mechanical relaxation model

A statistical mechanical model is used to analyze literature data regarding the restricted segmental dynamics of a number of crystallized polymers, as observed by means of broadband dielectric spectroscopy. A relationship between well defined physical quantities and the width parameter in the Havriliak- Negami representation of symmetric processes is established. It is found that, for materials crystallized from an isotropic amorphous state, the segmental relaxation process is associated to conformational changes within cooperatively rearranging regions of ~ 1 nm diameter. In case of chain orientation, the dimension of the rearranging regions along the chain direction increases up to 3-5 nm. It is argued that the average size of the rearranging regions may influence the thickness of the amorphous interlamellar layers in the stacks. It is also found in all cases that, at the end of the crystallization process, the average fluctuation component of the chemical potential within the confined amorphous regions, µ, is of the same order of the chemical potential drop µcryst associated to crystallization from the undercooled, relaxed melt. Except in one among the cases considered, it is found that µ ~ -µcryst, which is a hint towards the formalization of a thermodynamic criterion for crystallization arrest.