Recrystallization processes in cold-crystallized poly(ethylene terephthalate): Interplay between structure evolution and conformational relaxation

The changes induced in the amorphous regions of poly(ethylene terephthalate) as a consequence of recrystallization processes, taking place after cold-crystallization at Tc¼ 100 C, are analyzed by means of isothermal dynamical mechanical spectroscopy and microindentation hardness. Overall, a recrystallization process at either 115 or 125 C causes an increase of the rigidity within the amorphous domains confined by the crystals. Microhardness measurements carried out at room temperature reveal that recrystallization leads to an enhanced mechanical performance of the amorphous regions. The analysis of isothermal segmental relaxation patterns recorded in a frequency interval of 103e60 Hz indicates the appearance of two distinct contributions, which find correspondence with observations by broad band dielectric spectroscopy on the same systems. The faster one is ascribed to segmental relaxation within the amorphous domains where the confinement by crystals is relatively weak. The slow relaxation mode is associated with regions where the conformational dynamics is strongly restricted by the crystals. A relative increase of the slow process is detected upon recrystallization. A recently developed relaxation function model is employed to estimate the size of the static cooperatively rearranging regions for both, the slow and the fast modes. It is found that this size increases either upon decreasing the temperature or as an effect of recrystallization. In addition, the number of monomers involved in a conformational rearrangement turns out to be significantly larger in the regions associated to the slow mode process.