Re‐Plasticization by Confinement During Annealing Induced Phase Separation in Polycarbonate/Phthalate Plasticized Films

It is common practice during the analysis using optical microscopy to place the sample on a microscope slide and cover it with a cover slide or another microscope slide. The sample is thus sandwiched or confined between two glass surfaces. Such a set up is used in all disciplines of science to study the changes in the structure and morphology of materials caused by annealing and other processes in situ. We describe a case of annealing induced phase separation in polycarbonate plasticized with diphenyl terephthalate and diphenyl isophthalate, which unexpectedly showed a significant difference between the confined configuration as defined above and the unconfined state. This was traced to the large depression of the melting point of the plasticizer in the polymer. When annealing was performed with the composite film confined between two glass slides, with the temperatures used here, the phase-separated small molecule melts and re-plasticizes the polymer. As a result, there was no increase (recovery) of the T g of the polymer with annealing temperature. Such a confinement (or overcoating) could be a route to maintaining the T g of the composite in multi-layer devices, even if phase separation occurs during the operation of the device. However, when the covering glass slide was not used, sublimation of the plasticizer occurred, and the normal Tg recovery was seen. This is also the first reported case of replasticization of the polymer upon annealing in such a confined state. We also note significant differences in the phase separation behaviour of the terephthalate and isophthalate configurations.