Miscibility Studies of Coextruded Polycarbonate/Polymethyl Methacrylate Composites

Abstract : Miscibility and molecular relaxation measurements have been carried out using dynamic mechanical analysis (DMA) on the coextruded polycarbonate (PC)/polymethyl methacrylate (PMMA) multilayers with various compositions. Three distinct transitions are observed; two of these are corresponding to the glass transition of the PC-rich phase and PMMA-rich phase. The glass transition temperatures, T(g), of both phases shift slightly toward each other, compared to the T(g) of pure PC and pure PMMA, indicating limited miscibility. A third relaxation, which was not observed with differential scanning calorimetry (DSC), is present at a temperature between the T(g) of PC-rich and PMMA-rich phases. The transition temperature of this intermediate relaxation increases as the PC composition increases, and it also shifts to a higher temperature with an increase in the molecular weight of PMMA. The origin of the intermediate transition is attributed to the molecular relaxation response of an interphase resulted from interdiffusion between the PC and PMMA. In addition, this intermediate relaxation is more sensitive to the change in frequency compared to the response of glass transition of either pure component. As a result, the apparent activation energy values determined for the corresponding third relaxation are consistently smaller than those measured for the glass transition of pure PC and pure PMMA. The former values, however, remain relatively constant with respect to the variation in composition and molecular weight for all the PC/PMMA multilayers.