Blends of polycarbonate and poly(hexamethylene sebacate): II. Effect of molecular weight on compatibility

Blends of polycarbonate (PC) and poly(hexamethylene sebacate) (HMS) with two different molecular weights were prepared and their thermal properties were studied via differential scanning calorimetry. It was found that the high molecular weight PC (HPC) and high molecular weight HMS (HHMS) were partially miscible as evidenced by the decrease in glass transition temperature of HPC in the blends. This partial miscibility is attributed to the interaction of the carbonyl dipole of the ester group and the highly polarizable aromatic carbonate structure. When the low molecular weight PC or HMS was used, the compatibility was enhanced because of the increased entropic contribution to the Gibbs free energy of mixing. In all the blends prepared, the PC crystallized as a result of the plasticizing effect of HMS. Bisphenol-A diphenyl carbonate (dimer) was synthesized and used as the dimeric model of PC. This material was found to be an excellent diluent for HPC. A single glass transition was found in the HPC/dimer system and the temperature was dependent on the composition. The Couchman's equation was found to fit very well the glass transition temperature versus composition relationship for this system. A single glass transition was also found in the HHMS/dimer system. The melting point depression analysis was performed and resulted in a very low heat of fusion for the 100% crystalline HHMS. It may suggest that the dimer and HHMS are not completely miscible in the amorphous region.