Influence of Long-Chain Branching on the Crystallization and Melting Behavior of Polycarbonates in Supercritical CO2

The crystallization behavior of two long-chain branched bisphenol A polycarbonates (PC-Bs) was studied in supercritical CO 2 using differential scanning calorimetry (DSC), wide-angle X-diffraction, and atomic force microscopy (AFM), with a linear polycarbonate (PC-L) as reference. All the PCs had similar molecular weights and molecular weight distributions. Positron annihilation lifetime spectroscopy measurement indicated that the increase in free volume fraction of 13.1% (PC-B1) and 11.8% (PC-B2) was obtained with incorporating long-chain branches compared to PC-L. This increased segmental mobility decreased the energy barrier for PC crystallization and hence increased the crystallization kinetics for PC-Bs. Two melting peaks ( T m1 and T m2 ) observed during DSC heating scan were associated with the melting of secondary and primary crystallization, respectively. The T m2 of PC-Bs was about 2−4 °C lower than that of PC-L of at the same treatment time, pressure, and temperature. These results, qualitatively consistent with AFM results, mean that the incorporation of long-chain branches decreased the perfection or increased the lattice disorder of primary crystallites. The T m1 of crystallized PC-Bs was also much lower than that of PC-L under the same treatment conditions, which was attributed to the decrease in primary crystallization process and the resultant reduction in conformational constraints in the residual amorphous regions. AFM results also indicate that the branching structure might increase the crystallite density and sequentially the crystallinity degree of PC-Bs.