writer：翟文涛, 余坚*, 马伟明, 何嘉松*.
keywords：polycarbonate，long-chain branch，crystallization
source：期刊
specific source：Macromolecules, 2007, 40(1): 73-80.
Issue time：2007年

The crystallization behavior of two long-chain branched bisphenol A polycarbonates (PC-Bs) was studied in supercritical CO2 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 (Tm1 and Tm2) observed during DSC heating scan were associated with the melting of secondary and primary crystallization, respectively.
The Tm2 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 Tm1 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.