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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 (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.

The crystallization behavior of amorphous syndiotactic polystyrene (sPS) in supercritical CO2 was investigated in detail by using wide-angle X-ray diffraction, Fourier transform infrared spectroscopy, and differential scanning calorimetry. The pure γ form crystal was obtained directly after treating amorphous sPS in supercritical CO2 in a range from 8 to 20 MPa and from 35 to 100 °C. This transformation in supercritical CO2 was accelerated by increasing temperature. Supercritical CO2 provided a moderate condition to crystallize the amorphous into the γ form, while liquid CO2 could not induce the γ form even at the pressure as high as 20 MPa.

The crystal transition behavior of δ form syndiotactic polystyrene (sPS) in supercritical CO2 was investigated in detail by using Fourier transform infrared spectroscopy and differential scanning calorimetry. The empty δe form sPS was obtained after treating the δ form in supercritical CO2 of 12 MPa in a range from 35 to 80 °C. At higher temperatures, the δ form transformed into the δe form transiently and then into the γ form. The existence of the intermediate δe form lowered the temperature for the δ to γ transition. At 240 °C, supercritical CO2 of 12 MPa transformed the δ crystal form into more stable β form without passing through the δe form. Increasing the pressure of supercritical CO2 made the δ to δe transition occur easily and decreased the temperature for the δ to β transition.