The nonisothermal crystallization behaviors of poly(lactic acid) (PLA) under ambient N2 and compressed CO2 (5−50 bar) at cooling rates of 0.2−2.0 °C/min were carefully studied using high-pressure differential scanning calorimetry. The presence of compressed CO2 postponed the crystallization peak to a lower temperature region while effectively reducing the half-crystallization time and enhancing the crystallinity of the PLA specimen. On the basis of these findings, a new foaming strategy was proposed and utilized to fabricate PLA foams using the ordinary unmodified PLA. The upper and lower temperature limits of this foaming strategy were 105 and 90 °C, which were determined by the melt strength and crystallization behavior of the unmodified PLA specimen, respectively. In this temperature range, PLA foams with interconnected structures, porosity of 67.9−91.4%, and expansion ratio of 15−30 times are controllably produced. The obtained PLA foams have widely distributed average bubble size of 80−270 μm and CO2 enhanced crystallinity of 32−38%.

The solubility and diffusivity of carbon dioxide (CO2) in the solid-state isotactic polypropylene (iPP) were studied by using the pressure−decay method at temperatures of 373.15, 398.15, and 423.15 K and pressures ranging from 1 to 15 MPa. The solubilities of CO2 in the solid-state and amorphous regions of iPP were both obtained. They increased almost linearly with increasing pressure and decreased with increasing temperature. The Sanchez−Lacombe equation of state (S-L EOS) correlated the solubility in the amorphous regions of the solid-state iPP within 3% average relative deviation in conjunction with a temperature-dependent interaction parameter. The diffusion coefficient of CO2 in the solid-state iPP showed a weak concentration dependence and had an order of magnitude of 10−10−10−9 m2/s in the solid-state iPP.

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