Via batch process in an autoclave, we conducted foam processing on polypropylene (PP)/clay nanocomposite (PPCN) by using supercritical CO2 as foaming agent under 10 MPa at 134.7 °C. Through transmission electron microscopy observation, the biaxial flow-induced alignment of clay particles along the cell boundary was identified. Such aligning behavior of clay particles helps cells to withstand the stretching force from breaking the so thin cell wall and to improve the modulus of the foam.

Polypropylene (PP)/clay nanocomposites (PPCNs) were autoclave‐foamed in a batch process. Foaming was performed using supercritical CO2 at 10 MPa, within the temperature range from 130.6°C to 143.4°C, i.e., below the melting temperature of either PPCNs or maleic anhydride‐modified PP (PP‐MA) matrix without clay. The foamed PP‐MA and PPCN2 (prepared at 130.6°C and containing 2 wt% clay) show closed cell structures with pentagonal and/or hexagonal faces, while foams of PPCN4 and PPCN7.5 (prepared at 143.4°C, 4 and 7.5 wt% clay) had spherical cells. Scanning electron microscopy confirmed that foamed PPCNs had high cell density of 107–108 cells/mL, cell sizes in the range of 30–120 μm, cell wall thicknesses of 5–15 μm, and low densities of 0.05–0.3 g/mL. Interestingly, transmission electron microscopic observations of the PPCNs' cell structure showed biaxial flow‐induced alignment of clay particles along the cell boundary. In this paper, the correlation between foam structure and rheological properties of the PPCNs is also discussed.

The effect of CO2 on the isothermal crystallization kinetics of poly(L‐lactide), PLLA, was investigated using a high‐pressure differential scanning calorimeter (DSC), which can perform calorimetric measurements while keeping the sample polymer in contact with pressurized CO2. It was found that the crystallization rate followed the Avrami equation. However, the crystallization kinetic constant was changed depending upon the crystallization temperature and concentration of CO2 dissolved in the PLLA. The crystallization rate was accelerated by CO2 at the temperature in the crystal‐growth rate controlled region (self‐diffusion controlled region), and depressed in the nucleation‐controlled region. CO2 has also decreased the glass transition temperature, Tg, and the melting temperature, Tm. As a result, the CO2‐induced change in the crystallization rate can be predicted from the magnitudes of depression of both Tg and the equilibrium melting temperature. The crystalline structure and crystallinity of polymers crystallized in contact with pressurized CO2 were also investigated using a wide angle X‐ray diffractometer (WAXD). The resulting crystallinity of the sample was increased with the pressure level of CO2, although the presence of CO2 did not change the crystalline structure.

When CO2 dissolves into a polypropylene (PP), its crystallization kinetics changes These changes were studied, in the expectation that the information would reflect on the behavior of other semicrystalline polyolefins. The isothermal crystallizatior rate of the PP‐CO2 solutions was measured using a high‐pressure differential scanning calorimeter (DSC), which performed calorlmetric measurements while keeping the polymer in contact with pressurized CO2. Although the measured crystallizatior rate followed the Avrami equation, the value of the crystallization kinetic constant was different from that measured for PP crystallized in air under atmospheric pressure. The dissolved CO2 decreased the overall crystallization rate of PP within the nucleation dominated temperature region. This suggests that the dissolved CO2 decreases the melting and the glass transition temperatures and prevents formation of critical size nuclei.