Effect of compressed CO2 on crystallization and melting behavior of isotactic polypropylene

Compressed gases such as CO2 above their critical temperatures provide a highly tunable technique that has been shown to induce changes in phase behavior, crystallization kinetics and morphology of the polymers. Gas induced plasticization of the polymer matrix has been studied in a large number of polymers such as polystyrene, and poly(ethylene terephathalate). The knowledge of polymer--gas interactions is fundamental to the study of phenomena such as solubility and diffusivity of gases in polymers, dilation of polymers and in the development of applications such as foams and barrier materials. In this paper, we describe the interactions of compressed CO2 with isotactic polypropylene (PP). Crystallization of various PPs in presence of compressed CO2 was evaluated using a high pressure differential scanning calorimeter (HPDSC). CO2 plasticized the polymer matrix and decreased the crystallization temperature, Tc by approx8 deg C for PP at a pressure of 650psi CO2. The decrease as a function of pressure was -0.173 deg C/bar and did not change with the molecular architecture of PP. Both crystallization kinetics and melting behavior are evaluated. Since solubility and diffusivity are important thermodynamic parameters that establish the intrinsic gas transport characteristics in a polymer, solubility of CO2 in PP was measured using a high-pressure electrobalance and compared with cross-linked polyethylene. At 50 deg C, solubility followed Henry's law and at a pressure of 200psi about 1% CO2 dissolved in PP. Similar solubility was achieved in PE at a pressure of 160psi. Higher solubility of CO2 in PE is attributed to its lower crystallinity and lower Tg, than PP. Diffusion coefficients were calculated from the sorption kinetics using a Fickian transport model. Diffusivity was independent of pressure and PE showed higher diffusivity than PP. Preliminary foaming studies carried out using a batch process indicate that both PP and PE can be foamed from the solid state to form microcellular foams. Cell size and cell density were approx10 mu m and 10 exp 8 cells/cm exp 3 , respectively in PE. Differences in morphology between the foams for these polymers are attributed to the differences in diffusivity.