Physical supercritical fluid deposition of polymer films: controlling the crystallinity with pressure

Abstract

The self-assembly of isotactic polypropylene films from supercritical n-pentane was studied. The deposition technique relies on a detailed understanding of the solubility behavior of the system, and this was investigated as a function of temperature and pressure using gravimetric analysis. A peak in the isobaric solubility is observed, and a simple thermodynamic model was developed to describe both the temperature and pressure dependence. The nonmonotonic isobaric solubility allows films to be deposited from saturated solutions onto a heated substrate. Films were deposited at several pressures and their morphology was studied using polarized optical microscopy and grazing incidence wide angle X-ray scattering. We observe the alpha crystalline phase of the material with the dominant chain orientation parallel to the substrate. Contrary to expectations, we observe a decrease in crystallinity with pressure. In accord with visual observations of light scattering during deposition, we ascribe the decrease in crystallinity to increasing solution turbulence. We summarize our observations with a model of film growth that postulates a pre-aggregation step near the heated substrate surface, with an increase in turbulence disrupting solution-phase self-assembly. Following deposition, large scale spherulite formation is inhibited, which suggests a lack of chain mobility on the surface. The work demonstrates key insights necessary for optimizing thin-film morphologies and principles for understanding self-assembly in supercritical fluids in general.