The role of solvent additive in polymer crystallinity during physical supercritical fluid deposition†
We investigated the self-assembly of polymers in supercritical fluids by depositing thin films of isotactic polypropylene from n-pentane:acetone solutions and studying their structure. The deposition technique leverages a maximum in the isobaric solubility as a function of temperature to deposit films onto a resistively heated substrate. We characterized the solubility of isotactic polypropylene in n-pentane:acetone as a function of temperature and pressure using gravimetric analysis. A maximum in the mole fraction of isotactic polypropylene versus temperature is observed in all solutions studied. In addition, the peak in solubility narrows and shifts towards higher temperatures with greater acetone volume fraction. These observations can be understood using a simple model that highlights the interplay of intermolecular interactions and solvent entropy. Leveraging the nonmonotonic solubility behavior, films were deposited at several pressures and acetone concentrations. Their morphologies were studied using polarized optical microscopy and grazing incidence wide angle X-ray scattering. Irrespective of the amount of solvent additive, films show a decrease in crystallinity with increasing pressure. In accord with previous reports, we attribute this to an increase in turbulence near the substrate surface. Increased acetone concentration is correlated to an increase in the length scales of polymer crystals, leading to spherulite morphologies at the highest acetone concentrations. We interpret this behavior in terms of a two-step model for film formation. The first is pre-aggregation in solution whereby the local crystalline order is established. Longer length scale organization occurs on the substrate and is facilitated by the presence of acetone. The results not only establish key aspects of self-assembly in supercritical fluids but present a model that could be applied to self-assembly of materials in other contexts.