(640e) Density Functional Approach for Modeling Polymer-C02 Interfaces

Over the past decade supercritical CO2 has emerged as one of the most significant solvents for polymer processing. The success is attributed to fourfold advantage of scCO2 solvent system viz. ?tunability' to desired temperature and pressure, affinity to organic molecules, low viscosity leading to enhanced mobility, and environment friendly nature. Despite the advances, thorough understanding of polymer CO2 interfaces has remained an unresolved challenge. The problem becomes more significant considering the recent extension of scCO2 to nanoscale manufacturing. This study is aimed at bridging the gap between the theoretical and experimental knowledge through molecular level analysis of the system. We analyze the structure of polymer gas interface in the framework of statistical mechanics by using polymer density functional theory (PDFT). The basic rationale of this theory is to calculate the free energy of the binary system and the density profiles of the components at equilibrium. Free energy of the system in turn helps us calculate its equilibrium properties like solubility of polymer in CO2, glass transition at polymer gas interface, and Tg depression by CO2. Among the binary system properties, glass transition is the main focus of our study. We use Gibbs Di-Marzio theory (GDT) for glass transition calculation which says that the configurational entropy of a polymer becomes zero at glass transition. It is also interesting to note that the surface glass transition temperature (Tg) of a polymer is not a fixed quantity, rather there exists a distribution of Tg in the top layer (few nm). Due to size restrictions it is difficult to analyze such a distribution using experimental techniques. Evaluating the onset of glass transition in position space for different fixed temperatures (by using GDT) will help in overcoming this problem. Before studying the interfacial properties, it is essential to have a good insight into phase diagram of the binary system. In this talk we will present the phase behavior of the polymer CO2 system along with preliminary results for density profile and free energy calculations. We will validate the interaction parameters between the polymer chains and the gas molecules, and establish bulk system boundaries using our results. Since the size of polymer chains that we can incorporate into our model is restricted by computational cost, we will try to qualitatively depict the structure of polymer gas interface.