Towards Understanding Interfacial Phenomena in Polymer-CO₂ Systems

Supercritical (high pressure) CO₂, owing to its ability to make polymers pliable at temperatures much lower than the glass transition temperature (T_g), has been established as a very promising solvent for numerous macro scale polymer processing applications. In this work, we have tried to expand the scope of supercritical CO₂ assisted polymer processing to nano length scales with particular focus on manufacturing biomedical devices from polymer thin films. At such small length scales, however, the properties of the polymer become size dependent since the interfacial effects start dominating the bulk effects. As a result, adapting the macro and micro level fabrication technologies to nano level is not straightforward and requires integration of both theoretical and experimental tools.

We have used CO₂ assisted Nano-Imprint Lithography (CO₂-NIL) for fabricating nanochannels on polystyrene thin films. CO₂-NIL is a novel technique in which the features from a rigid mold are transferred on to a CO₂ pressurized polymer thin film by application of compressive force. We have explored efficiency of pattern transfer, resolution, and effects of molecular weight on transferability of patterns, and have thus established CO₂-NIL as a highly efficient and cost effective fabrication technique capable of transferring patterns as small as 20 nm in step height.

To understand the surface characteristics and the molecular level effects of CO₂ on polymer thin films, which are essential for optimizing the nanoscale experiments, we have used Polymer Density Functional Theory (PDFT) as our primary tool since it provides an adequate balance between the amount of details extracted and the computational costs involved. PDFT is a statistical mechanics based approach in which we express the free energy of the system as a functional of spatially varying density distributions of CO₂ and polymer segments. Equilibrium density distributions, free energy at equilibrium, and hence the equilibrium properties are then obtained by minimizing this functional. We have studied CO₂ solubility in polymer, surface adsorption of CO₂ on polymer surface, interfacial tension, and interfacial width for both polyethylene and polystyrene thin films. In addition, we have shown how the molecular level structural changes induced by CO₂ facilitate the nanoscale polymer processing, and that the causal mechanism of these changes is the enthalpic interactions between the polymeric segments and the CO₂ molecules. This work is the first instance of application of PDFT to polymer films pressurized by CO₂. We have also significantly enhanced the capability of PDFT by handling polymer chains much longer than previously reported using this theory, similar to the ones used commercially.

PDFT is a very powerful tool that can be used to study the thermodynamic behavior in several important technologies, especially the ones dominated by the surface phenomena. In addition to the nanoscale processing of polymer thin films, we have applied this theory to study the scaling approach to bubble nucleation in CO₂ based polymer foams and the surface tension in ‘petroleum hydrocarbon-CO₂ mixtures’ at the reservoir conditions. The data from the latter are significant for CO₂ assisted enhanced oil recovery applications.