GOALI: Microscopic Basis of Polymer-Supercritical Fluid Phase Behavior: Polymer-Solvent Interactions and Chain Stiffness

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ABSTRACT CTS-9729720 McHugh, M./Johns Hopkins U. This is a GOALI Collaboration project between industry and Johns Hopkins. The long-term objective of the research proposed here is to develop a new generation of supercritical fluid (SCF)-based technologies for polymer processing. However, rigorous models for SCF solvent-polymer solution behavior, both at the macroscopic and the microscopic levels, are still in the nascent stages of development. It is, therefore, exceptionally difficult to design reliably efficiently chemical processes that utilize the unique characteristics and capabilities of SCF solvents. Hence, the immediate research objective is to address the physics and chemistry of SCF-polymer solutions. In addition to high-pressure phase behavior measurements, a variety of scattering techniques will be utilized to investigate SCF-polymer solutions at elevated pressures and temperatures.. For each polymer-SCF pair, the cloud-point curve will be measured over a broad range of P-T space to modulate energetic interactions relative to entropic contributions to the Gibbs free energy of mixing. A coarse-grain indication of the balance of SCF-polymer interactions relative to SCF-SCF and polymer segment-segment interactions can be gleaned from an examination of the characteristics of the cloud-point curve in P-T space. Light scattering techniques will then be used to probe polymer dimensions and interactions in the single-phase region. Static light scattering will be used to determine polymer radii of gyration and second osmotic varial coefficients. The measured second osmotic virial coefficient defines an equivalent hard sphere radius for the polymers in solution. Dynamic light scattering will be used to measure the polymer hydrodynamics radius. A comparison of the hard sphere and hydrodynamic radii yields insight into the interactions that occur in the solution at submacromolecular length scales. Small angle neutron scattering will be used to determine the polymer persistence and contour lengths in an SCF solvent environment. Perrsistence length values are particularly useful for determining the influence of polymer conformational entropy on phase behavior and polymer segment-solvent interactions. This supremolecular to macromolecular length scale approach will enable us to develop a more detailed understanding of the molecular thermodynamic principles that govern the observed solution behavior. The phase behavior and scattering studies will focus on several different molecularly simple SCF solvents and two different classes of homopolymers: nonpolar poly(olefins) and polar polymers. The polymers will vary in backbone architecture to reveal the influence of polymer crystallinity, chain branching, polarity, and hydrogen bonding on the phase behavior and the chain dimensions in the single-phase region. We will also investigate the impact of chain "stiffiness" by focusing on polymers that have similar intermolecular potential functions, but different statistical segment lengths, such as poly(acrylates) and poly(methacrylates). The polymer-SCF solvent systems are chosen to isolate particular phenomena and, thus, allow a systematic exploration of the factors that govern polymer solubility and behavior in SCF solvents.