Neutron scattering studies of polymers in supercritical carbon dioxide

Above its critical point, carbon dioxide forms a supercritical fluid (SCF) that promises to be an environmentally responsible replacement for the organic solvents currently used in polymer synthesis and processing. Over the past two decades, small-angle neutron scattering (SANS) has provided a wealth of novel structural information on polymers and, more recently, the technique has been applied to characterize molecules dissolved in CO2 (e.g. fluoropolymers and siloxanes). When the interactions between the chain segments and the surrounding fluid are balanced, the chain trajectory is independent of both segment-segment and solvent-solute interactions. This phenomenon occurs at a 'theta temperature' (T-theta), where chain dimensions correspond to a volumeless polymer coil,`unperturbed' by long range interactions. For T > T-theta, the system exhibits a 'good solvent' domain, where the molecules expand beyond the unperturbed R-g in both organic solvents and in CO2. However, unlike organic solvents, this transition can be made to occur at a critical 'theta pressure' (P-theta) in CO2 and this represents a new concept in the physics of polymer-solvent systems. For T < T-theta, and P < P-theta, the system enters the 'poor solvent' domain, where diverging concentration fluctuations prevent the chains from collapsing and allow them to maintain their unperturbed dimensions. Other polymers such as polystyrene (PS) are insoluble in CO2, though they may be solubilized by means of PS-fluoropolymer stabilizers, which function as surfactants. In the case of such diblock-copolymers, aggregation occurs when the solvent is preferential for one of the blocks and SANS may be used to determine the structure of the aggregates (micelles), which consist of a CO2-phobic core surrounded by a CO2-philic shell. When CO2-insoluble material (e.g. PS) is added to the block-copolymer solutions, virtually all of the material is solubilized in the core. A unique attribute of SCFs is that the solvent quality may be adjusted over a wide range by varying the density (pressure), thus offering significant control over the solubility. This paper will illustrate the use of a tunable solvent to control the quality of the solvent-solute interaction of homopolymers and the self assembly of block-copolymer surfactants. SANS promises to give the same level of insight into polymers and amphiphiles in supercritical media that it has provided in the condensed state, organic solvents and in aqueous systems.