Small angle neutron scattering from polymers in supercritical carbon dioxide

Abstract

Above its critical point, carbon dioxide forms a supercritical (SC) solvent which promises to be an environmentally responsible replacement for the organic solvents currently used in polymer synthesis and processing. CO2–soluble molecules (e.g. tluoropolymers) have been charkterized using small angle neutron scattering (SANS). By analogy with light and small-angle X–ray scattering (SAXS) experiments on polymers in dilute solution, the radius of gyration (Rg), molecular weight (MW) and second osmotic virial coefficient (A,) are determined from SANS experiments in SC–CO2. However, many polymers do not dissolve in this medium and in order to promote the synthesis of such materials, a range of amphiphilic block copolymers has been developed. These molecules function as surfactants or emulsifying agents to solubilize the CO2–phobic polymers. In the case of diblock copolymers in SC–CO2, aggregation occurs when the solvent is preferential for one of the blocks, and SANS allows the size and shape of the aggregates (micelles) to be determined. Illustrative examples of these studies are given and neutron scattering promises to give the same level of insight into polymers and amphiphiles in supercritical media that it has provided in the condensed state and in aqueous systems.