Comparison of the behaviour of polymers in supercritical fluids and organic solvents via small angle neutron scattering

Small-angle neutron scattering has been used to study the effect of tempera ture and pressure on the phase behaviour of semidilute solutions of polymer s dissolved in organic and supercritical solvents. Above the theta temperat ure (T-Theta), these systems exhibit a "good solvent" domain, where the mol ecules expand beyond the unperturbed dimensions in both organic solvents an d in CO2. However, this transition can be made to occur at a critical "thet a pressure" (P-Theta) in CO2 and this represents a new concept in the physi cs of polymer-solvent systems. For T < T-Theta, and P < P-Theta, the system enters the "poor solvent" domain, where diverging concentration fluctuatio ns prevent the chains from collapsing and allow them to maintain their unpe rturbed dimensions.