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 temperature and pressure on the phase behaviour of semidilute solutions of polymers dissolved in organic and supercritical solvents. Above the theta temperature (T_Θ), these systems exhibit a "good solvent" domain, where the molecules expand beyond the unperturbed dimensions in both organic solvents and in CO₂. However, this transition can be made to occur at a critical "theta pressure" (P_Θ) in CO₂ and this represents a new concept in the physics of polymer-solvent systems. For T < T_Θ, and P < P_Θ, the system enters the "poor solvent" domain, where diverging concentration fluctuations prevent the chains from collapsing and allow them to maintain their unperturbed dimensions.