Excluded Volume and Solvency

Excluded volume scales as v ≈ b²d where b is the Kuhn monomer size (length) and d is the diameter (Kuhn segments are rod-like).

Athermal Solvents Here, v is independent of temperature. The system only features hard core repulsion and v ≈ b²d. Monomer-monomer contact is energetically indistinguishable from monomer-solvent contact, for example.
Good solvents Excluded volume is reduced due to monomer-monomer attraction. The effect of this attraction is greater at lower temperatures, causing a reduction in the excluded volume. 0 < v < b²d
Theta solvent The (positive) contribution to excluded volume from hard core repulsion is exactly balanced by that (negative) due to attractions and so v = 0. The chains thus have nearly ideal conformations. This occurs at a temperature called the theta temperature, Θ, which is analogous to the Boyle temperature in thermodynamics.
Poor solvents Excluded volume is negative due to large attractive interactions between the monomers, which prefer monomer-monomer contact strongly over monomer-solvent contact. Chain dimensions are reduced relative to ideal. -b²d < v < 0.
Non-solvents Here, v ≈-b²d and the polymer collapses into a very compact structure that excludes all solvent.