A polymer chain in solution undergoes a sharp conformational change as solvent quality varies. In a good solvent, monomer-solvent contacts are favored, the chain swells into an open random-coil with R ~ N0.588 (Flory exponent ν ≈ 3/5 in 3D). At the theta temperature Θ, excluded volume vanishes and R ~ N1/2 (ideal chain). In a poor solvent, monomers attract each other and the chain collapses into a compact globule with R ~ N1/3.
This transition is the polymer analog of a liquid-gas phase transition and is central to protein folding: most proteins are disordered (coil-like) in denaturing conditions and fold into compact globules (native state) under physiological conditions.