Theta solvent
= Theta solvent
A <solvent> under conditions where the leading effective pair <excluded volume> of a long dilute <polymer> vanishes. In the simplest <Flory–Huggins solution theory> this means $\chi=1/2$. The chain is approximately an <ideal chain>, while three-segment interactions can still affect the <osmotic pressure> of a concentrated solution. A particular polymer-solvent pair often meets this condition only near its <theta temperature>.