Theta solvent
A theta solvent is the name for a condition (sometimes known as the Flory condition) rather than an actual solvent. At the theta point, in the words of Paul Flory: "excluded volume interactions are neutralized" (Ref. 1). Thus when polymer is added to a theta solvent it will maintain the same average end-to-end distance, unlike in a poor solvent, where this distance is reduced, or in a good solvent where this length increases. An excluded volume of zero connotes a second virial coefficient of zero. The theta state also corresponds to the highest upper critical temperature of a given polymer-solvent system.