Phase formation of associating polymers: Gelation, phase separation and microphase formation

Polymers exhibit a variety of condensed phases when some of their segments are capable of forming weak bonds which can be created and destroyed by thermal motion. Transition from one phase to another caused by such 'segment association' is reversible by the change of the temperature and the concentration, so that it is called 'reversible phase transition'. What types of reversible phases are possible for a given associative interaction? How does molecular association interfere with phase separation? This paper surveys, as typical examples of reversible phases, macroscopic phase separation, microphase formation, solvation, and gelation from a unified point of view. It explores the possibility of new condensed phases and critical phenomena driven by the mutual interference of different phase transitions.