Phase Transitions in Asymmetric Rod−Coil Block Copolymers

Abstract

Poly(alkoxyphenylenevinylene-b-isoprene) is used to study the phase transitions of a weakly segregated rod−coil block copolymer system. In the high coil fraction region of the phase diagram, phase transitions are caused by relatively small changes in temperature and composition. Several different sequences of phase transitions are observed, allowing fine control of structure, orientation, and thermal history effects. Weakly segregated symmetric rod−coil block copolymers organize into lamellar microphases at low temperature and disorder into nematic and isotropic phases with increasing temperature. As the coil fraction is increased, the lamellar structure loses long-range order but the microphase structure does not change. In these asymmetric block copolymers, a distinct change in phase transitions upon heating is observed. The stable nematic window narrows as the coil fraction increases. As a result, while symmetric block copolymers transition from a lamellar to an intermediate nematic and finally to an isotropic phase upon heating, coil-rich block copolymers transition is directly from a lamellar to an isotropic. The division between these two regimes occurs at a coil fraction of ∼0.8.