Nonlamellar Phases in Asymmetric Rod−Coil Block Copolymers at Increased Segregation Strengths

Abstract

A new phase consisting of rectangular rod nanodomains packed onto a hexagonal lattice is observed in rod−coil block copolymers at the limit of both large volume fraction asymmetry and large geometrical asymmetry between the rod and coil. In moderately segregated poly(alkoxyphenylenevinylene-b-isoprene) (PPV-b-PI), an order−order transition is observed between hexagonal and lamellar phases for polymers near the phase boundary, and the lamellar phase is observed at high temperatures. The domain spacings of polymers in the lamellar phase collapse on to a simple scaling relationship where domain spacing is proportional to molecular weight. The proportionality constant is equal to the statistical segment length of the PPV rod block, suggesting that the angle between the rod director and the lamellar interface is nearly 90°. At higher temperatures, the block copolymers transition from ordered to nematic to isotropic states, with the intermediate nematic phase being observed for all coil fractions studied. A three-dimensional phase diagram shows the microphase and liquid crystalline transitions in rod−coil block copolymers as a function of temperature, geometrical asymmetry, and coil fraction.