Square Grains in Asymmetric Rod-Coil Block Copolymers

Unlike the rounded grains that are well known to form in most soft materials, square grains of microphase-separated lamellae are observed in thin films of a rod-coil block copolymer because of hierarchical structuring originating from the molecular packing of the rods. The square grains are oriented with lamellar layers parallel to the film interface and result from growth along orthogonal low-surface-energy directions as a result of the effects of the tetragonal crystalline lattice that forms within the rod-rich lamellar nanodomains of poly(2,5-di(2{prime}-ethylhexyloxy)-1,4-phenylene vinylene)-b-polyisoprene (PPV-b-PI). These grain shapes form only for a narrow range of coil volume fractions around 72% as a result of kinetic barriers at lower coil fractions and disordering of the lattice at higher coil fractions, and the polydisperse grain size suggests that growth is nucleation-limited. The grains form in both weakly and moderately segregated polymers at all annealing temperatures below the order-disorder transition, and they are observed for all thicknesses at which parallel-oriented grains are grown.