Crystalline Structure in Thin Films of DEH−PPV Homopolymer and PPV-b-PI Rod−Coil Block Copolymers

Abstract

The rod orientation and crystalline packing of a model semiconducting rodlike polymer, poly(2,5-di(2‘-ethylhexyloxy)-1,4-phenylenevinylene) (DEH−PPV), is shown to affect the self-assembly of weakly segregated rod−coil block copolymers. The in-plane packing of DEH−PPV rods in lamellar poly(2,5-di(2‘-ethylhexyloxy)-1,4-phenylenevinylene-b-isoprene) (DEH−PPV-b-PI) diblock copolymers is nearly identical to that observed in DEH−PPV homopolymers for compositions ranging from 0.42 to 0.82 vol % coil block. The crystal structure of DEH−PPV, characterized by grazing incidence X-ray diffraction and electron diffraction, consists of a tetragonal unit cell having c = 0.665 nm with a = b = 1.348 nm. The polymer chain axis is aligned along the [001] direction, and the nearest neighbor rod−rod spacing along 〈110〉 is 1.0 nm. As-cast thin films of DEH−PPV homopolymer demonstrate chain alignment primarily perpendicular to the substrate in 5100 g/mol homopolymer, while for 3500 g/mol homopolymer the chains align both perpen...