Near-surface and internal lamellar structure and orientation in thin films of rod–coil block copolymers

Abstract

Controlling the three-dimensional structure of rod–coil block copolymer thin films is critical for the optimization of the carrier transport properties and exciton dissociation or carrier recombination efficiencies at interfaces in optoelectronic polymers. Using a model rod–coil block copolymer based on poly(alkoxyphenylenevinylene-b-isoprene) (PPV-b-PI), the structure and orientation of thermally self-assembled rod–coil block copolymer thin films is investigated with grazing-incidence small-angle X-ray scattering (GISAXS), dynamic secondary ion mass spectrometry (DSIMS), scanning force microscopy (SFM), and cross-sectional transmission electron microscopy. Grains of lamellae are grown in films with one supported and one free interface by thermal annealing below the order–disorder transition from an initially structureless solution-cast state. In films only a few lamellar layers thick, parallel lamellar grains that span the entire film thickness are formed due to surface templating of lamellar orientation. As film thickness is increased, the lamellar block copolymers take on a bimodal orientation, with parallel and perpendicularly oriented microphases distributed uniformly throughout the center of the film. The vacuum interface is coated in a ∼10 nm thick layer of perpendicular lamellae, while the supported interface is covered with a single parallel oriented layer of block copolymer due to surface segregation of the polyisoprene block. Further increasing the thickness results in partial loss of orientational order as the bulk structure is approached. Strong suppression of the primary scattering peak is observed near a coil fraction of 0.7 for parallel lamellae, indicating changes in form factor from the bulk or perpendicularly oriented structures. Large dilations in the average perpendicular lamellar domain spacing are also observed in thin films as defects between perpendicular lamellar grains. As the film thickness increases, the lamellar period asymptotically decreases to the bulk value. The lamellae dilate rather than bending due to the high modulus of the PPV liquid crystalline nanodomains. Parallel lamellae show no dilation because of the free interface.