Lamellar phases and microemulsions in model ternary blends containing amphiphilic block copolymers

Abstract

The order–disorder transition (ODT), microdomain structures and the phase diagram for ternary blends of low molecular weight PS, PI and PS–PI have been determined by a combination of X-ray scattering and transmission electron microscopy. The distribution of the homopolymer within the layers of a related PS/PB/PS–PB system has also been determined by a combination of X-ray and neutron reflectivity. A series of nearly symmetric, ternary blends of polystyrene (PS), polyisoprene (PI) and polystyrene-block-polyisoprene (PS–PI) have been studied by small angle X-ray scattering, static light scattering and transmission electron microscopy. The molecular weight of the homopolymers and block copolymer were in the ratio NH/NBCP = 0.19, which gave a block copolymer ODT and a homopolymer blend TC that were similar (TC/TODT ≈ 1.1). The block copolymer and its blends showed a weakly first-order transition from a lamellar phase to a fluctuating disordered phase in the volume fraction range ΦH ≤ 0.77. A bicontinuous microemulsion was found between ΦH = 0.79 and ΦH = 0.93, and for ΦH ≥ 0.93 macrophase separation was observed. In a similar PS, polybutadiene (PB) and polystyrene-block-polybutadiene (PS–PB) system the distribution of the homopolymer diluent was studied by X-ray and neutron reflectivity with deuterium labelled PS. The initial microstructure formed on spin coating had a dry-brush structure with the homopolymer concentrated in the centre of the domains and on subsequent annealing a wet-brush morphology was observed with the homopolymers uniformly distributed.