Field and flow effects on tethered polymer chains

Abstract

Solvated brush layers formed by linear polystyrenes have been investigated under quiescent and solvent shear-flow conditions using neutron reflectometry. Cyclohexane and toluene were used as solvents, and the polystyrene chains were tethered by one end to macroscopically flat silicon substrates via short poly-4-vinylpyridine end- groups. The brush systems were studied using a purpose built flow reflectometry cell. The brush height was found to increase with improving thermodynamic quality of the solvent due to increasingly strong repulsive excluded volume interactions between chain segments. Model fitting of the reflectivity data revealed that the polymer volume fraction profile was well described by parabola-like functions in agreement with the predictions of self-consistent field theory. No changes in the reflectivity profiles were observed upon exposure of the brush layers to solvent flow-induced area average shear rates of up to 147 000 s(^-1). This observation has been rationalised through a number of considerations, including comparison with recent theoretical predictions. Aqueous micellar dispersions of diblock copolymers of styrene and ethylene oxide have been studied using small angle neutron scattering in the concentration range 0.05 to 6.5 weight % copolymer. The micelles formed were found to be spherically symmetric, consisting of a polystyrene core surrounded by a corona of highly stretched solvated polyethylene oxide chains. At higher concentrations, the intermicellar interaction has been interpreted using an analytic structure factor originally developed to describe macroion solutions. On subjecting the dispersions to Couette shear, no evidence for long-range ordering of the micelles was observed in the diffraction patterns.