Molecular architecture and functional group effects on segregation in polymers

Abstract

The properties of a polymer surface can be manipulated by the addition of a small quantity of surface active functionalised polymer to the bulk. On annealing above the glass transition these low surface energy functional groups attach to the air/polymer interface forming a brush like layer. To quantify this effect perdeuterated polystyrenes with fluoroalkane groups at specific locations in the polymer have been blended with unmodified polystyrene. Three key areas have been studied; effect of molecular architecture; effect of the molecular weight of the matrix polymer; and the rate of formation of the segregating layer. The complementary techniques of neutron reflectometry and nuclear reaction analysis have been used to determine the near surface depth profile of the deuterated polymer. Architectures studied were linear polystyrene with functional groups at both ends of the polymer chain, a 3-armed star with a functional core and linear polystyrene with functional groups evenly spaced along the chain. The architecture affected the shape of the composition profile but had little effect on the surface volume fraction and surface excess values obtained for the same bulk volume fraction. Self-consistent field theory simulations were carried out to determine the 'sticking energy' of the functional groups and good comparisons were obtained between the experimental volume fraction profiles and those predicted. Some segregation of functional polymer was observed during sample preparation and equilibrium segregation was obtained in less than one hour for 50000 M(_W) linear polymer up to eight hours for the 3-armed star after annealing under vacuum at 413K. For the difunctional polystyrene the functional groups did not have a significant affect on the rate of diffusion compared to non-functional polystyrene and diffusion coefficients obtained ranged from 6x10(^-16) to 9x10(^-15) cm(^2)s(^-1) The 3-armed star had the lowest diffusion coefficient value of 2xl0(^-16) cm(^-2)s(^-1) because of the inability for branched molecules to diffuse by reptation.