End Group Effects on Surface Properties of Polymers: Semiempirical Calculations and Comparison to Experimental Surface Tensions for α,ω-Functional Poly(dimethylsiloxanes)

Abstract

Current theoretical models for the surface behavior of polymers have stressed the importance of several factors such as the chain length (r), local chain stiffness, and the surface energy difference between chain ends and middle segments, χs. Here we assert that the critical parameter, which is affected by all of these factors, that controls thermodynamic properties is the surface composition of the different moieties in the macromolecular system. Composite surface properties, such as the surface tension, are calculated directly by assuming that the end group and repeat unit segments contribute to surface properties weighted by the composition in the lattice layer, which is immediately adjacent to the surface. We utilize the Scheutjens−Fleer lattice self-consistent mean-field model and Monte Carlo simulations to determine the surface composition of end groups for end-functionalized polymer chains. We find that end group segregation is primarily controlled by surface energetic differences between the chain ends and chain middle moieties and that entropic effects are effectively irrelevant in this context. Within the range of surface energy differences that are expected to be encountered in practice, the predicted surface segregation of chain ends is so small that the molecular weight dependence of the surface tension of an end-functionalized polymer melt is for all practical purposes determined by the direct relationship between the bulk end group concentration and chain length represented by φe = 2/r. Group contribution methods are employed to estimate the surface tensions of the end and middle groups, and no adjustable parameters are required. The simple model provides a facile method for determining the variation of surface tension with molecular weight and end group type and reproduces well experimental surface tension data for several α,ω-functional poly(dimethylsiloxanes).