Entropic attraction of polymers toward surfaces and its relationship to surface tension

摘要

Experimental methods for estimating entropic attraction parameters, U-e (attraction of chain ends) and U-j (repulsion of branched points), are important for quantifying surface enrichment of arbitrarily branched polymer components in blends. Measurements of Ue and Uj are also important for determining points of reversal of enrichment, where a small linear or branched species with higher surface energy is preferentially enriched at the surface of a blend with a chemically different, yet miscible polymer. In this article, we show that measurements of polymer surface tension as a function of molecular weight provides a simple tool for quantifying their entropic attraction to surfaces. Specifically, we show that in the limit of high molar mass the surface tension gamma of a branched polymer with n(e) ends and n(j) branch points varies as gamma[(Mw) congruent to gamma](infinity) + [rho bRT(n(e)U(e) + n(j)U(j))]/ M-w, where M-w is the molecular weight of the polymer, rho(b) is the bulk density, R is the gas constant, and T is the temperature. Surface tension values predicted using this expression are compared with values determined from self-consistent-field theory simulations of polymers on a lattice, including compressibility effects. We also estimate the value of Ue from published surface tension data for polystyrene and use the estimate to predict surface enrichment in linear polystyrene/deuterated polystyrene blends. Significantly, we find that the phenomenon of reversal of isotopic enrichment at PS/dPS blend surfaces reported by Hariharan et al. [J. Chem. Phys. 1993, 98, 4163- 4173] is quite well predicted using the linear response theory.