Measurement and modeling of end group concentration depth profiles for ω-fluorosilane polystyrene and its blends

Angle-dependent X-ray photoelectron spectroscopy (ADXPS) is used to measure end group concentration depth profiles for blends of surface active ω-fluorosilane polystyrene with nonfunctional polystyrene. The fluorine signal is in all cases enhanced at the surface, indicating surface segregation of the lower surface tension fluorosilane end groups. End group segregation is enhanced by an increase in the concentration of ω-fluorosilane polystyrene, an increase in the nonfunctional polystyrene molecular weight, or a decrease in the molecular weight of the ω-fluorosilane polystyrene. A self-consistent mean-field lattice theory is developed to model the surface structure and properties of blends containing end-functional polymers. Lattice model calculations provide estimates of concentration depth profiles as a function of the blend composition, the normalized chain lengths of the blend constituents, and the surface and bulk interaction parameters, χ_s, and χ_b, respectively. Two end-functional polystyrene architectures are considered: α-functional polystyrene for which the lattice reference volume is set equal to that of the entire fluorosilane end group and α,β-functional polystyrene where the fluorosilane end group is assumed to occupy two adjacent lattice sites at the chain end. The lattice model for both architectures provides excellent representations of experimental ADXPS data over a wide range of blend compositions and constituent molecular weights. The α,β-functional polymer model is shown to be superior on two accounts: the lattice reference volume and polymer repeat unit volumes are similar, and the optimal values of χ_s = -2.18 and χ_b = 1.59, obtained by regression of this model to ADXPS data, are consistent with group contribution estimates of these parameters.