The surface tensions of amine-, hydroxyl-, and methyl-terminated poly(dimethylsiloxane) (PDMS) oligomers with molecular weights ranging from 1000 to 75 000 are measured by pendant drop tensiometry. The surface tension increases with molecular weight for the methyl-terminated polymers, decreases with molecular weight for the amine-terminated polymers, and is nearly independent of molecular weight for the hydroxyl-terminated polymers. This behavior is attributed to differences in surface tension of the end groups and the repeat unit of the chain ( . -

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).

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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 meanfield lattice theory is developed to model the surface structure and properties of blends containing endfunctional 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 χb, respectively. Two end-functional polystyrene architectures are considered: R-functional polystyrene for which the lattice reference volume is set equal to that of the entire fluorosilane end group and R,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 R,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 χb ) 1.59, obtained by regression of this model to ADXPS data, are consistent with group contribution estimates of these parameters.

Dynamic water contact angle analysis is applied to characterize the surface reorganization of glassy polystyrenes (PS), terminated with either a fluorosilane or carboxylic acid end group, upon exposure to an atmosphere of saturated water vapor. Fluorosilane end groups are initially adsorbed preferentially at the surface and diffuse away from the surface when exposed to water vapor. Carboxylic acid end groups are initially depleted from the surface and are drawn to the surface when exposed to water vapor. The time dependence of the surface composition during reorganization, determined by application of Cassie's equation, scales with the square root of time, consistent with a diffusive process. Angle dependent X-ray photoelectron spectroscopy (ADXPS), applied to characterize the surface concentration depth profiles of the fluorosilane-terminated PS before and after exposure to water vapor, indicates that reorganization in the glassy state involves motions on a length scale of about 2 nm. Modified lattice model calculations, assuming that reorganization can occur only over length scales of this magnitude, are found to provide a reasonable representation of ADXPS surface composition depth profiles, supporting the conclusion that the length scale for surface reorganization of end-functional polymers in the glassy state is of the order of a few nanometers. When this length scale is coupled with the dynamic contact angle data, apparent diffusion coefficients in the range of 10 −13 to 10 −10 cm 2 /s are obtained. Analysis of the temperature dependence of the apparent diffusion coefficients, found to be an activated process following Arrhenius behavior, yields activation energies of 137 kJ/mol for fluorosilane-terminated PS and 39 kJ/mol for PS terminated with a carboxylic acid end group, considerably lower than experimental values determined from analysis of either polymer−polymer interdiffusion or free surface dynamics. These activation energies are a closer match to those of the βrelaxation of bulk PS than they are to the α-relaxation of either the PS surface or the PS bulk, suggesting that surface reorganization in glassy PS can occur by virtue of short-range motions characteristic of a surface activated β-relaxation occurring over length scales of a few nanometers.