We report neutron reflectivity measurements on the surface behavior of isotopic polystyrene blends of symmetric and disparate molecular weights near an air surface. For the symmetric blends we find, in agreement with past findings, that the segments of the deuterated polymer always partition to the air surface. These results, which are driven purely by energetic effects, can then be modeled in the framework of a mean-field lattice theory with a constant surface energy difference parameter. In contrast, for the asymmetric blends we find that the segments of either polymer can partition to the surface, and the controlling variable is the disparity in the molecular weights of the two components. These new results, which cannot be predicted with the constant density lattice models utilized for symmetric blends, can be modeled if one balances the energetic preference of placing the deuterated segments near the surface with entropic effects, which are caused by the presence of a density gradient at the air surface, preferring the surface segregation of the short chains. These findings emphasize the need for the inclusion of free-volume effects when modeling the segregation to a free surface, and we show that a recent mean-field compressible lattice model does capture these effects adequately.

The effects of polydispersity on the behavior of a polymer melt near a surface are considered in this work. To study these effects we have modeled an a thermal polymer melt with a bimodal molecular weight distribution in the vicinity of a neutral surface in the spirit of the mean-field lattice treatments of Sheutjens and Fleer20 and Theodorou.23 The results of the calculations show that purely entropic effects cause the shorter chains in the system to partition preferentially to the surface. This partitioning becomes more pronounced with increases in the difference between the molecular weights of the chains. The concentration profile of the segments of any one species in the vicinity of the wall can be described by a hyperbolic tangent function with a single correlation length that is a simple function of the mixture composition and the radii of gyration of the two chains. Implications of these results on the surface tension and orientation of chain segments in the interface of a bimodal polymer melt are also examined.

The entropy-driven surface segregation of polymer blends is investigated via computer simulations and integral equation theory. The model system is composed of a binary blend at a hard wall where one of the components of the blend is stiffer than the other. It is found that, at meltlike densities relevant to experiments, both simulations and microscopic theory predict the segments of the stiffer chains segregate to the surface.

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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).

We use molecular dynamics simulations to predict the eqUilibrium liquid-vapor interface structure and surface tension of two liquids, one comprised of short fluorocarbon-hydrocarbon diblock chains and the other of short fluorocarbon chains. Larger Lennard-Jones diameters and shallower well depths distinguish the perfluoromethyl segments from the methyl ones. In this model, realistic bond angle potentials, torsional potentials, and bond lengths describe the intramolecular interactions. At high temperatures, the density profile of the copolymer melt decays monotonically from the bulk liquid density to the vapor density and the structure of the free surface is similar to that of homopolymer melts. Increasing the chain length or decreasing the temperature causes the fluorocarbon segments to segregate to the free surface. Consequently, the constraint of connectivity between the two blocks results in oscillatory density profiles and a rich structure. Our model predicts that a copolymer can have a lower surface tension than either homopolymer of similar length. We also find that the simple Lennard-Jones based model is deficient in that it fails to explain the surface tension differences between decane and perfluorodecane.4156

We report neutron reflectivity and dynamic secondary ion mass spectroscopy measurements of surface segregation from symmetric, isotopic polystyrene blends, spin coated onto oxide covered silicon wafers, as a function of film thickness. The results of this analysis show that the segments of the deuterated polymer always partition to both the air and the substrate interfaces. Furthermore, the surface segregation is affected significantly if the film thicknesses are reduced below -four times the correlation length in the systems, and the segregation to both surfaces decreases with decreasing thickness. These results are in good agreement with the predictions of a mean-field lattice model which incorporates composition and chain length independent values of the surface energy parameter Xs at each surface.

The composition of the free surface of a binary polymer blend has been investigated in this work as a function of molecular weights, energetic parameters, and composition. The approach involved the use of a compressible mean-field lattice model that was developed in the spirit of the Scheutjens and Fleer theory of polymer solutions [J. Chem. Phys. 98, 6516 (1993)]. For symmetric polymer blends it was found that the surface segregation was driven by the degree of incompatibility of the blend, with the segregation increasing monotonically with the quantity χ1,2 r. These results are in qualitative agreement with conclusions obtained from an incompressible model for polymer blends near a surface, and suggests that the inclusion of compressibility effects do not change the predicted surface segregation significantly in these cases. In contrast, the behavior of isotopic polymer blends of disparate molecular weights, which could not be reproduced in the case of incompressible models, can be captured by the compressible model [J. Chem. Phys. 98, 4163 (1993)]. This fact therefore stresses the importance of compressibility effects in the context of segregation to free surfaces, especially for blends of disparate molecular weight polymers, and suggests that such models have to be utilized to incorporate all of the physics in these situations.