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.
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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