Characterizing Polymer Surfaces and Interfaces

The presence of a surface or interface can markedly alter the configuration and spatial distribution of polymer molecules. In the bulk, polymer molecules—comprised of numerous monomers covalently linked together—pervade 10s of nanometers spatially. However, packing such chains at an interface—under the constraint that a solid polymer is essentially incompressible—necessitates perturbations to the chain configurations near the interface. This may result in a collapse of coils at the surface or in a preferential orientation of the monomers with respect to the interface plane. For amorphous homopolymers, however, simulations indicate that, on a segmental level, surface effects are generally damped within several segment diameters from the surface. Interactions between the surface and the polymer chains place additional constraints on the molecular configurations. As the architecture of the polymer chain becomes more complex, as with diblock copolymers in which two chemically distinct polymer chains covalently bond together at one end, the packing of chains at the interface must take into account the relative interactions of the two portions of the chains with the interface. Due to the connectivity of the blocks, preferential interactions of the blocks with the interface can influence the spatial distribution of the chains far from the interface. As the number of components increases, as with a simple binary-polymer mixture, not only must packing constraints be satisfied, but also the interactions of the two chains with the surface and with each other must be taken into account. In the case of homogeneous mixtures, the preferential interaction of one chain with an interface can lead to a substantial excess of that chain at the interface which, depending upon the proximity to the demixing point, can lead to surface effects that propagate many molecular diameters into the sample. The key, however, is the connectivity of the monomers in the polymer chain, which can enhance surface effects, as in the case of block copolymers or polymer mixtures, or suppress surface effects, as in the case of homopolymers.