50th Anniversary Perspective: Putting the Squeeze on Polymers : A Perspective on Polymer Thin Films and Interfaces

Polymeric materials, used predominantly in their bulk form, are now finding increasing application as thin films or where the end-use dimensions are approaching the size of the individual polymer molecules. This introduces questions concerning the perturbations to the fundamental static and dynamic properties arising when the pervaded volume of the polymer must conform to the finite size limitations. These properties include the single chain dimensions, entanglement molecular weight, anisotropy of polymer molecules, and wetting. For block copolymers, where the disparity in the chemical nature of the constituent blocks gives rise to microphase-separated morphologies, confinement can introduce fundamental changes in the ordering transitions of the copolymer, changes in the fundamental length scales and morphologies typically seen in the bulk, and changes in the orientation of the microdomains. As the size scale decreases, the surface-to-volume ratio increases, and as such, interfacial interactions and surface energies become increasingly important. The dramatic density at a surface and preferential interactions at surfaces and interfaces can have a profound impact on the chain dynamics and in multicomponent systems and block copolymers the preferential segregations of one component to the surface or interface. While the orientation of morphologies relative to the interface will occur, the extent to which surface effects propagate into the film and the adhesion of one material to another are important to understand for end-use applications. For interfaces between disparate materials, as for example between an organic polymer and a metal, transport of electrons or holes from one medium to the next, where there is a disparity in the work function, reduces to the behavior and nature of the polymer immediately at the interface. Here we summarize some of the significant advances that have been made on polymer thin films and interfaces, where the polymer chains can be squeezed, and provide some insight into future and emerging areas in the field.