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(102b) Creation Of Nanocavities In Amphiphillic Block Copolymer Thin Films

Miller, A. C., Massachusetts Institute of Technology
Hammond, P. T., Massachusetts Institute of Technology
Bennett, R. D., Massachusetts Institute of Technology

Amphiphilic block copolymers have been the subject of significant recent research due to their ability to self-assemble on the nanometer length scale into a variety of morphologies, enabling a diverse range of applications. Previous work in our laboratory has focused on the poly(styrene-b-polyacrylic acid) (PS-b-PAA) block copolymer system which forms inverse spherical micelles in toluene solution. These micelles can be cast on substrates to form quasi-hexagonal micellar arrays. When exposed to appropriate aqueous solution, these films undergo a physical rearrangement, which we have termed cavitation. This process is driven by the swelling of PAA domains leading to the fracture of the glassy polystyrene corona. This results in the formation of nanometer size-scale cavities in the micelle film. The cavitation behavior and end-state polymer morphology of three different PS-b-PAA molecular weights in aqueous solutions of varying pH and ionic strength was characterized using atomic force microscopy (AFM). Behavior of the micellar films upon exposure to alkyl alcohols of varying chain length was also investigated to examine the effect of solvent quality. Cavitation of micellar films was found to depend on the pH and ionic strength of the aqueous medium, solvent quality and the molecular weight of the PS corona of the micelles. This process can be reversed by annealing the cavitated polymer films. We observed the rearrangement of thin films upon treatment using transmission electron microscopy (TEM) by selectively staining the PAA domains. In addition, we demonstrated the availability of PAA groups at the film surface by derivatizing cavitated films with fluorescent proteins. We have also shown this phenomena to occur in inverse spherical micellar films of poly(styrene-b-2-vinylpyridine) and a bioreadsorbable polymer, poly(epsilon-caprolactone)-block-poly(2-vinylpyridine). These results demonstrate that the organization of these micellar films can be tuned by changing the solution environment to which the micelles are exposed. Such environmentally-responsive films could be used to create nanopatterned arrays of a wide variety of molecules and have the potential to be used as a biomaterials platform.