Surface and bulk phase separations in block copolymers and their blends

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Virginia Polytechnic Institute and State University

Surface and bulk properties have been studied in terms of composition and morphology of siloxane containing block copolymers and their blends with homopolymers. X-ray Photoelectron Spectroscopy (XPS) has been used to obtain the compositional information from the top 60 angstroms or so at the surface. Transmission Electron Microscopy (TEM) was utilized to probe the bulk morphology. An attempt is made to compare the bulk and the surface and find possible mechanisms governing them. It is found that solvent-cast neat block copolymers have a uniform layer at the surface that is rich in siloxane whereas their bulk has a microphase-separated domain structure. In case of blends, siloxane enrichment is quite pronounced even at bulk concentrations as low as 0.05% w/w siloxane. Amount of surface siloxane as a function of bulk content is studied with the help of XPS. At the same time, the bulk morphology of these blends is studied by TEM. The changes occurring in the surface and the bulk are found to have similar patterns. It is shown that the observed surface behavior may be related to the bulk morphology. Molecular weight of the blocks in the copolymers is found to be a very important parameter governing both the surface and the bulk properties in the neat copolymers as well as their blends.