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Above, topographic (left) and phase (right) images of a 50:50 PS-PB blend sample are shown as a function of tapping force and before and after annealing. Increases in tapping force increased the relative phase contrast in the blend prior to annealing. This affect can be observed in the above images, where the top left image was taken using light tapping forces, the bottom left image was taken using moderate tapping forces, and the top right image was taken using hard tapping forces. Also prior to annealing, the surface mechanical properties of the PS and PB regions were quite different while their polarities were similar, in contrast to the PMMA-PB study. After annealing, the two polymers had similar mechanical properties and glass transition temperatures but different polarities due to oxidation of the PB. Consequently, the phase image contrast between the phase-separated regions of the blend samples is large before annealing and negligible after annealing. The bottom right image was taken after annealing using hard tapping forces. Nanoindentation was utilized to follow changes in the relative stiffness of the phase-separated regions and to determine that the domains that appear as circular were rich in PS and the surrounding matrix was rich in PB. Only the microphase separated regions within the PS domains still show phase contrast after annealing (see bottom right image).
This study was done in collaboration with Dr. Xiaohong Gu and Professor Dharmaraj Raghavan of the Howard University Department of Chemistry.