Synergistic influence of substrate wettability and topography on surface phase separation in PS/PMMA blend thin films

Polymer blend thin films undergo lateral and vertical phase separation, enabling the formation of domains with controlled composition and sizes ranging from sub-micron to nanoscale. To tune this phase-separated morphology further, various strategies have been employed, including modifications to substrate topography and surface wettability. In this study, we performed a comprehensive investigation of polystyrene (PS)/poly(methyl methacrylate) (PMMA) blend films by systematically varying the surface energy of polydimethylsiloxane (PDMS) substrates through a UV-oxidation method. The resulting morphological changes in the blend films were characterized using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). As the substrate surface energy increased, we observed a notable shift in morphology driven by the selective migration of PMMA toward the oxidized surface, confirmed through selective etching, XPS, and AFM analyses. To examine the combined effect of surface energy and topography, we studied phase separation on topographically patterned substrates fabricated via soft lithography. Our results reveal that topographic features play a dominant role over surface energy variations in determining the final phase-separated structure. Based on the combined insights from XPS and AFM, we propose a model structure for the PS/PMMA blend films that elucidates the effects of both surface energy and nanoscale topography. This parametric variation approach offers a valuable framework for understanding phase-separated morphologies in polymer blend films and can be extended to other multi-component systems.