Surface modification of versatile polymeric materials without changing their bulk properties is one of the essential techniques for regulating their physical and chemical characteristics, and this technique can improve the functions of materials. In this study, a bottlebrush-type poly[oligo(2-ethyl-2-oxazoline) methacrylate] (P[O(Ox)nMA]) (n = 7 and 19) was synthesized as a surface modifier. This compound was mixed into poly(methyl methacrylate) (PMMA) as a matrix (PMMA/P[O(Ox)nMA]) with a weight ratio of 15%, and the aggregation state in the surface region was examined under air and aqueous environments via atomic force microscopy, contact angle measurement, angular-dependent X-ray photoelectron spectroscopy, and neutron reflectivity. The surface of the PMMA/P[O(Ox)nMA] films was flat at the subnanometer level and covered with the PMMA-rich phase. However, once the films contacted water, the surface was reorganized due to the migration of P[O(Ox)nMA]. The extent of the surface segregation was more remarkable for P[O(Ox)7MA] than P[O(Ox)19MA] due to the entropic factor. Concurrently, NIH3T3 fibroblast adhesion and serum protein adsorption on the film were more strongly suppressed for P[O(Ox)7MA] than P[O(Ox)19MA] because it formed a thicker diffused interface in the film with water.
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