Surface-grafted assemblies of cyclic polymers: Shifting between high friction and extreme lubricity

The interfacial physicochemical properties of “brushes” constituted by cyclic polymers strongly depend on their surface density, and they can be modulated within a broader range with respect to those displayed by linear brush analogues of identical composition. This is especially valid for the nanotribological characteristics of poly(2-ethyl-2-oxazoline) (PEOXA) brushes, assembled on titanium oxide surfaces by grafting-to technique. At low surface coverage, cyclic PEOXA (C-PEOXA) grafts flatten down towards the grafting surface and provide high friction when sheared against an identical countersurface. In contrast, densely grafted C-PEOXA assemblies stretch vertically forming a molecularly smooth surface that hinders interpenetration with the opposing brush, and dramatically reduces friction, greatly surpassing the lubricious properties typically attained by applying linear grafts. Differently from their nanotribological properties, C-PEOXA brushes always show improved resistance towards the adsorption of proteins if compared to their linear counterparts, irrespective of their grafting density. The enhancement in biopassivity achieved by applying cyclic PEOXAs is ascribed to their intrinsic tendency to hinder protein penetration within a brush layer.