Engineering adhesion to thermoresponsive substrates: effect of polymer composition on liquid-liquid-solid wetting.

Adhesion control in liquid-liquid-solid systems represents a challenge for applications ranging from self-cleaning to biocompatibility of engineered materials. By using responsive polymer chemistry and molecular self-assembly, adhesion at solid/liquid interfaces can be achieved and modulated by external stimuli. Here, we utilize thermosensitive polymeric materials based on random copolymers of di(ethylene glycol) methyl ether methacrylate (x = MEO2MA) and oligo(ethylene glycol) methyl ether methacrylate (y = OEGMA), that is, P(MEO2MAx-co-OEGMAy), to investigate the role of hydrophobicity on the phenomenon of adhesion. The copolymer ratio (x/y) dictates macromolecular changes enabling control of the hydrophilic-to-lipophilic balance (HBL) of the polymer brushes through external triggers such as ionic strength and temperature. We discuss the HBL of the thermobrushes in terms of the surface energy of the substrate by measuring the contact angle at water-decane-P(MEO2MAx-co-OEGMAy) brush contact line as a function of polymer composition and temperature. Solid supported polyelectrolyte layers grafted with P(MEO2MAx-co-OEGMAy) display a transition in the wettability that is related to the lower critical solution temperature of the polymer brushes. Using experimental observation of the hydrophilic to hydrophobic transition by the contact angle, we extract the underlying energetics associated with liquid-liquid-solid adhesion as a function of the copolymer ratio. The change in cellular attachment on P(MEO2MAx-co-OEGMAy) substrates of variable (x/y) composition demonstrates the subtle role of compositional tuning on the ability to control liquid-liquid-solid adhesion in biological applications.