Side-group-modulated interfacial energy dissipation of amphiphilic copolymer-doped hydrogels

Poly(N-alkylacrylamide)-based copolymers provide a versatile platform for regulating interfacial behavior, in particular reduction of sliding dissipation through suitable lubrication, through their thermoresponsive and amphiphilic character. We hypothesize that controlling copolymer composition, side-chain architecture, and segmental distribution enables optimization of such lubrication against diverse countersurfaces while preserving mechanical resilience under shear.

Poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogels were modified with the random copolymers poly(2-methacryloyloxyethyl phosphorylcholine-co-N-isopropylacrylamide) (poly(MPC-co-NIPAM), PMN) and poly(2-methacryloyloxyethyl phosphorylcholine-co-N-propylacrylamide) (poly(MPC-co-nPAM), PMP), at different ratios of the two components. These copolymers differ only in the architecture of their N-alkyl substituents: branched isopropyl in PMN and linear n-propyl in PMP. Tribological measurements, atomic force microscopy (AFM), and cryogenic scanning electron microscopy (cryo-SEM) were used to elucidate the underlying lubrication mechanisms.

The PMN-doped hydrogel showed composition-dependent lubrication, with a 6:1 NIPAM:MPC ratio achieving near-optimal ultra-low friction (COF < 0.02) on both hydrophilic and hydrophobic countersurfaces up to ~1.1 MPa contact pressures. In contrast, the PMP-doped hydrogel, differing only by isomerism, failed on hydrophobic countersurface at high loads due to increased hydrophobic exposure, leading to aggregation, phase separation, and stronger surface interactions. Heating above the lower critical solution temperature (LCST) further reduced friction for both systems despite deswelling, attributed to lower viscoelastic dissipation and formation of a NIPAM-rich boundary layer that enhances adsorption on hydrophobic countersurface, while MPC maintains interfacial hydration. These results highlight design principles for such adaptive coatings: maintain interfacial MPC for hydration, tune NIPAM for hydrophobic interactions, and avoid substituents that promote aggregation.