Amplified responsiveness of multi-layered, polymer grafts: Synergy between brushes and hydrogels

The responsive properties of surface-grafted polymer films in aqueous media can be amplified by covalently layering thermosensitive brushes and hydrogels. This was demonstrated by synthesizing layers of linear poly(N-isopropylacrylamide) (PNIPAM) brushes, alternating with cross-linked, poly(hydroxyethyl)methacrylate (PHEMA) hydrogels via sequential surface-initiated atom-transfer radical polymerization (SI-ATRP) steps. Below the lower critical solution temperature (LCST) of PNIPAM, brush/hydrogel multilayered films swell similarly to linear PNIPAM homopolymer brushes, as measured by liquid ellipsometry. In contrast, above the LCST, the PHEMA hydrogel interlayer acts as stiffening element within the collapsed multilayered film, as monitored by atomic force microscopy (AFM) nanoindentation and lateral force microscopy (LFM). This translates into a 10-fold increase in Young’s modulus by the collapsed, layered films compared to PNIPAM homopolymer analogues. The (macro)molecular continuity between the brush main chains and hydrogel constituents thus enables a chemically robust layering to form graded, quasi-3D grafted polymer architectures, which display a concerted and amplified temperature-triggered transition.