Swelling behavior of multiresponsive poly(methacrylic acid)-block-poly (N-isopropylacrylamide) brushes synthesized using surface-initiated photoiniferter-mediated photopolymerization

Surface-initiated photoiniferter-mediated photopolymerization (SI-PMP) in presence of tetraethylthiuram disulfide is used to directly synthesize surface-grafted poly(methacrylic acid)-block-poly(N-isopropylacrylamide) (PMAA-b-PNIPAM) layers. The response of these PMAA-b-PNIPAM bi-level brushes to changes in pH, temperature and ionic strength is investigated by using in-situ multi-angle ellipsometry to measure changes in solvated layer thickness. As expected for a block copolymer architecture, PMAA blocks swell as pH is increased, with the maximum change in the thickness occurring near pH = 5, and PNIPAM blocks exhibit lower critical solution temperature (LCST) behavior, marked by a broad transition between swollen and collapsed states. The response of the bi-level brushes to changes in added salt at constant pH is complex, as the swelling behaviors of both the weak polyelectrolyte, PMAA, and thermoresponsive PNIPAM are affected by changes in ionic strength. This work demonstrates not only the robustness of SI-PMP for making novel, bi-level stimuli-responsive brushes, but also the complex links between synthesis, structure, and response of these materials.