Surface Microdynamics Phase Transition and Internal Structure of High-Density, Ultrathin PHEMA-b-PNIPAM Diblock Copolymer Brushes on Silicone Rubber

Abstract

Stimuli-responsive ultrathin diblock copolymer brushes (DCBs) composed of poly(2-hydroxyethyl methacrylate) (PHEMA) as bottom block and poly(N-isopropylacrylamide) (PNIPAM) as top block on the surface of silicone rubber (SR) were examined as high-throughput platforms to trace the thermally induced phase transition mechanisms in water. A high grafting density DCB was precisely prepared from SiOx substrate via surface-initiated atom transfer radical polymerization (SI-ATRP). Surface modification studies with different initiator concentrations were performed to optimize their attachment, producing surfaces with dense covalently attached initiator monolayers with up to 2.8 initiators/nm2. Changes in the physicochemical properties according to each step of surface modification were investigated using in situ analysis with the sample both in a liquid environment and on layer in dry state. The temperature-dependent contact angle of the air bubbles underneath the high-density DCB (0.53 chains/nm2) gradually increased up to around 24 °C in water with increasing temperature and then decreased up to around 30 °C, followed by a rapid decrease at LCST of PNIPAM top block, approximately 32 °C. To obtain a thorough understanding of the microdomain orientation in DCB, the structure of SR/PHEMA-b-PNIPAM brush was investigated by μ-focused grazing incidence small-angle X-ray scattering (μ-GISAXS). Upon exposure to higher temperature conditions (above the LCST of PNIPAM top block), the film showed a “matrix-island” structure, in which the PNIPAM nanodomains were distributed over the highly packed PHEMA bottom block. The thermally induced evolution mechanism of various interactions in PHEMA-b-PNIPAM DCB and water mixtures during the heating–cooling cycles was investigated by means of temperature-dependent FTIR in combination with a two-dimensional correlation (2Dcos) technique. Both conventional IR spectrum and 2Dcos had figured out the volume phase transition temperature for PNIPAM top block and PHEMA bottom block to be about 33 and 33.5 °C, respectively, close to the results obtained from turbidity measurements [Sun et al. Soft Matter2013, 9, 1807; Weaver et al. Macromolecules2004, 37, 2395] on the same bulk polymers.