Description
Polymer brushes offer patternable, robust, three-dimensional coatings that can be used for, among other applications, the capture and release of medicine and nanoparticles. High-density poly(2-hydroxyethyl methacrylate) (PHEMA) brushes are complex because interlaced combination between chain conformation, n-clustering-induced interactions near surface proximal layers, and proton donor-acceptor and proton acceptor side chain functional groups are regulated by the local aqueous environment temperature. In this contribution, we demonstrate robust PHEMA brush with a high grafting density that exhibits differential dewetting transitions in the broad temperature range 18-40 C which can be tentatively ascribed to the n-cluster-induced collapse of the polymer segments in the innermost region and so-called “pseudo reverse-solubility” behavior in the outermost region of archetypal high-density PHEMA brush. The polymer segment density profiles of the PHEMA brush in the directional normal to the silicon-D2O interface during a heating-cooling cycle were determined by using in situ neutron reflectivity (NR) measurement technique along with the corresponding degree of hydration. The time-resolved high temporal resolution NR profiles of the swollen brush in D2O had figured out a layered segregated structure with three identical hydrated states inside of the PHEMA brush, consisting of dense surface and underneath regions with ~100Å and ~10Å thick, respectively, in combination with a highly hydrated diffuse brush regime extended between upper and lower collapsed segments. This sandwiched structure is attributed to the skinning of the dense outer region during segment collapse that retards solvent egress. µ-focused grazing incidence small-angle synchrotron X-ray scattering (µ-GISASXS) measurements show that the well-equilibrated initial film exhibits highly ordered structure with isotropic vertical orientation; the film initially swell at the ambient temperature and maintain strongly stretched conformation. However, for temperatures above ~ 30 C, lateral segregation and mesostructural ordering in the dense skin was evidenced from GISASXS patterns, in which the collapsed segments microdomains were distributed over the highly packed diffusive regime.
