Thermal Response of Poly(N-isopropylacrylamide) Brushes Probed by Surface Plasmon Resonance

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Abstract
The thermally induced hydration transition of surface-grafted poly(N-isopropyl acrylamide) (PNIPAAm) brushes was probed by surface plasmon resonance spectroscopy (SPR) and contact angle measurements. Data are presented for a PNIPAAm brush film with a dry thickness of ∼50 nm that was synthesized by atom radical transfer polymerization on the surface of a self-assembled monolayer on gold. SPR measurements were taken as a function of temperature in two modes: the quasi-static mode, in which the sample was equilibrated at each temperature for ∼15 min prior to measurement, and the real-time mode, in which SPR reflectivity data were collected as the sample was heated and cooled at ∼4.5 °C/min. Both types of measurement indicate that the hydration transition for the PNIPAAm brush occurs over a broad range of temperatures (∼10−40 °C). This result is in accordance with theoretical predictions that have suggested that polymer brush structures on planar surfaces do not exhibit true critical solubility transitions. Contact angle measurements revealed a discontinuity in the surface wettability at a temperature (∼32 °C) that corresponds to the dilute aqueous critical solution temperature. Taken together, these results suggest that the polymer segments in the outermost region of the brush remain highly solvated until the dilute solution lower critical solution temperature (∼32°), while densely packed, less solvated segments within the brush layer undergo dehydration and collapse over a broad range of temperatures.