Optical characterization of salt-induced swelling behavior of poly($N$-isopropylacrylamide) terpolymer hydrogel layers

Authors: İLKE ANAÇ, ANIL BOZDOĞAN

Abstract: This study reports for the first time the effect of different sodium halide salts (NaClO$_{4}$, NaI, and Na$_{2}$CO$_{3})$ in the low concentration range (in the range of 10$^{-7}$-1 M) on the swelling behavior of photo-crosslinked, surface-immobilized poly($N$-isopropyl acrylamide) (PNIPAAm) terpolymer layers. PNIPAAm terpolymer (consisting of $N$-isopropylacrylamide, methacrylic acid, and 4-methacryloxybenzophenone) was synthesized via free radical polymerization. The PNIPAAm terpolymer layers were prepared on gold substrates via spin coating and crosslinked and surface attached via UV radiation. The swelling behavior of the layers in monovalent (NaClO$_{4}$ and NaI) and divalent (Na$_{2}$CO$_{3})$ solutions was monitored by surface plasmon/optical waveguide spectroscopy. The terpolymer layers did not swell significantly at very low salt concentrations (10$^{-7}$-10$^{-5}$ M), whereas an increase in swelling upon the addition of salt up to 10$^{-2}$-0.1 M depending on the salt was observed. Further increase in the salt concentration up to 1 M led to collapse of the terpolymer PNIPAAm layers due to the reduction in solvent quality of the salt solution. The full collapse of the terpolymer PNIPAAm layers with 1 M salt solution was observed only when divalent kosmotropic Na$_{2}$CO$_{3}$ salt was used.

Keywords: Salt effect, poly($N$-isopropylacrylamide), surface plasmon resonance spectroscopy, stimulus-responsive hydrogel

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