Characterization of poly(N-isopropylacrylamide)-grafted interfaces with sum-frequency generation spectroscopy

Sum-frequency generation vibrational spectroscopy was used to study the molecular structure of surface-grafted poly(N-isopropylacrylamide) (PNIPAM) in air and in D2O. It was found that the side chains of the PNIPAM were nearly standing at the air/PNIPAM interface, owing to the hydrophobic nature of the isopropyl groups. Total internal reflection geometry was employed to obtain SFG spectra of the D2O/PNIPAM interface. When the water temperature was increased, red shifts of the SFG peaks were observed. This can be explained by the dehydration of the alkyl groups at the water interface. From the quantitative analysis of the molecular orientation, restructuring of the main chain due to the dehydration occurs at the water/PNIPAM interface is suggested.