Hydration and dehydration behaviors of poly(N-isopropylacrylamide)-grafted silica beads

Abstract

The use of poly(N-isopropyl acrylamide) (PNIPAAm) in thermoresponsive interfaces for biomedical applications has been extensively investigated. Understanding the hydration and dehydration characteristics of PNIPAAm-grafted interfaces is crucial for the development of highly functional thermoresponsive interfaces. In this study, four PNIPAAm-grafted silica beads were prepared using atom transfer radical polymerization and radical polymerization techniques, with varying amounts of PNIPAAm grafted onto the beads. These PNIPAAm-grafted silica beads were used as samples, and their hydration and dehydration behaviors were analyzed using FTIR spectroscopy. Through the analysis of FTIR spectra obtained from the PNIPAAm-grafted silica beads prepared in this study, distinct observations were made under varying relative humidity and temperature conditions. Specifically, an elevation in the peak intensity of the broad peak at 3700–3000 cm−1, associated with the O-H group of hydrated water on PNIPAAm, was noted at higher humidity levels at 10°C. This increase in humidity resulted in an amplified peak intensity at 3700–3000 cm−1, indicative of the presence of the hydrated water molecules bonded to the PNIPAAm. Concurrently, shifts in the amide I and II peaks were observed with increasing relative humidity, signifying enhanced hydration and increased hydrogen bonding to C=O and N-H groups. Conversely, increasing the temperature at relatively high humidity led to a decreased in the intensity of the broad peak at 3700–3000 cm−1; this was attributed to the dehydration of the grafted PNIPAAm and subsequent water loss. The dehydration of PNIPAAm resulted in significant peak shifts for the amide I and II peaks, indicating a decrease in hydrogen bonding with the amide group. A significant change in peak intensity and shifts were evident, underscoring PNIPAAm brush's heightened propensity for hydration. These findings confirm the effectiveness of FTIR spectroscopy as a valuable approach for studying the hydration and dehydration behaviors of thinly grafted PNIPAAm layers on substrates.