Temperature-dependent behavior of poly(N-isopropylacrylamide) brushes via neutron reflectometry

Poly(N-isopropylacrylamide) (PNIPAAm)-modified interfaces are promising for various biomedical applications, however, the PNIPAAm brush structure must be investigated at various temperatures to enable the design of functional PNIPAAm interfaces. In this study, PNIPAAm brush structures with various densities and chain lengths are prepared via atom-transfer radical polymerization and analyzed using neutron reflectometry. The volume fraction of PNIPAAm exhibits temperature-dependent shrinkage; slight PNIPAAm shrinkage is observed below the lower critical solution temperature (LCST) of PNIPAAm, whereas large shrinkage occurs across the LCST of PNIPAAm. The volume of water in the PNIPAAm brush is 40% even when it is dehydrated at 37 °C and is slightly extended relative to its dry state. The dense PNIPAAm brush is thicker than the diluted PNIPAAm brush because the densely packed PNIPAAm chain tends to form an extended structure. A higher volume fraction of D2O is observed in the diluted PNIPAAm brush than in the dense PNIPAAm brush because the former can accommodate a larger volume of D2O. These results indicate that neutron reflectometry can provide essential information on the PNIPAAm brush configurations with various chain lengths and densities at different temperatures. These findings are applicable to the design of further functional PNIPAAm brush interfaces for biomedical applications. [Display omitted]