Evidence for vertical phase separation in densely grafted, high-molecular-weight poly(N-isopropylacrylamide) brushes in water

The detailed conformational change of poly($N$-isopropylacrylamide) (PNIPAM) brushes at high grafting density in ${\mathrm{D}}_2\mathrm{O}$ was investigated as a function of temperature using neutron reflection. PNIPAM chains were grafted at high surface density from gold and silicon oxide surfaces by atom transfer radical polymerization. Whereas single layer profiles were observed for temperatures below and above the transition region, bilayer profiles were observed for a narrow range of temperatures near the transition. This nonmonotonic change in the concentration profile with temperature is discussed in the context of theoretical models of vertical phase separation within a brush.