Clustering Induced Collapse of a Polymer Brush

Poly(ethylene oxide) (PEO) chains were grafted onto a spherical thermally sensitive poly( $N$-isopropylacrylamide) microgel with a low critical solution temperature of $\sim 32°\mathrm{C}$. The shrinking of the microgel at higher temperatures led to a dramatic decrease in its surface area, providing a convenient way to increase the grafting density of a polymer brush. As expected, we observed a repulsion-induced stretching of the PEO chains at the initial stage of the shrinking. However, as the grafting density increases, an unexpected collapse of the grafted PEO chains was observed, which can be attributed to a long predicted $n$-clustering attractive interaction among densely grafted polymer chains.