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We probe, using the Surface Forces Apparatus, the thermal response ofpoly(N-isopropylacrylamide) (PNIPAM) brushes of various grafting densities,grown from plasma-activated mica by means of surface-initiated polymerization.We thus show that dense thermoresponsive brushes collapse gradually astemperature is increased, and that grafting density greatly affects theirability to swell: the swelling ratio of the brushes, which characterizes thethickness variation between the swollen and the collapsed state, is found todecrease from $\sim 7$ to $\sim 3$ as the number of grafted chains per unitarea increases. Such a result, obtained with an unprecedent resolution ingrafting density, provides qualitative support to calculations by Mendez {\itet al.} [{\it Macromolecules} {\bf 2005} {\it 38}, 174]. We further show that,in contrast to swelling, adhesion between two PNIPAM brushes appears to berather insensitive to their molecular structure.

Density effects on collapse, compression and adhesion of thermoresponsive polymer brushes