  We probe, using the Surface Forces Apparatus, the thermal response of
poly(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 as
temperature is increased, and that grafting density greatly affects their
ability to swell: the swelling ratio of the brushes, which characterizes the
thickness variation between the swollen and the collapsed state, is found to
decrease from $\sim 7$ to $\sim 3$ as the number of grafted chains per unit
area increases. Such a result, obtained with an unprecedent resolution in
grafting density, provides qualitative support to calculations by Mendez {\it
et al.} [{\it Macromolecules} {\bf 2005} {\it 38}, 174]. We further show that,
in contrast to swelling, adhesion between two PNIPAM brushes appears to be
rather insensitive to their molecular structure.
