The effect of the grafting density on the molecular transport
through thermoresponsive brushes of poly(N-isopropylacrylamide)
(PNIPAM) grafted onto flat gold substrates was investigated using
voltammetry and impedance spectroscopy. PNIPAM brush layers were synthesized
at four different grafting densities using surface-initiated atom
transfer radical polymerization (SI-ATRP) from mixed self-assembled
monolayers of ω-mercaptoundecyl bromoisobutyrate and undecanethiol
chemisorbed on gold surfaces. Tethered PNIPAM layers with grafting
densities resulting from initiator concentrations lower than 25% in
the thiol monolayer show the same transport properties as the initial
self-assembled monolayer before brush synthesis. For higher grafting
densities, the diffusion coefficients, D, of the
K3[Fe(CN)6]/K4[Fe(CN)6] redox probe is 7 orders of magnitude smaller than those typically
measured in aqueous solutions and independent of whether the brush
is collapsed or swollen. The collapse of the PNIPAM brush drives a
hydrophilic/hydrophobic transition in addition to structural/conformational
transformations of the grafted layers, resulting in still smaller
values of D. However, these changes do not lead to
a blocking effect on the active area of the gold surface, which is
only determined by pinholes or discontinuities in the thiol initiator
monolayer. These results are only observed for thin PNIPAM brush layers.