We elucidate the sequence of events occurring during the collapse transition of thermoresponsive copolymer brushes based on poly(di(ethyleneglycol) methyl ether methacrylate) chains (PMEO2MA) grown by atom-transfer radical polymerization (ATRP). The collapse of the bulk of the brush is followed by quartz crystal microbalance measurements with dissipation monitoring (QCM-D), and the collapse of its outer surface is assessed by measuring equilibrium water contact angles in the captive bubble configuration. The bulk of the brush collapses over a broad temperature interval (approximately 25 degrees C), and the end of this process is signaled by a sharp first-order transition of the surface of the brush. These observations support theoretical predictions regarding the occurrence of a vertical phase separation during collapse, with surface properties of thermoresponsive brushes exhibiting a sharp variation at a temperature of T(br)(surf). In contrast, the bulk properties of the brush vary smoothly, with a bulk transition T(br)(bulk) occurring on average approximately 8 degrees C below T(br)(surf) and approximately 5 degrees C below the lower critical solution temperature (LCST) of free chains in solution. These observations should also be valid for planar brushes of other neutral, water-soluble thermoresponsive polymers such as poly(N-isopropylacrylamide) (PNIPAM). We also propose a way to analyze more quantitatively the temperature dependence of the QCM-D response of thermoresponsive brushes and deliver a simple thermodynamic interpretation of equilibrium contact angles, which can be of use for other complex temperature-responsive solvophilic systems.

Photoresponsive monolayers of hydrophilically substituted azobenzenes have been prepared by reaction on aminosilane monolayers on silicon surfaces. Grafting densities in the 0.2-1.0 molecule/nm(2) range were determined by X-ray reflectometry. The monolayers exhibit reversible photoisomerization, switching from a more hydrophilic trans state to a less hydrophilic cis state upon UV irradiation, in contrast with the usual behavior of most azobenzene monolayers that switch from a less to a more hydrophilic state. This indicates that the wettability is not dominated by the change in the dipole moment of the azobenzene moiety but originates from variations in the composition of the outer surface of the monolayers resulting from the reorientation of the substituent groups. The light-driven change in the water contact angle correlates linearly with the grafting density but remains small. However, the wettability contrast can be increased by forcing the molecules to stand in an improved vertical orientation, either by densifying the underlying aminosilane monolayer or by filling the voids left at the bottom of the layer of grafted azobenzene molecules.

We depict the collapse transition of adaptive thermo-and pH-responsive copolymer brushes based on poly(di(ethylene glycol) methyl ether methacrylate-co-methacrylic acid) random copolymer chains (P(MEO 2 MA-co-MAA)) by drawing bidimensional (2D) maps of the swelling ratio versus temperature and pH for different brush compositions. The collapse transition is probed by quartz crystal microbalance measurements with dissipation monitoring (QCM-D). While P(MEO 2 MA) brushes exhibit a thermocollapse transition around 22 °C and P(MAA) brushes display a pH-induced collapse transition at pH = 5.5, P(MEO 2 MA-co-MAA) brushes undergo a collapse transition modulated by either temperature or pH from a swollen state at low temperature and high pH to a collapsed state at high temperature and low pH. By varying the composition of the copolymer in MAA units from 4 to 14 mol %, the brushes switch from a pHmodulated thermo-responsive behavior to a temperature-modulated pH-responsive behavior in water. The 2D maps of swelling ratio also illustrate the complex interplay between pH and temperature, and provide a unique view of the response of adaptive brushes.

Catalytic surfaces have been prepared by complexation of palladium on self-assembled terpyridine monolayers on silicon. A reaction-based fluorogenic probe was developed to allow facile visualization of the catalytic potential of the surface. Superior activity of the immobilized catalyst compared with the homogeneous control reactions is demonstrated.

Photothermoresponsive polymer brushes were synthesized by coupling azobenzene derivatives to oligo(ethylene glycol) methacrylate (OEGMA) based copolymer brushes grown from silicon substrates. Varying the length and chemical ending of the lateral chains (using different OEGMA comonomers) afforded a collection of thermoresponsive copolymer brushes with predetermined fraction of hydroxyl side groups. These pendent hydroxyl functions were subsequently used to anchor the photochromic modules onto the brushes via classical activation/coupling chemistry, albeit in modest to moderate yields depending on the brush composition, thickness and grafting density. The extent of swelling of the functionalized brushes in water could be reversibly modulated by photoisomerization of the azobenzene motifs, even though the amplitude of the observed variation remained small. Decreasing the grafting density of the brush afforded improved photoresponse possibly through decreased steric crowding between the chains, facilitating azobenzene coupling and isomerization in the confined system.