Advances in Generation of Environmentally Responsive Surfaces pp.179-202
Authors: (Vikas Mittal, Institute of Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Switzerland)
Abstract: Ability to change the properties of the surfaces owing to changes in stimulus like temperature, salt concentration, pH etc. leads to the application of these surfaces in a number of specialty applications. Poly(N-isopropylacrylamide) (PNIPAAM) is one such polymer which is well known for its environmentally responsive characteristics which apart from being fast changing with stimulus are also very reversible. As an example, surfaces treated with PNIPAAM become totally hydrophobic if the temperature is raised above the lower critical solution temperature of ~32°C, whereas the same surfaces instantaneously become hydrophilic as soon as the temperature is decreased to 25°C or lower. A number of different substrates have been used to modify them with PNIPAAM and a number of techniques have also been used to graft polymer chains on he surfaces. Flat surfaces of glass, silicon or gold wafers have been used as planar surfaces to generate these characteristics. Various pre-modifications led to the generation of reactive groups on the surfaces which were subsequently reacted with NIPAAM monomer to generate PNIPAAM chains grated to the surface. Among the various techniques used for this purpose included atom transfer radical polymerization, electron beam polymerization, amidation reaction of the surfaces etc. The generated brushes were observed to be responsive to various stimuli indicating that the characteristic of pure PNIPAAM chains could also be transferred on the surfaces of these substrates, though lower critical solution temperature (LCST) of PNIPAAM chains was observed to change by a few degrees in some cases after immobilization on the surfaces. It was also observed that the roughness of the surface on which the PNIPAAM chains were grafted played an important role in the environmental responsiveness of the surfaces as the extent of this behavior was reported to significantly enhance on the rougher surfaces. Spherical substrates like glass beads, silica beads and polymer nanoparticles were also functionalized by PNIPAAM owing to their commercial potential in a number of applications especially in chromatographic separation columns. Similarly various techniques of free radical copolymerization, controlled free radical polymerizations like ATRP and reversible addition fragmentation chain transfer etc and simple amidation or other chemical reactions to graft PNIPAAM chains from the surfaces of particles. Apart from that, monoliths or crosslinked porous networks made from the polymer latex particles could also be successfully functionalized with PNIPAAM and were reported to be very effective in the temperature controlled separation of biological entities thus totally avoiding the use of harsh separation conditions of high salt concentration or pH which can sometimes deteriorate the qualities of biological media. These substrates, both planar and spherical, with environmentally response characteristics represent very high potential materials with a wide range of very special applications.