Polymer brushes examples

It is beyond the scope of this Chapter to discuss all kinds of various coating techniques, properties of the supports, properties of the coatings and the various fields of application of the composites in catalysis, separation techniques, materials science, colloid science, sensor technology, biocompatible materials, biomi-metic materials, optics etc. The scope had to be restricted to the fundamental properties of ultrathin organic layers on solid supports followed by some examples, outlining the benefit of the tailored functional surfaces such as SAM and polymer brushes for catalysis. 

Recalling the demands on the polymer architecture of a polymer brush and the projected properties in terms of swelling, wetting and friction, as described in the theoretical work, the brush has to consist of linear polymer chains of the same length at high grafting densities. The closest approximation to this can be obtained by the living anionic SIP (LASIP). The experimental difficulties outlined mean that only relatively few examples of LASIP are documented in the literature. 

SIP-driven polymer brush library fabrication leverages the fact that the polymerization initiation species are permanently bound to the substrate. Since the initiators are tethered, controlled delivery of monomer solution to different areas of the substrate results in a grafted polymer library. In NIST work, initiators bound via chlorosilane SAMs to silicon substrates were suitable for conducting controlled atom transfer radical polymerization (ATRP) [53] and traditional UV free radical polymerization [54, 55]. Suitable monomers are delivered in solution to the surface via microfluidic channels, which enables control over both the monomer solution composition and the time in which the solution is in contact with the initiating groups. After the polymerization is complete, the microchannel is removed from the substrate (or vice versa). This fabrication scheme, termed microchannel confined SIP ([t-SIP), is shown in Fig. 10. In these illustrations, and in the examples discussed below, the microchannels above the substrate are approximately 1 cm wide, 5 cm long, and 300-500 [tm high. 

Another example showing unique properties of high-density brushes concerns the miscibility of the polymer brush with a chemically identical polymer matrix [154]. Neutron reflectometry was applied to a series of deuter-ated PMMA (PMMAj) brushes with a constant chain length (Mn = 46000, Mw/Mn = 1.08) and differing graft density (a 0.7 and 0.06 chains nm ). 

Another attractive application of polymer brushes is directed toward a biointerface to tune the interaction of solid surfaces with biologically important materials such as proteins and biological cells. For example, it is important to prevent surface adsorption of proteins through nonspecific interactions, because the adsorbed protein often triggers a bio-fouling, e.g., the deposition of biological cells, bacteria and so on. In an effort to understand the process of protein adsorption, the interaction between proteins and brush surfaces has been modeled like the interaction with particles, the interaction with proteins is simplified into three generic modes. One is the primary adsorption. 

Recently multi-component polymer brush films have been synthesized in order to produce responsive substrates. As Fig. 10 illustrates, these so-called binary brushes contain two different polymers that may, or may not, be compatible with each other. Such a mixed brush may be synthesized by two strategies First, a mixed monolayer may be deposited with two different initiators (Fig. 10a). For example, one initiator could be activated thermally, while the other photochemically. Thus, a low to medium density polymer brush could be synthesized by selectively activating initiator A. The substrate could then be cleaned and immersed into a second monomer solution to activate B and grow a second polymer interdispersed with the first brush. 

Polymer brushes are polymers tethered to a surface via one end. The connection to the surface can be covalent or non-covalent, and the brushes can be made via grafting to or grafting from the surface. In the past few years, there has been considerable interest in the growth of polymer brushes via surface-initiated polymerisations from (patterned) initiator-functionalised SAMs.62,63 For example, we have recently shown that surface confined Atom Transfer Radical Polymerisations (ATRP) in aqueous solvents leads to rapid and controlled... 

Let us mention at this point that a number of studies have used block copolymers in order to test the Alexander-de Gennes description of polymer brushes. For example, surface force measurements have provided some global character-... 

Figure 1 Responsiveness in a polymer brush. In a good solvent (a) favourable interactions between polymer segments and solvent molecules lead to the chains being stretched - the loss of configurational entropy attributable to the chain stretching is outweighed by the lowering of energy due to the polymer solvent interaction. If this interaction becomes less favourable (b), due to a change in temperature or pH, for example, the chains reversibly shrink...

The collapse of a polymer gel in response to a change in environment can be scaled down to the single chain level. A layer of polymer chains grafted to a surface forms a polymer brush, for which the collapse transition can be nicely observed using neutron reflectivity. A pore lined with a responsive polymer brush will form a selective valve for example if the grafted polymer is a weak polybase, in aqueous acidic conditions the brush will be charged and will expand to close the pore, while in basic conditions the brush will be neutral. This principle has been used to create a selective membrane, which shows greatly reduced permeability in acidic conditions. 

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