Techniques for Surface Grafting

To enable well-defined, stimuli responsive polymer coatings, the surface chemistry needs to be understood. The modification of surfaces can be achieved by physical adsorption of the molecules of interest; however, it can be difficult to direct the adsorption to specific parts of the surface and only certain compounds and surfaces are compatible. When chemical grafting is used instead, the functionalisation may be targeted and the technique may be used in conjunction with surface structuration techniques.86 Within this subdivision, either self-assembled monolayers (SAMs) or polymeric/multilayers can be attached. A typical monolayer chemical grafting technique is the addition of thiols onto gold (and other lesser used metals including silver and platinum) to form a self-assembled, well orientated layer.87 The easy of formation of the Au-S bond is highly advantageous and excellent structural control is possible, however the bond is relatively weak and, unless a polydentate ligand is used, the SAM is typically unstable to physical stress.88 Due to the presence of the dithioester or trithiocarbonate RAFT end group on RAFT- synthesised polymers, they can undergo binding to gold surfaces. The RAFT group is often transformed into a thiol, using a nucleophile such as a primary amine, prior to gold binding. Alternatively, the polymers can bind to the surface through the RAFT group, which is particularly useful for polymers which are incompatible with

30 the reduction step, such as poly(acrylic acid). This process also removes the possibility of forming disulfides, rather than thiols, in the reduction process.89

Alternatively, should gold be an undesirable substrate, there are many possibilities when it comes to forming chemically grafted monolayers onto oxide surfaces. This opens up a greater range of modifiable substrates as the oxide surfaces include metals (e.g. Al, Fe, Cr), semiconductors (e.g. Si) or materials with surface bound hydroxyl groups. The hydroxyl groups, typically activated through wet etching, dry etching or plasma activation, act as anchoring points for the subsequent formation of dense monolayers. One of the most commonly used surface modifiers are the alkylsilanes, silicon based compounds containing alkyl group(s) and leaving group(s), such as chlorides, alkoxys or hydrides. The silane substrates can be grafted to the oxide anchoring group on the surfaces, via a very rapidly forming covalent linkage,90 resulting in a generally more stable layer than the thiol-gold system, however it is typically less ordered and less dense.91

The choice of silane group is important, not only should the linker contain the appropriate functionality for further modification but the functionality level is also important. Monofunctional-silanes, R3SiX, have the capability to form only one covalent grafting bond to the substrate, due to the presence of only one hydrolysable leaving group. The resulting surface structures are typically reproducible, but the surface coverage is limited due to the presence of the additional bulky R groups. In order to obtain more stable layers with greater coverage, trifunctional-silanes can be employed, RSiX3 (Figure 1.17). The presence of three leaving groups in the molecule allows intra-molecular cross-linking to occur and stabilise the system. Unfortunately, the trifunctionality also allows oligomerisation of the silane in solution either before

31 or during the attachment process and therefore increases the disorder of the structures. Better leaving groups on the silane (e.g. X = I) result in extensive multilayers, however, poor leaving groups (e.g. X = H) may not react and therefore a balance must be sought.

Figure 1.17: APDMES is an example of a monofunctional-silane, R3SiX, it has only one

hydrolysable leaving group and can therefore only form one bond to the substrate. APTES is a

trifunctional-silane, RSiX3, it can result in a more stable silane layer, due to the occurrence of

intramolecular cross-linking. APDIPES has been designed to improve the monolayer stability, without multilayer build-up

The mechanism of silane SAM formation proceeds via three steps, as seen in Figure 1.18. Initially, the organosilanes are hydrolysed by the water molecules that are bound to the oxide surface and converted to the corresponding hydroxysilanes. The hydroxysilanes can then hydrogen bond to the oxide surface, but due to the nature of the hydrogen bonds they are still able to migrate laterally across the surface and aggregate. The aggregated molecules can then condense onto the surface to form the Si-O-Si bonds. In the case of the multifunctional silanes, these bonds can form between neighbouring silanol groups in addition to between the silane and the surface, stabilising the layer. The key role of surface bound water in the first step is very important in the SAM formation. If too much water is present, the silane may polymerise in the bulk phase and rough layers will be obtained, however, if too little water is present, the monolayers will be incomplete. The optimal water concentration

32 has been found to be 0.15 mg of water in 100 mL of solvent92 and bicyclohexane, heptanes, toluene, cyclohexane, benzene and hexadecane have all been suggested as appropriate solvents to facilitate this requirement.88 Alternatively, the silane monolayers can be formed from the vapour phase, where solvent is not required. The advantages of this technique can include decreased formation of siloxane oligomers, reduced surface contamination and denser monolayers.

Figure 1.18: The mechanism of silane self-assembly on oxide surfaces, adapted from the work S.

P. Pujari et al.88 If the condensation proceeds ideally, a covalently bound and laterally cross-

linked monolayer is formed (bottom right). However, if the water content and silane concentration are not controlled, disordered multilayer may be formed (bottom left)

By coating the surfaces with silane containing a relevant functional moiety, it is then possible to attach on biomolecules, such as DNA,93 cross-linkers or other molecules of interest, including polymers. Covalently attaching organosilanes to silica and glass in order to attach antibodies or receptors, for biosensing applications,

33 has been seen and investigations into the glass preparatory methods have been reported.94 Additionally, silane linked immobilisation of carbohydrates has been demonstrated, using multistep synthetic routes to attach underivitised monosaccharides for array applications.32

The attachment of polymers to a surface to form polymer brushes; covalently tethered polymer chains which form an ultrathin coating can proceed by one of two different methods.40 Firstly, the grafting to technique involves attaching pre- synthesised polymer chains onto a prepared substrate by phys- or chem-isorption.95 This method can therefore be combined with the silanation process, should a chemical bond be required. It is experimentally simpler and allows full polymer characterisation to be undertaken prior to the grafting. However, the density of brushes formed using the grafting to method is limited, due to steric repulsion between the chains. The alternative technique is grafting from brush formation, both examples are seen in Figure 1.19. This involves surface-initiated polymerisation and can produce much denser and more controlled brushes, although it is experimentally more complex.96, 97

34

Figure 1.19: Grafting to and grafting from polymer brush formation techniques