Functionalizing surfaces with polymer brushes

When polymer chains are tethered to a surface in the presence of a good solvent, and are spaced closely together (separated by a distance, d, that is less than twice their radii of gyration, Rg, measured as the free molecule in a good solvent), they have a tendency to stretch out into the solvent in a brush-like configuration, rather than interacting closely with each other. 

Principles of Polymer Chemistry, P.J. Flory, Cornell University Press (1953). 

Efimenko, P. Vlcek, V. Subr, J. Genzer, Formation and Properties of Anchored Polymers with a Gradual Variation of Grafting Densities on Plat Substrates. Macmmolecules (2003) Vol. 36 pp. 2448-2453. 

Halperin, M. Tirrell, T.P. Lodge, Tethered Chains in Polymer Microstructures. Adv Polym Sci (1992) Vol. 100 pp. 31-71 Polymers at Interfaces, G.J. Fleer, M.A. Cohen Stuart, J.M.H.M. Scheutjens, T. Cosgrove, B. Vincent, Chapman HaU, London, 1993. 

Polymer Brushes Synthesis, Characterization, Applications, R.C. Advincula, W.J. Brittain, K.C. Caster, J. Riihe, Wiley-VCH, 2004. 

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A second example for the synthesis of a functional surface-attached polymer brush is the preparation of monolayers of a liquid-crystalline polymer (TCP) with mesogenic units in the sidechain [43]. Such a system could be of interest for the preparation of alignment layers for liquid-crystal displays (LCD) [44,45]. Alignment layers are key components for the production of LCDs [46-50]. As the orientation of the nematic director of the LC in contact with a surface is energetically degenerate, domains are formed in the LC layer that lower the contrast of the display. Thin films of polymers such as rubbed polyimide film ( alignment layers ) are frequently used to align the liquid-crystalline molecules in order to form extended monodomains. 

This section reviews studies on the synthesis of fine particles coated with polymer brushes by surface-initiated ATRP. SiP are among the most extensively studied particles for the application of surface-initiated ATRP [85-93]. Patten et al. first succeeded in the surface-initiated ATRP of ST and MMA on two SiPs with average diameters of 75 and 300 nm [85,86]. Several groups investigated the synthesis of hybrid SiP with different monomers. Maty-jaszewski et al. synthesized an initiator-functionalized SiP with a diameter of... 

The surface force trend resembles that of the interaction energy as a function of the distance between two mica surfaces bearing polymer brushes as obtained with the surface forces apparatus (SFA) (e.g. [242]). Such a similarity between polymer brushes symmetrically compressed by different phases is not unexpected. 

Polyacrylic acid (PAA)—P2VP mixed brushes were prepared by a similar synthetic procedure, by grafting of carboxyl-terminated poly(ferf-butyl acrylate) (PtBuA) and P2VP. Afterwards, PtBuA was hydrolyzed in the presence of p-toluene sulfonic acid. The same strategy was employed to graft mixed PEL brushes on polymer surfaces. In this case plasma treatment was used to functionalize surface of polymer substrates. We introduced amino groups on the surface of PA-6 and PTFE by treatment of the polymer samples with NH3 plasma. Then the carboxyl terminated homopolymers were grafted step by step from the melt to the solid substrate via amide bonds. 

The impact of the chemical functionality of grafted polymer brushes on the wettability of the substrate was investigated for a number of neutral polymers as well as for weak and strong polyelectrolytes (Figure 4.2) [2]. It is evident that the wettability of the hydrophobic ETFE substrate can be tailored over a wide range with polymer brushes. As a general trend, surfaces with more hydrophilic brushes... 

Although the previously discussed methods are applicable specifically to polymer substrates, there are also strategies for surface functionalization with polymer brushes that work on a broad range of substrates. A general method that allows producing polymer brushes on silicon, gold, perfluorinated poly(ethylene-co-propylene), and poly (styrene) has been recently introduced [21]. The first step of the modification sequence was not sensitive to the substrate used. An... 

Neuhaus S. Functionalization of polymer surfaces with polyelectrolyte brushes [dissertation]. Zurich ETH Zurich 2011. 

Xu Y, Chen X, Chen H, Xu S, Liu H, Hu Y Density functional theory for the selective adsorption of small molecules on a surface modified with polymer brushes. Mol Simul 38(4) 274-283, 2012. 

Several sessions were included in scientific program. The session Bio-inspired Polymers included 11 oral presentations. Self-assembly of an aquaporin mimic, tailoring surface properties with polymer brushes, bioinspired block copolymers, hierarchically structured conjugated polymers via supramolecular self-assembly, natural polymeric composites with mechanical function, macromolecular oxidation catalysts based on miniemulsion polymerization and some other problems were discussed on this session. 

The chain architecture and chemical structure could be modified by SCVCP leading to a facile, one-pot synthesis of surface-grafted branched polymers. The copolymerization gave an intermediate surface topography and film thickness between the polymer protrusions obtained from SCVP of an AB inimer and the polymer brushes obtained by ATRP of a conventional monomer. The difference in the Br content at the surface between hyperbranched, branched, and linear polymers was confirmed by XPS, suggesting the feasibility to control the surface chemical functionality. The principal result of the works is a demonstration of utility of the surface-initiated SCVP via ATRP to prepare surface-grafted hyperbranched and branched polymers with characteristic architecture and topography. 

Wang, D., et al., Graphene functionalized with azo polymer brushes Surface-initiated polymerization andphotoresponsiveproperties. Advanced Materials, 2011. 23(9) p. 1122-1125. 

The same aplies to polymer brushes. The use of SAMs as initiator systems for surface-initiated polymerization results in defined polymer brushes of known composition and morphology. The different polymerization techniques, from free radical to living ionic polymerizations and especially the recently developed controlled radical polymerization allows reproducible synthesis of strictly linear, hy-perbranched, dentritic or cross-linked polymer layer structures on solids. The added flexibility and functionality results in robust grafted supports with higher capacity and improved accessibility of surface functions. The collective and fast response of such layers could be used for the design of polymer-bonded catalytic systems with controllable activity. 

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