Polymer brush

Brush polymers are a class of polymers that are adhered to a solid surface^[1]. The polymer that is adhered to the solid substrate must be dense enough so that there is crowding among the polymers which then, forces the polymers to stretch away from the surface to avoid overlapping.^[2]

The stretched form of the polymers adhered to the solid surface often have different properties than the polymer chains in solution and therefore, often affects the behavior and results in the novel properties of several brush polymers. Brush Polymers are formed by an assembly of polymer chains adhered by one end to a surface or an interface. The assembly of polymer brushes allows for tailor-made surfaces for various applications. The diverse applications of brush polymers on solid surfaces and at liquid interfaces allows for several applications including new adhesive materials, protein-resistant biosurfaces, chromatographic devices, lubricants, polymer surfactants and polymer compatbilizers. ^[3] Polymer brush structures have existed in nature for quite some time. Examples of naturally occurring polymer brushes include extracellular polyssacharides on bacterial surfaces, neurofilaments and microtubules with associated proteins in neuritis, and the proteoglycans of cartilage. In particular, ultrathin polymer coatings are of particular interest for the surface modification of small (nano) particles. Some of these applications include antithrombogenic and anti-inflammatory which can allow for enhanced compatibility of biometerials. Cite error: Invalid <ref> tag; invalid names, e.g. too many; see the help page

Contents 1 History 2 Classifications of Linear Brush Polymers 3 Synthesis 4 Applications 4.1 Functionalized Thin Film 4.2 Nanoparticle Functionalization 5 See Also 6 References

History

Interest in brush polymers was gained in the 1950s when it was discovered that the aggregation of colloidal particles can be prevented by grafting polymer molecules to them. Van der Waarden found that by grafting hydrocarbon chains to carbon black particles, aggregation could be inhibited and thus, founded a new movement in polymer science. ^[4] In 1975, S. J. Alexander began focusing on theoretical study of polymer brushes. He noted the distinctive properties of polymer brushes through theoretical analysis of adsorption of functionalized polymers on flat surfaces. ^[5] The theoretical work started by Alexander was later elaborated on by de Gennes and by Cantor which stressed the application of tethered polymers in defining brush polymers. ^{[6] [7]}

Classifications of Linear Brush Polymers

There are several various types of brush polymers including:

• Flexible homopolymer brush

• A homopolymer brush refers to an assemblage of tethered polymer chains made up of a single type of repeat unit. This would entail a polymer solely made up of monomer A resulting in the polymer (-A-A-A-A-A-A-A-...) after polymerization.

• Mixed homopolymer brush

• Mixed homopolymer brushes refer to an assemblage of tethered chains made up of two or more different types of repeat unit. For example, in a mixed homopolymer brush system where there are only two different polymer chains, each chain would be made up of a single monomer repeat unit. Also, each of these polymers would be made up of a different repeat unit thus, conferring different physical properties to a brush polymer. Two possible polymers in a mixed homopolymer system could be made up of a monomer A and a monomer B leading to the polymerization of the polymers (-A-A-A-A-A-A-A...) and (-B-B-B-B-B-B-B-...).

• Random copolymer brush

• Random copolymer brushes refer to an assemblage of tethered polymer chains consisting of two different repeat units which are randomly distributed along the polymer chain. In a random copolymer system with the monomer A and monomer B, the resulting polymer would be completely random with no patterned distribution along the chain. An example of a resulting polymer could be (-A-B-A-A-B-A-B-...).

• Block copolymer brush

• Block copolymer brushes refer to an assemblage of tethered polymer chains consisting of two or more homopolymer chains covalently connected to each other at the ends. In a block copolymer system, "blocks" of single repeat units would be repeated along the polymer chain that is subsequently attached or polymerized from the target polymer or solid substrate. An example of a block copolymer brush with the monomers A and B could yield the resulting polymer, (-A-A-A-B-B-B-...).

Synthesis

There are several strategies that have been employed to synthesize brush polymers thus, increasing interest in their various possible applications. Brush polymers can be synthesized using any of the various polymerization techniques available including: cationic ^[8], anionic ^[9], radical ^[10], ring-opening metathesis ^{[11] [12]}, photochemical ^{[13] [14]}, and electrochemical ^[15] polymerization. There have also been recent advances using controlled radical polymerization (CRP) techniques such as atom transfer radical polymerization (ATRP) ^{[16] [17] [18]} , reversible addition-fragmentation chain transfer polymerization (RAFT (chemistry))^{[19] [20]} , and nitroxide mediated polymerization (NMP) ^[21] . These techniques have emerged as powerful synthesis techniques due to their ability to control polymer growth. Although there are several techniques for bush polymer synthesis, most people outline their synthesis strategy around the "grafting from" and "grafting to" approach and then decide which polymerization tools to employ based on their specific needs.

• One particular method which is very common in brush polymer synthesis is the “grafting to” approach. The "grafting to" approach is advantageous for adding pre-sythesized polymers to a common polymer backbone. Pre-synthesized polymers can be purchased relatively cheaply and attached to a common polymer backbone with a variety of synthesis methods. Some of the common methods used to synthesize brush polymers include anionic and cationic polymerization as well as adsorption methods such as chemisorption and physisorption. Although a wide variety of brush polymers can be quickly assembled using pre-synthesized polymers, a major disadvantage associated with brush polymer synthesis is that achieving high density polymer grafting becomes more difficult with increased size of the polymer. Polymers tend to no longer remain linear with increased size and aggregation of polymers near the polymer backbone blocks other potential sites for polymerization.

• Another method for synthesis of brush polymers is the “grafting from” approach. The "grafting from" approach is advantageous because you can "grow" polymers from the common polymer backbone. In order to carryout the "grafting from" approach, the polymer backbone needs to have a radical initiator site which can supply the radical necessary for radical polymerization or controlled radical polymerization(CRP). Using the "grafting from" approach is extremely advantageous because it gives every radical site on the polymer backbone an equal chance for polymerization. Therefore, much larger chains can be polymerized onto the polymer backbone without having to worry about the size of the polymer chain blocking other polymerization sites.

• A hybrid approach to brush polymer synthesis is the “multi-step grafting approach”^[2]. This is mostly used to create surface-grafted or polymer-grafted hyperbranched polymers. The synthesis of hyperbranched polymers can be achieved through a combination of the "grafting to" and "grafting from" approaches. This "multi-step grafting approach" is often advantageous when limitations of polymer chain density affect the "grafting to" synthesis strategy. Using the "multi-step grafting approach" synthesis allows for increased density of polymer attachment in the "grafting to" approach where the synthesis can be switched to a "grafting from" approach in order to increase branching of the previously attached polymer chains.

• One interesting case is to generate polymer film in the brush state on a soft support or a fluid membrane. There are examples of such polymer brushes in nature including bacterial cell wall. Over the last few decades a number of protocols have been proposed and used for generating solid supported fluid lipid membranes (SLB). However when lipids are linked to polymers like PEG, it was not possible to generate supported lipid membrane above the strongly interacting regime until recently. Recently Alireza Mashaghi from ETH Zurich developed a method to generate supported lipid membranes with associated brush polymer film on SiO2 surface. In Mashaghi method, this was achieved by the addition of 20 mM EDTA buffer agent, allowed self-organized formation of PEG-SLBs from PEGylated liposomes with significantly high PEG-lipid concentrations. The underlying mechanism is the increased affinity of methoxy-PEG(5k) to SiO2. SiO2 has a lower density of surface hydroxyl groups at pH 5.0 compared to pH 7.4 and the resulting weaker silica-water interaction correlates with increased adsorption of PEG from solution. The mechanical properties of such brush polymer films have been studied with Atomic force microscopy ^[22].

Applications

Brush polymers can be used in applications such as in adhesives, biosurfaces, lubrication, separations, compatibilizers, and coatings^[3]. The reported uses of polymer brushes include prevention of flocculation of colloidal particles^{[23][24] [25]}, new adhesive materials^[26], protein-resistant biosurfaces^[27], chromatographic devices^[28], lubricants^[29], polymer surfactants and polymer compatabilizers^[1]

Functionalized Thin Film

Polymer-functionalized thin films is one of the current applications that brush polymers have been adapted to. Polymer chains have been attached to functionalized thin film substrates including glass, silica, aluminum, and gold. The methods available for functionalizing thin film substrates include adsorption as well as anionic, cationic, and radical polymerization. Formation of brush polymers have also demonstrated the ability to create biocompatible thin films from previously incompatible thin film materials. By adsorbing biocompatible polymers to these previously incompatible thin films, a biocompatible thin film was created which can be directly applied to the field of implantable devices. Cite error: Invalid <ref> tag; invalid names, e.g. too many; see the help page

More specifically, Sergiy Minko and Evgeny Katz of the Department of Chemistry and Biomolecular Science at Clarkson University made a polymer brush which is responsive to pH with tunable redox properties. They used Poly(4-vinyl pyridine), functionalized with Os-complex redox units which was grafted to an indium tin oxide (ITO) conductive support to form a polymer brush. When pH = 4.0, the polymer brush is active electrochemically. However, when pH > 6.0, the polymer brush is not active. Through pH control they demonstrated that the polymer brush was a modified electrode. ^[30]

Another important application for thin films is their use as biofilms. Biofilms are particularly important in implant surgery because bacterial adhesion can cause infection and other serious complications. These infections and other complications can not only affect the living quality of the patient but also, dramatically increase the cost of healthcare. M. Reza Nejadnik in the Department of Biomedical Engineering at the University Medical Center Groningen recently demonstrated the potential advancement in the field of bacterial-resistant biosurfaces. They coated polyethylene oxide (PEO) and polypropylene oxide(PPO) on pure silicone rubber and exhibited bacterial growth inhibition on silicon rubber substrates. ^[31]

Nanoparticle Functionalization

Brush polymers have also found recent application in the field of nanoparticles. It has been demonstrated that nanoparticles made of various materials can be functionalized with polymers to become biologically relevant in the field of drug delivery. It has been demonstrated that by attaching polyethylene glycol spacers to gold nanoparticles, it is possible to attach target-specific ligands for specified cell and tissue drug delivery.^[32] The polymer chains of polyethylene glycol were capped with a thiol group on one end and another reactive functional group on the other. The thiol group on the polyethylene glycol chain was used to attach the polymer chains to the gold nanoparticle while the other reactive functional group is used to attach specific ligands for targeted drug delivery.

See Also

• Dendrimer
• Copolymer
• Block copolymer
• Thin Film
• Nanoparticle

References