Polymeric brushes

The structure formed when one end of a chain is tethered to d = 2 surface, is referred to as a grafted layer or more commonly as a brush . Brushes can be made by attaching a functional group to one end of a chain that can then bind to the surface. The binding energy can either be quite high (several hundred kaJ), in which case the end is chemically attached,or of order ten kaT, in which case it is physi-adsorbed. Diblock copolymers, in which one of the blocks (usually the shorter one) adsorbs strongly to the surface, while the other does not, also form brushes, as well as a 

Strongly damped by the repulsive interaction with other chains. This system, which closely resembles a pair of brushes brought into contact, has been reviewed by Binder.  

A large number of experimental, theoretical and numerical studies on brushes have been published during the last several years, including two excellent review articles. In this section, we shall discuss simulations on brushes and related systems, and compare their results with experiments and theoretical predictions. Although there has been some recent interest in brushes with chains of different chemical composition, we shall limit our discussion to brushes whose chains are of identical chemical composition. 

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Graft chains or long-chain polymer molecules attached by one end to a surface or the interface in contact with solvents are often called polymeric brushes . A number of studies on the structure of such polymer chains have been... 

Figure 4.2 Water contact angle as a function of chemical functionality on brush-covered surfaces. Brushes were grafted from ETFE via free radical polymerization. Brushes marked with an asterisk were obtained via postfunctionalization. Source. Adapted from Neuhaus et al. [2], with permission from John Wiley Sons Inc.

Tsarevsky has found that hypervalent iodine compounds can be used for the direct azidation of polystyrene and consecutive click-type functionalization [49]. In particular, polystyrene can be directly azidated in 1,2-dichloroethane or chlorobenzene using a combination of trimethylsilyl azide and (diacetoxyiodo)benzene. 2D NMR HMBC spectra indicate that the azido groups are attached to the polymer backbone and also possibly to the aryl pendant groups. Approximately one in every 11 styrene units can be modified by using a ratio of PhI(OAc)2 to trimethylsilyl azide to styrene units of 1 2.1 1 at 0 °C for 4 h followed by heating to 50 °C for 2 h in chlorobenzene. The azidated polymers have been further used as backbone precursors in the synthesis of polymeric brushes with hydrophilic side chains via a copper-catalyzed click reaction with poly(ethylene oxide) monomethyl ether 4-pentynoate [49],... 

Uses Surfactant, pigment wetting agent, dispersant, emulsifier, stabilizer for coatings emulsifier for emulsion polymerization, brush cleaners Features Solv.-free more hydrophilic than Nuosperse FN 1566 Properties SI. hazy Iiq. sol. 5% in water, propanol, xylene acid no. 0.1-0.15 cloud pt. 76-78 C (1% aq.) pH 5-7 (5% aq.) nonionic HLB 17.0 70% act. 

Uses Surfactant in wallpaper removers wetting agent for pigments in aq. systems, pigment pastes, gloss emulsion paints dispersant emulsifier for emulsion polymerization, brush cleaners stabilizer for emulsion paints Properties Clear Iiq. sol. 5% in water, 2-propanol, xylene HLB = 13.5 acid no. 0.2 max. pH 5-7 (2% aq.) nonionic 99-100% act. 

Dickman, R. and Hong, D. C. (1991) New simulation method for grafted polymeric brushes, J. Chem. Phys. 95,4650-55. 

Murat, M. and Grest, G. S. (1989) Interaction between grafted polymeric brushes A molecular dynamics study, Phys, Rev. Lett, 63, 1074-77. 

Murat, M., and Grest, G.S. 1993. Structure of grafted polymeric brushes in solvents of varying quality a molecular dynamics study. Macromolecules 26 3108-3117. 

Dense grafting of side chains onto linear backbones, and homopolymerization of macromonomers, are both used to synthesize macromolecular brushes. Steric repulsion of the side chains forces the main chain into an extended wormlike conformation, resulting in liquid-crystalline phases, and lower dynamic shear moduli than linear flexible coils in concentrated solutions [93, 94]. Densely grafted polymeric brushes on sliding surfaces have been found to reduce friction, and therefore have potential for providing biolubrication for artificial implants [95]. 

Platelet-like aggregates of polyethylene-polyethylenebutylene (PE-PEP) copolymers formed upon cooling a decane solution exhibit a polymeric brush of PEP-hairs on their surface. The fluctuation dynamics of this brush determined by a balance of solvent friction and entropic restoring forces has been observed by NSE spectroscopy. Besides, the direct dynamical information the NSE data allow for the separation of static scattering due to the average structure from the scattering contributions due to mobile fluctuating parts of the sample. 

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