Polyme brushes

Klein and co-workers have documented the remarkable lubricating attributes of polymer brushes tethered to surfaces by one end only [56], Studying zwitterionic polystyrene-X attached to mica by the zwitterion end group in a surface forces apparatus, they found /i < 0.001 for loads of 100 and speeds of 15-450 nm/sec. They attributed the low friction to strong repulsions existing between such polymer layers. At higher compression, stick-slip motion was observed. In a related study, they compared the friction between polymer brushes in toluene (ji < 0.005) to that of mica in pure toluene /t = 0.7 [57]. 

Kelly T W ef a/1998 Direct force measurements at polymer brush surfaces by atomic force microscopy Macromoiecuies 31 4297-300... 

Dhinojwala A and Granick S 1997 Surface forces In the tapping mode solvent permeability and hydrodynamic thickness of adsorbed polymer brushes Macromoiecuies 30 1079-85... 

Klein J ef a/1994 Reduction of frictional forces between solid surfaces bearing polymer brushes Nature 370 634-7... 

Grest G S 1996 Interfaoial sliding of polymer brushes a moleoular dynamios simulation Rhys. Rev. Lett. 76 4979-82... 

In cases when the two surfaces are non-equivalent (e.g., an attractive substrate on one side, an air on the other side), similar to the problem of a semi-infinite system in contact with a wall, wetting can also occur (the term dewetting appHes if the homogeneous film breaks up upon cooHng into droplets). We consider adsorption of chains only in the case where all monomers experience the same interaction energy with the surface. An important alternative case occurs for chains that are end-grafted at the walls polymer brushes which may also undergo collapse transition when the solvent quality deteriorates. Simulation of polymer brushes has been reviewed recently [9,29] and will not be considered here. 

K. Binder, P. Y. Lai, J. Wittmer. Monte Carlo simulations of chain dynamics in polymer brushes. Faraday Discuss Chem Sci 95 97-109, 1994. 

FIG. 4 Sterically stabilized colloidal particles are coated with short polymer brushes. A hard sphere-like interaction arises. 

Graft copolymers made by living polymerization processes are often called polymer brushes because of the uniformity in graft length that is possible. The basic approaches to graft copolymers also have some analogies with those used in making block and star copolymers. 

There have been several studies on the use of RAFT to form polymer brushes by polymerization or copolymerization of macromonomers 348-350. 

The preparation of polymer brushes by controlled radical polymerization from appropriately functionalized polymer chains, surfaces or particles by a grafting from approach has recently attracted a lot of attention.742 743 The advantages of growing a polymer brush directly on a surface include well-defined grafts, when the polymerization kinetics exhibit living character, and stability due to covalent attachment of the polymer chains to the surface. Most work has used ATRP or NMP, though papers on the use of RAFT polymerization in this context also have begun to appear. 

Highly branched polymers, polymer adsorption and the mesophases of block copolymers may seem weakly connected subjects. However, in this review we bring out some important common features related to the tethering experienced by the polymer chains in all of these structures. Tethered polymer chains, in our parlance, are chains attached to a point, a line, a surface or an interface by their ends. In this view, one may think of the arms of a star polymer as chains tethered to a point [1], or of polymerized macromonomers as chains tethered to a line [2-4]. Adsorption or grafting of end-functionalized polymers to a surface exemplifies a tethered surface layer [5] (a polymer brush ), whereas block copolymers straddling phase boundaries give rise to chains tethered to an interface [6],... 

The importance of polydispersity is an interesting clue that it may be possible to tailor the weak interactions between polymer brushes by controlled polydispersity, that is, designed mixtures of molecular weight. A mixture of two chain lengths in a flat tethered layer can be analyzed via the Alexander model since the extra chain length in the longer chains, like free chains, will not penetrate the denser, shorter brush. This is one aspect of the vertical segregation phenomenon discussed in the next section. 

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. 

Figure 4.6 shows an apparatus for the fluorescence depolarization measurement. The linearly polarized excitation pulse from a mode-locked Ti-Sapphire laser illuminated a polymer brush sample through a microscope objective. The fluorescence from a specimen was collected by the same objective and input to a polarizing beam splitter to detect 7 and I by photomultipliers (PMTs). The photon signal from the PMT was fed to a time-correlated single photon counting electronics to obtain the time profiles of 7 and I simultaneously. The experimental data of the fluorescence anisotropy was fitted to a double exponential function. 

Senaratne, W., Andruzzi, L. and Ober, C. K. (2005) Self-assembled monolayers and polymer brushes in biotechnology Current applications and future perspectives. Biomacromolecules, 6, 2427-2448. 

Pyun, J., Kowalewski, T. and Matyjaszewski, K. (2003) Synthesis of polymer brushes using atom transfer radical polymerization. Macromol. Rapid Commun., 24, 1043-1059. 

Tsujii, Y, Ohno, K., Yamamoto, S., Goto, A. and Fukuda, T. (2006) Structure and properties of high-density polymer brushes prepared by surface-initiated living radical polymerization. Adv. Polym. Sci., 197, 1-45. 

Lai, P. Y. and Binder, K. (1992) Structure and dynamics of polymer brushes near the theta point - a Monte-Carlo simulation. 

Semenov, A. N. (1995) Rheology of polymer brushes - Rouse model. Langmuir, 11, 3560-3564. 

He, G. L., Merlitz, H., Sommer, J. U. and Wu, C. X. (2007) Static and dynamic properties of polymer brushes at moderate and high grafting densities A molecular dynamics study. Macromolecules, 40, 6721-6730. 

Dimitrov, D. 1., Milchev, A. and Binder, K (2007) Polymer brushes in solvents of variable quality Molecular dynamics simulations using explicit solvent./. Chem. Phys., 127, 084905. 

Yakubov, G. E., Loppinet, B., Zhang, H., Ruhe, J., Sigel, R. and Fytas, G. (2004) Collective dynamics of an end-grafted polymer brush in solvents of varying quality. Phys. Rev. Lett., 92, 115501. 

Giannelis, E.P., Krishnamoorti, R., Manias, E, Polymer-Silicate Nanocomposites Model Systems for Confined Polymers and Polymer Brushes. VoL 138, pp. 107448. 

Grest, G.S. Normal and Shear Forces Between Polymer Brushes. Vol. 138, pp. 149-184 Grigorescu, G, Kulicke, W.-M.t Prediction of Viscoelastic Properties and Shear Stability of Polymers in Solution. Vol. 152, p, 1-40. 

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