Density grafting

The synthesis of end functional polymers by NMP, ATRP and RAFT has already been discussed in Section 9.7. The "grafting to approach involves the covalent attachment of an end-funetionalized polymer with reactive surface groups on the substrate. The approach is inherently limited by the crowding of chains at the surface and the limit this places on the final graft density. 

Fig. 4. Data of Auroy et al. [25] on polydimethyl-siloxane chains covalently grafted in the interior of a silica porous medium. L is the measured layer thickness from neutron scattering d is the average spacing per chain from the grafting density. The format of the plot is suggested by Eq. 5. The linearity of that relation is seen in these data...

In recent years, much attention has been paid to the use of controlled/ living polymerizations from flat and spherical surfaces [121,122],because this allows better control over the MW and MWD of the target polymer. By using these techniques, a high grafting density and a controlled film thickness can be obtained, as such brushes consist of end-grafted, strictly linear chains of the same length and the chains are forced to stretch away from the flat surface. Several research... 

Figure 4.7 Fluorescence anisotropy decay curves for the PMMA brush swollen in benzene (filled circles) and the free PMMA chain in benzene solution at concentrations of 0.33 (triangles) and 2.9 X 10 g (open circles). The graft density of the brush is 0.46 chains nm . The solid curve indicates the instrument response function. Reproduced with permission from the American Chemical Society.

Figure 4.8 shows the fluorescence anisotropy decay curves for PMMA brushes with various graft densities swollen in benzene and acetonitrile. Benzene and acetonitrile are good and 0 solvents for PMMA. As clearly shown in this figure. 

The chain dimension in the height direction was evaluated as the thickness of the brush layer, I, relative to the chain contour length, io, by atomic force microscopy (AFM). Figure 4.10 shows the solvent dependence of the conformation of the PMMA brush. Whereas the brush chain changes its conformation in response to the solvent quality at the low graft density, the high-density PMMA brush does not show... 

Figure4.11 Schematicdrawingofthe PMMA brush chain swollen in solvents, (a) brush with low graft density, (b) brush with high graft density. The left- and right-hand sides ofeach image illustrate the brush chains in poor and good solvents, respectively.

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. 

Fig. 7. Conformational states of polymer chains grafted at a surface. Influence of grafting density mushroom left) and brush right). 

Fig. 8. Adsorption of BSA on surfaces coated with polyethylene oxide (PEO). Influence of the grafting density of PEO at the surface for different polymer chain lengths (---700,-------- 445, and------148 ethylene oxide monomers) on the...

In contrast, the hydrodynamic radius distribution recorded for aqueous solutions of copolymers with a low grafting density is bimodal, with a contribution from small entities of = 5-30 nm, assigned to single polymer chains, and a contribution from larger particles of = 80 = 150 nm (Fig. 24b, c). The relative importance of the two populations depends on copolymer concentration the relative amount of the larger particles increases with increasing copolymer concentration. 

Turgman-Cohen, S. and J. Genzer, Computer simulation of controlled radical polymerization Effect of chain confinement due to initiator grafting density and solvent quality in "grafting from Method. Macromolecules, 2010. 43(22) p. 9567-9577. 

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