Polymer brush

In materials science, a polymer brush is the name given to a surface coating consisting of tethered to a surface. The brush may be either in a Solvation state, where the tethered polymer layer consists of polymer and solvent, or in a melt state, where the tethered chains completely fill up the space available. These polymer layers can be tethered to flat substrates such as silicon wafers, or highly curved substrates such as . Also, polymers can be tethered in high density to another single polymer chain, although this arrangement is normally named a bottle brush. Additionally, there is a separate class of polyelectrolyte brushes, when the polymer chains themselves carry an Electric charge.

The brushes are often characterized by the high density of grafted chains. The limited space then leads to a strong extension of the chains. Brushes can be used to stabilize , reduce friction between surfaces, and to provide lubrication in artificial .

Polymer brushes have been modeled with molecular dynamics, Monte Carlo methods, Brownian dynamics simulations, and molecular theories.

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Structure Polymer molecules within a brush are stretched away from the attachment surface as a result of the fact that they repel each other (steric repulsion or osmotic pressure). More precisely, they are more elongated near the attachment point and unstretched at the free end, as depicted on the drawing.

More precisely, within the approximation derived by Milner, Witten, Cates, the average density of all monomers in a given chain is always the same up to a prefactor:

$\backslash phi(z,\backslash rho)=\backslash frac\{\backslash partial\; n\}\{\backslash partial\; z\}$

$n(z,\backslash rho)=\backslash frac\{2N\}\{\backslash pi\}\backslash arcsin\backslash left(\backslash frac\{z\}\{\backslash rho\}\backslash right)$

where $\backslash rho$ is the altitude of the end monomer and $N$ the number of monomers per chain.

The averaged density profile $\backslash epsilon(\backslash rho)$ of the end monomers of all attached chains, convoluted with the above density profile for one chain, determines the density profile of the brush as a whole:

$\backslash phi(z)=\backslash int\_z^\backslash infty\; \backslash frac\{\backslash partial\; n(z,\backslash rho)\}\{\backslash partial\; z\}\backslash ,\backslash epsilon(\backslash rho)\backslash ,\{\backslash rm\; d\}\backslash rho$

A dry brush has a uniform monomer density up to some altitude $H$. One can show that the corresponding end monomer density profile is given by:

$\backslash epsilon\_\{\backslash rm\; dry\}(\backslash rho,H)=\backslash frac\{\backslash rho/H\}\{Na\backslash sqrt\{1-\backslash rho^2/H^2\}\}$

where $a$ is the monomer size.

The above monomer density profile $n(z,\backslash rho)$ for one single chain minimizes the total elastic energy of the brush,

$U=\backslash int\_0^\backslash infty\backslash epsilon(\backslash rho)\backslash ,\{\backslash rm\; d\}\backslash rho\backslash ,\backslash int\_0^N\backslash ,\{\backslash rm\; d\}n\backslash ,\backslash frac\{kT\}\{2Na^2\}\backslash left(\backslash frac\{\backslash partial\; z(n,\backslash rho)\}\{\backslash partial\; n\}\backslash right)^2$

regardless of the end monomer density profile $\backslash epsilon(\backslash rho)$, as shown in.

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From a dry brush to any brush As a consequence, the structure of any brush can be derived from the brush density profile $\backslash phi(z)$. Indeed, the free end distribution is simply a convolution of the density profile with the free end distribution of a dry brush:

$\backslash epsilon(\backslash rho)=\backslash int\_\backslash rho^\backslash infty\; -\backslash frac$

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