Mushroom chain conformation

An advancement in thin film polymers has appeared in the last 20 years. By chemically bonding one end of many polymer chains to a substrate, a monolayer of polymer chains can be manufactured. By tethering one end of a polymer chain to the surface, its motion is restricted. Several chain conformations have been proposed such as the mushrooms and clusters as seen in Figure 2.7-1. 

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

Fig. 18 Schematic of the conformational changes from mushroom to brush state that occur in graft-polymerized PDMAm during the chain propagation process...

Fig. 8. Schematic representation of the possible conformations of adsorbed (co)polymers prepared using macromonomer technique a brush adsorption of graft copolymer b terminally-attached adsorption c the mushroom-brush transition for strongly overlapping chains proposed by de Gennes [130] and Alexander [155]...

At low values of the surface coverage a, each anchored N-mer chain is essentially independent of its neighbors. It forms a mushroom (see Fig. 34, and region 7 in Fig. 35), the size of which is characterized by Rq. When mushrooms start to overlap, a continuous brush layer is created. This happens for the inter-anchor spacing X1/2=R0 and the overlap density C =N To consider the brush conformation in this regime, we re-write Eq. (57) in the limit of small volume fraction d> of N-mer chains [232] ... 

Figure 3 End-grafted polymer chains in a good solvent. If d > 2Rg, the chains assume a conformation similar to that of free chains in solution, for which Rg is the radius of gyration. This is known as the mushroom conformation. If d < 2Rg, the chains stretch out into the solvent to...

In other words, the free-energy cost incurred due to interaction between the chains exceeds the contribution due to the entropy elasticity of the chain (Figure 3). In the case of the widely spaced, (or mushroom ) conformation, the height of the polymer layer above the substrate is independent of the grafting density of the chains (number of attached chains per unit area). In the brush conformation, however, the height scales as the cube root of the grafting density (Figure 4), a, which itself is equivalent to 1/d. ... 

At a very low solution concentration, the surface concentration is also very low. Under these conditions the chains have a tendency to flatten out, or pancake] see Figure 12.21. In this concentration range much of the surface may be vacant. As the concentration increases, the competition for surface space increases. The chains may form a conformation known as mushrooms, where the chains are either bound at one end, or by a few mers at one end. At still higher solution concentrations, a brush structure appears. Also in this case each chain attached to the surface may only be bound at one end or by a few mers, the remainder of the chain sticking nearly straight up into the solution to avoid contact with neighboring chains. In these latter cases, of course, most of the chains remain entirely in solution, there being no space at the surface. 

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