Pseudo-brush

There are several ways of forming surface layers of polymer chains, and various solid/polymer systems have been used. The silica/PDMS system is quite convenient since both end-grafted layers with high grafting densities (i.e., brushes) and irreversibly adsorbed layers (i.e., pseudo-brushes) can be formed with controlled molecular characteristics (polymerization index of the tethered chains and surface density), allowing a detailed investigation of the structure and properties of these two different classes of surface anchored polymer layers. 

The internal structure of a Guiselin s pseudo-brush is schematically presented in Fig. 4. Similar arguments in the case of an irreversibly adsorbed layer pre-... 

Fig.4.a Schematic of a pseudo-brush. L is the layer thickness, b The monomer density profile vs distance from the grafting plane z according to Guiselin. Figure adapted from [66]... 

In order to understand the role played by surface-anchored chains in adhesion and friction, it is essential to understand under which conditions a surface layer, when in contact with a melt, is penetrated by free chains. The question has been addressed theoretically mostly for polymer brushes, and more recently for Guiselin s pseudo-brushes. We want to review here some of these analysis, and compare the predictions of the models with the available experimental data. 

In order to get an estimate of the adhesion energy between a pseudo-brush and an elastomer (cross-linked in the dry state), we can use the concentration profile calculated [65] for the case of a pseudo-brush exposed to a melt (Nc monomers per chain)... 

Fig. 15. Normalized enhanced adhesive strength (G0- W)/Wvs grafting density o for a pseudo-brush (with N=770,NC=176 and Nc= 176) according to Eq. (16) The optimum is indicated by a black dot...

These characteristics indicate in particular that one can improve the adhesion between the brush or the pseudo-brush and the elastomer by decreasing the number of monomers between crosslinks, Nc. This result, which is a direct consequence of the de Gennes analogy between a network and a melt (see Sect. 4.1), cannot be correct for too small values of Nc, that is for too highly reticulated networks. For pseudo-brushes, it has been conjectured [107] that Eqs. (20) and (21)... 

Figure 8. Results obtained for a polymer melt of molecular weight 9.6 10 flowing on a silica surface covered with a pseudo brush made from a melt of molecular weight 1.93 10. a) Vj as a function of Vt b) slip length b as a function of Vj. The friction in the low shear regime is comparable to that obtained on a low density surface layer, but the critical velocity V is much...

In certain cases a higher degree of irreversibility is observed. This is the case for the so-called pseudo-brushes where the monomer adsorption on the surface is supposed to be irreversible and where the loop size distribution remains fixed under swelling. A pseudobrush is obtained by placing the adsorbing surface in contact with a molten polymer (or a concentrated semidilute solution). All the chains within one Gaussian radius of gyration R N I a adsorb on the surface and the adsorbance is The adsorption is... 

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