Surface anchored chains

Fig.1. Dry thickness h0 of various PDMS layers irreversibly adsorbed on the silica surface of a silicon wafer, as a function of the scaling variable Nm 0m. N is the polymerisation index of the surface anchored chains and(])0 is the polymer volume fraction in the incubation bath. The triangles correspond to 4>0=1, i.e., to the maximum surface density of anchored chains for the particular molecular weight. The range of molecular weights used is 29.6 kg mol-1

In order to investigate the internal organization of the surface-anchored chains, we now consider the concentration profile of the layers when swollen by a good solvent. Such concentration profiles have been investigated in details theoretically, since the Alexander-de Gennes model, and we can hope to be able to dis-... 

Interdigitation Between Surface-Anchored Chains and a Polymer Melt... 

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. 

Fig. 11. Normalised enhanced adhesive strength wyw as a function of the surface density, a, for two PDMS elastomers in contact with silicon wafers covered with irreversibly adsorbed chains. Wis the thermodynamic work of adhesion, W=2y, with ythe surface tension of PDMS, 7=21.6 mN m"1 at 25 °C. The filled symbols correspond to a molecular weight between crosslinks in the elastomer Mc=24.2 kg mol-1 while Mc=10.2 kg mol-1 for the open symbols. The adhesive strength, G, has been measured by peel tests performed at a very low velocity of the propagation of fracture, 0.17 im/s. The molecular weight of the surface anchored chains is Mw=242 kg mol-1...

These surface-anchored layers can be used as adhesion promoters to enhance elastomer-solid adhesive strength. We have shown that there is then an optimum surface density of surface anchored chains to do so. At high coverage of the surface, the layers lose their efficiency, a tendency which can be rationalized in terms of interdigitation between the elastomer and the surface chains. A full characterization of the different regimes of adhesion enhancement associated with the different regimes of interdigitation between the surface chains and the elastomer is a difficult experimental task, not fully accomplished to date. 

An important question is now to understand how these two effects, adhesion enhancement and adjusted friction, can interplay. For example, when peel tests are used to estimate the adhesive strength, the curvature of the peeled ribbon implies a fracture advancing through both modes I and II of opening [72,155]. This means that in the presence of surface anchored chains, the connector... 

Results for surfaces with a low density of surface anchored chains ... 

We have investigated the way the typical features of Fig. 4 to 6 were dependent on the molecular characteristics of the system, i.e. the molecular weight of the polymer melt, Mw(P), the molecular weight of the surface anchored chains, Mw(N), and the surface density of surface attached chains, a parameter which is only qualitatively under control through the contact angle of dodecane with the surface. 

These ideas can be used to model the flow behaviour in the presence of surface anchored chains at low surface densities ( i.e. for N chains in the "mushroom" regime of grafting... 

The situation of a dense layer of surface anchored chains thus appears far less understood than the weak surface density case. It is, however, far more important practically and work is in progress to model it in details. 

---