ABSTRACT
FIG. 7 Evolution of the ratio of the dry thickness of the layer after immersion in a 0.5% TMSCl solution in octane to the initial thickness, h00 , as a function of the time of immersion, t, for layers formed at two different temperatures, respectively T = llOoC (filled symbols) and T = 20oc (open symbols), and different ratio r between the initial thickness of the layer, h00 , and the maximum initial thickness obtainable with that particular molecular weight, hmax·
now how the layer reacts when immersed in a good solvent. Two characteristics of the layer will be discussed: the shape of the concentration profile close to the surface which should be sensitive to the way the polymer molecules are attached to the surface (this concentration profile is expected to be very different for chains which are only attached to the surface by one extremity or for adsorbed chains [14]) and the swollen thickness of these layers. To measure any of these characteristics, concentration profile or swollen thickness, one needs a technique which gives a contrast between the polymer and the solvent. Neutron reflectivity can be used, taking advantage of the contrast for neutrons between hydrogen and deuterium. Neutron reflectivity allows the whole concentration profile inside the swollen layer to be characterized. Such experiments have already been reported [14,15]. The reflectivity data were adjusted with the calculated reflectivity curve of a layer made of 10 or 20 thin slabs, each having a constant concentration, the adjustable parameters being the concentrations in the different slabs. The adjustment procedure is efficient to determine the concentration profile, provided one fixes the surface excess, which for our layers is known independently. Such a procedure allows to extract the concentration profile from the reflectivity data without assuming a priori the shape of this concentration profile. A typical result is shown in Fig. 8. The full line gives the experimental concentration profile, while the dotted line is the predicted Guiselin's profile [13] for a layer having the same surface excess. The
FIG. 8 Example of a concentration profile of an adsorbed layer swollen in octane, for chains with a molecular weight 92 kg/mol and a surface density of adsorbed chains :E = 0.02. The full line is the profile determined by neutron reflectivity. The dashed line corresponds to the profile predicted by the Guiselin's description of "pseudobrushes," imposing to the pseudobrush the same surface excess as the layer. Guiselin's description has been corrected to take into account the finite length of the polymer chains [13].
