Sol-Gel Diagram of Silica-Siloxane Mixtures. Percolation Approach
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SOL-GEL DIAGRAM OF SILICA-SILOXANE MIXTURES.
PERCOLATION APPROACH
J.P. COHEN-ADDAD Laboratoire de Spectromdtrie Physique associd au CNRS, Universitd Joseph Fourier Grenoble I, B.P. 87, 38402 St Martin d'H~res Cedex, France
ABSTRACT The property of elasticity and the effect of swelling of mechanically induced silica-siloxane mixtures are interpreted within a percolation framework. This approach is illustrated by varying the specific amount of adsorbed polymer upon the silica surface of aggregates. INTRODUCTION Molten poly(dimethylsiloxane) is known to exhibit a low zero shear 3 rate viscosity n0o , 5 at room temperature (n 0 = 10 Pa.s. for a molecular weight equal to 10 g/mole). Viscoelastic properties are considerably improved by mixing the polymer with silica aggregates. The strength of the elasticity component of the response of such mixtures to a time dependent external constraint is increased both by the grappling of polymer chains with silica and by the effect of chain confinement induced by the presence of aggregates. The contact of poly(dimethylsiloxane) (PDMS) chains with the surface of silica gives rise to a process of adsorption through the formation hydrogen-bonds. Each hydrogen-bond is supposed to link one oxygen atom of the skeleton of a PDMS chain to one silanol groups of the silica surface. Several hydrogen-bonds can be formed between one end-methylated chain and the surface of one aggregate. This effect of uniform adsorption can induce a process of connection between silica aggregates to form clusters or gels. Silica-siloxane mixtures are characterized by two main features which are now well established. i) The first one concerns the kinetics of chain adsorption upon the particulate filler. This process is described as a progressive saturation of the silica surface ; it lasts over three years at room temperature while 0 the saturation is reached after three months at 70 C. The amount of polymer Q (t) left bound to silica at a time t, after eliminating all free, obeys the equation : Qr - Qr(t)
=
-
(Q)
Q is the limiting value of Q while Q (o) is the amount of polymer bound r r riigIl h to silica, right after the mechanical mixing [1]. The quantities Qr(o), Qr(t) and QZ are determined per gram of silica. ii) The other feature concerns the law of chain adsorption observed when the surface saturation is completed. It is clearly observed that the limiting value Qr. of the amount of adsorbed polymer is a linear function of the square root of the chain molecular weight Mn, Qr Xa n (2) The coefficient3 of linearity X has been determined experimentally ; it is equal to 4xlO(g/mole)-y when the specific area AT of silica is equal to 1 150 m2 g- [21. This law of adsorption has been given an interpretation [31; 1 it applies also when the specific area of silica is equal to 50 m2 g- . NMR properties of the fraction of monomeric units which are hightly bound to the surface can be discriminated from those of monomeric units which can Mat. Res. Soc. Symp. Proc. Vol. 248. 01992 Materials Research Society
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