Mechanically Induced Silica-Siloxane Mixtures. Structure of the Adsorbed Layer and Properties of the Network Structure
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MECHANICALLY INDUCED SILICA-SILOXANE MIXTURES. STRUCTURE OF THE ADSORBED LAYER AND PROPERTIES OF THE NETWORK STRUCTURE J.P. COHEN-ADDAD Laboratoire de Spectrometrie Physique associ6 au CNRS, Universit6 Joseph Fourier, Grenoble I, B.P. 87 - 38402 St Martin d'H~res Cedex, France ABSTRACT Properties specific to silica-siloxane mixtures are analysed and discussed. The effect of polymer adsorption is interpreted from the Gaussian statistics of chains : the amount of adsorbed polymer Qr is proportional to the square root of the chain molecular weight. The kinetics of adsorption is described as a process of surface saturation. It is discussed as a function of the silica concentration. The effect of swelling of the mixtures is interpreted within the statistical framework proposed by Flory for ordinary gels. I.
INTRODUCTION
This description deals with typical properties observed from siloxane polymers filled with silica particles. The addition of the particulate filler to poly(dimethylsiloxane) (PDMS) chains was achieved from a mechanical mixing . The contact of the polymer melt with the surface of silica gives rise to a process of adsorption through the formation of hydrogen-bonds. Each hydrogen-bond is supposed to link one silanol group of the surface to one oxygen atom of the skeleton of a PDMS chain. Several hydrogen-bonds can be formed between the silica surface and one end-methylated chain. This undergoes an uniform adsorption. An end-adsorption occurs when siloxane chains are hydroxyl terminated, in this case a double hydrogen bond is established between one chain-end and one silanol group of the surface. Investigations concerning properties of silica-siloxane systems are developed according to three main topics. i) The first one deals with the interface formed by monomeric units bound to the silica surface. This interface must be characterized from the number r of contact points of one chain with the surface. It is also characterized from the distribution of positions of these contact points upon the surface of particles. The number r contributes to the determination of the free enthalpy of adsorption of one chain. ii) The second topic concerns the interphase formed by loops and tails. The interphase determines the transition between the solid state of silica particles and the liquid state of the polymer defined from siloxane chains which are more freely because they are not adsorbed. The interphase must be characterized from the distribution of lengths of loops. Coated particles which are not bridged to one another govern the flow process of mixtures characterized by low silica concentrations. iii) An infinite cluster is formed when the concentration of particles is high enough and the polymer chains have an appropriate length. This infinite cluster results from a percolation process. It behaves like a permanent gel when all free chains have been removed from the initial mixture. It must be characterized from an effect of swelling induced by a good solvent or from an effect of uniaxial stretching. II.
THE INTERF
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