Siloxane group chains, adsorption

According to this model, the temperature dependence of molecular motions for adsorbed and non-adsorbed chain units in filled PDMS containing hydrophilic Aerosil is shown in Fig. 9 [9]. The lowest temperature motion is a C3 rotation of the CH3 groups around the Si-C bond (line 1 in Fig. 9). The rate of the a-relaxation (points 2 in Fig. 9) in filled PDMS is close to that for unfilled sample (line 2 in Fig. 9). It has been proposed that independence of the mean average frequency of a-relaxation process on the filler content in filled PDMS is due to defects in the chain packing in the proximity of primarily filler particles [7]. Furthermore, the chain adsorption does not restrict significantly the local chain motion, which is due to high flexibility of the siloxane main chain as well as due to fast adsorption-desorption processes at temperatures well above Tg. 

Two types of Aerosil with different surface activity have been studied hydrophilic and hydrophobic Aerosil. Hydrophilic Aerosil contains on its siirface hydroxyl groups which are the sites of adsorption. The surface groups of hydrophilic Aerosil are able to interact with the PDMS chain both through permanent dipoles in the partially ionic siloxane bond via permanent dipole-dipole interactions and through even weaker van der Waals forces. In contrast to hydrophilic Aerosil, non-polar trimethylsilyl groups on the surface of hydrophobic Aerosil effectively decrease the dipole-dipole interaction and mainly weak van der Waals forces are formed between methyl groups of PDMS and trimethylsilyl surface groups of Aerosil. 

This work deals with the statistical description of poly(dimethylsiloxane) chains adsorbed upon the surface of silica particles within silica-siloxane mixtures. In these systems, the incorporation of fumed silica into the PDMS polymer melt is obtained by mechanical mixing. Subsequently, the adsorption which occurs through the formation of hydrogen bonds between the oxygen atoms on the polymer chains and the silanol groups located on the silica s surface is a solvent free or melt adsorption process. Consequently, the law of adsorption observed from these systems is specific to such a process. 

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