This work was undertaken to study the effect of silica filler structure on the effective crosslink density and on mechanical properties. Vulcanizates of silicone, SBR, and EPM were included in the work. A compression modulus method was used for crosslink density determination. Samples were swollen prior to measurement in two environments : (a) toluene, (b) toluene in an NH8 atmosphere (designed to cleave polymer-filler linkages if they were present). The results showed that, in peroxide-cured silicone and EPM elastomers, the crosslink density increased with filler structure, but after cleavage of polymer-filler bonds by the NH3-modified swelling environment, there was very little change of crosslink density with filler structure increase. The effect of increasing filler structure on crosslink density in the SBR elastomers was very slight, showing a small increase in progressing from discrete particles to low structure and thereafter no change. The quanitative results of this study are important in that they provide excellent support for Medalia's theory of the role of occluded volume in reinforcement. The results show that polymer-filler bonds are required, in combination with occluded volume, in order to achieve a substantial increase in both 200% modulus and crosslink density.
Trimethylsilyl derivatives of silicate minerals-olivine, hemimorphitc, sodalite, natrolite, and lauiiiontitc-liavc bceii prepared in which the silicate structure of the parent mineral is retained. The same technique was used to study the distribution of silicate structures in aqueous solutions of sodium silicates.
Treated silica xerogels with controlled porosity and surface area were prepared by the in
situ treatment of hydrogels with hexamethyldisiloxane or alkylchlorosilanes in the presence
of isopropyl alcohol. The resulting hydrogels were hydrophobic and readily transferred to
nonpolar organic solvents upon contact. The surface area and porosity of the xerogel were
controlled by varying the pH, time, and temperature used to polymerize the hydrogel prior
to treatment. In general, the surface area decreased with increases in aging time or
temperature, whereas the total pore volume and pore size increased. Silylation of the hydrogel
in the aqueous phase retains the structure of the hydrogel and permits isolation with
minimum structure collapse. By using this technique, silylated xerogels with surface areas
ranging from 200 to >700 m2/g and pore volumes of 1.5 to 3.8 cm3/g were obtained.
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