Thermoreversible gelation of aromatic hydrocarbons
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Fumed silica of nominal size 20 microns is used to fill many different polymer and prepolymer mixtures creating filled composite materials. Of interest in this work is silica combined with low-viscosity non-polar hydrocarbon liquids (aryl-vinyl monomers). Unless they are continuously mixed, unmodified binary mixtures that have flowable viscosity (<75 wt% solids) phase separate over time scales of several days to pure liquid and a close-packed cake of silica. It is desired that this phase separation not occur for at least several months with minimal required physical or chemical modification. This work investigates the thermodynamic and rheological (mainly elastic) properties of binary solutions of ethylbenzene, styrene, toluene, xylene, and tertbutyl styrene each with low concentrations of various molecular weight syndiotactic polystyrenes (SPS). These systems exhibit a distinct thermally reversible phase transformation when cooled, going from a low-viscosity polymer solution to a translucent waxy solid of varying physical properties. Various methods have been used to characterize both the polymers used in generating this behavior, as well as the resultant gels. Analyses include differential scanning calorimetry, nuclear magnetic resonance spectroscopy, gel permeation chromatography, and dynamic viscoelastic rheometry. For a given binary system gel formation temperatures and physical properties were found to be primarily a function of polymer loading and molecular weight. The molecular weight of polymer studied was between 4,300 and 42,000 Mn; concentrations used typically at or below 5 wt% polymer; observed modulii varied greatly and somewhat erratically but were in the 2 kPa to 20 kPa range. Nearly all mechanical property measurements were done in the elastic regime (0.1-0.2% strain; yield point 2-4%). Elastic networks were observed at extraordinarily low concentrations.