Rayleigh-Taylor instability with polymerization

Figure 4.11 Rayleigh-Taylor instability with a descending front of butyl acrylate polymerization.

Figure 3. Rayleigh-Taylor instability with descending front of butyl acrylate polymerization. Although the polymer product is hot (> 200 °C) it still is about 20% more dense than the monomer below it.

Garbey et al. also predicted that for a descending liquid/liquid front, an instability could arise even though the configuration would be stable for unreactive fluids (29-31). This prediction has yet to be experimentally verified because liquid/liquid frontal polymerization exhibits the Rayleigh-Taylor instability. A thermal frontal system with a product that is less dense than the reactant is required. 

Frontal polymerization can be achieved with a variety of monomers and has been studied with thermosets and thermoplastics. Examples include n-butyl acrylate, benzyl acrylate, styrene, dodecyl acrylate and hexyl acrylate. If the front is ascending, the monomer inmiediately above the front is lower in density because of the temperature gradient than the bulk monomer and so simple convection can occur for a thermoset (/P) or a thermoplastic. 20) A descending front with a thermoset is stable but a thermoplastic is unstable because even though the polymer is very hot, it is more dense than the unreacted monomer. This leads to the Rayleigh-Taylor instability. 16,21)... 

Pojman et al. overcame the Rayleigh-Taylor instability by dispersing benzyl acrylate in a salt water solution whose density was greater than that of the polymer, an approach that had been considered theoretically. Fronts reached 200 °C, the same temperatme as benzyl acrylate fronts with a diacrylate to prevent fingering. This indicated that the dispersion broke relatively quickly, leaving the monomer to polymerize in bulk and the salt water to settle to the lower section of the tube. The polymer was insoluble in tetrahydrofuran (THF) and in DMSO, and so they concluded that the acrylic add formed then formed anhydride crosslinks. 

The most pernicious convective instability occurs with monomers that produce a molten polymer at the front, such as n-butyl acrylate, styrene and methyl methacrylate. A Rayleigh-Taylor instability (2, 25 ), which also appears as fingers as the more dense molten polymer streams down from the reaction zone and destroys the front (Figure 16). The only currently available methods to study frontal polymerization with thermoplastics are to add a crosslinking monomer to produce a thermoset or to increase the viscosity with a viscosifier such as ultrafine silica gel (CAB-O-SBL). To prepare pure poly(n-butyl acrylate) frontally, Pojman et al resorted to performing the reaction under weightless conditions of a sounding rocket (26 ). 

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