Frontal polymerization

 

Advances in polymer chemistry have led to the development of monomers and initiation agents providing propagating polymerization fronts driven by the exothermicity of the polymerization reaction and the transport of heat from the polymerized product to the monomer. The use of polymerization processes based on this mode of polymerization has many applications including rapid curing of polymers without external heating, uniform curing of thick samples, solventless preparation of some polymers, and filling/sealing of structures having cavities of arbitrary shape without having to heat the structure externally. One important limitation of this process is that the fronts extinguish when they try to propagate through channels that are too narrow (probably due to conductive heat losses) or too wide (for unknown reasons, which we propose to be convective heat losses driven by buoyancy-induced flow.) Even when extinction does not occur, convective and buoyant instabilities can affect the structure and properties of the resulting polymerized materials as well as the propagation rates of the fronts. The purpose of this work is to determine the mechanisms of extinction and instability and thereby determine means to obtain more useful product material at earth gravity and µg.

 

Experiments will be performed in two distinct geometries, specifically Hele-Shaw cells and round tubes (Figure 1), at earth gravity and microgravity. Our experiments in gas combustion in round tubes of varying diameter have demonstrated two distinct extinction limits due to these processes; it will be determined if the same applies to polymer fronts. Comparisons to instabilities and extinction mechanisms in flames and aqueous autocatalytic chemical reaction fronts will also be made. Effects of surface tension between miscible fluids (discussed above) will also be evaluated (see also Figure 2 below). Laser-induced fluorescence (Figure 3) will be used to obtain images of the polymerization fronts. Numerical simulations of frontal polymerization fronts in both Hele-Shaw cells and round tubes will be performed as well.

 

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Figure 1. Schematic diagram of experimental apparatuses, shown for upward-propagating fronts in Hele-Shaw cells and downward propagating fronts in tubes. Water bath and all diagnostics shown used for both Hele-Shaw and tube apparatuses. LDV system for 1g tests only. Not shown: Laser shearing interferometer.

 

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Figure 2. Schematic illustration of proposed effect of surface tension gradients on flow along polymer front (shown propagating upward). Note flow direction is opposite conventional thermocapillary flow.

 

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Figure 3. Images of polymerization fronts. (a) LIF image using 20 ppm (by mass) BODIPY 493503 fluorescent indicator (from Molecular Probes, Eugene, OR) illuminated by a sheet of argon-ion laser light 0.5 mm thick, upward propagation, no Cab-o-sil (note thermal plumes rising from turns of igniter wire); (b) LIF image using BODIPY 493503 indicator, downward propagation, no Cab-o-sil (note finger of downward-spreading non-fluorescent products); (c) LIF image using BODIPY 493503 indicator, downward propagation, 0.75 g Cab-o-sil; (d) same as (c) but direct image (not LIF). All images: tube diameter (w) 18 mm, mixture composition 1.5g AP, 15 ml HEMA, 15 ml DMSO.

 

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