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.
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.
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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