Spherically Propagating Thermal Polymerization Fronts
Burcu Binici, Dionne I. Fortenberry, Kayce C. Leard, Marcus Molden, Nesrin
Olten, Sam Popwell and John A. Pojman
Binici, B.; Fortenberry, D. I.; Leard, K. C.; Molden, M.; Olten, N.; Popwell, S.; Pojman, J. A. " Spherically Propagating Thermal Polymerization Fronts," J. Polym. Sci. Part A: Polym. Chem. 2006, 44, 1387-1395.
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We can create spherically-propagating fronts using liquid-solid systems. By
adding silica gel we can suppress convection but still have an initially transparent
system. Figure 2 shows such a system in which a drop of photoinitiator was
injected
in the center of the HDDA-DMSO-persulfate solution that contained enough silica
gel to make it highly viscous. By exposing the system to 365 nm radiation,
the front ignites in the center. No spin modes were observed, however.
A front propagating in a solution of 1,6 hexanediol diacrylate (35%),
DMSO (53%), diethyl phthalate (12%) and CAB-O-SIL (6.7%). Ammonium persulfate
was the thermal initiator and Irgacure 184 the photoinitiator.
We developed a novel system that supports spherically-propagating thermal
polymerization fronts. A gel is prepared by reacting a trithiol with a triacrylate
in a 1:4 molar ratio by an amine-catalyzed Michael addition. A small piece
of gel containing photoinitiator can be inserted into the mixture as it gels.
When gelation is complete, fronts were initiated by UV illumination and fronts
propagated from the center. The gel could be cast into different shapes. This
approach works much better than using a multifunctional acrylate in DMSO with
dissolved persulfate, which does not produce bubbles. The thiol-triacrylate
gel prevents the appearance of bubbles from the peroxide initiator.
Spin modes were observed on the surface of the spherical fronts – a novel
phenomenon never before observed with thermal fronts in condensed media. Using
a gel containing a photoinitiator, it was possible to initiate a front from rectangular
initial conditions. The gel could be formed in rectangular slabs to observe fronts
in quasi-two-dimensional media.
A spin mode in a descending front of the thiol-ene system in a 1.5 cm diameter test tube. The horizontal arrow indicates the direction of the spin mode. The vertical arrow indicates the direction of the overall front propagation.
Spin modes on the surface of front. Diameter of reactor is 8 cm.
Using a cube of initiating gel it is possible to initiate fronts with non-spherical initial conditions. Diameter of reactor is 8 cm.