Free-radical frontal polymerization: self-propagating thermal reaction waves

Abstract

Frontal polymerization is a mode of converting monomer into polymer via a localized reaction zone that propagates. Such fronts can exist with free-radical polymerization or epoxy curing. The necessary conditions for the existence of the free-radical frontal polymerization regime are considered. The physicochemical properties of monomers are classified in connection with complications arising in the experiments, including low conversion, convection and bubbles. The density gradient in the reaction zone leading to front decay is the most insidious problem. The analysis of forces that can withstand the Taylor instability allowed us to overcome experimental difficulties and develop approaches to polymerize low-viscosity monomers in the frontal mode. The factors affecting front shape, velocity, conversion, and molecular weight are considered.Historically, frontal polymerization has been performed in neat monomer but we have been able to obtain fronts of very reactive monomers in high-boiling-point solvents including water, DMSO and DMF. Periodic frontal polymerization modes have been observed, including single and multi-point spin modes in which 'hot spots' (local high-temperature regions) migrate around the front as it propagates. We investigated experimentally the occurrence of the different modes as a function of the initial temperature and the intensity of heat losses and made a qualitative comparison to the results of theoretical analyses based on infinitely narrow reaction zone models.The future directions of research with frontal polymerization are considered, especially with regard to applications to materials synthesis.

Propagating waves of self-assembly in organosilane monolayers

“…The growth of urban sprawl is another near 2D growth phenomenon for which detailed land use records and satellite imaging provide information about the front structure and dynamics (42). From a material science standpoint, it would be interesting to pursue studies of frontal polymerization (43), the corrosion of metallic films (44), and polymer dissolution (45), fronts in thin films to determine whether our results apply broadly to these phenomena as well. The general finding that fluctuation effects tend to make these fronts increasingly incoherent in time (diffuse) can be expected to have a large impact on the interaction of these frontal patterns under conditions where species (or different types of ordering) are competing for supremacy.…”

Miscible density fingering of chemical fronts in porous media: Nonlinear simulations

“…The only striking evidence of chemically driven viscous fingering for miscible autocatalytic systems occurs in polymeric systems where the monomer solution and the polymer matrix can have quite strong differences in viscosity. 9 In miscible systems, chemical reactions are more prone to provide density differences that can drive buoyantly unstable situations as soon as a heavier fluid lies on top of a lighter one. Experimental evidence [10][11][12][13][14][15][16][17][18][19][20] and theoretical studies [20][21][22][23][24][25][26][27][28][29][30][31][32] on density driven instabilities have shown that the coupling between buoyancy driven flows and chemical reactions can strongly affect the properties of the fingering instability.…”

Synthetic PEG Hydrogels as Extracellular Matrix Mimics for Tissue Engineering Applications

“…The reaction front is created by an external energy source, such as heat or light, and propagates through the system via consumption of the monomeric species. The PBFB technique is distinct from the previously published methods of frontal polymerization (Chechilo et al, 1972;Chekanov & Pojman, 2000;Pojman et al, 1996) in that it is based on the perfusion of a photoinitiator resulting in localized initiation and subsequent formation of crosslinked PEGDA hydrogels with gradients of mechanical properties and of immobilized YRGDS. In PBFP, gradients are generated by the controlled delivery of eosin Y, a photoinitiator, to the base of the monomeric solution via a perfusion pump and a glass frit filter disk.…”