Frontal bulk polymerization: Convection, dynamics, kinetics, and reactor design
Thermal frontal bulk polymerization is a method of converting neat monomer into polymer rapidly, via a narrow reaction-diffusion zone that propagates in space. Free radical polymerizations are highly exothermic, and therefore support self-propagating polymerization fronts by autocatalytic generation of heat. The exothermicity of polymerization gives rise to large temperature gradients both radially and along the direction of front propagation. This in turn gives rise to thermal and convective instabilities in the medium. For kinetic studies and synthesis of uniform materials, it was necessary to suppress the instabilities. Therefore, the design and construction of a high pressure reactor that allowed suppression of bubbles and the vigorous fluid motion (the hydrodynamic instability) associated with bubbles is discussed. The reactor provided precise control of pressure, temperature, and heat loss regulation for frontal kinetic studies. The convective instabilities are the result of the density changes caused by compositional and thermal changes. The simple convection restricts the orientation of front propagation. The Rayleigh-Taylor instability and hydrodynamic instability also interfere with a stable front propagation. By increasing medium viscosity along with high pressure, these instabilities were suppressed. A sounding rocket experiment was launched to ascertain the effect of microgravity on frontal polymerization of n-butyl acrylate. The microgravity environment helped in suppression of the buoyancy driven convection and the Rayleigh-Taylor instability. In the absence of these two instabilities, the effort was made to study themocapillary convection, and to test the validity of the proposed mechanism for hydrodynamic instability. The analysis of control and microgravity samples revealed insignificant differences in the molecular weight.