TRANSPORT PHENOMENA ASPECTS OF THE FREE-RADICAL RETROGRADE-PRECIPITATION POLYMERIZATION (FRRPP) PROCESS

We have been studying free-radical polymerization that is accompanied by phase separation above the lower critical solution temperature. In the past, we have experimentally shown evidence of hot regions in the reactive system. We have also shown in the past that eventually the system exerts control over the rate of propagation as well as termination. In this work, we invoke a concept in polymer physics (the coil-to-globule transition) to help explain the mechanism of thermal trapping within the polymerization zones. The diffusivities of polymer chains at different stages in the reaction are calculated using appropriate methods. From the diffusivities, the propagation and termination rate coefficients are calculated using the Achilias-Kiparissides gel effect model. With experimental kinetic data, we then estimate rates of monomer consumption within polymer-rich particles. Using a pseudo-steady-state heat transfer model, we are able to show that interior temperatures of polymer-rich particle domains greater than about 1 mm can reach spinodal temperature values at the early stage of polymerization. Polymer-rich particle sizes are obtained from the same reactor system whereby a small amount of crosslinker is added to preserve particle morphology. This experiment indicates that even under turbulent flow conditions, relatively large particles can exist in the reactor fluid. This agrees with the physical implications of the coil-to-globule transition. However, since these particles were obtained during the period of slow conversion rate, our heat transfer calculations indicate that interior particle temperatures would be almost the same as surface temperatures. This points to an unknown radical-trapping mechanism at this stage of the polymerization process.