NON-FICKIAN DIFFUSION WITH REACTION IN GLASSY-POLYMERS WITH SWELLING-INDUCED BY THE PENETRANT - EFFECTS OF CONSECUTIVE AND PARALLEL REACTIONS

A mathematical model is presented for non-Fickian diffusion of a penet rant A into a granular glassy polymer containing a reactive group B, r esulting in tile desired product P. Further, both a consecutive reacti on between A and P (producing X) and a parallel reaction between A and C (producing Y) are incorporated, with C initially present in the par ticle. The swelling of the polymer, induced by the penetrant, is descr ibed by power-law kinetics for the velocity of the swelling front. Kin etics are considered to be first order in each of the two reactants. C oncentration profiles in the particle and selectivity to desired produ ct are calculated as function of the swelling behavior of the polymer grain. In case of a consecutive reaction the local concentration of P reaches a maximum value independent of the swelling rate. However, the position of the maximal concentration of P moves towards the center o f the grain with a rate depending on the kinetics of swelling. For Cas e II diffusion this velocity equals the velocity of the advancing fron t between glassy and rubbery polymer. The selectivity of the desired r eaction decreases with decreasing swelling rate. A low swelling rate a lso results in an inhomogeneous product distribution within the granul e. A criterion is derived predicting under what conditions the consecu tive reaction can be neglected and a pure product is obtained. The ana lysis further reveals that both a more homogeneous product and a highe r selectivity toward a desired product can be obtained by realizing pr eswelling of the polymer with an inert swelling agent. For Case II dif fusion the concentration profiles of the side product of the parallel reaction, Y, are flat in the rubbery part of the polymer. This is caus ed by the relatively low swelling rate allowing Y to redistribute in t he swollen polymer. If additional C is continuously supplied from the gas phase, then the selectivity decreases continuously with increasing conversion of B.