Zero migration of monomers in glassy polymers: A possible artifact of thermal depolymerization

Abstract

AbstractEfforts to remove traces of monomers contained in polymers by stripping with steam have resulted in reports that a small concentration of monomer is inextractable, or “locked‐in,” the polymer. Any concomitant depolymerization to monomer and diffusive elution of the formed monomer would, in the steady state, result in a constant concentration of monomer within the confines of the polymer. If only the polymer phase were sampled and depolymerization were ignored, this result could be interpreted as an apparent “zero migration” level of monomer. A model that describes the coupled processes of depolymerization and diffusion is presented; predictions and estimates of an apparent “zero‐migration” concentration are offered for various polymer/monomer systems considering a variety of kinetic schemes. The effects of temperature, geometry, and sample size on the transient and apparent steady‐state, residual‐weight fraction of monomer in the polymer are considered. Also, the weight fraction of monomer, neglecting diffusive elution, is estimated for comparison with the steady‐state weight fractions calculated for the case in which diffusive elution significantly lowers the residual steady‐state monomer level. The results predicted by the model presented are compared with experimental data reported in the literature. Transient approaches to the steady state are included in the model analysis. The steady‐state monomer concentrations predicted for polystyrene and poly(acrylonitrile) at 100°C vary from 0.0001 ppm to 100 ppm as the diameter of the respective polymeric spheres varies between 0.5 μ and 500 μ. The predicted steady‐state concentrations for vinyl chloride monomer in poly(vinyl chloride) are, under comparable conditions, seven orders of magnitude lower, reflecting the extraordinarily small tendency of polyvinyl chloride to degrade to monomer.