Kinetics of the Surface Degradation of Polymethylmethacrylate

The surface degradation of both linear and crosslinked polymethylmethacrylate (PMM) has been studied over the surface temperature range of 550° to 910°K by means of a hot-plate pyrolysis technique. It was demonstrated that surface gasification due to the high heat flux at the decomposing PMM surface involves a depolymerization process and surface desorption of methylmethacrylate monomer. The apparent activation energy for the linear rate of regression of the solid PMM surface (linear pyrolysis rate) was found to decrease with increasing surface temperature, approaching a limiting constant value of 11.2±0.6 kcal/mole at ∼650°K for linear PMM and at ∼770°K for crosslinked PMM. The mechanism for the surface degradation is depicted as (1) formation of monomer in the surface substrate, (2) diffusion of monomer to the surface, and (3) desorption of monomer from the surface. The linear pyrolysis rate data are correlated by means of an absolute rate theory treatment of surface decomposition. The experimental results are in good agreement with the theory.