Data are presented for the surface pyrolysis temperature, T(s), and for the devolatilization mass loss rate per unit area, m, for polymethylmethacrylate (PMMA) exposed to a laser beam at incident flux levels of 15-500 W/cm2. After a brief induction time during which the surface is being heated to T(s), the measured m values are thereafter steady state in time, and are linearly related to the net absorbed flux, I(abs). The measured values for m are in good agreement with that derived from the first law of thermodynamics: m = I(abs) [integral(T0)T(s)C(T) dT + DELTA-H(v)(T(s))]-1. Kinetically, this equation represents a devolatilization wave front whose propagation rate and maximum temperature are both heat transport controlled, and determined by the magnitude of the driving flux, I(abs). The available data for ammonium perchlorate and for coal are also shown to obey a similar relationship, and the same heat transport-controlled process appears to govern their pyrolysis and devolatilization kinetics. At high fluxes, the measured m and T(s) values reported here for PMMA, and those reported by other investigators, can be interrelated to one another through an Arrhenius plot whose slope gives an "activation energy" that is essentially equal to the heat of vaporization, DELTA-H(v)(T(s)). That measured relationship between m and T(s) is thus a reflection of the Clausius-Clapeyron equation for the equilibrium vapor flux emanating from the polymer melt at its surface decomposition temperature, T(s). However, at low fluxes, the measured m values are significantly lower than those predicted by the Clausius-Clapeyron line. The departure is attributed to the additional presence of mass transport limitations. Photographic data are presented for bubble structure and bubble location within the quenched PMMA samples that independently confirm the increasing significance of mass transport limitations at the lower fluxes.
