Gas phase oxygen effect on chain scission and monomer content in bulk poly(methyl methacrylate) degraded by external thermal radiation

The effect of atmospheric oxygen on the thermal decomposition of poly(methyl methacrylate), PMMA, in a slab-like configuration was investigated. Blackbody irradiation of 12 mm thick PMMA slabs on one side was used to simulate the thermal decomposition and gasification pf the polymer in a fire environment. Results are reported for chain scission number obtained from molecular weight measurements and for residual monomer content at various levels below the slab surfaces irradiated at 17 and 30 kW/m(2) in atmospheres containing 0, 10, 21, and 41% oxygen in nitrogen. The scission number and polydispersity of surface layers, about 0.1 mm thick, were found to increase linearly with the mole fraction of oxygen in nitrogen. Over this range (0 to 41% O-2) the scission number increased from 1.5 to 5.0 and the polydispersity increased from 3.6 to 11.3 when the PMMA was degraded at the lower flux, while at the higher flux, the scission number increased from 5.0 to 14.4 with a concomitant polydispersity change from 2.0 to 4.5. These results show that gas phase O-2 reacts with the polymer chains, enhancing random scissions and generating functional groups from which depropagation is initiated. This enhanced decomposition increases the transient gasification rate leading to ignition and flame spread.