Abstract
This work examines the thermal degradation of polybenzoxazines by monitoring the thermal degradation and evaluating the physical properties of the materials during the process. Polymers from five chemically similar bis-benzoxazine monomers have been prepared and characterised. One of the major successes of this project has been developing a reliable method to produce polybenzoxazines that are free of voids. DSC and DMA have been used to study the thermal and mechanical properties of the polybenzoxazines revealing that the phenolphthalein based bisphenol linkage (PBP-a) gives rise to the highest glass transition temperature at 200°C with the dicyclopentadiene linkage (PBD-a) giving the lowest (150°C), a trend which has been shown to relate to crosslink density. Thermal degradation of the materials has been studied primarily via TGA to reveal that under nitrogen polybenzoxazines (generally) undergo three stages of thermal degradation. These were identified by pyrolysis-GC/MS as: breakdown of Mannich bridges (releasing aniline), collapse of bisphenol linkages (releasing phenolic species) and finally degradation of the remaining char (releasing more complex structures). Char yield (at 800°C) was greatest for PBP-a and least for PBD-a. Experimental parameters such as particle size, heating rate and atmosphere have also been found to influence thermal degradation. Pyrolysis-GC/MS has, in addition demonstrated how degradation processes vary with polybenzoxazine backbone, atmosphere and temperature. Some polybenzoxazines have been found to expand dramatically as they are heated with some forming irregular shapes (PBP-a) whilst others (PBT-a) retain their shape well. As they char, polybenzoxazines have been found via X-ray micro-CT scanning to form porous networks dependent on backbone structure. Nanoindentation has also revealed that these materials become harder as they are charred at increasing temperatures. Computer models have been developed to simulate and predict Tg, onset of thermal degradation temperature and volumetric changes of the polybenzoxazines used in this work.