The thermosonimetry of polymers

Thermosonimetry is the measurement of sound (acoustic emission) emitted from a substance as a function of temperature, while the substance is heated in a controlled manner. In this work the thermo-oxidative decompositions of poly(vinyl chloride), polyethylene, polypropylene, poly(ethylene terephthalate) and ethylene-vinyl acetate have been studied using thermosonimetry. The amount of acoustic emission produced was dependent on the polymer, the heating rate and the sample mass but not the molecular mass of the polymer. Thermosonimetry was also used to obtain apparent activation energies and the reaction orders for the thermo-oxidative decomposition processes under a variety of experimental conditions. Typically, the activation energies obtained were higher than values reported in the literature obtained using thermal analysis methods such as thermogravimetry. An apparatus has been developed that permits simultaneous thermogravimetric analysis (TG) and thermosonimetry (TS). The acoustic emission commenced at the maximum rate of mass loss, and confirmed that thermosonimetry was not suitable for measuring the relative thermal stability of polymers. Extensive chemical analysis of the polymer residues was used to study the decomposition mechanisms but did not reveal any direct link between the chemical structure of the polymers and the acoustic emission. Microscopic analysis of the residues, using scanning electron microscopy, suggested that the physical nature of the residues and the physical processes occurring during thermo-oxidative decomposition determined whether acoustic emission was produced. For PVC and PET, gas evolution and fracture processes were linked to the production of acoustic emission. For PE and PP, which did not produce significant amounts of acoustic emission, these processes were not observed to the same degree. Further studies sought to investigate whether acoustic emission was produced during other polymer processes. Polymerization and phase transitions did not produce acoustic emission. The crystallization of isotactic polypropylene was found to produce acoustic emission. The acoustic emission had been linked to the formation of cavities between the polymer spherulites. Thermosonimetry was also used to measure the rates of crystallization from the isothermal crystallization of polypropylene.