Studies of coating degradation and polymer chemical environment using positron annihilation spectroscopy.

摘要

Positron annihilation spectroscopy has been demonstrated to be a powerful tool in studying polymer properties. Injunction with slow positron beams, it is becoming a state-of-art technique in investigating polymer surface properties. In this dissertation, the development of slow positron beam, study of coating degradation, measurement of the glass transition temperature (T_g) near a polystyrene surface, and investigation of chemical environment in some polymeric materials are presented in detail.; A slow positron beam with a computer-controlled system was developed and constructed at University of Missouri-Kansas City. The beam was designed to produce the positrons with variable energies ranging from 0.1 to 30 keV. This variable mono-energetic positron beam provides an ideal way to measure the depth profile of sub-nanometer defect properties for polymeric materials.; By using the constructed slow positron beam, the degradation of polyurethane based coating was studied. The influence of two basic factors, UV wavelength and oxygen concentration, on the coating degradation was investigated systematically. In addition, correlations between the free-volume change and the chemical processes as well as the cross-link density of polymers were also explored to provide a further understanding for coating degradation.; Glass transition temperature (T_g) near the polystyrene surface was measured by using this novel technique. It was found that T_g was depressed as large as 70°C when approaching polystyrene surface. This result is consistent with the theoretical prediction based on a multilayer model.; Hydrocarbon polymers, incorporated with different chemical groups, were studied thoroughly with both traditional positron annihilation spectroscopy and slow positron beam technique. Two groups of polymers, vinyl polymer and styrene polymer, were investigated. Their physical properties, e.g. crystallinity, molecular weight, and chemical environment, e.g. bond strength, electronic properties, were examined and discussed. The results were interpreted quantitatively with some fundamental theories and concepts.; In summary, through the research in this dissertation, it is further demonstrated that positron annihilation spectroscopy, especially when coupled with slow positron beam technique, has a great application to investigate coating degradation and chemical environment of polymeric materials.