الفهرس 
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Abstract
Positron annihilation lifetime spectroscopy (PALS) and Doppler broadening energy spectroscopy (DBES) are well established as novel methods currently available for the study of polymers at a molecular level because of their sensitivity to the microstructural changes in the polymer nanocomposites materials. These techniques provide unique information of the solid state structure – primarily on the nature of the free volume (or unoccupied space) in the polymer due to the less dense packing of polymer chains relative to other solid materials.
The free-volume properties in a system of zinc oxide (ZnO) nanoparticles (20 nm) dispersed in waterborne polyurethane (WBPU) were measured using positron annihilation lifetime spectroscopy. Two glass-transition temperatures (Tg), lower Tg ~ 220 K and higher Tg ~ 380 K of the ZnO/WBPU nanocomposites, were found and both increase with increasing zinc oxide content from 0% to 5%. These two glass transitions are interpreted from two segmental domains of WBPU; the lower Tg is due to soft aliphatic chains and high Tg is due to polar hard microdomains, respectively. The increase in Tg with the addition of ZnO fillers is mainly attributed to interfacial interactions through hydrogen bonding, van der Walls forces, and electrostatic forces between the polymer matrix and zinc oxide nanoparticles. These results are supported by the data from the dynamic mechanical thermal analysis (DMTA). The relationship between the free volume obtained from nanoscopic positron method and the physical cross-link density from macroscopic DMTA method as a result of microphase separation of hard and soft segments in polyurethane is found to follow an exponential function. Chemical properties and surface morphology of nanocomposites were examined by Fourier transform infrared spectroscopy (FTIR) and by atomic force microscopy (AFM).
Positron annihilation spectroscopy coupled with a variable mono-energy positron beam has been applied to study nanoscale polymeric nanocomposites. New information about multilayer depth profiles and structures, interfacial free-volume and open space properties have been obtained in WBPU/ZnO nanocomposites. The S parameter in Doppler Broadening Energy Spectra combined slow positron beam is used to quantitatively represent the free volume, open spaces, and interactions in the interface between WBPU and Zinc Oxide nanoparticles. The S parameter was found to decrease with increasing ZnO loading in WBPU matrix which is consistent with the results obtained from PALS discrete data.
Further more, Positron annihilation lifetime spectroscopy (PALS) was employed to study the free volume properties of polystyrene (PS) containing three different types of carbon nanoparticles: polystyrene-grafted single wall carbon nanotubes (SWCNTs-g-PS), single wall carbon nanotubes (SWCNTs), and carbon nanofibers (CNFs). The glass transition temperature measured via PALS was significantly lower than that from differential scanning calorimetry (DSC), although qualitatively the two methods agreed in that the Tg measured increased as nanotubes were added to the material. There were some specific differences between the two measurements which may have been related to the fact that DSC does not measure Tg of a portion of the material which is immobilized on the surface of the particle, while PALS measures all polymer, whether immobilized or not.