The hyperbranched polyester based on poly dimethyl 5-(4 hydroxybutoxy) isophthalate ate has been successfully fractionated and a complete analysis of the solution properties has been carried out. Fractions with an average polydispersity of 1.8 over a range of molecular weight from 5x10(^3) g mol(^-1) were produced starting from an initial polymer with an approximate polydispersity of 7 and average molecular weight of l.3x10(^5) g mol(^-1). A linear analogue of the hyperbranched polyester has been also synthesised and different molecular weight samples were obtained by the use of a transesterification reaction. The hyperbranched fractions were investigated in the dilute regime by size exclusion chromatography, viscometry and light scattering techniques, while the semi-dilute regime has been studied by small-angle neutron scattering. Dilute solution properties in chloroform and in THF have been investigated, whilst D-THF was the solvent for the semi-dilute regime. In the dilute solution regime a whole range of physical parameters have been determined for the hyperbranched fractions and compared, where possible, with the linear analogues. From these results it is concluded that chloroform is a good solvent and THF a poor solvent for the hyperbranched polyester. The molar mass dependence of the radii of gyration has been interpreted by fractal dimension analysis and for the hyperbranched polyester the exponents obtained gave a fractal dimension d(_f)=2.5 ± 0.3 in both the solvents. The exponents obtained by the molar mass dependence of the radius of gyration in the semi-dilute regime have confirmed these values. These exponents also fit with those obtained from analysis of the intermediate g range of the small-angle neutron scattering cross sections, where an average slope 2.5 ± 0.1 was obtained. The values suggested the hyperbranched polymer is a mass fractal object with a rough surface.

No description available.

No description available.

The scattering of laser light by laboratory plasmas has been observed. When the scattering was from a plasma formed in a θ-pinch, with scattering angle of 90° , a nearly Gaussian profile of the scattered intensity as a function of wavelengths was observed, corresponding to scattering by non-interacting electrons. When the scattering was from a plasma jet, with scattering angle of 45° from the forward direction distinct satellite peaks were observed on both sides of a narrow central peak at the laser frequency as predicted by theory, 1,2,3,4 indicating a strong collective scattering effect between the electrons and the ions. The widths of the satellite lines were greater than the values predicted by theory. The discrepancy is ascribed to spatial variations in the electron density in the volume of the observed plasma. The intensities and frequencies at-which they occur these peaks also vary with the current of the plasma jet in a manner consistent with theory. The scattered intensity of the central peak was measured approximately and it agrees with theoretical prediction. Some indication of perturbation of the plasma by the incident laser light has also been observed. 1. E.E; Salpeter, Phys. Rev. 120, 1528(1960). 2. J.A. Fejer, Can. J. Phys. 38, 1114(1960). 3. J.P, Dougherty and D.T. Farley, Pro. Roy. Soc. (London) A259, 79(1960). 4. M.N. Rosenbluth and N. Rostoker, Phys. Fluids 5., 776(1962). / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The enhancement of the scattered light signal due to the mixing of two optical beams in a plasma has been observed. The two beams are produced in a dual-cavity organic dye laser pumped by a Q-switched ruby laser. The plasma is generated by a Helium plasma jet. In normal scattering a distinct satellite is observed on either side of the incident ruby wavelength 6943Å. The presence of enhanced oscillations is demonstrated by a significant increase in the intensity of one of these satellites when the dye lasers are tuned such that the difference in their frequencies is equal to the normally-observed resonant frequency of the plasma. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1995.

Carbon nanotubes, and particularly single wall carbon nanotubes (SWNTs) have attracted much attention for their unique structure, as well as for their excellent mechanical, electrical and thermal properties. Most properties of carbon nanotubs are closely related with its anisotropic structure and geometry factor. Characterization of carbon nanotube length is critical for understanding their behavior in solutions as well as in polymer composites. Microscopy, particularly atomic force microscopy, has been used for their length measurement. Microscopy, though straightforward, is quite laborious, particularly for statistically meaningful sampling. Light scattering can be used to measure particle dimensions. In this study, light scattering has been used to study polyvinyl pyrrolidone (PVP) wrapped SWNTs surfactant assisted aqueous dispersion and SWNT dispersion in oleum. To determine the length of SWNTs, Stokes - Mueller formalism was used, which is a universal model for particles with any size and shape. The Mueller matrix for an ensemble of long, thin cylinders proposed by McClain et al. was used in this study. This Mueller matrix includes the information of size (length and radius) and optical constants (refractive index and extinction coefficient) of cylinders. In this matrix, extinction coefficient, radius and length of SWNTs are unknown. By normalizing scattering intensity I(theta) (theta from 30 to 155 degree) to that at 30degree , the effects of radius and extinction coefficient were cancelled out. Thus, the effect of SWNT length on scattering intensity could be studied independently. A series of curves of normalized scattering intensity of SWNTs (I(theta) /I(30degree)) with varied length as a function of wave vector were predicted. A curve of normalized scattering intensity of SWNT as a function of wave vector was also obtained experimentally. By comparing experimental and predicted curves, average SWNT length in the dispersion has been determined. Scattering intensity at a given angle initially increases with concentration, and then reaches a critical concentration(C*), above which the scattering intensity decreases. This phenomenon has been attributed to the competition between scattering and absorption of light by the presence of SWNT. By using Beer-Lambert law, this phenomenon has been used to determine the molar absorption coefficient of SWNTs.

No description available.

No description available.

No description available.