| The natural polysaccharide xanthan gum (XG) produced by the microorganism Xanthomonas campestris finds wide applications in oil recovery because of its excellent properties like thickening, salt tolerance, shear endurnce and stability over a wide pH. Beside these advantages, XG has drawbacks, like the limit of temperature resistance and the large consumption, which limit its use. By introducing various functional groups and increasing the molecular weight of XG, chemical modification usually give it better thermo-and degradation-resistant properties, high viscous and shear-resistant properties, etc. The main work and the results of study are as followes.1. A slightly crosslinked xanthan gum (named as PX) was prepared by esterification of XG with poly(maleic-anhydride/1-octadecene) under heterogeneous conditions. The structure, thermal stability and molecular weight of the PX samples were characterized by FTIR, 1H-NMR spectra, thermal gravimetric analyses and light scattering. The thermal transition temperature of XG was at about 40℃ while PX samples at about 50℃, which indicating that the thermal stability of PX solutions were enhanced by ester linkage and the hydrophobic association of long alkyl chain. All PX samples exhibited higher viscous properties than XG, expecially for PX-4, the viscosity of which reached to 1946mPas at lOg-L’1, while XG just reached 790mPas at the same concentration. In addition, the influences of salts and mechanical shear degration on viscosity of polymer solutions clearly showed that PX samples had better performance on salt tolerance and shear endurance. The emulsification performance of polymer/crude oil system was tested, and PX samples formed smaller multiple oil globules than XG, which suggesting that PX samples had better emulsifying effectiveness.2. Amphiphilic xanthan gum (named as MXG) was prepared by one-pot reaction of XG with octadecyl isocyanate under heterogeneous conditions. The successful modification process was verified and monitored by FTIR,1H-NMR spectra, elemental analysis and thermal gravimetric analysis. Light scattering measurements found apparent molecular weight of MXG was increased to 6.58 X106 dalton while XG was 2.37×106 dalton. Thermal degradation experiments evidently showed that MXG-1 and MXG-2 performed better temperature tolerance than XG After aging 2160h at 65℃, the viscosity of MXG-1 and MXG-2 decreased to 138mPas and 124mPas, respectively, while the viscosity of XG decreased to 79mPas. The influences of temperature and NaCl concentration on apparent viscosity of polymer solutions clearly showed that properly modified xanthan gum had better performance on salt tolerance and temperature resistance. In addition, five MXG samples performed stronger surface activity than XG, expecially for MXG-5, the surface tension of which reached to 52mN·m-1 at Sg·L-1, while XG was 63mN·m-1 at the same concentration. The emulsification performance of polymer/crude oil system showed that MXG samples had better emulsification ability due to the grafted long alkyl chain.3. Amphiphilic xanthan gum (named as BX) was synthesized by etherification of xanthan gum with 1-bromotetradecane under heterogeneous conditions. The structure, thermal stability and molecular weight of the BX samples were characterized by FTIR, 1H-NMR spectra, thermal gravimetric analysis and light scattering. Three BX samples performed stronger interfacial activity (polymer acqueous solutions/n-heptane) than XG, expecially for BX-3, the interfacial tension of which reached to 37mN·m-1 at 5g·L-1, while XG was 48mN·m-1 at the same concentration. Thermal degration determinations clearly showed that the properly modified xanthan gum samples had better thermal tolerance and the optimal feeding weight of 1-bromotetradecane was 0.5g per 4g XG... |
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