| Summary: | 碩士 === 國立臺北科技大學 === 有機高分子研究所 === 94 === Xanthan gum is a natural high molecular weight extracellular polysaccharide with exceptional rheological properties produced by the bacterium Xanthomonas. Xanthan gum solutions are non-Newtonian and highly pseudoplastic. The viscosity of Newtonian fluids depends on temperature, shear rate, time. Due to its exceptional rheological properties this polymer is actually employed in many industrial processes
such as food,textile,and cosmetics.
The effects of temperature, pH value and salt on viscosity of xanthan solutions are of great importance. The xanthan gum have different properties in low and high polymer concentration. The rheometry SR5 was performed in this study for trying to find the sol/gel transition and measuring the viscoelasticities at high and low polymer concentration range respectively. Accroding to the result we found that the effects of the addition of salt depend on the xanthan concentration. At sol status, the addition of salt reduces the viscosity of the preparation; however, at gel condition , the salt is seen to increase the viscosity of the formulation.
Xanthan gum in gel condition still have the properties of liquid crystal. Xanthan gum, which possesses lyotropic properties.and it was confirmed by optical studies that showed samples in this region to consist of a nematic liquid crystalline phase dispersed in an isotropic phase. The structure of xanthan gum have changed with heating, it is anisotropic in low temperature , but it is become isotropic with heating until the special temperayure .The polarized optical microscopy was performed in this study for trying to find the temperature range of xanthan gum’s liquid crystal. We also discuss the activation energy of flow and hysteresis. Accroding to the conclusion, the viscosity and hysteresis are influence by structure in anisotropic , but in isotropic are influence by temperature.
Xanthan gum is modified with formaldehyde, the structure of main chain have changed. The liquid crystal phases are unapparent , and no matter the viscosity or hysteresis in anisotropic are not changed a lot
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