I have prepared Potato distarch phosphate(PDSP) by crolinking potatostarch(PS) at pH=11 using POCl3 as cross linking agent.Delebrately i have used higher concentration of POCl3 to study change in the morphology.During this study what i observe is the decrease in viscosity,shear and loss modulas than native potatostarch.What could be the best possible reason for the same.
Starch is a granular material. Each granule swells if temperature and water content have become sufficiently high (gelatinization). In this process part of the starch goes into molecular solution but the thickening action relies on the presence of swollen granules. So, each gelatinized granule acts as a microgel particle and a gelatinized starch suspension can be considered as a microgel suspension. Cross-linking serves to suppress molecular dissolution of starch, thereby enhancing the granular nature of gelatinized starch suspensions.
It is known from fundamental principles of polymer science that the modulus G ( = rigidity) of a polymer gel network increases at increasing degree of cross-linking. At the same time the swelling capacity q of the network decreases (P.J. Flory, Principles of Polymer Chemistry, 1953).
The viscosity of particle suspensions is described in first order approximation by the Einstein equation: suspension viscosity increases with increasing volume fraction (= C q, with C = concentration). Because q is lower in the cross-linked starch than in the native starch, the viscosity of gelatinized cross-linked starch suspension is expected to be lower than that of the unmodified starch, as was observed.
However, the above reasoning only applies in excess water. At higher starch concentration, the amount of water may not be sufficient for complete swelling of individual granules and jamming will occur. In this case the degree of granule swelling becomes independent of the starch type and depend only on the starch concentration. Viscosity will now depend on swollen granule rigidity G which, as explained above, increases with cross-linking degree.
To summarize, the viscosity of cross-linked starch suspensions is expected to be lower than that of native starch at low concentration and higher at high concentration with a cross-over at the close packing concentration. See the attached file.
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