I causticized starch by NaOH and analysed the Molecular weight by GPC. I thought the causticization was breaking down the starch structure, but the molecular weight has increased. what could be the possible reason? and the proposed structure of causticized starch?
It can not be, because it can not be ever. The molecular mass of starch after dissolving in alkali can not increase. Theoretically, the proportion of linear chains (amylose) from the hydrolysis of amylopectin may increase and this factor may affect your analysis. In this case, the molecular weight of starch treated with alkali will be lower than that of the original starch. The point is that the analysis of the gel permeation chromatography (GPC) method is based on measuring the hydrodynamic volume of the macromolecule in solution and this must be taken into account.
Normally not much, strong alkali may degrade polysaccharides but from the reducing end. However I think the question is more about how you dissolved your starch BEFORE causticization, are you sure you did not have selective dissolution of the smaller molecules?
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