| Starch is one of the most abundant natural polymeric materials in the world and has the advantages of low cost,renewability and biodegradability.Recently starch-based materials have gained increasing attentions due to the rapid resource depletion and severe environmental pollution.Compared to traditional polymers,starch has more complex super molecular structure and usually needs physical or chemical modification before processing.However,starch is insoluble in most solvents,which certainly restricts its further development and application in various fields.In the past decades the application of urea and urea/water mixtures in the starch fields are mainly focused on their plasticization and gelatinization role during processing.To the best of our knowledge,there is few paper that systematically studied the change of morphological structures and phase transitions of starch in the urea/water blending solvent system,especially using one type of starches with different ratios of amylose/amylopetin.The objective of the present work is to systematically study the effect of urea/water mixtures on the morphological changes and phase transition of starch,which in turn would provide information about starch structure at the molecular level.In addition,corn sarches with different amylose/amylopectin ratios(0/100,23/77,50/50 and 80/20)were also employed to compare and interpret the relative results.Basing on the study of two-phase model of liquid water and the hydration mechanism of urea in aqueous solution,the relationship among urea,water and starch was discussed further and the relative mechanism of action was explored.Results showed that:In the blending system of urea/water,the morphologies,internal structure and thermal behavior of starch were significantly influenced by the ratios of urea/water and the amylose content.For low amylose corn starch(waxy and normal starch),when the urea/water ratio was lower than 20/80,during heating process,the phase transition temperature,observed by both differential scanning calorimeter and microscopy with hot-stage,moved to low temperature with the increasing urea content.Liquid water is regarded as containing two phases,one of which comprises highly ordered and hydrogen-bonded clusters of molecules,which are surrounded and interspersed by continuous phase consisting of disordered and less fully hydrogen-bonded molecules.The equilibrium is believed to be highly dynamic.When the content of water in the solvent system was high,urea formed hydrogen bond with the nearest non-hydrogen-bonded water molecules,which certainly strengthened the structure of watermolecules and reduced its energy.It induced the dynamic balance moving to the continuous phase and increasing the amount of non-hydrogen-bonded water molecules.Then starch molecules formed hydrogen bonds with these water molecules and consequently caused the dissolubility of starch in water.When using such mixture to process starch at room temperature,no evident difference was observed on the morphologies and internal structures of starch.When the urea/water ratio was 30/70,during heating process,the peak of phase transition shifted to lower temperature with an increase in urea content.Apparently starch granules were destroyed and the relative crystallinity(RC)decreased significantly after being treated with the blending solvent at room temperature.In such environment,the influence of urea on the water structure no longer played a leading role,and the interaction between urea and starch gradually strengthened.The combining of carbonyl of urea molecule and hydroxyl of starch molecule formed stronger hydrogen bonding,damaging intramolecular and intermolecular hydrogen bonds of starch,which reasonably caused a weakened structure of starch molecules.When urea/water radio was 40/60,gelatinization of low amylose starch occurred at room temperature with a complete loss of polarization cross.For waxy starch,its crystalline structure was obliterated and the granules started to fall apart from the hilum to the periphery,and finally was broken completely and the fragments formed into large clumps.Normal starch had similar change as the waxy starch,but the granule appearance still was remained after destruction,accompanying with the disappearence of polygonal structure of granule and the shape distortion.Moreover,V type diffraction peaks were found clearly at the diffraction angle 13 o and 20 o by X-ray observation.Similar change was also observed for high amylose starch.Due to the high amylose content,G50 and G80 have compact granule structure and smaller particle size,which certainly need more energy to damage the intramolecular and intermolecular hydrogen bonding.As a result,these starches still kept granule intergrity with a decrease in RC and had higher phase transition temperature in the same experimental condition. |
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