Solid-state 1H and 13C NMR of corn starch plasticized with glycerol and urea

H-1 and C-13 NMR spectroscopy was used for the study of corn starch plasticized with urea and/or glycerol. The information on the interactions between plasticizers and starch macromolecules as well as the arrangement of starch macromolecules depending on the storage time (from 1 day to 52 weeks), were obtained from the broad line and magic-angle spinning H-1 NMR spectra measured at 22 degrees C and within the range from 30 to 140 degrees C and the cross-polarization magic-angle spinning (CP MAS) C-13 NMR spectra measured at 30 degrees C. The H-1 chemical shifts for the groups of the plasticizer molecules enabled characterization of the interactions between urea and glycerol molecules in starch plasticized with a mixture of urea and glycerol. With increasing temperature, the signal related to water molecules gradually vanished due to water evaporation, and resolved H-1 peaks associated with starch macromolecules were detected above 100 degrees C. The free water amount in the thermoplastic starch samples was also estimated using MAS H-1 NMR spectra. Formation of double helices due to sample ageing was observed through the CP MAS C-13 NMR spectra.