Effects of Water Content and Molecular Weight on Spin Probe and Water Mobility in Malto-oligomer Glasses

Abstract

Spin probe rotational mobility has been studied by means of saturation transfer ESR in malto-oligomer−water glasses and has been compared to water mobility as measured by 1H NMR. Increasing the water content from 10 to 30 wt % leads to a decrease in spin probe mobility at Tg while the water mobility increases at Tg. From FTIR data we conclude that this decrease in spin probe mobility is caused by the fact that the overall packing of water and carbohydrate molecules in the hydrogen-bonded network becomes denser upon increasing water content. From proton density measurements we conclude that the concomitant increase in mobility of water molecules is caused by the fact that the carbohydrate molecules become more separated from one another. Both water and spin probe mobility at Tg increase upon increasing the molecular weight of the malto-oligomers (ranging from glucose up to maltoheptaose), while keeping the water content constant. This can be explained by the fact that larger oligomers form less densely packed networks.