Molecular mobility of sorbitol and maltitol: A C-13 NMR and molecular dynamics approach

Molecular mobility in two similar organic glass formers, namely sorbitol an d maltitol, is studied in order to understand their difference in the cross -over between alpha and beta relaxations, far above their respective glass transition temperatures. In this goal, the individual mobility of each carb on atom of the 6 carbon chain present both in sorbitol and maltitol is stud ied by means of C-13 nuclear magnetic resonance and molecular dynamics simu lations. Both techniques imply that the mobility of carbons located at the end of the 6 carbon chain is greater than that of any other carbon of this chain and that the difference in carbon mobility is greater within the sorb itol moiety of maltitol than in sorbitol. The relaxation time obtained by m agnetic resonance for carbons at the end of the 6 carbon chain is related t o the beta relaxation time and the one of carbons in the middle of the chai n is in relation with the value of the alpha relaxation time. This result m ay suggest that the merging between the alpha and beta relaxation processes in both sugars would be related to the decrease of the differences in mobi lity between the atoms of 6 carbon chain.