Mobility in maltose-water glasses studied with 1H NMR

We have studied the molecular mobility of the water and carbohydrate protons in maltose samples as a function of water content and temperature using H-1 NMR. In the NMR signal, slow decaying and fast decaying fractions of protons are distinguished as arising from mobile and immobile (tau(c), > 3 mu s) protons, respectively. The assignment of these fractions in terms of water and maltose protons is temperature dependent. By analyzing the relaxation behavior of the mobile protons, the mobility of the water molecules is determined. The mobility of water molecules increases with water content and temperature, and at the glass transition, a small break in mobility is observed, indicating that the water molecules slightly sense the glass transition. The method of second moments gives information about the mobility of the immobile protons. Upon cooling, the glass transition is marked by a decrease in the temperature dependence of the mobility of the hydroxyl protons of maltose. This suggests that a stable hydrogen-bond network between the sugar molecules is formed at the glass transition temperature that immobilizes the hydroxyl groups. Water disrupts this network, and this results in a higher mobility of the hydroxyl protons of maltose. The more water the stronger is this plasticizing effect.