Films of arabinoxylans and β-glucans extracted from cereal grains: Molecular motions by TD-NMR

To mimic the lamellar organisation of polymers within cereal cell walls, films of arabinoxylan (AX) and β-glucan (BG) were prepared and characterized using Time-Domain (TD) 1H NMR at different water contents and temperatures of measurement. The glass transition temperature (Tg) of the films was measured using differential scanning calorimetry (DSC). The investigation of M2, i.e. the second moment of proton dipolar interactions, and T2, i.e. the water spin–spin relaxation times, as a function of temperature and water content emphasized the complementary mechanisms involving the mobility of the polysaccharide chains both below and above the glass transition temperature, and the mobility of water molecules in interactions with the hydroxyl groups of the polysaccharides. In spite of the complexity of these mechanisms, we found that BG films featured higher M2 values than AX films, which is consistent with higher proton intra- and inter-molecular dipolar interactions. These results, which are in agreement with the higher Tg obtained for BG films, were assigned to the smaller nanopores in the BG films which reduce the kinetics of the exchange between water molecules.