1H and 2H NMR mobility in cellulose

Abstract

Mobility of water in cellulose was studied by solid-state 1H and high-resolution 2H NMR as a function of moisture content within the unfreezable moisture range (0–19% dry basis). Measurements of relative mobilities were based on relative intensities, transverse and longitudinal relaxation times and lineshape analysis. At 2–16% moisture content (dry basis), water molecules reoriented anisotropically, suggesting an interaction with cellulose fibers. At moisture content below the monolayer value (2.8%, dry basis), 90% of the protons were immobile and no liquid deuterium signal was detected. A sharp increase in liquid or mobile 1H intensity (accompanied by a decreased LW) and increases in 2H NMR T 1 and T 2 relaxation times were observed as moisture increased above 9% (dry basis). At this moisture content the molecular mobility approached the fast exchange regime. The data confirmed earlier reports that unfreezable water could be highly mobile and not in a rigid state. No glass transition was observed by DSC. However, NMR showed a significant mobility transition as the material transformed from a slow exchange (retarded mobility) to a fast exchange regime.