The molecular dynamic in hydrated cellulose has been investigated by a combination of thermal analyses and dielectric spectroscopy. Differential Scanning Calorimetry (DSC) shows the dependence upon hydration of the glass transition temperature T g . A physical ageing phenomenon has been observed. At the molecular scale, bound water is hydrogen bonded to polar sites of cellulose macromolecules. At the macroscopic scale, water molecules play the role of a plasticizer for cellulose lowering its T g . Dynamic Dielectric Spectroscopy (DDS) combined with Thermostimulated Currents (TSC) have allowed us to follow more localized molecular mobility. The β relaxation mode is characterized by activation entropies that vanish for higher water contents indicating molecular mobility localization. It is plasticized by water like the glass transition. This analogy is explained by a common origin of both mechanisms: the mobility of the cellulose backbone. The evolution of the γ mode upon hydration follows an anti-compensation law. Water acts as an anti-plasticizer in a hydrogen bonded network.

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a b s t r a c tThe influence of hydration on cellulose molecular mobility is investigated by two dielectric methods at different molecular scale. The mobility of side groups, assigned to c mode, for dried cellulose increases. The water molecules have an anti-plasticizer effect on c mode due to the water-polymer hydrogen bonding. For the b relaxation mode, only observed by the Thermo Stimulated Current technique, the hydration plays a role of plasticizer. The a relaxation mode assigned to the delocalized cooperative mobility of long chain segments of cellulose is plasticized by water. The study of activated parameters deduced from fractional polarization procedure, shows an increase of the activation enthalpy range with dehydration. It permits to conclude that reduction of hydrogen bonds density leading to a more extended cooperative mobility.

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. Any correspondence concerning this service should be sent to the repository administrator: staff-oatao@listes-diff.inp-toulouse.fr Abstract The low-temperature molecular mobility of three different wood species was analyzed, with the two major constituents-cellulose and lignin-as reference. Mechanical and dielectric dynamic techniques were used. In order to observe the fine structure of the broad relaxation modes of wood, a very low-frequency analysis was carried out by thermostimulated current technique. Low-temperature relaxations of rosewood were assigned to low-temperature relaxations of cellulose. There was no dielectric response of lignin in rosewood. Contrarily, both cellulose and lignin responses were distinguished in ebony and varongy. Thermostimulated currents analyses exhibit the specific behavior of lignin in the various wood species. Moreover, the relaxation mode of cellulose observed at lower temperature remains localized in rosewood, while it tends to delocalize in varongy and ebony. The nature and intensity of physical interactions that induce variation of phase miscibility might be responsible for the observed differences. Even at the scale of the c relaxation, physical interactions modify molecular mobility.

Arabidopsis Thaliana is a plant composed mainly of cellulose and lignin. Geneticists need techniques able to make differences at the molecular level between modified plants (DML6, CAD C/D) and non-modified ones. Thermo-stimulated current (TSC) analysis is a promising route to identify gene mutations. For the non-modified plant, at low temperatures, TSC thermograms highlight three dielectric relaxation modes. From −150 to −110 • C, γ Cellulose is attributed to CH 2 OH and-OH groups of cellulose. Between −110 and −80 • C, β Lignin is detected. From −80 to −40 • C, β Cellulose is characteristic of the molecular mobility of glycosidic linkages. For the CAD C/D modified plants, only γ Cellulose and β Lignin are observed; due to analogous enthalpy values, those modes have the same molecular origin as in the non-modified plant. So, the β Lignin mode is associated with the molecular mobility of the lignin-OH groups. The CAD C/D gene mutation changes the chemical structure of lignin, which promotes hydrogen bonds in the network and inhibits molecular mobility of glucosidic rings. It is also interesting to note that the DML6 gene mutation induces a higher cooperativity of this β Cellulose relaxation than in wild vegetal composites. In fact, this mutation promotes molecular mobility of glycosidic rings thanks to β 1-4 glycosidic linkages.

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