SECONDARY MECHANICAL RELAXATIONS IN AMORPHOUS CELLULOSE

Amorphous cellulose exhibits two mechanical relaxation processes, so-c alled gamma and beta, below its glass transition temperature T-g. Thou gh much work has already been published on the subject, the origin of these relaxations is still uncertain, especially because most of the d ynamic mechanical data were obtained at constant frequency and mixed t ogether with dielectric data. In order to reach more definitive answer s, two main approaches were taken in this work, namely (i) the use of a low-frequency mechanical spectroscopic technique and (ii) the compar ison with data of different polysaccharides, having different lateral groups and different intramolecular links along the main chain. In add ition, preliminary molecular modeling based on molecular mechanics was performed on isolated chains in vacuum. The work is focused on dried cellulose, to minimize the effect of relaxation overlap, which becomes negligible for moisture content lower than 2%. The results show that the gamma process occurs without cooperativity and confirm its origin in the rotation of primary hydroxyl groups. In fact, in contrast with the beta relaxation, its activation energy does not involve significan t entropic contribution, and molecular modeling suggests that the rota tion of those lateral groups does not lead to conformational change in the rest of the chain (no cooperativity). The activation energy of th e beta relaxation involves an entropy contribution that varies with th e water content.