THE MECHANICAL BETA-RELAXATION IN AMORPHOUS CELLULOSE

Amorphous cellulose exhibits two secondary relaxations, so-called gamm a(cell) and beta(cell), at temperatures below its glass transition tem perature. The activation energy of gamma(cell) indicates that it corre sponds to noncooperative motions and the comparison with other polysac charides leads to the conclusion that it corresponds to the rotation o f CH2OH lateral groups. This proposal has been confirmed by preliminar y molecular modeling. On the contrary, experimental data indicates tha t the activation energy of the beta(cell) has an entropic contribution which leads to the conclusion that molecular motions at its origin sh ould be cooperative. Preliminary molecular modeling calculations perfo rmed on isolated chains in vacuum with random conformations, indicate conformational changes can occur between equivalent energy conformers. Such hopping jumps seem to involve a limited number of repeat units ( about six according to these results). (C) 1998 Elsevier Science B.V. All rights reserved.