Polymerdynamics of cellulose and other polysaccharides in solid state-secondary dielectric relaxation processes

Abstract

Dielectric relaxation spectroscopy (DRS) separates different molecular groups of a repeating unit of a polymer with respect to the rate of its orientational dynamics. In the case of dry solid polysaccharides, four modes of relaxation processes can be observed in the sub-Tg range, which we interpret in the following way. The local main chain motion forms the β-relaxation, and the side groups motion in the repeating unit generates the γ-relaxation. Additionally, the so-called δ-relaxation can be observed in the low frequency side of the β-relaxation for well dried samples and a further βwet-relaxation occurs only in wet samples in the room temperature range, but the origins of this last process are not clear up to now. In the high temperature range (T>80°C), the σ-relaxation can be measured which is associated with the hopping motion of ions in the disordered structure of the biopolymeric material. For all these processes, we give experimental evidence. In addition, further relaxation processes are detected in the electrical inhomogeneous polysaccharide samples, which are associated with internal interfaces and the interface to the electrode and are well known as Maxwell–Wagner–Sillars and the electrode polarisation. The influence of the type of the glucosidic linkage to the β-relaxation is discussed by comparing the dynamic dielectric behaviour of different polysaccharides. Small amounts of water or other swelling solvents in the sample modify the relaxation processes in a characteristic manner and increase the activation energy and the cooperativity of the local chain motion. The morphological structure of the cellulose affects the dielectric spectra in the low frequency range below the β-loss peak. This spectral range in the DR spectra correlates with the chemical accessibility and the water retention capacity of chemical pulps. In the case of derivatives of cellulose or starch, we can show that the relaxation of side groups can be separated depending on the type of the side group and its position in the anhydro-glucose unit (AGU). Results are presented both in the form of dielectric spectra and as fit parameters calculated with the help of the Havriliak–Negami function and also in the form of the activation energies and the pre-exponential factors resulting from the Arrhenius representation. Essential literature concerning relaxation processes in polysaccharides is reviewed and the results given are compared with our findings.