Bernd Morgenstern

On the particulate structure of cellulose solutions

A simple model for aggregate formation in cellulose solutions is presented. Owing to a strong tendency of hydrogen-bond formation among the cellulose molecules, aggregation does not lead to a completely random arrangement of the molecules. In a core, parts of the chain molecules are laterally aligned. This core is surrounded by disordered regions that give rise to the formation of coronas under the action of solvent molecules while the core is completely immiscible with the solvent. Therefore, the aggregates can be seen as fringed micelles. The equilibrium structure of these micelles, number of aggregated chains and size of the coronas, is discussed as a function of the interfacial tension between core and solvent. It turns out that both number of aggregated chains and thickness of the coronas increase with increasing interfacial tension. In perfect solutions of the micelles, these quantities also increase with cellulose concentration. If one admits attractions between coronas of different micelles, as a small perturbation, clustering of micelles might be induced. This may cause phase instability of the particle phase which results in the coexistence of a diluted and a more concentrated solution.

Changes in cellulose structure during dissolution in LiCl:N,N-dimethylacetamide and in the alkaline iron tartrate system EWNN

The influence of different solvents on the morphology of cellulose during the dissolution process was studied. Spruce sulfite pulp, cotton linters and hydrolysed cotton linters were treated for a short time with lithium chloride:N,N-dimethylacetamide (LiCl:DMAc) and an alkaline solution of iron sodium tartrate (EWNN), respectively. The changes occurring at the fibre surfaces and within the cell walls were observed by scanning as well as by transmission electron microscopy. The cellulose fibres show significant differences in the dissolution behaviour when comparing the reaction of the two solvents. Using LiCl:DMAc, the cotton linters fibres become lamellar separated and within the spruce sulfite pulp fibres solvent channels appear in the first step with the fibrils becoming separated. In contrast, EWNN has a swelling effect on the surface of the cellulose fibres. Both solvent systems predominantly affect the ends of the fibres and places where the wall structure has been damaged.