Dissolution of cellulose in aqueous hydroxide bases

Sammanfattning: As we move towards a circular bioeconomy, new and advanced materials based on cellulose are constantly developed. Unlike most plastics or metals, cellulose cannot be melted, and therefore dissolution is an important tool for processing of cellulose but also for analytical purposes. There is, however, both a knowledge gap in understanding the mechanisms behind dissolution as well as a continued search for new and improved solvents. Aqueous solutions of hydroxide bases are a group of solvents with considerable variation in both dissolution capacity and stability of cellulose solutions, and their properties need to be improved to be useful solvents. Despite this, they are interesting because they have the potential to be cheap and non-toxic, depending on the base of choice. Therefore, the purpose of this thesis has been to further understand the interactions governing cellulose dissolution and properties in aqueous solutions of hydroxide bases, so that in the future, new and improved solvents can be designed. In order to achieve this, cellulose dissolution at low temperatures in aqueous solutions of NaOH and selected quaternary ammonium hydroxide bases has been investigated. The effect of combining NaOH with a quaternary ammonium hydroxide was also investigated, along with the influence of the commonly used additive urea. Results based on light scattering measurements revealed that dissolution in NaOH(aq) is poor, with relatively large aggregates present already at very dilute concentrations and a fraction of undissolved cellulose always present. Upon comparing NaOH to more hydrophobic quaternary ammonium hydroxides, it was observed that the dissolution capacity of the bases increased with increasing hydrophobicity of the cation, alongside their ability to act as hydrogen bond acceptors. Rheology measurements showed that compared to pure NaOH(aq) or pure tetramethylammonium hydroxide (TMAH)(aq), combining NaOH with TMAH improved the stability of the solutions over time and against increasing temperature. It was therefore proved that combining bases can have a similar effect as an additive, but the results were highly dependent on the base pair employed and indicated that both bases need to be able to dissolve cellulose on their own, within the same temperature interval and be miscible with each other in order to improve solution properties.

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