Understanding overlooked molecular interactions in the cellulose/NaOH(aq) system
Doctoral thesis, 2019
The cold NaOH(aq) system is a promising process for the production of textile fibres. Although the process has been studied for almost a century, there remain unsolved research issues, especially regarding the dissolution mechanism. The focus of the thesis is to investigate the fundamental molecular interactions that govern the behaviour of a pure cellulose/NaOH(aq) system and systems containing selected additives. Nuclear magnetic resonance (NMR) spectroscopy in combination with complementary methods, such as infrared spectroscopy and pH, was used as characterisation technique, including substantial work on method development.
The main findings of the thesis show that magnetisation transfer NMR has great potential for studying the dynamics of water exchange in a cellulose/aqueous alkali system, which gives valuable insights into the dissolution mechanism of cellulose in NaOH(aq). Evaluation of 3JHH and 1JCH couplings of the cellulose model compound methyl β-D-glucopyranoside (β-MeO-Glcp) showed that conformational changes occur in NaOH(aq), but not as a function of temperature. However, upon the addition of urea, temperature-dependent conformational changes were observed. Steady-state heteronuclear Overhauser effect NMR measurements confirmed a non-specific interaction between urea and cellulose. This non-specific interaction suggests that urea facilitates a chemical environment that promotes a conformational change in the β-MeO-Glcp, which is most likely one of the mechanisms behind the positive effect of urea on the dissolution of cellulose in NaOH(aq). CO2 from the surrounding air was found to dissolve readily in cellulose/NaOH(aq) solutions, which leads to the formation of a cellulose carbonate intermediate and the introduction of substantial CO32- that is capable of interacting with cellulose. Cellulose-CO32- interactions were found to be disrupted in the presence of urea in NaOH(aq). Taken together, it is suggested that the cellulose-CO32- interactions are one of the factors that promote gelling. Urea is likely able to delay gelation due to the disruption of cellulose-CO32- interactions. The discovery of carbonate chemistry in cellulose/NaOH(aq) solutions reveals a new dimension of these solutions with high relevance for the stability and implementation of the cold NaOH(aq) process.
KC-salen, Kemivägen 4, Chalmers University of Technology
Opponent: Prof. Patrick Navard, CEMEF, Mines Paristech, Sophia Antipolis, France