Cellulose dissolution and gelation in alkaline-based solvents

• Autor(a) principal: Pereira, Ana Catarina Dias
• Data de Publicação: 2021
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10400.1/16711
• topic: Celulose; Gelificação; NaOH; Cristalitos; TBAH; Difusão; Domínio/Área Científica::Engenharia e Tecnologia::Outras Engenharias e Tecnologias

Issues of environmental concern and sustainability are pressing for an efficient and growing use of biomass and naturally occurring polymers to create new materials and sustainable opportunities and solutions. An effective utilization and sourcing of cellulose, as the world’s most abundant renewable material, not only reduces the consumption of our limited fossil resources but also protects the environment. To produce cellulose based materials for various applications, cellulose requires dissolution at some point, and this task can be challenging. The traditional methods of dissolution have some limitations: some require high investments others create occupational health and environmental issues. Therefore, it is important to develop inexpensive and environmentally “friendly” alternatives to these solvents. With this purpose in mind, the main goal of this work was to study alkaline-based solvents regarding their interaction and effect on cellulose dissolution, while accessing the aggregation and gelation phenomena. This work was divided in two parts: The first part focus on cellulose dissolution and gelation in sodium hydroxide studied by time resolved rheology, turbidimetry and wide-angle X-ray scattering. The kinetics of gelation are observed to be strongly temperature dependent, changing from several hours down to few seconds when the temperature is increased from 25 to 30 ºC. The obtained gels are irreversibly formed. Wide angle X-ray scattering data reveals the formation of ordered domains, as the sample gels, and it is suggested the gelation is due to the crystallization and precipitation of cellulose. This strongly indicates that the gelation phenomena can be understood as cellulose precipitation/crystallization where an effectively cross-linked network and gelation results from that cellulose chains may participate in more than one crystallite. This part of the work resulted in a published article that can be found in Appendix B. The second part focussed on the molecular self-diffusion coefficients measured in cellulose solutions dissolved in 30 wt. %, 55 wt. % and mixtures of 40 wt. % TBAH (aq.) and DMSO at different ratios, using pulsed field gradient stimulated echo NMR. The results show that dissolution efficiency of TBAH (aq.) is not compromised significantly even when high amounts of the organic co-solvent (DMSO) are present, in agreement with previous studies. Compared with the standard TBAH (aq.) solvent - 55 wt. % TBAH (aq.), the systems with DMSO presented in this work are highly advantageous since they use much less TBAH (substituted by DMSO) making the dissolution process much less expensive and possibly viable for large scale applications. Additionally, the molecular self-diffusion coefficients obtained highlight that TBAH can “bind” to cellulose within an interval of 0.5 TBA+ to 2.3 TBA+ per anhydroglucose unit depending on the TBAH amount in solution. The main driving force for this binding may be the favourable electrostatic interactions between the TBA+ cations and the deprotonated hydroxyl groups on the cellulose molecules coupled with hydrophobic interactions. Moreover, temperature seems to not have a significant effect on the relative diffusion coefficient of the different species either in the presence or absence of DMSO in the mixture, at least in the temperature range studied.