The degree of polymerization of cellulose is very relevant for physical and chemical properties of highly engineered biomaterials. The ability to control the level of depolymerisation to a final specific value opens new opportunities to design cellulose-based nanostructured materials. In this paper, the controlled hydrolysis of cellulose in 0-96% ethanol environment and with nine chosen acids (pKa-10-4.7) was studied in order to tailor the pretreatment of dissolving and kraft pulps for various applications. The acid hydrolysis of cellulose in aqueous environment decreased the viscosityaveraged degree of polymerisation (DPν) and relative cellulose content. However, the addition of small amounts of ethanol preserved the cellulose content nearly at the original level, while decreasing the DPν. Furthermore, when the ethanol concentration increased, the DPν decreased manifoldly. The treatment with strong mineral acids in ethanol environment decreased the DPν by 75-80%, regardless of the initial DPν of the pulps. The correlation between the pKa of the acid and the final DPν was notable, and organic acids yielded a much higher DPν. Also, the electron microscopy study revealed that the outer cell wall layers were intact after the treatment with weak acids, whereas mineral acids in ethanol environment weakened the outer cell wall layers. This caused the dissolution mechanism to change when fibres were immersed in 0.2 M cupriethylenediamine. Fibres with intact outer cell wall layers dissolved via the ballooning mechanism, whereas fibres with ruptured outer cell wall dissolved via either fragmenting or uniform swelling.
Jani Trygg, Poonam Trivedi, Pedro Fardim. Cellulose Chem. Technol., 50 (5-6), 557-567 (2016)