¹³C NMR and XRD studies on the enhancement of cellulose II crystallinity with low concentration NaOH post-treatments

In this study, methods of completing the structural conversion of partly crystalline cellulose II were studied by solid-state ¹³C CP/MAS NMR spectroscopy and X-ray diffraction. The study incorporated post-treatments of samples that had been mercerized or regenerated to yield somewhat crystalline cellulose II. These post-treatments used different temperatures, lower concentrations of NaOH than used in, for example, the original mercerization, and different neutralization methods. The cellulose employed was originally filter paper from wood pulp that was subsequently mercerized or dissolved and regenerated to create partly crystalline cellulose II. The variations in rate of each component were monitored at different NaOH concentrations and reaction temperatures during the treatment; these components include the crystalline and accessible surface as well as the inaccessible surface/amorphous region. Subsequently, neutralization was performed for the crystallization of cellulose II and the effects of the acid employed were also evaluated. It can be thought that controlling the partial Na-cellulose construction is important for improving the assembly of cellulose II; this is achieved by optimizing the concentration of NaOH, treatment temperature, and selection of acid for neutralization. NMR spectroscopy of the intermediate state of the NaOH detected the partial generation of Na-cellulose; further, another domain exhibited the spectrum of highly crystallized cellulose II. It is believed that the selective Na-cellulose construction in the inaccessible surface cellulose is the result of an increase in the size of cellulose II crystals. During the NaOH treatment, at a concentration of 10%, the crystallinity index evaluated by both NMR and X-ray was maximized; further, at the treatment temperature of 80 °C the crystallinity of the cellulose II evaluated by NMR improved. Moreover, the neutralization, which was conducted after the alkaline treatment with H₂SO₄, established the most suitable conditions for the crystallization of cellulose II.