Crystallinity improvements in cellulose II after multiple posttreatment cycles with a dilute NaOH solution

The low crystallinity of cellulose II makes industrial use a major challenge. In our previous report, we proposed a mechanism for improvement of the crystallinity of cellulose II during posttreatment with a dilute NaOH solution. Specifically, rearrangements of the cellulose molecules occurred only near inaccessible surfaces through which NaOH penetrated, resulting in conversion into crystalline regions. To verify our proposed mechanism and to improve the crystallinity further, we performed both long-term single-cycle and multicycle posttreatments with dilute NaOH solutions. Multiple posttreatment cycles under optimal conditions increased the crystallinity by up to approximately 20%. In contrast, long-term single-cycle posttreatment led to crystallinity improvements of only up to approximately 10%. Changes in the proportions of the regions during multiple cycles after treatment supported our proposed mechanism. During multiple posttreatment cycles, the products were converted to crystalline regions only on the inaccessible surfaces enlarged by the previous posttreatment cycle, which increased the crystal sizes in each cycle. Optimization of the treatment times used during the multiple posttreatment cycles led to crystallinity improvements with fewer posttreatment cycles. Our investigations of the mechanism for crystallinity improvement will contribute to structural elucidation of cellulose II and the efficient preparation of highly crystalline cellulose II. The study on the improvement of the crystallinity of cellulose II by post-treatment with dilute NaOH solution showed that the crystallinity was significantly improved by post-treatment with multiple cycles. The NaOH in an aqueous NaOH solution penetrated only inaccessible surface regions, and cellulose rearrangement occurred only in these regions during post-treatment, improving crystal size. In the second and subsequent posttreatment cycles, cellulose rearrangement occurred only at the inaccessible surfaces expanded during the previous post-treatment cycle, crystallization progressed toward amorphous regions away from the initial crystalline regions.