Optimizing crystal transitions in low-temperature, low-concentration NaOH solutions to prepare cellulose I and II composite materials

Cellulose II exhibits exceptional attributes, including flexibility, high stainability, and gloss. However, its strength is lower than that of cellulose I due to reduced crystallinity during the crystal transition. In this study, we devised a novel method to regulate the proportion and distribution of cellulose I and II crystals. This was achieved by employing a low-concentration alkaline solution and liquid nitrogen, resulting in a high-strength composite material that retained the excellent properties of cellulose II. When cellulose powder was immersed in an 8 wt% NaOH solution and quenched with liquid nitrogen, the crystal transition from cellulose I to II occurred outward from the sample periphery to its center. The percentage of cellulose II increased proportionally with treatment time. This technique was extended to cellulose I-rich cotton fibers, facilitating the creation of a composite fiber with cellulose I at the core and cellulose II on the surface. The tensile strength and Young’s modulus of the composite fiber were greater than those of the mercerized cellulose II fiber. Additionally, the elongation at break and toughness of these fibers surpassed those of conventional cotton fibers. This innovative method allows for the preparation of cellulose I and II composite materials with diverse properties.