Objective To promote the high-value utilization of cellulose, it is necessary to address the cellulose degradation caused by traditional high-concentration NaOH mercerization treatment, as well as the complex solvent systems and difficult solvent recovery associated with homogeneous reaction systems. In this study, a low-concentration NaOH freeze-thaw pretreatment method was developed to achieve efficient cellulose swelling, maintain the degree of polymerization, and improve cellulose reactivity. Meanwhile, the mechanism of cellulose structural activation and reactivity enhancement during the freeze-thaw process was clarified.

Methods Defatted cotton fibers were used as the raw material, and cellulose was activated by low-concentration NaOH freeze-thaw pretreatment. The swelling behavior, morphology, crystalline structure, thermal stability, and water adsorption-desorption behavior of cellulose before and after pretreatment were characterized to analyze structural changes and molecular-chain accessibility. Fock reactivity tests and acetylation reactions were further conducted to evaluate the improvement in cellulose reactivity and clarify the mechanism of NaOH freeze-thaw pretreatment.

Results (1) NaOH freeze-thaw pretreatment promoted cellulose swelling, resulting in spherical or bead-like swollen morphology, while helping to maintain a relatively high degree of polymerization. (2) After NaOH freeze-thaw pretreatment, the crystallinity of cellulose decreased significantly, and the NaOH concentration required for cellulose crystal transition was reduced. When the NaOH concentration increased from 0% to 10%, the crystallinity decreased from 80.21% to 48.55%; a cellulose crystal transition from cellulose I to cellulose II occurred at a NaOH concentration of 4%. (3) NaOH freeze-thaw pretreatment significantly improved cellulose reactivity. Under the same NaOH concentration, Fock reactivity values of the samples treated with 4% and 8% NaOH freeze-thaw pretreatment were 77.43% and 81.78%, respectively, representing relative increases of 52.27% and 50.08% compared with the corresponding samples treated with NaOH at room temperature. (4) Mechanistic analysis indicated that low-concentration NaOH weakened the original hydrogen-bond network of cellulose and promoted initial fiber swelling. During the freeze-thaw process, local enrichment of NaOH, ice-crystal expansion, and the formation of water channels after thawing further promoted alkali penetration and molecular-chain relaxation, thereby increasing the exposure of reaction sites and improving cellulose reactivity.

Conclusion Through the synergistic effect of low-concentration NaOH-induced swelling and the freeze-thaw process, mild structural activation of cotton fibers was achieved under non-dissolving conditions. This method reduced the crystalline order of cellulose, enhanced molecular-chain accessibility, and significantly improved Fock reactivity and the degree of substitution after acetylation while maintaining a relatively high degree of polymerization. This study provides an effective approach for the mild pretreatment of cellulose prior to chemical modification.