Effects of NMMO swelling-extrusion treatment on the structural deconstruction and material properties of rice straw fibers

This study aims to examine the effects of N-methylmorpholine-N-oxide (NMMO) swelling-extrusion treatments on the deconstruction behavior and material properties of rice straw fibers. Three strategies were compared: simultaneous swelling and extrusion (CLNE), swelling followed by extrusion (CLNWE), and extrusion followed by swelling (CLEN). A systematic investigation was implemented to clarify the influence of the different treatment sequences on the fiber composition, morphology, crystallinity, hydrogen-bond networks, energy consumption, and tensile performance. The results showed that extrusion alone (CLE) had little effect, whereas the CLNE treatment increased the cellulose and lignin contents, while the hemicellulose was reduced to reflect the preferential disruption of amorphous polysaccharides. CLNWE pretreatment also enhanced the crystallinity to retain the higher hemicellulose, while CLEN facilitated the removal of neutral detergent-soluble fractions at low concentrations. Particle size analysis revealed that CLNE samples also exhibited the concentration-dependent shifts: The moderate NMMO levels promoted the fiber deconstruction, while the high concentrations reduced the mechanical separation, due to the lubrication. CLNWE fibers showed a decreasing mean particle size, indicating the irreversible hydrogen-bond disruption, whereas CLEN fibers remained largely unaffected. Microscopic observations confirmed that CLNE also generated abundant high-aspect-ratio fibers, though excessive NMMO weakened mechanical force transmission. In the CLNWE group, the fibers tended to cluster together at higher NMMO concentrations, whereas the CLEN group maintained the relatively uniform morphology. The XRD analysis demonstrated that all samples retained cellulose I structure. CLNE treatment increased crystallinity by up to 10.41%, compared with the CLE, though the values declined at higher concentrations. CLNWE samples exhibited steadily rising crystallinity, while CLEN samples showed the overall higher crystallinity than that of CLE, with the decreasing trends at elevated concentrations. FTIR analysis confirmed hydrogen-bond restructuring: CLNE promoted the conversion of intrachain to interchain bonds, while CLNWE facilitated chain separation and reformation after water washing, and CLEN primarily altered surface hydrogen-bond networks. Energy consumption indicated that CLNE reduced the average extrusion power by 56.4%~66.77% compared with CLE, due to the decreasing friction with the high fiber mobility. CLNWE and CLEN also lowered the energy demand with the concentration dependence. Mechanical testing revealed that CLNE was achieved in the highest tensile index at 8% NMMO concentration, which was improved by 64.27% over CLE, while simultaneously reducing energy demand. CLNWE and CLEN groups shared the smaller improvements. The CLEN fibers improved the tensile index by 17.21%~23.98% after surface-level regulation rather than bulk structures. Furthermore, the low-concentration trials (0~16% NMMO) confirmed that CLNE also exhibited the most sensitive mechanical response. The tensile index followed a quadratic relationship with the concentration (T₁=3.86+0.66x-0.04x², and R²=0.999), thus reaching the maximum of 6.33 (N·m)/g at 8% NMMO, which was 64.27% higher than CLE, superior to values reported for the rest fiber composites. The tensile index decreased linearly (T₂=5.94 - 0.05x, R²=0.982) at the higher concentrations (12%~75%). Excessive lubrication weakened fiber separation and reduced performance. Overall, the simultaneous NMMO swelling-extrusion treatment (CLNE) provided synergistic chemical and mechanical effects for the oriented deconstruction of cellulose microfibers, fiber morphology, and tensile strength, with reduced energy consumption. These findings can offer a strong reference for the green and efficient preparation of rice straw fiber composites. The agricultural residues can be valorized into sustainable bio-based materials. Importantly, the insights can be gained for the scalable process in the biomass fiber modification. Practical pathways toward renewable composites can also offer the broader development of the circular bioeconomy.