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

This study examined 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). The aim was to clarify how different treatment sequences influence fiber composition, morphology, crystallinity, hydrogen-bond networks, energy consumption, and tensile performance.Component analysis showed that extrusion alone (CLE) had little effect, whereas CLNE treatment increased cellulose and lignin contents while reducing hemicellulose, reflecting preferential disruption of amorphous polysaccharides. CLNWE pretreatment enhanced crystallinity but retained higher hemicellulose, while CLEN facilitated removal of neutral detergent-soluble fractions at low concentrations.Particle size analysis revealed that CLNE samples exhibited concentration-dependent shifts: moderate NMMO levels promoted fiber deconstruction, while high concentrations reduced mechanical separation due to lubrication. CLNWE fibers showed decreasing mean particle size, indicating irreversible hydrogen-bond disruption, whereas CLEN fibers remained largely unaffected. Microscopic observations confirmed that CLNE generated abundant high-aspect-ratio fibers, though excessive NMMO weakened mechanical force transmission. In the CLNWE group, fibers tended to cluster together at higher NMMO concentrations, whereas the CLEN group maintained a relatively uniform morphology.Crystallinity analysis by XRD demonstrated that all samples retained cellulose I structure. CLNE treatment increased crystallinity by up to 10.41% compared with CLE, though values declined at higher concentrations. CLNWE samples exhibited steadily rising crystallinity, while CLEN samples showed overall higher crystallinity than CLE but decreasing trends at elevated concentrations. FTIR analysis confirmed hydrogen-bond restructuring: CLNE promoted conversion of intrachain to interchain bonds, CLNWE facilitated chain separation and reformation after water washing, and CLEN primarily altered surface hydrogen-bond networks.Energy consumption analysis indicated that CLNE reduced average extrusion power by 56.4%～66.77% compared with CLE, due to decreased friction and enhanced fiber mobility. CLNWE and CLEN also lowered energy demand but showed concentration-dependent effects. Mechanical testing revealed that CLNE achieved the highest tensile index at 8% NMMO concentration, improving by 64.27% over CLE, while simultaneously reducing energy demand. CLNWE and CLEN groups showed smaller improvements, with CLEN fibers improving tensile index by 17.21%～23.98% mainly through surface-level regulation rather than bulk structural changes.Further low-concentration trials (0～16% NMMO) confirmed that CLNE exhibited the most sensitive mechanical response. The tensile index followed a quadratic relationship with concentration (T₁ = 3.86 + 0.66x – 0.04x², R² = 0.999), reaching a maximum of 6.33 N·m·g^-1 at 8% NMMO, which was 64.27% higher than CLE and superior to values reported for other fiber composites. At higher concentrations (12～75%), tensile index decreased linearly (T₂ = 5.94 – 0.05x, R² = 0.982), confirming that excessive lubrication weakened fiber separation and reduced performance.Overall, simultaneous NMMO swelling-extrusion treatment (CLNE) provided synergistic chemical and mechanical effects, enabling oriented deconstruction of cellulose microfibers, enhancing fiber morphology and tensile strength, and reducing energy consumption. These findings establish a mechanistic foundation for the green and efficient preparation of rice straw fiber composites, supporting the valorization of agricultural residues into sustainable bio-based materials. Importantly, the insights gained here can guide the design of scalable processes for biomass fiber modification, offering practical pathways toward renewable composites and contributing to the broader development of circular bioeconomy strategies.