Effect of PLA-g-MAH Content on the Interfacial Structure and Properties of Wheat Straw/PLA Biocomposites

Wheat straw/poly(lactic acid) (WS/PLA) biocomposites provide a promising route for agricultural residue valorization, but poor interfacial compatibility limits their performance. This study investigated the effect of maleic-anhydride-grafted PLA (PLA-g-MAH) on WS/PLA biocomposites containing 50 wt% wheat straw. PLA-g-MAH was incorporated at 0–10 wt% of the total composite mass by replacing an equivalent proportion of neat PLA, while the wheat straw content was maintained at 50 wt%. The composites were evaluated through mechanical testing, density and water absorption measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). Mechanical properties improved with increasing compatibilizer content and then declined at excessive dosages. At 7 wt% PLA-g-MAH, tensile and impact strengths reached 17.24 MPa and 15.56 kJ/m2, increasing by approximately 72% and 55%, respectively, compared with the uncompatibilized control. Density increased to 1.22 g/cm3, while water absorption decreased by about 43.2%. XRD indicated that moderate PLA-g-MAH addition promoted more favorable crystallization, whereas excessive addition reduced peak intensity and broadened diffraction peaks. SEM showed that 7 wt% PLA-g-MAH produced the most compact fibre–matrix interface, with fewer voids and less fibre pull-out. Thermal analyses further confirmed changes in thermal stability and crystallization behaviour after PLA-g-MAH addition. Overall, approximately 7 wt% PLA-g-MAH provided the best balance of interfacial integrity, mechanical performance, and moisture resistance.