PLA vitrimerisation and flax fiber pretreatment: Multi-scale characterization of mechanical, thermal, and rheological properties in flax-based biocomposites

Bio-based vitrimers represent a promising class of sustainable structural materials, uniquely combining the mechanical robustness and dimensional stability of thermosets with the reprocessability and recyclability of thermoplastics. Within this framework, the presented study develops high-performance, recyclable biocomposites built on an improved polylactic acid (PLA) vitrimers matrix (PLA _v ). Converting neat PLA into a vitrimeric network yielded a matrix with enhanced thermal and mechanical stability, enabling the feasibility assessment of flax fiber (FF)-reinforced biocomposites. Rheological characterization confirmed vitrimers-like behavior, revealing a topology-freezing transition temperature (T _v ) of approximately 184°C. Four unidirectional composite systems were fabricated to evaluate the individual and combined effects of matrix vitrimerisation and fiber silanisation, including PLA/FF, PLA/SiFF, PLA _v /FF, and PLA _v /SiFF, where SiFF denoting silanized flax. Profilometry revealed homogeneous microstructures with an average layer thickness of 42 μm, while void content varied with treatment from 2.93% (PLA/SiFF) to 16.62% (PLA _v /SiFF); the lowest porosity, observed in PLA/SiFF, was attributed to enhanced fiber–matrix interfacial adhesion. Tensile testing (ISO 527-5) showed that PLA/SiFF achieved superior tensile performance and ductility, indicating efficient stress transfer across the interface. Three-point bending tests (ASTM D7264-15) confirmed that PLA/SiFF reached the highest flexural strength (130 MPa), whereas PLA _v /FF exhibited greater strain and energy absorption prior to fracture, reflecting the flexibility imparted by the vitrimerised matrix combined with natural fiber reinforcement. These results demonstrate that fiber silanization and PLA vitrimerisation act synergistically to enhance mechanical performance, offering a viable, sustainable pathway toward high-performance, recyclable biocomposites for industrial applications.