Tuning APTES Silanization Window in PLA/Cellulose Nanofiber Biocomposites: Optimizing Interfacial Adhesion, Mechanical Strength, and Thermal Stability

Polylactic acid (PLA) is limited by inherent brittleness and poor thermal stability, hindering its engineering applications. This study systematically investigated cellulose nanofibers (CNFs) silanized with 3- aminopropyl-triethoxysilane (APTES) at mass ratios of 2:1 (R5) and 4:1 (R7), incorporated into PLA at 0.5 wt% to 1.5 wt% levels. Unmodified 1.0 wt% CNFs (R2) enhanced impact strength (10.15 ± 0.50 kJ·m⁻²) via pull-out toughening. Moderate silanization (R5) improved interfacial adhesion, achieving balanced tensile strength (50.2 ± 1.4 MPa), modulus (321 ± 16 MPa), and elongation (5.0 ± 0.2%). Stronger silanization (R7) increased modulus (545 ± 26 MPa) but reduced elongation (4.04 ± 0.18%), inducing brittleness. Fourier transform infrared analysis confirmed reduced hydroxyl groups and Si–O–Si/Si–O–C bond formations. Uniform interphases were observed in R5, while R7 exhibited voids and heterogeneity in scanning electron microscopy. Thermogravimetric analysis revealed higher onset (Tonset 352 ± 2 °C) and maximum decomposition (Tmax 360 ± 2 °C) temperatures for R5 compared to R2 (335 ± 2 °C, 342 ± 2 °C). This study validates an "optimized silanization window" (2:1 ratio), enabling simultaneous enhancements in stiffness, toughness, and thermal stability for sustainable PLA/ CNFs biocomposites, suitable for industrially compostable packaging and biomedical applications.