Low-temperature liquid-phase citric-acid-assisted fabrication of poly(vinyl alcohol) nanocomposite films reinforced with rice husk-derived nanoparticles

Poly(vinyl alcohol) (PVA) is a water-processable film-forming polymer, but its use in moisture-exposed environments is limited by water-assisted plasticisation and swelling. This study evaluates a low-thermal-budget, pre-casting route in which PVA, citric acid, and rice husk-derived nanofillers were held at 80 °C in the liquid phase before casting, avoiding a separate high-temperature post-curing step. Silica, lignin, lignin–silica hybrid, and quaternized lignin–silica hybrid nanoparticles were incorporated at 1 wt% to compare filler chemistry under fixed processing conditions. Increasing liquid-phase residence time from 0 to 3 h progressively increased tensile strength, with the strongest response in silica-containing systems. The silica-filled film reached the highest ultimate tensile strength of 61.22 MPa, followed by the quaternized hybrid-filled film at 57.48 MPa and the lignin–silica hybrid-filled film at 52.57 MPa, compared with 45.66 MPa for the unfilled citric-acid-modified film. Dynamic mechanical analysis showed that silica-containing fillers increased storage modulus and shifted the tan δ-derived glass transition temperature from 36 to 57.5 °C, indicating restricted segmental mobility. One-hour water uptake decreased from 87.86% for neat PVA to 24.80% after citric acid addition, and further to 16.57% and 18.02% with silica and quaternized hybrid fillers, respectively. ATR-FTIR supported citric acid incorporation and bonding-environment changes, while UV–Vis analysis showed that lignin-containing fillers enhanced UVC, UVB, and UVA attenuation. Overall, matrix structuring and rice husk-derived filler chemistry jointly controlled the mechanical, viscoelastic, water-uptake, and optical responses of PVA nanocomposite films.