Influence of Kraft Lignin on the Structure and Cross‐Linking of Poly(Vinyl Alcohol)/Chitosan Composite Membranes

ABSTRACT

Poly(vinyl alcohol)/chitosan (PVA/CS) composite membranes containing kraft lignin (KL) were prepared by solvent casting followed by thermal cross‐linking with sulfosuccinic acid (SSA) in a hot press. The effect of incorporating varying amounts of KL filler on the composite structure, properties, and cross‐linking behavior was thoroughly studied. Interactions between KL and the PVA/CS matrix limit the incorporation of SSA due to a reduced availability of functional groups, particularly at higher KL contents. Consequently, the ion exchange capacity and water uptake decrease, ultimately reducing the proton conductivity and diffusion of the membranes. Higher KL loadings promote stronger covalent cross‐linking, resulting in denser surface morphologies, reduced dimensional swelling, and restricted polymer chain mobility. In addition, disruption of the hydrogen‐bonding network is observed at high lignin contents due to the increased cross‐linking density and possible lignin aggregation, leading to brittle membranes with reduced stability. In contrast, composites with low lignin content favor the formation of strong hydrogen bonds, thereby enhancing oxidative stability, onset thermal degradation, and tensile strength. Controlled lignin incorporation prior to thermal cross‐linking allows balancing the intermolecular interactions within the composite and tailoring the membrane properties for use in energy‐related applications.