Thermally Stable and Tough Coatings and Films Using Vinyl Silylated Lignin

We modified lignin, a renewable biomacromolecule with high carbon density, with silicon-containing vinyl groups via a highly efficient silylation reaction that achieved ∼30% substitution of lignin's hydroxyl units. This exothermic process was carried out in the melt state, in situ, in a reactive extruder. ¹H, ¹³C, and ³¹P NMR and FTIR confirmed the success of the silylation and were used to access the reactivity of the vinyl silylated lignin for copolymerization with polyacrylonitrile (PAN). Copolymers of the unmodified lignin and PAN were also produced as a reference. Importantly, the rheological behaviors of the copolymers of lignin and PAN were suitable for application in surface coating and films that were not possible if lignin or physical mixtures of lignin and PAN were used. Glass surfaces were treated via solution casting followed by oven drying, yielding films that were evaluated regarding their morphology (SEM) and thermal properties (TGA and DSC). The films produced with copolymers based on vinyl silylated lignin displayed a Young modulus of 486 MPa, an ultimate stress of 18 MPa, and a strain of 55% (DMA). These results indicate a much tougher system than that achieved after copolymerization of the unmodified lignin with PAN (ultimate stress and strain of less than 10 MPa and 10%, respectively). Overall, the introduced SiC and SiOC bonds improved lignin macromolecular mobility and facilitated its processability due to the remarkably low viscosity of the system. Our results show promising opportunities for lignin utilization as a precursor of copolymers, as illustrated here with PAN, for the synthesis of flexible films and other advanced materials.