Polymerization of α-Lipoic Acid on Silica Particles for Adhesive Applications

Structural adhesives have attracted increasing attention as sustainable alternatives to petroleum-based adhesives because of their strong interfacial bonding and environmentally friendly processing. However, natural adhesive systems often exhibit insufficient mechanical strength and poor structural stability, limiting their practical applications. Silica nanoparticles (SiNPs) have been widely explored as reinforcing materials owing to their high surface area, chemical stability, and versatile surface chemistry.

α-Lipoic acid (ALA), a naturally occurring disulfide-containing molecule, is an attractive natural adhesive component because of its dynamic disulfide chemistry and strong interfacial interactions. However, ALA readily polymerizes, while poly(α-lipoic acid) undergoes depolymerization, resulting in poor structural stability and reduced adhesive performance. To overcome this limitation, N-hydroxysuccinimide (NHS) functionalization was introduced onto the SiNP surface to stabilize the polymer network through covalent interactions and suppress depolymerization.

In this study, silica nanoparticles (~400 nm) were functionalized with amine groups via silane chemistry and subsequently modified with NHS. Surface functionalization was confirmed by FT-IR, DLS, and SEM analyses. FT-IR spectra showed characteristic peaks of amine and NHS groups, DLS revealed an increase in hydrodynamic diameter after each modification step, and SEM confirmed that the nanoparticles maintained a uniform spherical morphology. The adhesive performance of ALA-incorporated NHS-functionalized SiNPs will be evaluated using a universal testing machine. This study presents a strategy for improving the structural stability of poly(α-lipoic acid) and developing sustainable natural adhesive systems for structural bonding, paper lamination, and advanced packaging