Bushy-surface hybrid nanoparticles for developing epoxy superadhesives

Hydroxyl-rich halloysite nanotube (HNT)/silica nanosphere (SiO₂) core/shell particles were functionalized with multi-arm hyperbranched polyethylenimine macromolecule to develop epoxy superadhesives for metal/polymer composite interfaces. A variety of techniques including FTIR, TGA, XRD, XPS, and TEM were employed to precisely monitor the surface and bulk chemistry of HNTs flecked with silica particles and subsequently with hyperbranched polymer. Amine groups of the functionalized core/shell nanoparticles were exchanged with imide ones through the reaction with pyromellitic acid dianhydride to make the system thermally stable. Incorporation of the developed reactive bushy-surface hybrid nanoparticles into epoxy resulted in a highly crosslinkable superadhesive with exceptional thermal and mechanical properties; so that addition of only 0.5 wt% of core/shell nanoparticles caused a rise of about 31 °C in glass transition temperature with respect to the blank epoxy, suggesting very high potential of particles to cure with epoxy. Cure of adhesives containing 2 wt% of bushy-surface hybrid nanoparticles via nonisothermal differential scanning calorimetry was indicative of facilitated crosslinking, as detected by a higher enthalpy of ca. 106 J/g at low heating rate of 5 °C min⁻¹ in a narrower temperature interval of ca. 13 °C compared to blank epoxy, visualized by the Cure Index universal dimensionless criterion. From property standpoint, a rise of ca. 130% in lap shear strength was surprisingly observed for adhesives containing bushy-surface hybrid nanoparticles.