PEDOT:PSS nano-gels for highly electrically conductive silver/epoxy composite adhesives

In this work, we report a methanol-facilitated approach to directly use aqueous Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) (PEDOT:PSS) in the silver/epoxy composites for preparation of highly electrically conductive adhesives (ECAs) and an investigation of the interaction between PEDOT:PSS nano-gels and silver microflakes. PEDOT:PSS nano-gel (18 < d < 30 nm) aqueous dispersion is immiscible with epoxy resin and difficult to incorporate into the conventional silver-filled ECAs. To overcome this challenge, we used methanol to facilitate the dispersion of PEDOT:PSS and silver microflake in epoxy resin. The synergetic interactions between PEDOT:PSS and silver and the effect of methanol were investigated using dynamic light scattering (DLS), atomic force microscopy, Kelvin probe force microscopy, and scanning electron microscope. When PEDOT:PSS was exposed to methanol, its morphology changed from coil to coil/linear structure; the contact potential difference between silver microflake and PEDOT:PSS increased from 9.47 to 22.56 mV, showing an increased conductivity between PEDOT:PSS and silver microflake. It was found that the introduction of a small amount of PEDOT:PSS (0.1 wt%) to the conventional ECA with 60 wt% silver microflake remarkably improved the electrical conductivity from 104 to 386 S/cm. A significantly high conductivity of 2526 S/cm was achieved by further increasing the PEDOT:PSS concentration to 1 wt%. The impact of PEDOT:PSS on the adhesive bonding strength towards copper substrate was also examined; the bonding strength slightly decreased when < 1 wt% PEDOT:PSS was used, but abruptly dropped when PEDOT:PSS content was further increased beyond 1 wt%. The incorporation of the optimal 1 wt% PEDOT:PSS into conventional ECAs with 60% silver microflake greatly increased the electrical conductivities by 25 times with limited impact on the shear strength. The results provide insights to the synergetic interplay of conductive polymer and metallic fillers, and might have profound technical implications on the development of advanced conductive composites.