Intercalation of poly(3,4-ethylenedioxythiophene) within halloysite nanotubes: Synthesis of composites with improved thermal and electrical properties

Abstract

Improvements in thermal stability and electrical conductivity of poly(3,4ethylenedioxythiophene), or PEDOT, were observed by bonding it to an acidified nano-structured tubular silico-aluminate clay, Halloysite (HNT), obtaining the corresponding doped composite (PEDOT:HNT). The composites were synthesized via suspension polymerization in aqueous media using ammonium persulfate (APS) as oxidant. Changes in the hydrodynamic conditions and APS/EDOT ratio were studied. PEDOT was also doped with hydrochloric acid and poly(p-styrene sulfonic acid). PEDOT:HNT composites were characterized by Scanning Transmission Electron Microscopy with Energy-Dispersive X-ray spectroscopy (STEM-EDS), Thermogravimetric analysis (TGA), X-Ray Diffraction (XRD), Four Point Electrical Conductivity Tests and Nitrogen Adsorption Analysis in order to obtain the surface area and the pore size distribution (PSD) of bare and coated halloysite. We observed that the halloysite, which is made of multilayer nano-tubes, does not appear to work only as a template. Halloysite nanotubes interact with PEDOT as functional dopants, resulting in PEDOT:HNT composites with high thermal stability and electrical conductivity. XRD analysis revealed intercalation of PEDOT inside the halloysite nano-tubes. XRD analysis also showed that the polymer/nano-clay interaction does not merely take place at the surface or inside the lumen, but does also modify the properties at the HNT walls, restricting PEDOT mobility. STEM-EDS of a transverse observation of a single PEDOT:HNT nano-tube confirmed that carbon and sulfur (associated to PEDOT) are found inside the halloysite walls. Moreover, STEM, XRD, measured PSD and TGA tests demonstrate a strong influence of hydrodynamic synthesis variables on the amount of free and intercalated PEDOT in HNT walls and, as a consequence, on its electrical conductivity.