Multifunctional thermostable oligomer based on para-phenylenediamine and resorcinol with hierarchical micro- and submicron structure

Abstract

This work reports the synthesis and comprehensive characterization of semiconductive, π-conjugated oligomers based on 1,3-benzenediol (resorcinol) and p-phenylenediamine (PPD), obtained via oxidative polycondensation under ultrasonic irradiation in acidic medium, initiated by potassium persulfate and performed without surfactants. The synthesized materials were characterized by different physico-chemical methods. UV–Vis spectroscopy revealed distinct absorption bands corresponding to π–π* and n–π* transitions, indicating extended electronic delocalization along the conjugated oligomer backbone. Extrapolation of the linear region of the (αhν)² versus photon energy (hν) plot yielded a direct optical band gap of 3.59 eV, confirming the wide-bandgap semiconducting nature of the synthesized material. FTIR, NMR, and elemental analyses confirmed the formation of co-oligomers with an approximate PPD-to-resorcinol ratio of 4:1. Thermogravimetric analysis demonstrated high thermal stability of the materials up to 550 °C. XRD analysis indicated a predominantly amorphous structure with a minor semicrystalline contribution. SEM imaging revealed that the co-oligomer exhibits hierarchical micro- and submicron particle sizes in the range of 0.1–3 μm. The surfactant-free co-oligomer pellets exhibited an average electrical conductivity of 7.94 × 10⁻⁸ S·cm⁻¹. The broader ESR signal of the oligomer, exhibiting a mixed Lorentzian–Gaussian character, suggests more localized unpaired electrons and weaker exchange interactions compared to DPPH. The synthesized copolymers, obtained as black powdery materials, contain amine (–NH₂) and hydroxyl (–OH) functional groups and are soluble in polar organic solvents.