Nanocomposite polyphenyleneoxide with amino-functionalized silica: structural characterization based on thermal analysis

A polymer nanocomposite based on sulfonated polyphenylene oxide with amino-functionalized mesoporous silica was designed, synthesized, and tested as a new material for proton exchange membrane (PEM preparation. Characterization of the intermediate and final products of synthesis was realized by Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and thermal analysis. Broadband Dielectric Spectroscopy (BDS) was used to determine dielectric properties including ionic conductivity. Thermogravimetric analysis has provided important information regarding the composition and thermal stability of the three compounds, subject to thermal degradation: 1) the amino-silica with cetyltrimethylammonium bromide (CTAB) template inside the pores (MS-NH₂ I), 2) the mesoporous amino-silica after removing the template (MS-NH₂ II) and 3) the polymer nanocomposite (sPPO-MS-NH₂). The thermal decomposition of the composite samples occurs in three stages: in the first, up to 150 °C, water and organic solvents were lost; the second stage, between 200-300 °C, was due to breaking the organic functionalities (-NH₂, amino and -SO₃H, sulfonic acid), and the third stage, above 400 °C was due to polymer chain degradation. The final residue at 700 °C reflects the contribution of inorganic silica. The proton conductivity, for polymeric (sPPO) and composite (sPPO-MS-NH₂) membranes was determined from BDS dates, both in dry and hydrated states. For dried samples, the higher values of proton conductivities were: 0.16 mS cm⁻¹ (sPPO, 70 °C) and 0.03 mS cm⁻¹ (sPPO-MS-NH₂, 120 °C), and the higher values of proton conductivity increased for the hydrated samples with two orders of magnitude: 36.5 mS cm⁻¹ (sPPO, 40 °C) and 22.4 mS cm⁻¹ (sPPO-MS-NH₂, 50 °C). However, the proton conductivity is still dependent on the hydration state, even for the composite membrane.