Synthesis and Characterization of Triphenylamine Based Polyimides and their Nanocomposites

Abstract

Five new triphenylamine based amine monomers with benzoxazole (DA-1), carbazole (DA-2), 4-(quinoline-8-yloxy) (DA-3) and benzimidazole pendant groups (DA-4 and DA-5) respectively and a triphenylamine based tetramine monomer (TA) were successfully prepared in high purity and good yield. The synthesized amine monomers were then polymerized with four commercially available dianhydrides such as pyromelleticdianhydride (PMDA), 3,3’,4,4’-benzophenonetetracarboxylic dianhydride (BTDA), 4,4’-(hexafluoroisopropylidine) diphthalicanydride (6FDA) and 4,4’- oxydiphthalic anhydride (ODPA) to prepare five new series of linear polyimides (1- PIs,2-PIs,3-PIs,4-PIs ad 5-PIs) and a series of hyperbranched polyimides (TA-PIs). The polyimides were synthesized by two step conventional thermal imidization method. All the synthesized precursors and amine monomers were characterized by their physical data, elemental analysis, FTIR, 1HNMR and 13CNMR spectroscopic techniques which confirmed the presence of all expected moieties in their proposed structures. GPC analysis demonstrated the attainment of reasonably high molecular-weight PIs. TGA and DSC analytical techniques evaluated the thermal stability of prepared polyimides. The photophysical aspects of the polyimides were explored by UV-Visible and photoluminescence spectroscopy which revealed their violet-blue and blue-green emissive properties with reasonable quantum yields. Cyclic voltammetry demonstrated reasonably low onset oxidation potentials (Eonset 0.32 -1.3 V) with high lying HOMO energy levels (-4.7 to -5.7 eV) suggesting the hole transport properties of prepared materials. To widen the scope of prepared PIs and enhance their electrical conductivity polyimide graphene oxide (PI-MGO) composites were prepared. Two different series of MGO-PI composites were prepared by using a linear polyimide (1-PMDA) and a hyperbranched polyimide (TA-PMDA) as polyimide matrices. The dispersion of GO was improved by developing like-like chemical interactions ensured by the presence of similar structural units e.g. TPA and imide linkages in both the matrix and the reinforcement phase. A reasonable shift in the glass transition temperature (Tg) of the PI-MGO to higher temperature confirmed the restricted segmental motion of polymer backbone due to increased interfacial interactions. The enhanced electrical conductivity values reaching 6.48 x10-5 S/m to 9.36 x10-4S/m at 30 oC and the presence of high lying HOMO levels and intense blue emissive solution property were attributed to the improved interphase interactions and uniform dispersion of modified GO nanosheets into the PI matrix.