Preparation And Properties Of Polyimide (Pi)Nanofibers, Pi Compiste Proton Exchange Membranes And Carbonized Pi Membranes

Polyimide (PI) is of a high performance polymer with excellent high temperature performance, good mechanical performance, high dimensional stability, as well as high radiation resistance, lower temperature performance and chemical resistance. Soluble PI possess excellent processing properties against traditional PI which can be dissolved in DMF, DMAc, NMP, DMSO and other organic solvents. So that its processing performance is greatly improved compared with the traditional PI materials.In this paper, one kind soluble PI was synthesized and the electrospinning process was studied in detail. The electrospun PI membrane from optimized process parameters was used to prepare composite proton exchange membrane and carbonized membrane, and properties of the resultant membranes were studied. The main results are as follows:1. A diamine monomer,3,3’5,5’-tetramethyl-4,4’-diaminodiphenyl-butyltoluene, was synthesize via one step from2,6-dimethylaniline and benzaldehyde. The obtained diamine was subsequently employed to prepare a soluable polyimide (3,3’5,5’-tetramethyl-4,4"-diaminodiphenylbutyltoluene-co-3,3’,4,4’-biphenyltetracarboxylic dianhydride). The chemical structure and purity of monomer were confirmed by means of element analysis, viscosity test and FT-IR analysis. The obtained PI polymer processed high molecular weight (the inherent viscosity is1.13dL/g), excellent thermal stability and solubility. The electrospinning solution was prepared by dissolving PI in DMAc, and the parameters such as concentration, extrusion rate, spinning voltage, receiving distance, and salt/surface active agent were studied in detail. PI mats with a diameter of249nm and diameter deviation of0.047could be prepared with the optimized electrospinning parameters of18wt.%concentration,5μl/min extrution rate,18kV voltage,18cm distance, and1wt.%CTAB content. By high speed rotating drum receiver and adding MWCNTs, PI and PI/MWCNTs electrospun mats with good alignment, higher crystallinity, and mechanical properties were prepared. The tensile strength of PI mat could be improved from58.37MPa to91.64MPa by adding3wt.%MWCNTs at a drum speed of9m/s.2. PI/SiO2/Nafion composite proton exchange membranes were prepared by electrospinning of PI/SiO2/DMAc followed by immersing in Nafion solution, or by immersing PI membranes in SiO2/ethanol and Nafion solution. PI/SiO2/Nafion composite membranes had good mechanical properties, for instance the tensile strength of PI-SiO2(300wt.%)-Nafion was42.34MPa, the elasticity modulus was2.36GPa. Proton exchange rate of PI-SiO2(300wt.%)-Nafion under100%R.H. and30℃was0.067S-cm-1, the water absorption rate was less than30%, the expansion rate was less than15%, which indicated a good dimensional stability. In the constant temperature and humidity box, under the circumstance of10%R.H., the capability of proton exchange keep rate of composite membranes were much better than Nafion115. It was found the rate of chemical water absorption in composite membrane increased obviously with the increasing of SiO2content, chemical absorption rate of PI-SiO2(300wt.%)-Nafion was67.1%, while for Nafion115it was only19.7%. The composite membranes possessed a good ability of maintaining proton exchange rate under low humidity or high temperature, due to the water absorption ability of nano-SiO2particles. PI/SiO2was a good candidate as the base material for proton exchange membrane.3. Carbon nano fiber (CNF) membranes were prepared by the carbonization of electrospun PI membranes in tube furnace at1200℃in N2atmosphere with varying acetyl acetone and iron (AAI) content. The diameter of obtained CNF ranged from132nm to250nm. With the increasing of AAI content, fusion of nanofibers decreased, carbon yield increased to40.12%maximum. The mechanical properties of CNF membrane was improved slightly, for example, the tensile strength of3wt.%AAI/CNF was11.21MPa and the modulus was1.25GPa. XRD, FT-IR, and Raman analysis indicated the CNF had turbostratic graphite structure. BET test showed the AAI catalytic CNF membranes processed higher specific surface area up to199.98cm2·g-1. Conductivity test illustrated the conductivity increased as the content of AAI increasing. For AAI3wt.%PI carbonized membrane the conductivity was17.7S·cm-1, which indicated the PI based CNF membranes could be used as electrode materials for super capacitor.