Modification Of Polyimide Membranes With Silicon Compounds And Its Pervaporation Performance

Polyimides (PI) have unique physicochemical properties: strong resistance to high temperature, radiation and chemical resistance, good mechanical strength, superior insulation properties, etc. The properties make polyimides valuable materials which can be used widely in different branches of industry, such as aviation, aerospace, electron, chemical industry and mechanical industry, etc. With the rapid development of modern science and technology some new materials with special properties are required. Hybrid materials emerged in this context. Hybrid materials which combine the advantages of their components can play the multifunctional role. For its unique properties polyimide hybrid materials received much attention. In this dissertation focusing on the hybrid modification with silicon compounds four studies were conducted.1. Determination of process conditions in Synthesis of polyamic acid and thermal imidization. The polycondensation of HQDPA-ODA polyimide were carried out in two stages. The optimum conditions for synthesis of polyamic acid are present as: HQDPA was added into ODA solution in batches, the molar ratio of HQDPA: ODA is 1.01:1, the solid content of this solution is 10wt%, reaction temperature is 10℃, and reaction time is 6h. Imidization was carried out in successive isothermal steps of 1 h for each 80, 150, 240℃, and 0.5h for 280℃. Fourier transfer infrared spectrophotometer (FT-IR) spectra and element analysis show that the film nearly finished a complete imidization. The reaction mechanisms and kinetic models for imidization were investigated by Thermogravimetric analysis (TGA). The results suggest the imidization is a first, two step reactions. Activation energies for the fast process and the second slow process are 38.14 KJ/mol and 37.99 KJ/mol, respectively. Preexponential factors for the two steps are 172.31S^-1 and 22.08S^-1, which account for the difference in the rate constants.2. Preparation of polyimide/SiO₂ hybrid membranes and the effects of coupling agents on properties of the polyimide/SiO₂ hybrid membranes. Polyimide and polyimide/SiO₂ hybrid membranes were prepared via sol-gel process while Tetraethyl orthosilicate(TEOS) was added as inorganic precursor. Results show the thermal stability of PI/SiO₂ increases with the increasing of SiO₂ content, but this increase of Td is not remarkable. Elongation at break of the hybrid membrane containing 10wt% SiO₂ increases by 32.5%, strength 31.3%, and modulus 26.1% in contrast to that of the pristine PI. But more introduction of SiO₂ decreases the strength and modulus. Scanning electron microscopy (SEM) indicates that the decrease may be caused by the aggregation of silica, which would lead to stress concentration at the silica/PI interfaces.In order to improve the compatibility between polyimide and silica, inter-molecular coupling agent, GOTMS was introduced into PI/SiO₂ hybrid membranes. The aggregation of silica particle was efficiently prevented. The particle size in PI/SiO₂ hybrid membranes containing 10wt% SiO₂ was reduced from 1～1.5μm to 400nm, and that containing 10wt% SiO₂ reduced greatly too. The Td of PI/SiO₂ hybrid membranes decreased by 10～25℃after the introduction of GOTMS. The strength of pristine PI and hybrid membranes containing 10wt% SiO₂ declined while that containing 20wt% and 30wt% SiO₂ heightened. The elongation at break and modulus lowered.After the introduction of inter-molecular coupling agent GOTMS and intra-molecular coupling agent APrTEOS at one time, the compatibility between organic and inorganic phase was further improved. The particle size in PI/SiO₂ hybrid membranes containing 10wt% SiO₂ was reduced from 1～1.5μm to 100nm, and particle in that containing 20wt% and 30% SiO₂ nearly disappeared. The Td of PI/SiO₂ hybrid membranes decreased markedly by 120℃or more. The strength of pristine PI and hybrid membranes containing 10wt% SiO₂ declined while that containing 20wt% and 30wt% SiO₂ heightened. The elongation at break and modulus lowered. However comparing with these hybrid membranes with equal SiO₂ the elongation at break lowered, and modulus rise.3. Preparation and characterization of polyimide-block-polydimethylsiloxane (PI-PDMS). To facilitate the processibility of PI-PDMS, we designed and synthesized polyamic acid ester-block-polydimethylsiloxane which has a better solubility and processibility than polyamic acid-block-polydimethylsiloxane. Experiments proved the synthesis route is feasible. Comparing with the pristine PI Td of PI-PDMS decreased mildly by 22～37℃, but still was above 550℃. Investigation of morphology of PAE-PDMS and PI-PDMS provided that micro-phase separation in the course of solidification, and that the micro-phase separation had a tendency to be vertical with surface of membranes. The domain of phase separation became bigger with the increasing PDMS. When content of PDMS rise to 20wt% spherical particles isolated from the continuous matrix. The successive thermal imidization made the domain of phase separation bigger further and structure looser. An interpenetrating network polymer was developed which should be a promising membrane materials for pervaporation.4. Pervaporation performance of PI/SiO₂ and PI-PDMS membranes. In separation of 85wt% aqueous ethanol solution at 65℃the pristine PI had preferential permeability to water over ethanol, and exhibited a flux of 60.7g/m2·h and a separation factor of 59. The apparent permeation activation energy for ethanol and water was 33.6 and 76.2kJ/mol respectively. PI/SiO₂ hybrid membranes had lower flux and higher separation factors than the pristine PI. Apparent activation energy of water and EtOH through polyimide hybrid membranes increases with increasing of SiO₂. When PDMS was blocked in the PI, PI-PDMS membranes exhibited high separation performance. The flux became 2～3 times high than the pristine PI while separation factor is sill in a high level.