Preparation And Performance Of Low-temperature Pyrolytic Carbon Membrane From Polyimide

Membrane separation is a new technique developed in recent decades. It has got a widely research and used due to its high separation efficiency, simple operation and low energy consumption. However, the large scale applications of polymeric membrane which was widely utilized in the market are greatly limited by their some shortcomings. For example, the low thermal and chemical stability, low gas permeaselectivity which did not exceed the "Robsen" upper bound and so on. Carbon membrane as a novel membrane has greatly attracted people's attention since their nanoporous structure and excellent gas separation performances.Low-temperature pyrolytic membrane was prepared from the polymeric precursors in the atmosphere of inert gas, vacuum or air (only occurring part oxidizing reaction). It has the better application prospects because it combined the polymeric membrane's high toughness with carbon membrane's high gas permeability properties. In addition, Low-temperature pyrolysis is the key stage of polymer transferring to carbon during the carbonization. Therefore, the low-temperature pyrolysis directly affects the carbon structure, pore structure and gas separation property of carbon membrane.In this work, Kapton type polyimide was selected as the precursor to prepare low temperature carbon membranes. ATR-FTIR, TG, TG-MS, DSC, XPS, EA, XRD and Gas Permeability were used to investigate the effects of pyrolytic temperature, atmosphere on the chemical structure, microstructure and gas permeability. Results shows that low temperature pyrolysis can be divided into two stages. The first stage is from room temperature to 300℃. The desorption of water, remove of solvent and imidization were happened in this stage. The second stage is from 300℃to 550℃. The main reaction happened in this stage is crosslinking reaction which caused by radical came from polyimide ring crack and ether bond ruptures. In addition, O2 in the air promotes the pyrolysis and crosslinking reaction. Therefore, more developed crosslinking structure was formed in air atmosphere. Microstructure changed because of the chemical structure variation.That is the polyimide tends to aromatic heterocyclic and carbon structure, Amorphous carbon structure was formed at the temperature of 525℃(air) and 550℃(nitrogen). Moreover, the microstructure changing further affects the gas separation performance of Low-temperature pyrolytic membrane. Before 450℃, There is unchanges in gas permeability and little increase in gas selectivity for Low-temperature pyrolytic membrane. After 450℃, gas permeability and selectivity of Low-temperature pyrolytic membrane was enhanced obviously. This implies that the separation mechanism of Low-temperature pyrolytic membrane was changed from solution diffusion to surface diffusion. In addition, the gas permeability of membrane prepared in air is larger than that prepared in nitrogen. But the selectivity is.contrary to gas permeability. Gas permeability and selectivity of Low-temperature pyrolytic membrane for O2/N2 prepared at 500℃in air and nitrogen are 7.86, 1.13Barrer and 1.32,4.16 respectively.