Xiao Youchang, Chung Tai-Shung, Chng Mei Lin, Tamai Shouji, Yamaguchi Akihiro
Department of Chemical & Biomolecular Engineering, National University of Singapore, Singapore 117602.
J Phys Chem B. 2005 Oct 13;109(40):18741-8. doi: 10.1021/jp050177l.
The main objective of this study is to investigate the factors of the chemical structure and physical properties of rigid polyimides in determining the performance of derived carbon membranes through both the experimental and simulation methods. Four polyimides made of different dianhydrides were pyrolyzed at 550 and 800 degrees C under vacuum conditions. The resultant carbon membranes exhibit excellent gas separation performances beyond the traditional upper limit line for polymer membranes. The thermal stability and the fractional free volume (FFV) of polyimides were examined by a thermogravimetric analyzer and a density meter. The chain properties of polyimide, such as flatness, chain linearity, and mobility, were simulated using the Cerius(2) software. All above characterizations of polyimides were related to the microstructure and gas transport properties of the resultant carbon membranes. It was observed that the high FFV values and low thermal stability of polyimide produce carbon membranes with bigger pore and higher gas permeability at low pyrolysis temperatures. Therefore, polyimides with big thermally labile side groups should be preferred to prepare carbon membranes at low pyrolysis temperatures for high permeability applications. On the other side, since the flatness and in-plane orientation of precursors may lead carbon membranes to ordered structure, thus obtaining high gas selectivity, linear polyimides with more coplanar aromatic rings should be first choice to prepare carbon membranes at high pyrolysis temperatures for high selectivity applications. The location of the indan group also affects chain flatness and in-plane orientation. As a result, carbon membranes derived from the BTDA-DAI precursor have superior separation performance to those derived from Matrimid.
本研究的主要目的是通过实验和模拟方法，研究刚性聚酰亚胺的化学结构和物理性质在决定衍生碳膜性能方面的因素。由不同二酐制成的四种聚酰亚胺在真空条件下于550和800摄氏度进行热解。所得碳膜表现出优异的气体分离性能，超出了聚合物膜的传统上限线。用热重分析仪和密度计检测聚酰亚胺的热稳定性和自由体积分数（FFV）。使用Cerius(2)软件模拟聚酰亚胺的链性质，如扁平度、链线性和流动性。聚酰亚胺的所有上述表征都与所得碳膜的微观结构和气体传输性质相关。观察到聚酰亚胺的高FFV值和低热稳定性在低热解温度下产生具有更大孔径和更高气体渗透率的碳膜。因此，对于在低热解温度下制备用于高渗透率应用的碳膜，应优先选择具有大的热不稳定侧基的聚酰亚胺。另一方面，由于前驱体的扁平度和面内取向可能导致碳膜形成有序结构，从而获得高气体选择性，对于在高热解温度下制备用于高选择性应用的碳膜，具有更多共面芳环的线性聚酰亚胺应是首选。茚满基团的位置也会影响链的扁平度和面内取向。因此，由BTDA-DAI前驱体制备的碳膜比由Matrimid制备的碳膜具有更优异的分离性能。
