ABSTRACT
Polyimides have been important membrane materials for gas separation for many years. Nevertheless, there is still a strong need for further improved transport properties of these polymers particularly regarding small molecule permeabilities and the selectivities for certain gas pairs. One problem is that improving the permeability goes normally along with a loss of selectivity. Numerous experiments have been performed to žnd out which factors might improve the permeability of a polyimide. Higher permeabilities can be, for example, reached by increasing the ortho alkylation in the diamine; inserting –CF3 groups, SiO(Me)2, Si(Me)3, or C(Me)3 groups into the dianhydride; increasing rigidity; decreasing the diamine length; or inserting bulky groups. It also turned out that producing a higher free volume leads to a higher permeability [1-18]. Kim and coworkers [19-23] designed and examined various bulky silicon-containing structures and their carbon-containing counterparts (Figure 11.1) with regard to their oxygen permeabilities (Table 11.1). The permeabilities between each pair of polyimide (e.g., Si-containing structure and C-containing
11.1 Introduction .................................................................................................. 153 11.2 Computational Details .................................................................................. 155 11.3 Results and Discussion ................................................................................. 157 11.4 Conclusions ................................................................................................... 167 Acknowledgment ................................................................................................... 167 References .............................................................................................................. 167
counterpart) are, except for one case, higher for the Si than for the respective C case. With regard to the diffusion (D) and the solubility (S), no experimental data for these types of polyimides are available. To validate the chosen models, experimental wide angle x-ray diffraction (WAXD) measurements reported in the literature [21] are available for two polyimides (A-1 and B-1). The major model properties considered to assess the critical structural differences between the polymers are the fractional
free volume distribution (FFV) and the mean squared displacement (MSD) of the respective polymer atoms. These properties should be closely related to the transport properties of interest.
