An understanding of the fine structures of glassy polymers is very important for the development of gas-barrier and gas-separation membranes. 129Xe NMR porosimetry is a useful technique for this type of investigations. In this study, polyimide (PI) was used as a glassy polymer. A PI sample (about 150 mg) was packed into a 5-mm thck-wall Pyrex NMR sample tube, and then the desired pressure of Xe (129Xe isotope enriched at about 81 %) was introduced into the tube. After a sufficient time for sorption equilibrium, 129Xe NMR spectra were recorded. By analyzing the peak intensity, Xe sorption isotherms of PI at various temperatures were obtained. Then, the dual-mode sorption parameters were determined. CH’ (the Langmuir saturation constant) decreased with increasing temperature. For other glassy polymers, it had been reported that the expected temperature at CH’ = 0 was very close to glass transition temperature (Tg). Tg of PI was expected to be about 430 oC, although it could not be determined by DSC. Figure 1 shows the intensity-normalized 129Xe NMR spectra of PI under various pressures at room temperature. An increasing pressure (i.e., increasing sorption amount) led to a low-field shift. In view of these chemical shift data, PI contained relatively large spaces among polymer chains, when compared with other polymers. These structural features of PI are well able to explain the gas diffusion properties.

Characterization of glassy state of polyimide by 129Xe NMR

ACS Fall 2020 Virtual Meeting

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