Incorporating benzimidazole units into soluble polyimides for improving properties and application in organic solvents' water content detection

Abstract

To investigate the influence of benzimidazole units on the properties and application of soluble polyimides (PIs), three cyano-containing triphenylmethane-(TPM-) and benzimidazole-based co-polyimides (co-PIs, namely BPADA-CN-NH, ODPA-CN-NH and 6FDA-CN-NH) and three cyano-containing TPM-based homo-polyimides (homo-PIs, namely BPADA-CN, ODPA-CN and 6FDA-CN) for comparison were prepared. Results show that three co-PI films possess some better physical properties, such as higher glass transition temperature (T_g) and mechanical properties than those of their corresponding homo-PI films without benzimidazole units, due to the rigidity of benzimidazole units and the formation of intermolecular H-bonding. Another unique feature, after BPADA-CN-NH reacts with fluoride ion (F⁻), the resulting BPADA-CN-NH plus F⁻ system (BPADA-CN-NH•F) can be developed into a ratiometric sensor suitable for detection of trace water in various organic solvents such as DMSO, THF and DMF by means of UV-vis absorption spectra and color change. The quantitative detection of the lower solvents' water content is available using a good linear relationship between the ratio of the intensities of the two UV-Vis absorption bands and the lower solvents' water content region. The detection limits (DLs) of BPADA-CN-NH•F for water are calculated as low as 0.00183 vol% in DMSO, 0.00267 vol% in THF, and 0.00433 vol% in DMF, respectively. The sensing mechanism based on F⁻-induced deprotonation of BPADA-CN-NH to BPADA-CN-NH•F followed by re-protonation with water was confirmed by ¹H NMR studies. Finally, the BPADA-CN-NH film was also used for visual water detection in CH₃CN.