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

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) were prepared and compared. Results show that the three co-PI films possess 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 is that after BPADA-CN-NH reacts with the fluoride ion (F⁻), the resulting BPADA-CN-NH plus F⁻ system (BPADA-CN-NH·F) can be developed into a ratiometric sensor suitable for the 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 low water content of three solvents can be quantitatively detected by utilizing the good linear relationship between the ratio of the intensities of the two UV-vis absorption bands and the low water content range. The detection limits (DLs) of BPADA-CN-NH·F for water are calculated to be as low as 0.00183 vol% in DMSO, 0.00267 vol% in THF, and 0.00433 vol% in DMF. 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 detection of water in CH₃CN.