Sulfonation of Polyvinylidene fluoride: Investigation of the Microstructure by {¹H, ¹³C, ¹⁹F} NMR Spectroscopy and Mechanisms

The cycle between the preparation of sulfonated polyvinylidene fluoride (PVDF) membranes and the study of their properties allows several groups to make critical observations that lead to a hypothesis-driven process for sulfonation optimization. To facilitate this previously poorly understood process, we studied an example of PVDF sulfonation by chlorosulfonic acid (ClSO3H). We prepared sulfonated polyvinylidene fluoride (S-PVDF_X) samples with different sulfonation reaction times (X = 3, 5, and 16 h) and confirmed the sulfonation by means of Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis and NMR spectroscopy. The microstructure of the sulfonated sample S-PVDF_5 h was determined using a multinuclear {1H, 13C, 19F} NMR study. The results obtained allowed, for the first time, highlighting the dehydrofluorination of the PVDF chain in a super acidic medium and the formation of a sequence due to the esterification reaction between the alcoholic and acidic chain ends. In addition, we identified the two primary grafting sites of the sulfonic acid groups in the S-PVDF chain and demonstrated the importance of the head-to-head and tail-to-tail defects in the dehydrofluorination and sulfonation mechanisms.