Sulfonation of Polyvinylidene\nfluoride: Investigation\nof the Microstructure by {<sup>1</sup>H, <sup>13</sup>C, <sup>19</sup>F} NMR Spectroscopy and Mechanisms

The cycle between the preparation of sulfonated polyvinylidene\nfluoride (PVDF) membranes and the study of their properties allows\nseveral groups to make critical observations that lead to a hypothesis-driven\nprocess for sulfonation optimization. To facilitate this previously\npoorly understood process, we studied an example of PVDF sulfonation\nby chlorosulfonic acid (ClSO₃H). We prepared sulfonated\npolyvinylidene fluoride (S-PVDF_<i>X</i>) samples with different\nsulfonation reaction times (<i>X</i> = 3, 5, and 16 h) and\nconfirmed the sulfonation by means of Fourier transform infrared spectroscopy,\ndifferential scanning calorimetry, thermogravimetric analysis and\nNMR spectroscopy. The microstructure of the sulfonated sample S-PVDF_5\nh was determined using a multinuclear {¹H, ¹³C, ¹⁹F} NMR study. The results obtained allowed, for the\nfirst time, highlighting the dehydrofluorination of the PVDF chain\nin a super acidic medium and the formation of a sequence due to the\nesterification reaction between the alcoholic and acidic chain ends.\nIn addition, we identified the two primary grafting sites of the sulfonic\nacid groups in the S-PVDF chain and demonstrated the importance of\nthe head-to-head and tail-to-tail defects in the dehydrofluorination\nand sulfonation mechanisms.