Hydration of sulfonated polyimide membranes.: II.: Water uptake and hydration mechanisms of protonated homopolymer and block copolymers

The hydration of sulfonated polyimide membranes in their protonated form is probed by infrared spectrometry using a recently described method. The membranes considered are the homopolymer, made of identical sulfonated repeat units, and two block copolymers composed of these units plus similar ones with no sulfonic groups in two different proportions. The experiments consist of registering series of spectra of these membranes at various hygrometries of the surrounding atmosphere. The quantitative analysis of the evolution of these spectra allows one to measure precisely the water uptake and to define in terms of chemical reactions the various hydration mechanisms that are active at a definite value of the hygrometry. It shows how the dried homopolymer significantly differs from the two dried block copolymers: in the homopolymer, a good proportion Of SO3H groups that represent 83% of sulfonate groups, cannot establish H-bonds on C=O groups that are in a relatively small number. As a consequence, all coexisting SO3- groups are H-bonded to single H3O+ cations with no extra H2O molecules. In both dried block copolymers, each SO3H group (60% of the sulfonate groups) establishes H-bonds on C=O groups that are in a sufficiently great number. These H-bonds stabilize these SO3H groups, and coexisting SO3- groups are H-bonded to cations that are found in the form of H5O2+ or H7O3+ that contain several H2O molecules. When the hygrometry increases, these differences get less marked but can be precisely defined.