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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 SO(3)H 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 SO(3)(-) groups are H-bonded to single H(3)O(+) cations with no extra H(2)O molecules. In both dried block copolymers, each SO(3)H 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 SO(3)H groups, and coexisting SO(3)(-) groups are H-bonded to cations that are found in the form of H(5)O(2)(+) or H(7)O(3)(+) that contain several H(2)O molecules. When the hygrometry increases, these differences get less marked but can be precisely defined.