The state of water in styrene‐grafted and sulfonated poly(vinylidene fluoride) membranes

Abstract

The state of water in proton exchange membranes prepared by pre-irradiation (electron beam, 100 kGy) grafting of styrene onto poly(viny1idene fluoride) films (PVDF-g-PS), followed by sulfonation (PVDF-g-PSSA), has been studied with thermal analysis, Raman spectroscopy and small angle X-ray diffraction (SAXS). Raman spectra show that, in addition to free liquid water in the membranes, single water molecules are weakly bound to the polymer backbone. Thermal analysis shows that there are three types of water molecules in the membrane; non-freezable water associated with the ionic sites, freezable free water, and freezable bound water. The amount of non-freezable water is around 10 H20/S03H (mol/mol), and is independent of the degree of grafting (d.0.g.). The amount of freezable water is strongly dependent on the d.0.g. as long as the grafting has not penetrated the whole of the film, and reaches a value of around 40 H20/S03H (mol/mol) above a d.0.g. of 50%. The conductivity of membranes containing only the non-freezable water is low, i. e. the ionically bound water alone does not form the domains necessary for proton and water transport. SAXS measurements show that water/sulfonic acid clusters in hydrated PVDF-g-PSSA membranes with a Bragg distance of 25 A are formed; these form the ion conducting channels in the membrane.

ASAXS study of styrene-grafted sulfonated poly(vinylidene fluoride) membranes

“…Our previous investigations 7 have shown the existence of sulfonic acid/water aggregates in styrene-grafted and sulfonated PVDF. The purpose of this work was to investigate with ASAXS how functionalizing the polymer into a proton-conducting membrane affects its structure.…”

“…A more detailed description of the sample preparation can be found in related articles. 7,9,10 In Table I, the main characteristics of the membranes before metal neutralization are listed. The level of sulfonation, that is, the number of sulfonate groups per number of styrene groups (N S /N PS ), is calculated from ion-exchange capacities.…”

Neutron and X‐ray Scattering: Suitable Tools for Studying Ionomer Membranes

“…[87] These membranes have been extensively studied for a possible use in fuel cells. However, the chemical stability of sulfonated polystyrene is not sufficient to permit long-term use in fuel cell conditions despite the use of divinylbenzene as crosslink agents to diminish the effect of polymer chain scission due to degradation.…”

Increases in the proton conductivity and selectivity of proton exchange membranes for direct methanol fuel cells by formation of nanocomposites having proton conducting channels

“…The bound water is the state of water associated with the membrane matrix whereas the free water is not. These two states of water exhibit different calorimetric behaviors and can be detected with DSC measurements [51,52]. After cooling the membrane to below 0 • C, free water will freeze whereas bound water is nonfreezing.…”