FTIR–ATR studies of diffusion and perturbation of water in polyelectrolyte thin films. Part 4. Diffusion, perturbation and swelling processes for ionic solutions in SPEES/PES membranes
Section snippets
Introduction and background
Extensive studies [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] have been made over the last two decades of the swelling processes (volume transitions) in polyelectrolyte membranes in relation to their application in industry as separation membranes, as sensors and actuators, and for their extensive use in drug encapsulation and delivery [11], [12], [13]. A wide range of polyelectrolyte membranes have been studied, both in the absence and in the presence of electrolyte solutions with the
Chemicals
SPEES/PES with the following structure, was obtained from NWW Acumem Limited and used as received.
The copolymer is sulphonated only at hydroquinone residue ∗ so that ion exchange capacity of sulphonated polymer is pre-determined by copolymer composition. Sulphonation occurs in 98% H2SO4 without degradation.
Degree of sulphonation is characterised by ‘S–No’, whereMost of the measurements reported here were made on S8 material which
Measurements and data treatment
The diffusion of water into the SPEES/PES films was measured by observing the ν(OH) stretching band, Fig. 2, in the presence of a 0.2 M solution of each of the electrolytes studied. This peak was integrated from 2945 to 3800 cm−1 in order to obtain the integrated absorption at each mean time. The integrated intensities as a function of time, are shown in Fig. 3 for the counter cations studied. It is seen that the intensity of the OH band of diffusing water increases rapidly and reached an
Results and discussion
The diffusion of ionic species in solution (in water) into a polyelectrolyte film shields the repulsion of the polymer chains (carrying SO3− groups) [1], [2], [3], [4], [5], [6], [7], [8]. This should lead to a dramatic reduction in the degree of ‘swelling’ of the polymer (which leads to an increase in the interchain average separation and therefore an increase in volume diffusion rate). Thus, an influx of C+A− species is expected to lead to a decrease in the water diffusion rate and a
Summary and conclusions
We have demonstrated that the major factor affecting the differential diffusion rate of water from electrolyte solutions into SPEES/PES polyelectrolytes is the degree to which repulsion-induced swelling is mediated by SO3−/SO3− screening by different cations. This effect broadly follows the hydration shell stability of the ions concerned but detailed ‘order’ variations are found in the measured swelling for different cations. This is probably due to variations in the –SO3H dissociation
Acknowledgements
The provision of polymer samples from NWW (Acumen) Ltd and the valuable discussions with Dr P. Cardew are gratefully acknowledged.
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