Abstract
pH-responsive membranes prepared by treating chitosan (Ch) with glutaraldehyde (GA) or with GA and sulfuric acid (SA) were studied. The structure and properties of the membranes were characterized by Fourier transform infrared (FTIR) spectroscopy, wide-angle X-ray diffraction (WAXD), scanning electron microscopy (SEM), atomic force microscopy (AFM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and elemental analysis. The effect of the cross-linking process both on the dynamic swelling behaviour of the hydrogel membranes and on the mechanism of water transport through those membranes was investigated in buffer solutions of different pH (1.2–9.5). The mechanism of water transport through the hydrogel chitosan membranes was affected by membrane composition and pH of the swelling medium. Non-cross-linked Ch membrane showed a non-Fickian swelling behaviour in the pH range 6.5–9.5. Chitosan membrane cross-linked with GA (Ch/GA) showed less-Fickian or Fickian swelling behaviour in all buffer solutions. In the case of chitosan membrane cross-linked with GA and SA (Ch/GA/SA), at low pH (lower than the pK a of the hydrogel) the water transport was controlled more by polymer relaxation than by penetrant diffusion. The experimental data clearly suggested that the swelling process in all buffer solutions obeyed second-order kinetics. Values of an apparent swelling rate constant for Ch/GA and Ch/GA/SA membranes were of the same order of magnitude for acidic and neutral swelling media but they increased for alkaline solutions.
Highlights
In recent years, stimuli-responsive membranes have received much attention owing to their many potential applications in drug delivery systems, chemical separations, water treatment, chemical sensing, bioreactors, etc
Two main bands that can be seen in the Fourier transform infrared (FTIR) spectrum of cross-linked with GA (Ch/GA)/sulfuric acid (SA) (Fig. 2) in the frequency range of 1650– 1500 cm−1 derive mainly from the asymmetric and symmetric N–H deformation vibrations in protonated amines, but the initial amide I, amide II and imine bands are possibly overlapped by these vibrations [7]
The new band observed at 617 cm−1 in the FTIR spectrum of Ch/GA/SA can be attributed to the S–O bending vibration in SO42− ions [7]
Summary
Stimuli-responsive membranes have received much attention owing to their many potential applications in drug delivery systems, chemical separations, water treatment, chemical sensing, bioreactors, etc. Stimuli-responsive membranes change their physical properties in response to changes in environmental conditions, such as pH, solution ionic strength, temperature, concentration of specific chemical species, electric and magnetic field [1,2,3]. Changes in the physical properties of the membrane in response to changed environmental conditions can lead to changes in the mass transfer and interfacial properties of the membrane. Responsive membranes are formed from stimuli-sensitive materials (polymers or copolymers) or by modification of existing membranes by various chemical/physical processes to incorporate stimuli-responsive polymers [1, 2]. An example of a pH-sensitive polymer is the natural polymer chitosan
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