Swelling and Diffusion in Polymerized Ionic Liquids-Based Hydrogels

Abstract

Hydrogels are one of the emerging classes of materials in current research. Besides their numerous applications in the medical sector as a drug delivery system or in tissue replacement, they are also suitable as irrigation components or as immobilization matrices in catalysis. For optimal application of these compounds, knowledge of the swelling properties and the diffusion mechanisms occurring in the gels is mandatory. This study is focused on hydrogels synthesized by radical polymerization of imidazolium-based ionic liquids. Both the swelling and diffusion behavior of these hydrogels were investigated via gravimetric swelling as well as sorption experiments implemented in water, ethanol, n-heptane, and tetrahydrofuran. In water and ethanol, strong swelling was observed while the transport mechanism deviated from Fickian-type behavior. By varying the counterion and the chain length of the cation, their influences on the processes were observed. The calculation of the diffusion coefficients delivered values in the range of 10−10 to 10−12 m2 s−1. The gravimetric results were supported by apparent diffusion coefficients measured through diffusion-weighted magnetic resonance imaging. A visualization of the water diffusion front within the hydrogel should help to further elucidate the diffusion processes in the imidazolium-based hydrogels.

Optical mapping of the evolution of water content during the swelling of hydrophilic polymers

“…According to the previous report, the diffusion mechanism of water molecules in hydrogels can be identified from the slope ( k ) of the plot log ( W t / W ∞ ) versus log (time), where the W t and W ∞ represent the water content at time t and at the final equilibrium state, respectively. 17 Here, the slope was determined to be smaller than 0.5 (Fig. 3c).…”

“…Of the feasible methodologies for determining the water content of hydrophilic polymers, the most classical experimental method is gravimetry, which is based on measuring the weight change of hydrophilic polymers before and after water absorption. 17 Although simple and effective, the accuracy of the results is limited by the sensitivity of the balance (B0.1 mg), especially when the sample size is small. In addition, the result reflects the average water content of the bulk sample, which masks the spatial heterogeneity of water content within the hydrophilic polymers.…”

“…The diffusion coefficient (D) of water molecules within this hydrogel was finally calculated to be 6 Â 10 À12 m 2 s À1 according to the linear diffusion model (Length = Sqrt(2Dt)), 30 which was consistent with previous literature reports. 17 In summary, we have developed a new optical methodology to quantitatively image the local water content of hydrogels as a typical hydrophilic polymer by advanced total internal reflection optical microscopy. The dynamic evolution of the water content of the hydrogels during swelling was further mapped.…”

Swelling and Mechanical Characterization of Polyelectrolyte Hydrogels as Potential Synthetic Cartilage Substitute Materials

“…One of the main properties of hydrogels is the ability to swell and expand in hydrophilic environments without losing their three-dimensional shape. In general, the swelling of a gel depends on the solvent mobility, solvent-polymer-interaction, polymer relaxation time, structure of the used monomer, the chain length of the crosslinker, and the degree of crosslinking [20]. In Figure 1, exemplary swelling kinetics of the investigated hydrogels are shown.…”

“…The kinetics of the polymer swelling was investigated in more detail an according to the calculations of one of our previous studies [20]. In short, the process can be described as a second order relation:…”