Swelling kinetics and rheological behaviour of microwave synthesized poly(acrylamide-co-acrylic acid) hydrogels
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Swelling kinetics and rheological behaviour of microwave synthesized poly(acrylamide-co-acrylic acid) hydrogels Tamara Erceg 1 & Tamara Dapčević-Hadnađev 2 & Miroslav Hadnađev 2 & Ivan Ristić 1 Received: 25 May 2020 / Accepted: 28 September 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Poly(acrylamide-co-acrylic acid) hydrogels are synthesized by free-radical polymerization in a microwave oven, in a one-step procedure that provides reduction in time, energy and resources in comparison with conventional heating in aqueous solution. Hydrogel microstructures are analysed by scanning electron microscopy (SEM). Dynamic swelling tests are carried out at three pH values (3.0, 10.0 and 7.0) and two temperatures (25 and 37 °C) considering the conditions for hydrogel application. SEM pictures indicate irregular cellular and porous structure of microwave synthesized hydrogels, obtained as a result of simultaneous polymerization and water evaporation. The results of swelling measurements show that the swelling ratio at pH 7.0 and 10.0 increases with increasing in acrylic acid amount, while at pH 3.0 decreases. Swelling rate is higher at physiological temperature, and solvent diffusion is relaxation-controlled. Swelling of hydrogels follows the second-order kinetics. Rheological behaviour of hydrogels has been assessed using the results of frequency and amplitude sweep tests applied at the systems in equilibrium swollen state. Obtained data reveal a dominant elastic character and a long linear viscoelastic region (up to 2000 Pa), as well as a similar three-dimensional internal structure of investigated hydrogels. Keywords Acrylate hydrogels . Microwave synthesis . Cellular structure . Swelling kinetics . Rheological properties
Introduction Hydrogels are three-dimensional cross-linked hydrophilic polymer chains which swell in water but do not dissolve in it [1, 2]. Using appropriate monomers and building units, it is possible to obtain hydrogels that respond to environmental stimuli, such as changes in temperature, pH, ionic strength of solution, pressure, light, electric and magnetic field. The ability of special types of hydrogels to respond to changes in external conditions classifies them as intelligent soft materials [3, 4]. The most important feature of hydrogels is swelling, i.e. the ability to absorb water and increase their volume [5]. Swelling behaviour of hydrogels is affected by polymer chemical composition, cross-linking
* Tamara Erceg [email protected] 1
Faculty of Technology Novi Sad, University of Novi Sad, Bulevar cara Lazara 1, Novi Sad 21000, Serbia
2
Institute of Food Technology, University of Novi Sad, Bulevar cara Lazara 1, Novi Sad 21000, Serbia
degree, charge density, hydrogel microstructure (porosity), polymer-water interaction and environmental (medium) conditions [6]. Due to the presence of ionizing groups in their structure, polyelectrolyte hydrogels show the dependence of the swelling degree on pH and ionic strength of the surrounding me
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