Thermodynamics of Interaction between Water and Lightly Crosslinked Hydrogels of Acrylic and Metacrylic Acids
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HE 100th ANNIVERSARY OF URAL FEDERAL UNIVERSITY
Thermodynamics of Interaction between Water and Lightly Crosslinked Hydrogels of Acrylic and Metacrylic Acids L. V. Adamovaa,* and A. P. Safronova,b a Yeltsin
b
Ural Federal University, Yekaterinburg, 620002 Russia Institute of Electrophysics, Ural Branch, Russian Academy of Sciences, Yekaterinburg, 620016 Russia *e-mail: [email protected] Received March 3, 2020; revised March 25, 2020; accepted April 14, 2020
Abstract—Equilibrium isothermal sorption and microcalorimetry are used to study the effect the ratio of units of different nature in the macromolecular network of hydrogels of copolymers of acrylic (AA) and methacrylic (MAA) acids have on the thermodynamic parameters of interaction with water. Isotherms of the sorption of water vapor on dried hydrogels and the concentration dependences of the enthalpy of swelling of partially swollen hydrogels in excess of water are measured experimentally. The values of effective Flory–Huggins integral parameter χ is determined, along with its components χH of enthalpy and χS of entropy. The values of the Flory–Huggins parameter grow naturally upon an increase in the content of MAA units in the copolymers, which indicates weaker interaction with water and is consistent with a reduction in their degree of swelling. In contrast, the values of χH fall as the content of MAA units in the copolymers grows. This suggests that water interacts more energetically with the PMAA network than with PAA, despite the more hydrophilic nature of the latter. The values of χS are positive for all systems, which corresponds to the negative noncombinatorial entropy of gel swelling. Results are discussed in terms of the balance of hydrophilic and hydrophobic hydration in AA and MAA gels. Keywords: hydrogels, polyacrylic acid, polymethacrylic acid, sorption, calorimetry, Flory–Huggins parameter DOI: 10.1134/S003602442012002X
INTRODUCTION Hydrophilic gels of weakly crosslinked polymers have been objects of great interest to researchers in recent years. Of particular interest are gels of a polyelectrolyte nature, the polymer networks of which contains ionic groups that can dissociate in water. One of the strongest manifestations of the polyelectrolyte nature of gels is their ability to absorb and retain huge amounts of a solvent and reversibly alter its volume under the influence of external factors. Due to these properties, polyelectrolyte gels are used in medicine as biocompatible polymers, in pharmacology for the delivery of drugs with controlled release, in chromatography, in solving environmental problems, and for immobilizing water [1–6]. Gels based on polyacrylic acid are widely used for these purposes. They are often modified using synthetic or natural polymers, and via copolymerization [7–11]. One means of such modification is the production of hydrogels of copolymers of acrylic CH2=CH–COOH (AA) and methacrylic CH2=C(CH3)–COOH (MAA) acids [1, 3, 12, 13]. Polymer acids polyacrylic (PAA) and polymethacrylic (PMAA) have a fairly linear chai
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