Abstract
Hydrogels are synthesized by the method of radical polymerization of monomers: N-isopropylacrylamide (NIPAM) and acrylic acid (AA). Characterization of poly(N-isopropylacrylamide- co-acrylic acid) hydrogels, p(NIPAM/AA), has been performed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and by determination of the swelling behaviour in aqueous solutions at different temperatures (25, 31 and 37?C) and pH values (2.2, 4.5, 6 and 6.8). After lyophilisation in the solution at pH 6 and temperature of 25?C, p(NIPAM/AA) hydrogels have rapidly reached equilibrium degree of swelling, ?e, in comparison to non-lyophilized samples. The mechanism of solvent transport within matrix in lyophilized samples corresponds to less Fickian diffusion, whereas Super case II diffusion is characteristic for non-lyophilized samples. p(NIPAM/AA) hydrogel with 1.5 mol% of ethylene glycol dimethacrylate (EGDM) at the temperature of 25?C and pH 6.8, has reached the highest swelling equilibrium degree, ?e = 259.8. The results of swelling studies have shown that p(NIPAM/AA) hydrogels can be classified as superabsorbent polymers (SAPs). For the evaluation of pH and temperature influences on synthesized hydrogels swelling, a full three-level experimental design has been used. Two-factor interaction model (2FI) is the most optimal model of a full three-level experimental design for representing the swelling equilibrium degree of p(NIPAM/AA) hydrogels as a function of investigated parameters, i.e., temperature and pH.
Highlights
Hydrogels are synthesized by the method of radical polymerization of monomers: N-isopropylacrylamide (NIPAM) and acrylic acid (AA)
The first number represents mol% of NIPAM, the second number is mol% of AA, and the third one is mol% of ethylene glycol dimethacrylate (EGDM) cross-linker
The results of Fourier transform infrared spectroscopy (FTIR) analysis have shown that the reaction of polymerization has been carried out by breaking the double bonds, C=C, of monomers and that intramolecular hydrogen bonds in p(NIPAM/AA) copolymer have been probably accomplished through OH and NH groups and C=O groups of the monomers
Summary
Hydrogels are synthesized by the method of radical polymerization of monomers: N-isopropylacrylamide (NIPAM) and acrylic acid (AA). After lyophilisation in the solution at pH 6 and temperature of 25 °C, p(NIPAM/AA) hydrogels have rapidly reached equilibrium degree of swelling, αe, in comparison to non-lyophilized samples. Two-factor interaction model (2FI) is the most optimal model of a full three-level experimental design for representing the swelling equilibrium degree of p(NIPAM/AA) hydrogels as a function of investigated parameters, i.e., temperature and pH. Characteristics of swelling and contracting of temperature-sensitive hydrogels can be significantly influenced by incorporation of hydrophilic or hydrophobic co-monomers [8]. Polymers with weak ionisable functional groups which can bond and release protons as a response to change in pH in the environment are called pH-sensitive polymers [9,10]. Incorporating a hydrophilic monomer like acrylic acid, AA, into the structure of the homopolymer p(NIPAM), hydrogels with higher degree of water absorption and lower critical solution temperature (LCST) are obtained. The p(NIPAM/AA) copolymer has been chosen due to its sensitivity to pH and temperature changes in the surrounding medium, which enables its use in drug delivery [19,20], adsorption of heavy metals from sol-
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