Hydrogels copolymers N,N-dimethylacrylamide (DMA) and maleic acid (MA) were prepared by free-radical polymerization at 56°C in aqueous solution, using N,N-methylenebisacrylamide (NMBA) as cross-linking agent and potassium persulfate (KPS) as initiator. The effects of comonomer composition, cross-linker content, and variation of pH solutions on the swelling behavior of polymers were investigated. The obtained results showed an increase of the swelling of poly(N,N-dimethylacrylamide-co-maleic acid) (P(DMA-MAx)) as the content of maleic acid increases in the polymeric matrix, while they indicate a great reduction of the degree of swelling as the cross-linking agent ratio increases. It was also shown that the swelling of copolymer hydrogels increased with the increase of pH and the maximum extent was reached at pH 8.7 in all compositions. Fourier transform infrared spectroscopy (FTIR) revealed the existence of hydrogen bonding interactions between the carboxylic groups of MA and the carbonyl groups of DMA. Differential scanning calorimetry analysis (DSC) showed an increase of the glass-transition temperature ( ) as concentrations of MA and NMBA increased. Thermogravimetric analysis (TGA) of copolymers was performed to investigate the degradation mechanism. 1. Introduction Hydrogels are composed of hydrophilic homopolymer or copolymer network and can swell in the presence of water or physiological fluids. Chemical cross-links (covalent bonds) or physical junctions (e.g., secondary forces, crystallite formation, and chain entanglements) provide the hydrogels, unique swelling behavior and three-dimensional structure [1–3]. Volume changes in hydrogels occur in response to changing environmental conditions such as temperature [4–6], pH [7–9], solvent composition [10], and ionic strength [4, 11]. Hydrogels have been a topic of extensive research in the past decades and their properties as, for example, their high water content and the possible control over the swelling kinetics make them very attractive for biomedical applications [12–16]. The pH sensitive hydrogels containing pendant acidic or basic groups such as carboxylic acids, sulfonic acids, primary amines, or ammonium salts which change ionization in response to change in the pH have become the subject matter of major interest for use as carriers in drug delivery research [17–19]. Poly(N,N-dimethylacrylamide) is a hydrophilic polymer, due to its remarkable properties, such as water solubility and biocompatibility, and it is very useful in the biomedical applications including polymer supports for protein
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