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Item type:Publication, Synthesis and characterization of stiff, self-crosslinked thermoresponsive DMAA hydrogels(MDPI, 2020-06-01)Stiff thermosensitive hydrogels (HG) were synthesized by self-crosslinking free radical polymerization of N, N-dimethylacrylamide (DMAA) and N-isopropylacrylamide (NIPAAm), adjusting the degree of swelling by carboxylate-containing sodium acrylate (NaAc) or a 2-oxazoline macromonomer (MM). The formation of hydrogels was possible due to the self-crosslinking property of DMAA when polymerized with peroxodisulfate initiator type. The MM was synthetized by the ring-opening cationic polymerization of 2-methyl-2-oxazoline (MeOxa) and methyl-3-(oxazol-2-yl)-propionate (EsterOxa), and contained a polymerizable styryl endgroup. After ester hydrolysis of EsterOxa units, a carboxylate-containing MM was obtained. The structure of the hydrogels was confirmed by 1Hhigh-resolution (HR)-MASNMRspectroscopy. Suitable conditions and compositions of the comonomers have been found, which allowed efficient self-crosslinking as well as a thermoresponsive swelling in water. Incorporation of both the polar comonomer and the macromonomer, in small amounts furthermore allowed the adjustment of the degree of swelling. However, the macromonomer was better suited to retain the thermoresponsive behavior of the poly (NIPAAm) due to a phase separation of the tangling polyoxazoline side chains. Thermogravimetric analysis determined that the hydrogels were stable up to ~ 350 ffiC, and dynamic mechanical analysis characterized a viscoelastic behavior of the hydrogels, properties that are required, for example, for possible use as an actuator material.7 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Removal of Methylene Blue by Hydrogels based on N, N-Dimethylacrylamide and 2-Oxazoline macromonomer(Springer, 2020-09-01)New hydrogels (HG) were synthesized in aqueous medium by free radical polymerization of N,N-dimethylacrylamide (DMAA) and 2-oxazoline macromonomer (MM) initiated by potassium persulfate (KPS) and catalyzed by N,N,N,N′-tetramethylethylenediamine (TEMED). In this polymerization, the monomer DMAA was also used as a crosslinker because it has the ability of self-crosslinking in the presence of peroxodisulfate initiator type. The macromonomer (DP = 24) was a gradient copolymer of 2-methyl-2-oxazoline and methyl-3-(oxazol-2-yl)-propionate with a styryl end group. 1H high-resolution (HR)-MAS NMR spectroscopy allowed to confirm the structure of hydrogels and to determine the molar content of DMAA and MM in each of them. Hydrogels (HG-H) containing carboxylic groups were obtained by basic hydrolysis of HG. Hydrolyzed hydrogels (HG-H) were used for the adsorption of methylene blue (MB) in aqueous medium. It was found that the MB adsorption increased as the initial MB concentration increased, and maximum adsorption capacities were found. The influence of pH value on MB adsorption was evaluated, showing that the MB adsorption capacity of the hydrogels was higher at pH value ≥ 5.7. Adsorption isotherms were studied using Langmuir and Freundlich models. The latter model describes best the process suggesting a possible adsorption mechanism through electrostatic interactions between MB and HG-H hydrogels. The MB adsorbed inside the hydrogels, could be removed with an acidic solution and therefore the hydrogel could be applied to adsorb MB again. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Synthesis and characterization of new interpenetrated hydrogels from N-isopropylacrylamide, 2-oxazoline macromonomer and acrylamide(Elsevier Ltd, 2022-08-15)New interpenetrated hydrogels (IPN), sensitive to pH and temperature, were synthesized by sequential free radical polymerizations in aqueous medium. In the first stage, a thermosensitive hydrogel of poly(N-isopropylacrylamide) (HG-PNiPAAm) was prepared, and in the second stage a hydrogel of acrylamide and 2-oxazoline macromonomer (MM) containing carboxylic acid ester groups was synthesized in the presence of the PNiPAAm hydrogel. In both stages, bisacrylamide was used as a crosslinker. The 2-oxazoline macromonomer was a random copolymer of methyl-3-(oxazol-2-yl)-propionate (EsterOxa) (23 % mol) and 2-methyl-2-oxazoline (MeOxa) (77 % mol) with a polymerization degree of 21 and contained a vinylbenzene end group for radical polymerization. Five different IPN-hydrogels were synthesized, the amount of the oxazoline was varied systematically, and the EsterOxa units were finally hydrolyzed to carboxylic acid groups. The structure of the IPNs was characterized by 1H HR-MAS NMR spectroscopy. All IPN hydrogels showed a conformational transition when varying the temperature or the pH value and these transitions were a function of the composition of the IPN hydrogel. While pure HG-PNiPAAm resulted in a transition temperature of 31 °C, this value rose to 50 °C and higher for MM-H containing IPNs. This property was shown macroscopically as a contraction or expansion of the hydrogel but also in the 1H HR-MAS NMR measurements. The sensitivity to pH in the IPN hydrogels was manifested as a contraction of the volume of the hydrogel at low pH. While introducing poly(acryl amide) PAAm increased the degree of water absorption, increasing the amount of hydrolyzed EsterOxa macromonomer within the hydrogel decreased this absorption at high pH values. These features were attributed to the formation of hydrogen bonds between the acid and amide or protonated amino groups. A lower initial swelling at elevated temperatures but constant switching pH value (pH = 6) supported this reasoning. Importantly, at 20 °C and pH = 5.7 all IPN had a similar degree of swelling Q of 34 to 39, strongly reduced due to the IPN structure compared to a PAAmMM hydrogel (Q > 200). The reported IPNs result from a straight forward synthesis and are thus an interesting material for future applications as potent actuator and sensor materials.