Synthesis of smart polymeric hybrid hydrogels, rheological investigations and their catalytic performance in reduction of dyes

Abstract

Novel biopolymer based hydrogels of Xanthan gum (Gx), acrylic acid (AAc) and N-Isopropyl acrylamide (NIPAM) (i.e. MG07, MG0 and MG09) were synthesized via graft copolymerization technique. The blending of Zwitter-ionic terpolymeric hydrogels coded as ZHG-01 and ZHG-02 with varying concentrations of 2-Acrylamido-2-methylpropane sulphonic acid (2AMPS) was done by adopting free radical polymerization method. The prepared sets of hydrogels were characterized through FT-IR, SEM, rheology and UV-visible spectroscopy. FT-IR analysis verified the successful copolymerization of hydrogels by appearance of respective peaks at certain wave-numbers. FT-IR results confirmed the compatibility of monomers in both series of hydrogels. SEM results highlighted hydrogels as suitable materials for absorption of dyes and incorporation of heavy metal ions due to their rough surface texture. Detailed rheological investigations were carried out to study the flow behavior of hydrogels and explore their possible applications in various fields. Most widely practical rheological models like Bingham model, modified Bingham model and Ostwald power law model were taken into consideration which uncovered hydrogels with elastic, shear thinning and non-Newtonian behavior. Modified Bingham model presented best fit understanding to our manufactured hydrogel materials. Comparing the calculated activation energies (Ea) for both systems of hydrogels expressed resilient association with viscosity. The highest degree of activation energy with measured value 13.87 kJ/mol was obtained for MG09 with greatest concentration of Gx. However, Ea in case of ZHG-02 calculated was 204.085 kJ/mol much greater value than 132.282 kJ/mol for ZHG-01. Dynamic mechanical study (frequency sweep tests) showed dominant elastic character of hydrogels than viscous behavior due to greater storage moduli than loss moduli. Moduli versus shear stress endorsed the elastic behavior in dominance over viscous nature. xiv Zwitter-ionic terpolymeric hydrogels were passed through swelling studies at different pHs to determine swelling capacity upto their maximum. It was learnt that both ZHG-01 and ZHG-02 hydrogels displayed pH dependency and swelled at any pH showing maximum percent swelling i.e. 2068% and 3710% at pH near 7 for ZHG 01 and ZHG-02 respectively. Hybrid hydrogels were produced simply by incorporating silver ions which then reduced to metal nanoparticles by in-situ reduction process. Incorporation of metal ions followed by their reduction was done to discover the catalytic performance of hybrid hydrogels during the reduction of organic dyes like methylene blue (MB) and Congo red (CR). The catalytic reduction efficacy of hybrid hydrogels for MB followed the order of (MG09)-Ag > (ZHG-02)-Ag > (ZHG-01)-Ag with 92.2%, 84.2% and 82.4% efficiency correspondingly. For reduction of CR, catalytic performance of hybrid hydrogels followed somewhat different order i.e. (ZHG-02)- Ag > (MG09)-Ag > (ZHG-01)-Ag with 97.53%, 89.6 and 69.4% competence respectively. The catalyst with higher values of apparent rate constant (Kapp) suggested that the catalyst is more operational than rest of other catalysts with lower Kapp values during the reduction of MB and CR. If comparison among catalysts is made when dyes are desired to reduce, then (MG09)-Ag is the finest choice for reduction of MB. However, when CR is required to get reduced in the presence of an appropriate catalyst, then (ZHG-02)-Ag hybrid hydrogels with maximum linear Kapp value project itself the most suitable catalytic material among the three catalytic systems. The prepared materials are expected to find wide range applications in adsorption/absorption of heavy metal ions and dyes from aqueous media, cosmetics, pharmaceuticals, and sensing devices. Keywords: Hydrogels, Hybrid hydrogels, Rheology, Elastic behavior, Viscous behavior, Activation energy, Congo red, Methylene blue, Zwitter-ionic terpolymeric hydrogels, Bingham model, modified Bingham model, Ostwald power law model, Free radical polymerization