Responsive Polymer Hybrid Gel Cross-Linked by N,n-(1,2-Dihydroxyethylene) Bisacrylamide for Catalytic Application

Abstract

Copolymer microgel composed of N-isopropyl acrylamide and methacrylic acid, poly(NIPAM-MAA), was synthesized by free radical emulsion polymerization using N,N-(1,2-dihydroxyethylene) bisacrylamide as cross-linking agent. The hybrid microgels poly(NIPAM-MAA)-Au and poly(NIPAM-MAA)-Ag, were synthesized by in-situ reduction of metal ions using sodium borohydride (NaBH 4 ) as reducing agent. The temperature and pH sensitivity of pure and hybrid microgels were studied by dynamic light scattering (DLS) and UV-Visible spectroscopy. The existence of gold and silver nanoparticles inside the microgel network was confirmed by transmission electron microscopy (TEM). Poly(NIPAM-MAA)-Au and poly(NIPAM-MAA)-Ag hybrid microgels were tested for the comparative catalytic activity towards the reduction of 4-nitrophenol to 4-aminophenol. The apparent rate constant (k app ) for poly(NIPAM-MAA)-Au was found superior than poly(NIPAM-MAA)-Ag. Keywords:Free radical emulsion polymerization, in-situ reduction method, hybrid microgels, surface plasmon resonance, catalytic activity . However, the enhanced surface energy causes aggregation of these NP's and hinders their applications in many fields. INTRODUCTIONIn recent decades, many attempts have been made to overcome this problem. It has been proven by various studies that anchoring metal NP's on supports is an effective approach to solve the aggregation problem. Therefore, many materials have been designed for the stabilization of metal NP's, such as surfactants 6 , micelles 7 , microspheres 8 , hollow capsules 9 , polymer brushes 10 , polymer microgels 11 and dendrimers 12 . Of the wide range of materials, polymer microgels have attracted considerable interest due to their high surface area and quick responsive behavior towards environmental stimuli. Agrawal et al 13 prepared N-vinylcaprolactam/acetoacetoxyethyl methacrylate/acrylic acid microgels for the entrapment of colloidal gold nanostructure and studied their catalytic properties. Zheng et al 14 synthesized chitosan and acrylic acid based granular hydrogels for the stabilization of silver nanoparticles and used it for the catalytic reduction of organic dyes. We have successfully fabricated silver nanoparticles inside the pure thermo responsive poly(N-isopropyl acrylamide) microgels synthesized with different crosslinking density 15 and dual responsive cationic poly(N-isopropyl acrylamide-co-vinylimidazole) microgels 16 . The hybrid microgels were used as a catalyst for the reduction of 4-nitrophenol at entire pH range and the catalyst was found more efficient at pH < 6. In all these studies N, N-methylene bisacrylamide used as a crosslinking agent. To best of our knowledge very little work has been done using N,N-(1,2-Dihydroxyethylene) bisacrylamide (DHEBA) as cross linking moiety 17 for 3D microgel formation and used as a templet for immobilization of metal nanoparticles.In this work we have carried out the synthesis of stable poly(NIPAM-MAA) microgels has been explored by using DHEBA as cross-linker for the fabrication ...

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