Synthesis of poly(2-hydroxyethyl methacrylate-co-acrylic acid)-grafted graphene oxide nanosheets via reversible addition–fragmentation chain transfer polymerization

Abstract

Natural graphite was oxidized via the Hummer's method and sonicated to obtain graphene oxide (GO). A three-step modification process was performed to attach a reversible addition–fragmentation chain transfer (RAFT)-agent onto the GO surface and obtain GO-RAFT nanosheets. RAFT polymerization of 2-hydroxyethyl methacrylate (HEMA) and acrylic acid (AA) was performed by using azobisisobutyronitrile (AIBN) as initiator. After separation of free chains, GO-P(HEMA) and GO-P(HEMA-co-AA) were obtained. Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), X-ray diffraction (XRD) analysis, Raman spectroscopy, proton nuclear magnetic resonance (¹H NMR), size exclusion chromatography (SEC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the structure of modified nanosheets. These modified nanosheets showed dual pH- and thermo-sensitive properties as measured by UV-visible spectroscopy. Due to the dark nature of GO-P(HEMA) and GO-P(HEMA-co-AA) nanosheets, the UV-visible absorption of their solutions is an effective parameter to examine the stimuli responsive behavior as measured here at different pH (3–12) and temperature (25–55 °C) values. According to the results, UV-visible absorption of GO-P(HEMA) decreases slightly as pH decreases while the decrease is more significant for GO-P(HEMA-co-AA). Also, a lower critical solution temperature (LCST) of 30 and 34 °C is observed for GO-P(HEMA) and GO-P(HEMA-co-AA) nanosheets respectively.