EXPERIMENTAL

2.1 Materials

Graphite with particle size quoted as +100 mesh (Product number 332461) was supplied by Aldrich. VIB-co -VI-co -Py with VIB/VI/Py = 47/50/3 (mol/mol) was synthesized according to a reported method.40 Dimethyl acrylamide (DMAA), methylene bisacrylamide (MBA), tetramethyl ethylenediamine (TEMEDA), and ammonium persulfate (APS) were purchased from Aladdin. In all relative experiments, the ultrapure water (18.25 MΩ cm) was used.

Exfoliation of graphite into FLG

Into a 250 mL beaker, 150 mL of aqueous VIB-co -VI-co -Py solution and desired amount of graphite were added in turn. Subsequently, a high-shear mixing process was performed by a Silverson L5M mixer with a 4-blade rotor (32 mm diameter) and a square-hole screen (96 holes, each 2 × 2 mm2) separated from each other by a 135 μm gap. After mixing for a preset time and sitting for 6 h, the mixture was experienced 90 min of centrifugation at 2,000 rpm, and 2/3 of supernatant was gently sucked out. Thus collected dispersion was directly used for determining the concentration of FLG and evaluating its stability. For other studies, the excess VIB-co -VI-co -Py was eliminated by repetitiously filtering the dispersion through a 0.2 μm hydrophilic membrane and redispersing filter-cake in fresh water until no VIB-co -VI-co -Py was monitored in filtrate by UV spectroscopy. The resultant filter-cake was vacuum-dried for 10 h at 60 oC and named asFLG-P hybrid.

Synthesis of hydrogels

To synthesize the poly(N ,N -dimethyl acrylamide)/FLG (PDMAA/FLG) hydrogel, FLG-P hybrid (1.0 wt% of DMAA) was first dispersed in DMAA to yield a uniform dispersion by 5 min sonication. Then, 1.0 mL of such dispersion was injected into a 25 mL beaker containing 3.8 mL of aqueous solution of MBA (0.1 wt% of DMAA) and 7.7 μL of TEMEDA (0.6 wt% of DMAA), followed by 30 s sonication. After being kept in an ice-water bath and bubbled with high-purity N2 for 15 min, 0.2 mL of aqueous solution of APS (0.6 wt% of DMAA) was introduced, bubbled for another 5 min, and transferred into a plastic syringe (2 mL) or a home-made glass mold (50 × 50 × 1 mm3). Finally, the syringe or glass mold was sealed and kept at 35 °C for 24 h to complete the hydrogel synthesis. Following the same procedures, the PDMAA hydrogel was also synthesized.

Characterization

UV-VIS and Raman spectra were collected respectively from TU-1901PC and Renishaw inVia micro-Raman (with a 532 nm laser) spectrometers. Both dynamic light scattering (DLS) and zeta potential tests were conducted on a Zetasizer Nano-ZS system (Malvern) equipped with a 633 nm laser. Thermogravimetric analyses (TGA) were performed by TA-Instruments Q500 in N2 at the heating rate of 10 oC min-1. Tapping-mode atomic force microscopy (AFM) and transmission electron microscopy (TEM) were done using Bruker Multimode 8 and JEOL JEM-2100 microscopes, respectively. Electrical conductivities of FLG films were evaluated on a Model RTS-8 multimeter using the four-probe method, whereas those of hydrogels were measured on a Keithley 2636B multimeter connected with a test fixture using the two-probe method.42 For mechanical testing, the hydrogel samples were cut into 50 mm × 10 mm × 1 mm specimens and assessed by Zwick/Roell Z005 at a tensile speed of 30 mm min-1.