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.