FIGURE 9 Raman spectra of a) graphite, b) FLG-P-80hybrid, and c) FLG-P-20 hybrid.
Although polymer molecules residing on the FLG surface may be unwanted for some uses, they are really helpful for preparation of the flexible FLG films. Figure 10a shows a typical one prepared by vacuum filtration of aqueous dispersion of FLG-P-80 hybrid (0.5 mg mL-1) over a 25 nm hydrophilic membrane followed by drying in an oven at 70 oC. AFM analysis (Figure 10b) reveals that FLG sheets involved in the film are aligned parallel to the film plane. The obtained free-standing film (47 mm-diameter and 28 μm-thick) is robust and can be bent to an angle of ~90o without breaking (Figure 10c). Even after 100 bendings, no cracks or delamination are observed by naked eyes. In microscopic scale, no discernible morphological changes occur before and after bending tests; average surface roughness only slightly increases from 151 to 160 nm (Figure 10d). As a sharp contrast, the film prepared from shear-exfoliated FLG in NMP or aqueous solution of SDBS breaks under the same bending condition. The toughening effect of VIB-co -VI-co -Py on FLG film is thus demonstrated. Moreover, the as-prepared film shows an electrical conductivity of 9.3 × 103 S m-1. After annealing at 200oC for 10 h in N2, it further increases to 4.0 × 104 S m-1 with no obvious weakening in bending tolerance. The combination of good toughness and excellent electrical conductivity in a single material system makes FLG films developed here very promising find some important applications, such as flexible and portable electronic devices.59