# Highly Controlled Integration of Graphene Oxide into PAN Nanofiber Membranes
## 3 Results and Discussion  
Figure 1 shows the digital photos, water contact angles, and SEM images of representative bare PAN, PAN-bare GO, and a series of PAN-mGO (mGO: CATC-modified GO) composite membranes as a function of the GO content. In order to show the change color based on the various GO amount clearly picture of bare mGO sheet (Figure 1h) was prepared. It can be easily seen that the more GO in the PAN the darker color was obtained. Based on our previous strategy, the loading amount of bare GO into PAN nanofibers by electrospinninging generally reached up to 4 wt % with respect to the polymer concentration due to the limited dispersity of GO in a polymer precursor solution [1]. Simply by using the surface modified GO, the GO filler was reliably loaded up to 30 wt % under the same electrospinninging conditions. This surface modification involved the electrostatic interactions between the CTAC surfactant and GO which greatly improved the dispersity and compatibility of GO in polymer precursor solution as well as reliable electrospinningning of nanofibers without a clogging problem. The final composite membranes loaded with excess GO did not show notable defects related to the random bead formation, which is often caused by poor dispersity of fillers [6]. The photos also show the uniform color changes from white to dark gray throughout the membranes, implying the systematical loading of GO. However, in the case of 40 wt % of GO it was found that the nanofiber mat could not be obtained due to the too much loading of GO, resulting in the difficulties of nanofiber formation [7]. So as to investigate the successful preparation of mGO the FTIR of GO, mGO, and CTAC was measured (Figure 2). It was easily recognized that mGO was successfully prepared due to the stronger specific CTAC peaks (2850 cm\({}^{-1}\) for C-H stretch and 2915 cm\({}^{-1}\) for CH\({}_{2}\)) in the mGO FTIR spectra. From the wettability test, a bare GO sheet exhibited a water contact angle of \(\sim\)26\({}^{\circ}\)[1], and a CTAC-modified GO sheet showed an angle of \(\sim\)15\({}^{\circ}\). The bare PAN membrane initially exhibited a water contact angle of 41\({}^{\circ}\), but the series of composite membranes systematically decreased the angle as low as 15\({}^{\circ}\) with the increase of GO content. The improved hydrophilicity of GO by the surface modification allowed for the excess incorporation of GO fillers into PAN nanofibers and significantly improved the wettability of the membranes. SEM images show the diameter distribution of composite nanofibers where the average diameter of nanofiber strands and its distribution gradually increased as a function of GO content. Particularly, more than 10 wt % of GO containing composite membranes led to larger diameters of the nanofibers with a wider distribution due to the systematic loading efficiency of GO into nanofibers. In addition, these composite nanofibers did not show beaded structures throughout the membrane surface. Figure 3 shows the stress-strain curves of the bare PAN and the series of composite nanofiber membranes as a function of GO content. Unlike the composite membranes loaded with bare GO, the composite membranes integrated with the modified GO significantly improved the mechanical strength due to the reinforcement effect of GO filler [12; 13]. This enhancement could be explained  
Figure 1: Bare polyacrylonitrile (PAN), surface-modified graphene oxide (GO), and series of PAN-GO composite membranes as a function of GO content. Figure 2: FTIR of GO, mGO, and cetyltrimethylammonium chloride (CTAC) indicates that the mGO is successfully prepared. by the uniform loading of GO filler throughout the PAN nanofibers where the external stress can be possibly distributed to the embedded GO to improve the overall stress [14]. Upon loading the amount of GO over 8 wt % of polymer, the composites still displayed increased strength, but notably decreased strain % possibly caused by the formation of a thicker diameter of the nanofibers and a slight reduction in the packing density [6; 15].