## 3 Results and Discussion  
### Characterization of the Membrane  
Crosslink density of the membrane increases as the sulfur loading increases from 0.5 to 1.5 phr (Table 1). Addition of sulfur forms a three-dimensional network between NBR macromolecular chains. This network holds the chains together and prevents the chains from slipping past each other when subjected to force, thus increasing the strength of NBR [39]. Previous studies also reported that sulfur crosslinking could produce NBR films with high resistance to biodiesel [40], gasoline and oil [41]. Thus, the sulfur crosslinking introduced in the NBR membrane in this study is expected to provide the membrane with strength and chemical resistance properties. However, the effect of the sulfur loading on the morphology of the membrane has to be understood to optimize the performance of the membrane in oily wastewater filtration. SEM images of all the membranes with GO exhibit a clear wrinkled and folded surface, Figure 1b,d,f compared to pure NBR membrane as shown in Figure 1a,c,e. This is due to the low-density nature of GO that causes it to migrate up towards the membrane surface during the curing period. GO sheets contains not evenly distributed oxygen functional groups which attribute to the scoring and folding effect of the GO sheet edges [42]. This deformation of GO sheets will bring about the corrugated, scrolling and wrinkle surface in the NBR matrix membrane upon casting [43]. As a result, the folds create ridges and grooves which increases the effective surface area for filtration of oily wastewater. Thereby, enhanced performance in terms of permeation flux is expected. Formation of wrinkles on membranes with GO was also reported in poly(L-lactic acid) membrane coated with GO [44] and polyvinyl alcohol membrane with GO [45]. For membrane with 0.5 phr sulfur content, some micro-cracks or discontinuity could be observed, Figure 1b. This is attributed to low sulfur loading that leads to less crosslink networks which affected continuous strong membrane film formation. At sulfur loading of 1.0 phr, no micro-crack could be observed on the surface of the membrane which suggest the formation of strong and continuous membrane, Figure 1d. The wrinkles and ridges are more apparent at 1.0 phr sulfur loading. In NBR/GO membrane with 1.5 phr sulfur, Figure 1f, less wrinkles and ridges are observed which suggest less migration of GO nanosheets to the surface of the membrane during casting and curing processes. More micro-cracks can also be observed at high magnification which suggest discontinuity on membrane surface. Phase inversion method is the most common method used to produce porous polymer matrix membrane in which polymer resins are being dissolved in a solvent and casted into films based on phase separation of polymer rich phase from polymer poor phase. In this method, the structural integrity of the membrane is influenced by the solvent type, non-solvent precipitation bath type and precipitation time [46]. However, in the proposed latex compounding and curing method, to produce polymer membrane, the latex membrane integrity and strength is obtained only by sufficient coalescence between latex particle during membrane casting process. Previous studies suggested that an optimum level of latex chains crosslinking inside the latex particle is required to obtain sufficient coalescence between latex particles. Too low or too much crosslinking would retard the extent of coalescence and cause the formation of discontinuous films with micro cracks [47]. Very low crosslinking will cause the particles to aggregate together due to stickiness, while very high crosslinking will cause the particle to become hard and bounces back during coalescence. From the SEM images, it can be concluded that membranes with