# Suppressing Salt Transport through Composite Pervaporation Membranes for Brine Desalination
## 3 Results
### Incorporation of GO into PVA Matrix  
#### 3.2.1 Characterizations  
The addition of a small amount of GO into PVA formed small protrusions on the membrane surface, and this became more significant with the increase of GO loading within the coating layer  
Figure 4: Cross-sectional SEM and AFM surface height images of PVA/PVDF composite membrane with MR of (**a**) and (**b**) 0.025; (**c**) and (**d**) 0.1; (**e**) and (**f**) 0.2. Figure 5: Schematic diagram of salt passage through the swollen PVA membrane (not to scale). (Figure 6). Similar surface morphology was observed after incorporating GO into polyether block amide (Pebax) matrix using cast coating method [43]. In terms of the cross-sectional images (Figure S4), they showed good agreement with the surface SEM images. The pure PVA coating was even with a thickness of around 0.8 \(\upmu\)m. The incorporation of GO reduced the smoothness for the PVA-GO coating layer, especially when the GO loading was relatively high. During the formation of PVA-GO surface coating, the gradual evaporation of the solvent (water) allowed the cross-linking of the PVA chains, forming the final coating layer of 0.5-1 \(\upmu\)m. In terms of the GO, the evaporation-induced capillary flow during the drying process, together with the viscous PVA solution, disrupted the entropy-driven phase transition of GO, leading to the formation of surface protrusions [44]. The XRD pattern can reveal the GO nanosheets arrangement within the hybrid coating layer. As shown in Figure 7, for the pure GO coated membrane, it had a clear peak at 10.6\({}^{\circ}\), suggesting the GO nanosheets were piled together with an average interlayer distance of 0.83 nm. After incorporating the GO into PVA matrix, no clear GO peak can be observed. This observation was in good agreement with the previous researches [14; 45]. This can be attributed to the good interfacial compatibility between GO and PVA: the presence of carboxyl groups and hydroxyl groups on GO can form hydrogen bonding with hydroxyl groups on PVA. As a result, the GO nanosheets can be well dispersed within the PVA matrix at the molecular level [26]. The successful incorporation of GO was further evidenced by the FT-IR patterns, as shown in Figure 8 The characteristic peaks of GO powder at around 3280 cm\({}^{-1}\) and 1725 cm\({}^{-1}\) corresponded to O-H and C=O stretch, which indicated the existence of hydroxyl groups and carbonyl groups on GO. Besides, the peak at around 2950 cm\({}^{-1}\) confirmed a large amount of C-H stretch on GO. Compared to pure PVA coating, the emerging peaks for the PVA-GO composite coating layer at the region of 3100-3010 cm\({}^{-1}\) can be ascribed to alkenyl C-H stretch due to the presence of GO. In addition, the right shift of the O-H stretching peaks were detected with the addition of GO in PVA matrix (from 3375 cm\({}^{-1}\) for pure PVA to 3345 cm\({}^{-1}\) for PVA with GO of 0.3 wt. %). This observation confirmed the formation  
Figure 6: Surface SEM images of composite membranes fabricated with cast coating method: (**a**) pure PVA coating; and PVA-GO hybrid coating with (**b**) 0.1 wt. %, (**c**) 0.2 wt. %, (**d**) 0.3 wt. % GO loading. of hydrogen bonding between hydroxyl/carboxylate groups on GO and hydroxyl groups on PVA chains, which was consistent with those of other studies [14; 45; 46]. Regarding the contact angle shown in Table S1, the addition of GO into PVA matrix slightly increased the contact angle of the membrane surface. For example, the pure PVA surface coating has a contact angle of 30.2 \(\pm\) 3.5\({}^{\circ}\), compared with the 37.1 \(\pm\) 2.3\({}^{\circ}\) for the composite membrane containing 0.2 wt. % GO. This can be attributed to the presence of a large amount non-polar benzene rings on GO surface, as well as the formation of hierarchical surface morphology after the addition of GO [47].