# Suppressing Salt Transport through Composite Pervaporation Membranes for Brine Desalination
## 3 Results
### Incorporation of GO into PVA Matrix
#### 3.2.1 Characterizations
To better understand the effect of GO incorporation on the coating layer properties, freestanding PVA-GO films were also prepared and characterized to explore their surface morphology, thermal and swelling properties. The thickness of the freestanding films was around 100 \(\upmu\)m. From the digital photo (Figure S5), with higher GO loading, the freestanding film color changed from almost transparent to yellowish brown. Similar to the composite membrane fabricated via cast coating, the addition of GO introduced wrinkles and protrusions onto the membrane surface, as shown in Figure S6. As the freestanding films were fabricated via the solvent evaporation method, without the shear force induced alignment during the casting process, the GO nanosheets randomly oriented within the original membrane casting solution, and eventually led to the wrinkled paper-like structure on the surface due to the capillary flow during the solvent evaporation process. Based on the SEM images, GO was homogeneously dispersed within the PVA matrix. Same as the composite membranes, no GO peak was observed in freestanding films after the incorporation of GO in PVA matrix from the  
Figure 8: FT-IR curves of PVA-GO/PVDF membranes with GO loading of 0.1, 0.2 and 0.3 wt. %. Figure 7: XRD curves of composite membranes. XRD patterns (Figure S7), indicating a good interfacial compatibility between the GO nanosheets and the PVA matrix. As shown in Table 3, the DSC results exhibited an increased Tg for the freestanding films after the incorporation of GO nanosheets (from around 48 \({}^{\circ}\)C to around 60 \({}^{\circ}\)C). This further confirmed the formation of hydrogen bonding between GO nanosheets and PVA matrix, which reduced the polymer chain mobility. This could be also lead to the rigidification of polymer chains near the GO sheets [15]. Similar trends were also observed by incorporating a small amount of GO into PVA matrix [45; 48]. The freestanding films' swelling properties were also studied as it is closely related to the membrane's cross-linking density and free volume [12]. The equilibrium water content (EWC) values of freestanding films (Table 3) in both Mill-Q water and 100 g/L NaCl aqueous solution were investigated. Among different membranes, the pure PVA freestanding film exhibited the highest EWC values for both Milli-Q water and salt water. The initial addition of GO (0.1 wt. %) significantly reduced the EWC value. This can be attributed to the impermeable feature of GO and formation of hydrogen bonding between GO and PVA, which reduced the solubility of water in the PVA polymer chains. However, with the increase of GO loading (0.2 and 0.3 wt. %), the EWC values increased again due to the water adsorption on the nanosheet surface, but they were still lower than the pure PVA films. Another potential reason could be the aggregation of GO sheets in PVA matrix created new pathways for water adsorption and diffusion. Smaller free volume fraction was also obtained by adding GO into PVA matrix from other studies [45]. By comparing the EWC value in Milli-Q water and salt water, it was found that for pure PVA films the EWC value in salt water was significantly lower than that in Milli-Q water. While for the films with GO incorporated, the difference is less significant. This indicated that the swelling ability of PVA with GO was less affected by the salt content, compared to pure PVA. Furthermore, with the addition of GO into the PVA matrix, the salt diffusivity tended to decrease. This was caused by much longer and more torturous transport pathways for salt to pass through the film, since GO is an impermeable nanosheet to salt ions. It is worth mentioning that in the salt diffusivity test, with the addition of GO, the amount of salt absorbed by the membrane decreases significantly. After immersing the hydrated films into Milli-Q water, the equilibrium solution conductivity dropped from around 120 \(\upmu\)S/cm for pure PVA films down to 20 \(\upmu\)S/cm for PVA with 0.1 wt. % GO.