MS-3-P-1809 Direct observation of functional layer structure for reverse osmosismembrane by scanning electron microscopy

Known as the "water purification technology in twenty-first Century", reverse osmosis(RO) membrane separation technology has the advantages of high purifying rate, low cost and environment protection, widely used in seawater desalination, ultra pure water preparation and industrial wastewater treatment etc..At present, most commercial RO membrane was prepared through interfacically polymerizing m-phenylene-diamine(MPD) and trimesoyl chloride(TMC) on top of a poly-sulfone substrate to form an ultrathin active polyamide layer. The surface property and microstructure of ultrathin functional layer greatly influence the water flux, salt rejection of RO membrane. Our previous work has proved that scanning electron microscopy (SEM) is suitable for direct observation of membrane structure, especially for superfine structure of ultrathin functional layer. The skin layer of RO membrane is much denser than ultrafiltration membrane and its microstructure is more difficult to observe by SEM with regular way. But the extreme thinness of the skin layer, which is the key to the success of thin-film composite membranes, constitutes a major obstacle to understanding their structure and function.
So in our research, we observed RO membrane microstructure using a special and simple sample preparation method to overcome the difficulty.Firstly, the RO sample is swelling in deionized water and ethanol at 30 degrees for several minutes, and then we put the sample in liquid nitrogen and fracture it with a certain slope. The surface, back and cross section morphologies of the resulting functional layer can be observed directly and effectively by means of SEM as shown in figure1. Figure 2, 3, 4 are the details of amplification in Fig.1. From the surface morphology in Fig.2, we can see the large blades and leaflets structure as the so-called "peak and valley" morphology; Fig.3 shows the cross-section structure, it can be clearly seen the sharp boundary between skin layer and support layer. The thickness of functional layer is about 300nm (blue region), and dense structure is about 100nm thick (yellow region); Fig.4 reflects the hole shape and convex structure on the back of functional layer.
This research has solved the sample-preparation difficulties with transmission electron microscopy(TEM) for RO membrane cross section, which has been examined almost exclusively by TEM; Moreover, we observed the bottom dense structure and the backside structure of RO functional layer in- situ by SEM for the first time. It has confirmed the “self-limiting” formation mechanism of interfacial polymerization, which also provided the basis for RO separation mechanism. In addition, our work has given important guidance in theory for the preparation of high performance RO membranes.