Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802.
The Dow Chemical Company, Lake Jackson, TX 77566.
Proc Natl Acad Sci U S A. 2018 Aug 28;115(35):8694-8699. doi: 10.1073/pnas.1804708115. Epub 2018 Aug 13.
As water availability becomes a growing challenge in various regions throughout the world, desalination and wastewater reclamation through technologies such as reverse osmosis (RO) are becoming more important. Nevertheless, many open questions remain regarding the internal structure of thin-film composite RO membranes. In this work, fully aromatic polyamide films that serve as the active layer of state-of-the-art water filtration membranes were investigated using high-angle annular dark-field scanning transmission electron microscopy tomography. Reconstructions of the 3D morphology reveal intricate aspects of the complex microstructure not visible from 2D projections. We find that internal voids of the active layer of compressed commercial membranes account for less than 0.2% of the total polymer volume, contrary to previously reported values that are two orders of magnitude higher. Measurements of the local variation in polyamide density from electron tomography reveal that the polymer density is highest at the permeable surface for the two membranes tested and establish the significance of surface area on RO membrane transport properties. The same type of analyses could provide explanations for different flux variations with surface area for other types of membranes where the density is distributed differently. Thus, 3D reconstructions and quantitative analyses will be crucial to characterize the complex morphology of polymeric membranes used in next-generation water-purification membranes.
随着世界各地许多地区的水资源供应日益成为一个挑战，通过反渗透（RO）等技术进行海水淡化和废水回收变得越来越重要。然而，关于薄膜复合 RO 膜的内部结构仍存在许多悬而未决的问题。在这项工作中，使用高角度环形暗场扫描透射电子显微镜断层扫描技术研究了全芳香族聚酰胺薄膜，该薄膜可用作最先进的水过滤膜的活性层。3D 形态的重建揭示了复杂的微观结构中从 2D 投影中看不到的复杂方面。我们发现，压缩商业膜活性层的内部空隙不到总聚合物体积的 0.2%，与以前报道的两个数量级更高的值相反。电子断层扫描测量聚酰胺局部密度的变化表明，在所测试的两种膜中，聚合物密度在可渗透表面处最高，并确定了 RO 膜传输性能中表面积的重要性。对于其他类型的膜，其中密度分布不同，如果表面积与通量变化存在差异，那么同样类型的分析可以提供解释。因此，3D 重建和定量分析对于表征用于下一代水净化膜的聚合物膜的复杂形态至关重要。
