The Void Structure of Fully-Aromatic Polyamide and Its Influence on the Effective Thickness of the Barrier Layer and Permeability of TFC Membranes

The void structure of fully-aromatic polyamide and its influence on the effective thickness of the barrier layer and permeability of TFC membranes

 

Yiqun Liu*, Jian Xu

SINOPEC Beijing Research Institute of Chemical Industry, Beijing, China

Abstract

        The thickness of the fully-aromatic polyamide (PA) barrier layer of the TFC membrane plays an important role in controlling the water permeability of the membrane. Typically this PA layer is not a homogenous dense layer, and thus the thickness of this separation layer measured with TEM or other characterization techniques is actually an apparent thickness. Our previous research [1] has proved that the PA layer has complex porous structure. Other researchers [2] also confirmed the existence of voids in the PA active layer and measured the volume fraction of these voids recently. Considering the void structure in the PA layer, the effective thickness of the PA layer through which water really penetrates during the filtration process is much thinner than the apparent thickness. In our opinion, the dense layer near the PA top surface with thickness of ca. 20 nm is the key barrier layer for PA layer possessing pores on its backside due to the connection between the voids with the backside of the PA layer.[1] In this case, the effective thickness of the PA layer with apparent thickness of several hundred nm should equal the thickness of the key barrier layer, ie. ca 20 nm.

        In this study, a series of TFC membranes with different void structures in the PA layer were synthesized by only changing the concentration of m-phenylene diamine (MPD) and trimesyol chloride (TMC) respectively. The apparent thickness of the PA layer of these TFC membranes was measured by using SEM, TEM and Film Thickness Probe. The void structure of the PA layer was characterized by observing the morphology of the top surface, cross-section and backside of the PA layer. The effective thickness of the PA layer was analyzed by combining the water permeability results and the membrane void structure. Based on the membrane structure characterization and the membrane permeability performance, the mechanism of the formation of PA layer with different void structures was proposed and discussed.

References

[1] Hao Yan, Xiaopei Miao, Jian Xu, Guoyuan Pan, Yang Zhang, Yuanteng Shi, Min Guo, Yiqun Liu, The porous structure of the fully-aromatic polyamide film in reverse osmosis membranes, Journal of Membrane Science, 475 (2015) 504–510

[2] Lin Lin, ReneLopez, GuyZ.Ramon, OrlandoCoronell, Investigating the void structure of the polyamide active layers of thin-film composite membranes, Journal of Membrane Science, 497 (2016) 365–376