Development of self-assembled polyelectrolyte membranes for pervaporation applications

Abstract

Electrostatic self-assembly is a simple, yet versatile and environmentally friendly technique. This technique has been widely used in different areas and recently it has also been used to make nano-structured separating layers for composite polyelectrolyte pervaporation membranes. Non-porous substrates are usually employed for electrostatic self-assembly depositions, but porous substrates have to be used for membrane applications because the composite membranes fabricated with non-porous substrates will have low permeation fluxes. When porous substrates were used to make composite membranes for pervaporation, it was reported that 60 double-layers were needed to get a membrane with suitable separation performance. The deposition of each double-layer needed about one hour, and the fabrication of reported self-assembled membranes with porous substrates was time-consuming and, from an industrial point of view, not practical. <br /><br /> The aim of this work was to make self-assembled composite membranes in a more practical way. The methodology used here is to find appropriate materials and suitable preparation conditions to make self-assembled composite membranes that have less than 10 self-assembled double layers but still have good performance for the dehydration of isopropyl alcohol (IPA)/ water mixtures by pervaporation. <br /><br /> A hydrolyzed polyacrylonitrile (PAN) ultrafiltration membrane is a permanently charged porous material. In this work, this porous material was, for the first time, used as a substrate for the fabrication of a composite self-assembled membrane. It was found that the hydrolyzed porous PAN membranes were good substrates for making self-assembled membranes for pervaporation. <br /><br /> In order to reduce the number of the depositions required for making composite membranes with suitable separation performance, a new deposition technique, concentration-changing deposition technique, has been developed. To obtain more extended conformations of polyelectrolytes to prevent them from going into the pores on a porous substrate, dilute deposition solutions were used for the first several depositions. After these first depositions, the pore size of the porous substrate had been reduced and more concentrated solutions (but still dilute solutions) could be used for the subsequent depositions. By using more concentrated deposition solutions, the number of the polyelectrolyte coils adsorbed by the charged substrate was increased and the thickness of each deposited layer was increased. In this way, the total number of deposition layers needed for a good membrane would be decreased. It has been proved in this work that the number of deposition layers in a composite membrane can be reduced by using the concentration-changing deposition technique. <br /><br /> By selecting appropriate materials and by selecting proper preparation conditions, composite polyelectrolyte membranes with less than 10 self-assembled double layers have been successfully fabricated. The obtained membranes had good performance for the dehydration of IPA/water mixtures by pervaporation. The lowest number of double layers in a composite membrane was 2 and this composite membrane had both a high flux and a high selectivity. It was also found that using polyelectrolytes with high molecular weights and a porous substrate with fine pores were the prerequisites for making composite polyelectrolyte membranes with less than 10 self-assembled double layers, while using a polyelectrolyte pair with high charge densities was the prerequisite for making composite membranes with a high selectivity. The successful fabrication of polyelectrolyte membranes with less than 10 double layers makes self-assembled membranes more practical because self-assembled composite membranes can be easily fabricated. <br /><br /> The data reproducibility and the stability of self-assembled composite membranes with less than 10 double layers have been discussed in this work. Random defects in the self-assembled separating layer and low repeatability of thickness in the first several deposition layers are believed to be the major reasons for the relatively low data reproducibility of single composite membranes, while the conformation change of adsorbed polyelectrolytes is one of the reasons for the flux reduction of composite membranes with less than 10 self-assembled double layers. Though the flux reproducibility of single membranes is barely acceptable (relative error about 25%), the average fluxes of several membranes made under the same conditions show good reproducibility. All composite membranes with less than 10 self-assembled double layers, from a structure point of view, were stable because the fluxes of polyelectrolyte membranes didn?t increase as time passed. <br /><br /> The separation performance of the self-assembled composite membranes developed in this work is not as good as it was originally expected, but it is still better than that of commercial poly(vinyl alcohol) (PVA) membranes for the dehydration of IPA/water mixtures, which indicates that new self-assembled composite membranes could be used for practical dehydration of IPA. The flux of the self-assembled composite membrane with 2 double layers was two times higher than that of reported self-assembled membrane in the literature when an IPA/water feed mixture with 10. 0 wt% of water was used at 60&deg;C. The composite membrane with 2 self-assembled double layers is a high performance membrane for IPA dehydration. <br /><br /> The formation of a single self-assembled layer on a non-porous substrate has been studied, but nothing has been reported about the formation of a self-assembled multilayer on a porous substrate. Based on the separation performance of different self-assembled composite membranes made from different materials and at different fabrication conditions, a two-stage process is proposed to explain the formation of a self-assembled multilayer on a porous substrate. Polyelectrolyte molecules, in the first stage, will deposit on the non-porous portion of the surface of a porous substrate while polyelectrolyte molecules will go into and fill the pores on the surface of a porous substrate to change a porous substrate into a "non-porous" substrate. In the second stage, polyelectrolyte molecules will deposit on a "non-porous substrate" to form a multilayer. This process can also be used to explain the formation of a multilayer on a non-porous substrate.