Fabrication of hollow fiber membranes using amphiphilic block copolymers as pore-forming additives

The use of additives is an important mean to tailoring the membrane structures and properties during membrane fabrication. Amphiphilic Pluronic triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) have received much attention recently in membrane preparation due to their unique behaviors, such as high surface activity and micellization in water, which potentially change the surface properties and pore structure of membranes. However, the mechanism of membrane formation with the presence of amphiphilic polymers is still not fully understood. Therefore, a more comprehensive investigation on the role of amphiphilic polymers in membrane formation is required before they can become potential alternatives to the conventional additives. A systematical study has been firstly carried out to investigate the effect of Pluronic copolymers with different molecular architectures and contents as a pore-forming additive on the fabrication of polyethersulfone (PES) ultrafiltration (UF) hollow fibers, and to identify the most preferential features of Pluronic for making high-performance membranes. It is observed that the pure water permeation (PWP) and molecular weight cut-off (MWCO) of the as-spun hollow fibers are dependent on the structure of the additives. Among all the membranes spun with 5 wt% additives, the hollow fibers spun using Pluronic F127 (EO100–PO65–EO100, where EO is ethylene oxide, PO is propylene oxide) and F108 (EO132–PO50–EO132) as the additives possess the highest PWP, the lowest MWCO, and the narrowest pore size distribution. It is possible that due to the long PPO and PEO blocks in these Pluronics, PEO brush layer that reduces the apparent membrane pore size has formed on the internal pore surface and hence the solute rejection of the resultant membranes is improved. When Pluronic F127 concentration was 10 wt%, the as-spun hollow fiber exhibited the highest PWP of 114 L/m2•h•bar and the lowest MWCO of 9 kDa. Due to the good performance of Pluronic F127 in PES membrane fabrication, it has been selected as the amphiphilic additive to further study the formation of poly(vinylidene fluoride) (PVDF) membranes. In this work, the fundamental study has been carried out on flat sheet membranes instead of hollow fibers because the former involve less fabrication parameters. The surface tension – concentration curve reveals that no micelles are formed in Pluronic/N-methyl-2-pyrrolidone (NMP) solutions while the thermodynamics and kinetics are found to be similar for the systems containing Pluronic and polyethylene glycol (PEG), respectively. Based on the characteristics of the prepared membranes, Pluronic shows a stronger pore-forming ability than PEG. Instead of the commonly used concept of instantaneous or delayed demixing, it is believed that the stronger pore-forming ability of Pluronic is caused by the hydrophobic-hydrophobic interaction between Pluronic and PVDF that delays the solidification process. The loss of Pluronic after 2-propanol-treatment causes an increase in water permeation and a decrease in rejection, further proving that the presence of Pluronic on the membrane pore surface can narrow down the pore size. Pluronic F127 has also been used as the additive to fabricate PVDF hollow fiber membranes. However, tremendous growth of macrovoids is observed in the fibers prepared with Pluronic, even with a concentration as low as 1 wt%. The big macrovoids in hollow fibers are unfavorable to the mechanical strength. Therefore, a second additive, which is either LiCl or water, has been added together with Pluronic in the spinning dope to examine the synergetic effects of mixed additives on macrovoid formation. The macrovoid growth is suppressed when either LiCl or water is used as the second additive. An increase in PWP and MWCO and an enhancement in mechanical properties can be observed for the fibers prepared with water as the second additive. On the other hand, the addition of LiCl causes an increase in PWP yet a decrease in MWCO of the resultant fibers. However, the mechanical strength is not improved by the addition of LiCl. A stability study has been carried out on PVDF/Pluronic membranes and Pluronic is observed to be not completely stable in PVDF membrane matrix in long term. Considering this stability issue, as well as the strong pore-forming ability of Pluronic F127 and the effectiveness of Pluronic/LiCl mixed additive to form smaller surface pores as observed in the above PVDF study, the Pluronic/LiCl mixed additive consisting of a very low concentration of Pluronic has been used in further study which investigated the effects of spinning conditions on PVDF hollow fiber fabrication. It is found that the use of merely 0.2 wt% of Pluronic is sufficient to impose significant impacts on the morphology, filtration performance, and mechanical properties of the resultant fibers. Hollow fibers spun using higher coagulant temperature, higher take-up speed, and water as the bore fluid exhibit better mechanical properties and filtration performance. PVDF hollow fiber membranes with PWP of 118 L/m2•h•bar and MWCO of 5 kDa were obtained when spun with optimized spinning conditions and using 3 wt% of LiCl and 0.2 wt% of Pluronic as the mixed additive. The fibers can withstand a pressure of 5.5 bar from the lumen. In conclusion, this study provides a better understanding on the role of amphiphilic Pluronic block copolymers as an additive in membrane formation based on the studies on the fundamental mechanisms of membrane formation, the effects of block copolymers on apparent membrane pore size, the synergetic effects of mixed additives, and the influence of spinning conditions. This work expands the current list of additives used in membrane fabrication, where amphiphilic polymers are revealed to be potential alternatives to the conventional additives for preparing membranes for various applications.Doctor of Philosophy (CEE