Study on the macrovoid formation of Poly(ether sulfone) membranes
Date Issued
2012
Date
2012
Author(s)
Lai, Yi-Ling
Abstract
In the present study, polyether sulfone (PES) membranes were prepared by nonsolvent- induced phase separarion (NIPS, also called wet inversion process) method and using N-methyl pyrrolidinone (NMP) and 2-Pyrrolidinone (2P) as the solvent. In the present work we found out that the morphology of the PES membranes prepared by these two different solvents were widely divergent, and the type of macrovoid formation in the PES membranes were also dissimilar.
As the casting solution was using NMP as the solvent, the PES membranes prepared by NIPS were mainly filled with cellular structure, and the macrovoid was form just beneath the skin layer. But as the casting solution was using 2P as the solvent, the PES membranes prepared by NIPS were mainly filled with bicontinuous structure, and the macrovoid was form under the skin with a little distance. In general, the bicontinuous structure occurs in the system which is not easy to nucleation and growth. Because 2P is a poor solvent to PES, poor solvent can cause the PES polymer chains to have more entanglements and increase the resistance to nucleation of the casting solution. For this reason, it need more time to nucleation and growth after the composition phase separation.
In the first part, increasing the polymer concentration in casting solution could increase the viscosity of the casting solution, and also increase the resistance to nucleation. Although increasing the viscosity could increase the macrovoid initiation position and decrease the macrovoid length successfully, but the morphology in PES/NMP system was still filled with cellular structure, only a thin layer of bicontinuous was beneath the skin layer. Therefore, in second part we tried to use water as a nonsolvent additive. Adding water can decrease the solubility of PES in solvent and speed up the phase separation rate, lead to increase the entanglement of polymer chains and the hard to nucleation. As the result, even under the same viscosity, the more water additive made the higher macrovoid initiation position and the lower length. But the morphology of the membranes with adding water had no obvious difference between the membranes altered by concentration, PES/NMP system was still filled with cellular structure. Hence, in the third part, we used two different molecular weight of polyethylene glycol (PEG, Mw=10000 and 20000) as additive. PEG is a kind of weak nonsolvent to PES , the solubility of PES will decrease and result in the inter- and intra-molecular aggregations and entanglements of the polymer chains in the casting solution. Also, adding PEG will made the casting solution more thermodynamics unstable, the phase separation rate will increase, it will large increase the nucleation resistance. Consequently, when the concentration of PEG was over 10wt% in casting solution, the more PEG additive could make the morphology of PES/NMP system was more likely to PES/2P system, over half upper part is filled with bicontinuous structure and the macrovoid is more closely to the membrane bottom.
Subjects
polyether sulfone (PES)
nonsolvent- induced phase separarion (NIPS)
wet inversion process
macrovoid
additive
Type
thesis
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