Abstract

The control of surface roughness of polyvinylidene fluoride (PVDF), polyethersulfone (PES), polysulfone (PS) and cellulose (CE) membranes was attempted by changing the rate of nonsolvent influx in the phase inversion process. PVDF and CE were chosen to represent membranes of high hydrophobicity and hydrophilicity, respectively, while PES and PS were chosen to represent membranes of intermediate hydrophobicity/-philicity. The concentration of sodium chloride (NaCl) in the aqueous coagulation medium was increased from 0 to 1.9 mol/L to decrease the rate of nonsolvent (water) influx in the solvent/nonsolvent exchange process. As well, the effect of polymer concentration and solvent on the surface roughness was investigated with respect to PVDF and PES. It was observed that the membrane surface roughness increased and decreased, respectively, for the hydrophobic PVDF and hydrophilic CE membrane as the rate of nonsolvent influx was decreased. For the PES and PS membranes of intermediate hydrophilic/-philicity, no significant roughness change was observed. The surface roughness tended to increase as the solution viscosity decreased. It was also observed that the pattern wave length of the hydrophobic membrane did not change significantly while that of the hydrophilic membrane increased significantly as the solvent influx rate was reduced. This trend is predictable by considering the shrinking or swelling of the cast polymer solution during the solvent/nonsolvent exchange process.

Highlights

  • It has been known for a long time that surface properties such as surface roughness, pore size, pore size distribution, etc., play an important role in the membrane performance

  • After the invention of Atomic Force Microscope (AFM) in 1981, a large amount of work has been reported on the measurement of surface roughness by AFM and attempts have been made to correlate the results with the membrane performance, as summarized in the work of Khulbe et al [2] and Yao et al [3]

  • The results showed that the membrane performance depends primarily on the nature of the thin skin layer in which the roughest membrane has the highest water flux

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Summary

Introduction

It has been known for a long time that surface properties such as surface roughness, pore size, pore size distribution, etc., play an important role in the membrane performance. After the invention of Atomic Force Microscope (AFM) in 1981, a large amount of work has been reported on the measurement of surface roughness by AFM and attempts have been made to correlate the results with the membrane performance, as summarized in the work of Khulbe et al [2] and Yao et al [3]. Some of the examples are; Hirose et al used Scanning Electron Microscope (SEM) and AFM to investigate the relationship between the surface structure of the skin layers of polyamide thin film composite (TFC) reverse osmosis (RO) membranes and their performances [4]. Vrijenhoek et al showed a strong correlation between fouling and surface roughness for some RO and nanofiltration (NF) membranes [8]. Valleys become blocked and fouling becomes more severe for the rough membrane

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