The influence of the density of crystalline nuclei in casting solution on PVDF membrane morphology
Date Issued
2009
Date
2009
Author(s)
Deng, You-Tzung
Abstract
In this work, the crystallization behavior for poly(vinylidene fluorides) (PVDF) membranes via non-solvent induced phase separation (NIPS) was investigated. Formation mechanism of crystallizable polymer membranes is governed primarily by the competition between two types of phase separation, the solid-liquid demixing, or so called crystallization, and liquid-liquid demixing. Recent studies have shown that the nuclei density for initiation of crystallization also plays an important role in determining membrane morphology. The effect of nuclei density on the morphology of membranes was investigated by changing different conditions of dissolution temperature, solvent, and additives. Membranes were fabricated from PVDF/N-methyl-2-pyrrolidinone (NMP) and PVDF/Triethyl phosphate (TEP) solutions using vapor induced phase separation (VIPS) or wet immersion in EtOH bath. In those processes the composition path stayed in the crystallization longer because of low mass transfer rate between solvent and non-solvent.
Result shows that at high dissolution temperatures, the nuclei density was low because the polymer chains were well-dispersed. For PVDF/NMP membranes prepared by VIPS, water moisture entered the casting solution slowly, and the composition path stayed in S-L demixing region longer so that crystallization occurred. In the case of low nuclei density, nuclei were not affected by each other in the growing stage and finally large particulate structures were formed. As for the case of high nuclei density, crystallization was suppressed because of high degree of polymer chain entanglement. As the composition entered the L-L demixing region, lacy structures were formed via L-L demixing. For membranes prepared by wet immersion in EtOH, the composition path entered the L-L demixing and S-L demixing region quickly because of fast mass transfer rate between solvent and non-solvent. In the case of low nuclei density, lacy structures formed first and then followed by crystallization. Particulate structures with bi-continuous surface were formed. As for the case of high nuclei density, crystallization was suppressed and lacy structures were obtained.
Solvent and the additives also dominated the nuclei density, which was decreased in a solvent with higher solubility or by the addition of additives with better affinity for PVDF.
Besides, the difference in the form of crystal from different preparation process was discovered. Membranes by liquid-liquid demixing followed by crystallization were of α-type crystal structure. However, membranes of β-type crystal were obtained by the domination of crystallization.
In practical application, membranes with interconnected globule structures can be applied in some process because of high inter-connectivity and porosity. However, with the increase of globule diameter, the mechanism strength decreased. In this work, bi-continuous membranes were fabricated by increasing the initial nuclei density. The porosity was about 75 to 85%, and the pore size was in the range of 0.11 to 0.48μm.
Subjects
non-solvent induced phase separation (NIPS)
nuclei density
solid-liquid demixing
liquid-liquid demixing
Type
thesis
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