Studies on pervaporation characteristics of polyacrylonitrile–b-poly(ethylene glycol)–b-polyacrylonitrile block copolymer membrane for dehydration of aqueous acetone solutions

doi:10.1016/j.memsci.2007.12.023

Journal of Membrane Science

Volume 311, Issues 1–2, 20 March 2008, Pages 284-293

https://doi.org/10.1016/j.memsci.2007.12.023 Get rights and content

Abstract

A novel polyacrylonitrile–b-poly(ethylene glycol)–b-polyacrylonitrile (PAN–b-PEG–b-PAN) block copolymer was synthesized by water-phase precipitation copolymerization, using ceric ammonium nitrate–PEG pair as an initiator. Structure and composition of the block copolymer were characterized by FT-IR, ¹H NMR, differential scanning calorimetry and elemental analysis. Block copolymer membranes were prepared by solution casting method and morphologies of the membranes were examined by transmission electron micrographs. Dehydration of aqueous acetone solution with 5 wt% water is considerably improved by using block copolymer membrane and pervaporation performance is greatly dependent on the PEG molecular weight (M_PEG) and weight content (W_PEG) in the block copolymer. Maximum separation factor and minimum permeate flux displayed for the block copolymer membrane with M_PEG = 6000 and W_PEG = 7.3 wt%. Effect of feed temperature and feed composition on pervaporation was analyzed in two pairs of parameters, separation factor, flux and selectivity, permeability and it was found that the latter pair can better correlate pervaporation performance with membrane structure. These pervaporation characteristics of the membrane were explained mainly in terms of PEG micro-phase separation behavior in the PAN–b-PEG–b-PAN block copolymer and PEG solubility characteristics in water with temperature.

Introduction

Pervaporation is a promising technique for separating liquid mixtures including dehydration of aqueous organic solutions, separation of organics from water and separation of binary organic mixtures since it is energy saving and environment protective, etc. [1]. Aqueous solutions of organic compounds are major targets for pervaporation separation and the main industrial application of pervaporation is organics dehydration [2], [3]. Finding excellent membrane materials has always been an intense focus in the area of membrane separation. For separating organics from its dilute aqueous solution, multicomponent polymer membranes were widely studied and it has been elucidated that permselectivity of multicomponent polymer membranes was dependent on the morphology of their micro-phase separation [4], [5]. However, for pervaporation dehydration of aqueous organics solutions, effect of micro-phase separation in block copolymer on pervaporation performance is less understood. Usually, special attention is paid to control the swelling degree of the membrane, and hence cross-linked polymers and polymer-inorganic hybrids are commonly favored [6], [7], [8], [9], [10].

Separation of acetone from its aqueous solution as well as its dehydration is of importance in biochemical process and there have already been some studies on it with moderate flux and separation factor [11], [12], [13], [14]. Ray and Ray [11] studied dehydration of aqueous acetone solutions with PAN copolymer membranes. In their study, acrylonitrile was copolymerized with three second monomers, i.e. 2-hydroxy ethyl methacrylate, vinyl acetate and methyl methacrylate and a separation factor of 12 and a flux of 120 (g/m² h) were obtained for their membrane when used for pervaporation dehydration of an aqueous acetone solution with 10 wt% water. Judging from their research, in order to improve the pervaporation performance, acrylonitrile monomer should copolymerize with a second monomer which is more hydrophilic. Poly(ethylene glycol) is biocompatible, highly hydrophilic and anti-fouling, making it a proper candidate for organics dehydration [15]. But it is not easy to make membrane from PEG alone due to its brittleness and crystallinity. Polyacrylonitrile is a widely used membrane material with good membrane forming ability, high stability, chemical resistance and physical strength [11]. So, the combination of PEG's hydrophilicity and polyacrylonitrile's good membrane forming ability should be desirable for making a dehydration membrane with good performance. However, due to the incompatibility of PEG and polyacrylonitrile, their blend membranes are not suitable for pervaporation application and it is desirable to copolymerize PEG with acrylonitrile monomers. Besides, to our knowledge, block copolymer membranes for pervaporation dehydration were rarely studied even though a considerable amount of block polymers have already been studied in pervaporation separation of organics from its aqueous solution and separation of binary organics.

Besides the material used in pervaporation, operation conditions such as feed temperature and feed composition also have a heavy effect on pervaporation performance by influencing both the driving force for pervaporation and the structure of the membrane. Traditionally, separation factor and permeate flux were adopted to evaluate pervaporation performance and these two parameters are directly correlated with membrane structure in spite of the fact that doing this ignored the contribution made by driving force on pervaporation. In order to take into consideration the contribution of driving force made on flux, Wijmans [16], [17] recommended another two parameters, permeability and selectivity, to assess pervaporation performance of membranes. Obviously, doing this is good for properly correlating membrane structure with pervaporation, especially when the driving force of each component changes.

In this work, our objective is to improve the pervaporation performance of acetone dehydration and elucidate the effect of aggregation structures of PAN–b-PEG–b-PAN block copolymer on pervaporation performance of acetone dehydration. Meanwhile, aimed at properly correlating pervaporation performance with membrane structure, pervaporation performances of PAN–b-PEG–b-PAN block copolymers were analyzed in terms of both permeability, selectivity and permeate flux, separation factor when the driving force was changed, i.e., when feed temperature or feed composition was changed.

Section snippets

Materials

Four poly(ethylene glycol) (PEG) samples with molecular weight 2000, 6000, 10,000 and 20,000 were purchased from Shanghai Chemical Company, China. Analysis grade acrylonitrile was purchased from Dongrui chemical plant, shanghai. Ceric ammonium nitrate initiator was purchased from Shanghai First Chemical Reagent Factory. N,N-Dimethylformamide and acetone were all of analysis grade and used with no further purification. KBr for FT-IR measurement and DMSO-D6 for ¹H NMR were from Aldrich and of

Characterization of block copolymer PAN–b-PEG–b-PAN

Fig. 2 shows the ¹H NMR spectrum of PAN–b-PEG–b-PAN block copolymer with M_PEG = 20,000 and W_PEG = 7.3 wt%. It is clear from Fig. 2 that chemical shifts at 2.03 and 3.17 ppm are attributed to the hydrogen in CH₂ and CH on the PAN segments, the appearance of chemical shift at 3.51 ppm is due to the –CH₂CH₂O– group of PEG, and chemical shift at 2.51 ppm is for the solvent DMSO-D6.

Fig. 3 illustrates the FT-IR spectrum of PEG20000 and PAN–b-PEG–b-PAN block copolymer with M_PEG = 20,000 and W_PEG = 7.3 wt%. From

Conclusions

PAN–b-PEG–b-PAN block copolymers with different PEG molecular weight and PEG weight content were synthesized by water-phase precipitation copolymerization using Ce(IV)–PEG as a redox initiate system. PAN–b-PEG–b-PAN block copolymers membranes were made and used in the pervaporation dehydration of aqueous acetone solution. The pervaporation characteristics of PAN–b-PEG–b-PAN block copolymer membranes were as follows:

(1)
Maximum separation factor of 412.8 and minimum total flux of 274.4 (g/m² h) is

Acknowledgements

This research was financially supported by the NNSFC (20376068, 20606028) and the Major State Basic Research Program of China (9732003C8615700).

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Studies on pervaporation characteristics of polyacrylonitrile–b-poly(ethylene glycol)–b-polyacrylonitrile block copolymer membrane for dehydration of aqueous acetone solutions

Abstract

Introduction

Section snippets

Materials

Characterization of block copolymer PAN–b-PEG–b-PAN

Conclusions

Acknowledgements

J. Membr. Sci.

J. Membr. Sci.

J. Membr. Sci.

J. Membr. Sci.

J. Membr. Sci.

Prog. Polym. Sci.

J. Membr. Sci.

Polymer

J. Membr. Sci.

J. Membr. Sci.

J. Membr. Sci.

Selective pervaporation of water through a nonselective microporous titania membrane by a dynamically induced molecular sieving mechanism

Langmuir

Removal of benzene from an aqueous solution of dilute benzene by various cross-linked poly(dimethylsiloxane) membranes during pervaporation

Macromolecules

Effects of morphology of multicomponent polymer membranes containing calixarene on permselective removal of benzene from a dilute aqueous solution of benzene

Macromolecules