Grafting copolymer of thermo-responsive and polysaccharide chains for surface modification of high performance membrane

https://doi.org/10.1016/j.seppur.2020.116585Get rights and content

Highlights

  • A thermo-responsive copolymer is fabricated for membrane modification.

  • The hydrophilic chains of dextran contain a large amount of hydroxyl groups.

  • The pure water flux is up to 118.5 L m−2 h−1, and the BSA rejection ratio is above 95%

  • From 25 to 40 °C, the pure water flux of the modified membrane increases by about 20 L m−2 h−1.

  • The pore size of the modified membrane can be adjusted to achieve molecular separation.

Abstract

In membrane separation technology, designing and preparing membranes with excellent hydrophilicity and separation performance remains a great challenge. In this study, a thermo-responsive copolymer (DexPNI) is made by RAFT polymerization and used as an additive for polyethersulfone membrane modification to improve the hydrophilicity of the membrane also impart thermal response behavior. The water contact angle of the modified membrane is significantly decreased from 89.3° to 58.8° compared to pristine polyethersulfone membrane. The pure water flux of the modified membrane is found to be 118.5 L m−2 h−1, and has a high rejection for BSA above 95%. The obtained results show that the modified membrane has excellent hydrophilicity and rejection properties. Moreover, the pore size of the modified membrane can be adjusted for molecular separation by simply controlling the extension and contraction of the thermally responsive polymer chains.

Introduction

Nowadays, with the rapid development of membrane technology, membrane separation technology has become an important technology to solve current energy, resource and environmental problems, including seawater desalination, environmental protection, petrochemical, food processing, protein purification, and so on [1], [2], [3], [4]. Separation membranes can be divided into two major categories: inorganic membranes and organic polymer membranes [5], [6], [7], [8]. Organic polymers have been widely used in separation membrane materials due to their diversity, good processing performance and low membrane forming cost [9], [10].

Usually, cellulose, polyamides, polysulfones and polyolefins are well known membrane materials [11], [12], [13]. Among them, polyethersulfone (PES) has received extensive attention due to its excellent chemical durability, thermal stability, and good membrane forming capability [14], [15]. Unfortunately, the inherent hydrophobic of PES may cause membrane pores to be blocked by proteins or other retentate in the membrane separation process, can cause decrease in water flux and anti-fouling properties during membrane filtration [16], [17], [18]. These inadequacies of the membrane incarcerate the development and applications of membrane separation technology.

Permeability and selectivity are two important parameters for evaluating membrane performance [19], [20], [21], [22]. The separation mechanism of the membrane is mainly the sieving mechanism, which screens molecules of different sizes. Under the static pressure, micro-molecules in the solution flow through the pores of membrane to the other side, and macromolecules larger than the pores of the membrane are intercepted at the surface of the membrane, thereby achieving the purpose of separation [23], [24], [25]. Researchers have made considerable efforts to improve the properties of the membrane. In order to increase the permeability and selectivity of the membrane, functional modification of membrane surface has become an attractive and effective method. [26], [27]. The functional separation membrane is mainly fabricated by combining functionalized polymers or functionalized particles with membrane materials [28], [29], [30]. Stimuli-responsive membrane, also called environment sensitive membrane, belongs to functional membrane and has great significance [15], [31], [32], [33]. In general, stimuli-responsive membranes has been fabricated by using stimuli-responsive polymers. The response behavior of these membranes depends on the unique characteristics of the responsive polymer which is built-in the membrane [34], [35]. The stimuli-responsive membrane can make the membrane technology become controllable and intelligent by responding to the change of external environment [36], [37]. Researchers have been developed variety of polymers that can respond according to external environment changes.

These respond polymers have been classified into different kinds, such as temperature responsive, pH responsive, redox responsive, photoresponsive, electric and magnetic field responsive polymers, etc [38], [39], [40], [41]. Among these responsive polymers, temperature-responsive polymers have been widely used in membrane separation technology due to the simplicity of temperature control [42], [43]. Critical phase transition temperature is one of the unique and important properties of temperature responsive polymer. Near the critical phase transition temperature, the dissolution state of the polymer has been altered drastically since the change in temperature creates hydrogen bonding effect and hydrophobic/hydrophilic effect. This in turn leads to a phase transition caused by a change in the hydrophilic-hydrophobic stability of the polymer [33], [44], [45], [46].

A class of polymers which endure phase separation at prominent temperatures have been referred as polymers with low critical solution temperatures (LCST) [15], [47]. This concert of phase separation near the critical phase transition temperature is called LCST behavior. For instance, as a thermal sensitive polymer, poly(N-isopropylacrylamide) (PNIPAAm) is extensively studied and used to construct thermal-responsive membranes. When the temperature reaches to LCST and the chain segments of PNIPAAm in the modified membrane change from the expanded state to the contracted state, then a phase separation takes place. Therefore, by changing specific environmental conditions, the chemical or physical properties of the stimuli-responsive membrane have been changed, which makes stimuli-responsive membranes more flexible for special applications in diverse fields [33], [48], [49].

In this study, the thermo-responsive copolymer (DexPNI) was fabricated by RAFT polymerization of NIPAAm using macromolecular chain transfer agent (DexDTM), which was synthesized via esterification of 2-(dodecylthiocarbonothioylthio)-2-methylpropanoic acid (DTM) with the hydroxyl groups of dextran [50], [51], [52]. The thermo-responsive copolymer has been used as an additive for modification of PES membrane to improve the hydrophilicity of the membrane and impart thermal response behavior. The intention of present study is mainly grafting the dextran and PNIPAAm by RAFT polymerization to synthesize a dextran-based temperature responsive polymer to modify the PES membrane also achieve membrane fictionalization. As a natural polysaccharide, dextran is widely used in the field of biomedicine due to its biological compatibility, excellent hydrophilicity and anti-fouling properties. Therefore, the hydrophilic chains of dextran improved the hydrophilicity and anti-fouling properties of the modified membrane. And the segments of PNIPAAm can make the modified membrane has temperature response characteristics. When the temperature is higher than the LCST, the PNIPAAm chains of the modified membrane start to indenture, so that the pore size of the customized membrane has been increased, and thus the permeability of the modified membranes has also increases. Hence, the permeability of the modified membrane has been controlled by change in temperature, and separation of different molecules would be achieved according to their molecular sizes.

Section snippets

Chemicals

Polyethersulfone (PES, Ultrason E6020P, Mw = 58, 000), and N-isopropylacrylamide (NIPAAm, 98%) were purchased from BASF, Germany. 1-Dodecanethiol (C12H26S, 98%), 2-Dimethylaminopyridine (97%), 2, 2′-azobis(2-(2-imidazolin-2-y) propane) dihydrochloride (VA-044, 98%), methyltrioctylammonium chloride, and dextran (Mw = 70000) were purchased from Shanghai Macklin Biochemical Co., Ltd. N, N-dimethylacetamide (DMAc, AR, 99.0%) was purchased from Sino-pharm Chemical Reagent Co., Ltd, China, and used

Results and discussion

The thermo-responsive copolymer DexPNI was fabricated by RAFT polymerization of NIPAM by using macromolecular chain transfer agent DexDTM, which was synthesized via esterification of DTM with the hydroxyl groups on dextran, as shown in Scheme 1. The dextran chain segment in copolymer DexPNI exhibited excellent hydrophilicity and biocompatibility, and the DTM in DexPNI was the hydrophobic chain segments.

The branch segment of PNIPAAm in DexPNI has thermo-responsive behavior; when the temperature

Conclusions

The thermo-responsive copolymer (DexPNI) has been successfully fabricated by using RAFT polymerization method, which is used to modify the PES membrane and achieve membrane functionalization. The flux experiments demonstrated that the permeability of the modified membrane showed obvious thermal responsive properties: when the temperature raised from 25 to 40 °C, the water flux of the modified membrane significantly increased, and the flux increased to about 20 L m−2 h−1, corresponding to the

Acknowledgments

This work was partly supported by the National Natural Science Foundation of China (51763014), Joint fund between Shenyang National Laboratory for Materials Science and State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals (18LHPY002), and the Program for Hongliu Distinguished Young Scholars in Lanzhou University of Technology.

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