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New Concepts at the Interface: Novel Viewpoints and Interpretations, Theory and Computations
Zwitterionic Membrane via Non-solvent Induced Phase Separation: a Computer Simulation Study Jinhao Huo, Zheng Chen, and Jian Zhou Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01786 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018
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Zwitterionic Membrane via Nonsolvent Induced Phase Separation: a Computer Simulation Study Jinhao Huo, Zheng Chen and Jian Zhou* Guangdong Provincial Key Laboratory for Green Chemical Product technology, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
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ABSTRACT: Dissipative particle dynamics (DPD) was adopted to study the nonsolvent induced phase separation (NIPS) process during a pH-responsive polyethersulfone membrane preparation with a zwitterionic copolymer polyethersulfone-block-polycarboxybetaine methacrylate (PES −
b − PCBMA) as the blending additive. The membrane formation process and final morphology were analyzed. Simulation results show that the hydrophilic PCBMA segments enrich on the membrane surface by surface segregation and exhibit pH responsive behavior, which is attributed to the deprotonation of the carboxylic acid group. With the polymer concentration increasing, the shrinkage of the membrane decreases and the flexibility of the system decreases, which also reduce the effect of surface segregation. By adjusting the blend ratio of PES − b −
PCBMA with PES from 5% to 15%, the surface coverage of PCBMA segments on the membrane can be regulated. This work contributes to a better understanding on the mechanism of NIPS and can serve as a guide for the design of polymer blend membrane. KEYWORDS: dissipative particle dynamics, zwitterionic, pH-responsive, polymer blend membrane, nonsolvent induced phase separation
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I． ．INTRODUCTION Nonsolvent induced phase separation (NIPS) method has been widely applied in polymer membrane preparation for its great advantage of easy operation.1-2 During the NIPS process, casting solution is immersed into a nonsolvent bath, phase separation occurs along with the interphase mass transfer between the solvent and the nonsolvent. As a result, polymer membrane with asymmetric morphologies is obtained. It is well known that surface hydrophilization is an effective strategy to resist membrane fouling.3-4 Generally, to fabricate antifouling membrane, amphiphilic copolymers consisting of hydrophilic and hydrophobic segments as additives are blended with the bulk membrane material (BMM) in a good solvent to prepare the casting solution, which is known as surface segregation.3, 5 The enrichment of hydrophilic segments on the membrane surface can significantly enhance the fouling-resistant property against protein and oil foulants.6-7 Compared with the conventional PEG-based polymers, zwitterionic polymers can form stronger hydration shell via electrostatic interactions8 and do not have the oxidative degradation problem9-10. They have been recognized as promising antifouling materials. Zhao et al.11 synthesized a zwitterionic polymer with poly(sulfobetaine methacrylate) (PSBMA) segments and low surface energy polydimethylsiloxane (PDMS) segments, and blended it with polyvinylidene difluoride (PVDF) BMM to develop a membrane with both biofouling-resistance and biofoulingrelease abilities. Kaner et al.12 prepared PVDF blend membranes with zwitterion-containing polymer as the hydrophilic additives based on sulfobetaine-2-vinylpyridine, the membrane was endowed with properties of superior antifouling and high flux during the utilization in oil/water separation. Among zwitterionic polymers, polycarboxybetaine methacrylate (PCBMA) has been extensively used since it has shown pH-responsive capability based on the deprotonation state of
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the carboxylic acid group in the side chain.13 A material which can switch between fouling and anti-fouling properties by changing its surface charge is very important on practical membrane applications. PCBMA surfaces were shown to be non-fouling to bacteria cells in intermediate to high ionic strength solutions and at pH values that ranged from neutral to basic.14 Thus, PCBMA is highly desirable to construct a membrane with the smart responsive surface. Though some experiments and theoretical studies have been reported on polymer membranes preparation by NIPS method15-16, an analysis at molecular level is imperative to detect and speculate the phase separation process and the surface composition of the membrane. As a mesoscale simulation method, dissipative particle dynamics (DPD) simulation coincides with the time scale of the self-assembled behavior of polymers, it can be utilized as a complement to understand the formation process of polymer membrane at the molecular level.17-18 It has been used as an important tool to study complex soft matter systems and the dynamic process of phase separation in membrane formation.19-21 Esteves et al.22 put forward a experimental-DPD simulation approach to investigate the surface segregation and the bulk distribution of fluorinated polymeric dangling chains in a cross-linked poly(urethane) network. The combined results show great agreement with the segregation of the low surface energy fluorinated chains to the air/polymer interface. The behavior of nanoparticle in an athermal polymer system was studied by Teng et al..23 Nanoparticles transited from surface segregation to bulk aggregation as the particle size changes. Recently, Wang et al. simulated the NIPS process and explored the influential factors of porous membrane formation.24 DPD is becoming a more and more important tool to study the complex self-assembled behavior of polymer systems. In this study, DPD simulations will be performed to study the NIPS process of the PES
polymer membrane with an amphiphilic copolymer PES − b − PCBMA serving as a blending
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additive. A nonsolvent water bath is established for solvent/nonsolvent exchange with the casting solution. The process of membrane formation was simulated and the effect of polymer concentration, the blend ratio of PES − b − PCBMA/PES and pH-responsive ability on the membrane morphology were studied. This study aims at analyzing the phase separation mechanism of NIPS process and provides reference for the design of pH-responsive hydrophilic surface for polymer membrane.
II． ．SIMULATION METHOD DPD Method. Generally, coarse-grained (CG) model is used in DPD simulations25-27 and each polymer monomer is simplified into one or more DPD beads. Each DPD bead has the same volume and mass.28 All beads obey Newton's equations of motion:
d d
, = = d d
(1)
where , , , represent bead ’s position, velocity, mass and total force, respectively. All the physical quantities in DPD are in reduced units to simplify the calculation. Each bead’s mass is equal to 1. The conservative force ( ), dissipative force ( ), random force ( ), spring force ( ) and electrostatic force( ) constitute the total force acting on bead .29-30 So the
total force can be expressed as:
' ( + = ! "$# + "& # + "# + "# + "#
(2)
)*
The interaction between two beads will be neglected when the distance among them exceeds a certain cut-off radius (, ). In DPD, , = 1, which is the unit length in reduced unit. The direction and strength of the first three forces can be calculated by the position and velocity of the beads with the given expressions:
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"$# = .
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/) 01 − ) 1̂) () < , ) 0 () ≥ , )
(3)
:
"&
# )(̂) ) # = −γ9 () )(̂)
(4)
"'# = σ9< () )=) ̂)
(5)
# =
−
# , ) = in Eqs. (3-5), /) is the repulsive parameter between beads and > ,
# /?) ? ,
# =
−
# . σ is the amplitude of noise; γ is the coefficient of ?) ? , ̂) =
dissipative force; =) is a random fluctuation variable with an average value of 0; 9: and 9< are weighting functions. Eq. 6 expresses the selection of weighting function and Eq. 7 determines the relationship between γ and σ: B
01 − ) 1 () < , ) 9 () ) = @9 () )A = . 0 () ≥ , ) :
