Thermoresponsive Ultrathin Membranes with Precisely Tuned

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Thermo-Responsive Ultrathin Membranes with Precisely Tuned Nanopores for High-Flux Separation Yuzhang Zhu, Shoujian Gao, Liang Hu, and Jian Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03389 • Publication Date (Web): 13 May 2016 Downloaded from http://pubs.acs.org on May 18, 2016

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ACS Applied Materials & Interfaces

Thermo-Responsive Ultrathin Membranes with Precisely Tuned Nanopores for High-Flux Separation Yuzhang Zhu†, Shoujian Gao†, Liang Hu†, Jian Jin†,* †

i-Lab and CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-Tech

and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China. Abstract: With the more and more growing demands for small or large-scale bio-process, advanced membranes with high energy efficiency are highly required. However, conventional polymer-based membranes often confront the contradiction of selectivity and permeability. In this work, we report the fabrication of a thermo-responsive composite ultrathin membrane with precisely controlled nanopores for high-throughput separation. The composite membrane is made by grafting PEG analogue thermo-responsive copolymer onto ultrathin single-wall carbon nanotubes (SWCNTs) membrane via π-π interaction with no use of the common “grafting from” synthesis approach. The composite membrane exhibits ultrahigh water permeation flux as high as 6430 Lm-2h-1 at 40°C and more importantly, the pore size of the membrane could be finely adjusted by utilizing the thermo-responsive property of grafted copolymer. With the temperature changing below and above the lower critical solution temperature (LCST) of the copolymer, the effective pore size of the membrane is tuned precisely between ~12 nm and ~14 nm, which could be applied to effectively separate the matters with very small size difference through size sieving. Keywords: ultrathin membrane, high-flux separation, high-selective separation, thermo-responsive, SWCNTs 1 ACS Paragon Plus Environment


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1. Introduction With the more and more growing demands for small or large-scale bio-process, advanced membranes with both high selectivity and high permeability are highly required.1-5 In principle, filtration membranes are based on “size-sieving” effect to allow mass of certain size to pass through its pores. Selectivity and permeability are thus the two important performance parameters. The advanced membranes should possess both high selectivity and high permeability. However, conventional polymer-based membranes, prepared by either phase separation, or track-etched process, often confront the contradiction of the permeating flux and selectivity.6,7 High selectivity always correlates with low intrinsic permeability. Moreover, there is lack of an effective means to achieve pore size uniform and meanwhile maintain high porosity, especially to the pores down to nanometer scale. The self-assembly of block copolymers via phase separation in bulk or in film has been proved to be a powerful approach to produce regular nanopore.8-14 Numerous works have demonstrated their potential in the application of membrane separation with high selectivity.15,16 However, self-assembly of block copolymer to form vertically oriented micro-domains in large area is still difficult. A specific surface treatment of the substrate or precisely controlled annealing process is thus always requested.17,18 To address this issue, K. V. Peinemann and V. Abetz et al. recently developed a novel one-step phase conversion process to fabricate asymmetric membrane of diblock copolymer, poly(styrene-b-4-vinylpyridine) ( PS-b-P4VP) in a mixture of high- and low-volatile solvent.19-24 As tailoring the proportion of the high2 ACS Paragon Plus Environment

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and low-volatile solvent and the exposed time in air, an integral structure with well-tuned cylindrical pores in ultrathin top layer could be achieved. This work marks a great achievement of block copolymer based membranes toward realistic practical production. However, the high demanding for the block copolymer itself and restricted synthesis method (mainly anion polymerization) might limit the field-of-use in practical applications in a more cost-effective way. Therefore, developing realistic path to reach the new generation of advanced membranes with both precisely controlled nanopores and high permeation is rather significant and highly required. Herein, we report the fabrication of an ultrathin membrane composed of PEG analogue thermo-responsive copolymer grafted SWCNTs with precisely controlled nanopores. Although surface initiated polymerization could effectively immobilize functionalized polymer onto the sidewall of single-wall or multi-wall carbon nanotube, it always required harsh condition to produce initiated sites which do cause the destruction the sidewalls of CNT.25-28 In contrast, non-covalent functionalization of CNT is particularly attractive because it offers the possibility of attaching chemical handles without affecting the inherent structure of the tubes via non-covalent interactions by either van der Waals force or π-π stacking interaction.29,30 Aromatic molecules, such as pyrene, porphyrin, and their derivatives, can do interact with the sidewall of single-wall or multi-wall carbon nanotubes by means of π-π stacking interaction.31-33 Dai and co-workers

32

found that the pyrene group could irreversibly

adsorbed on the sidewall of single-wall carbon. With the aid of pyrene, they successfully immobilized a wide range of biomolecules on the SWCNT sidewalls. 3 ACS Paragon Plus Environment


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After this work, surface functionalization of CNT with polymers, metal complex and biomolecules by means of the affinity between pyrene and CNT were widely reported. These works convince us that non-covalent functionalization surface of SWCNT film is feasible. Thus, in this work, the thermo-responsive copolymer was immobilized on the surface of SWCNTs membrane via simple π-π interaction with no use of the common “grafting from” approach (as shown in Scheme 1a). Besides the ultrahigh permeation flux that the ultrathin membrane displays, the pore size of the membrane could be finely adjusted by utilizing the thermo-responsive property of the grafted copolymer. With the temperature changing below and above the lower critical solution temperature (LCST) of the polymer, the effective pore size of the membrane is tuned precisely from ~12 to ~14 nm (Scheme 1b). It represents a new type of advanced membrane for achieving high-precision and high-flux separation. 2. Results and Discussion Stimuli-responsive polymers have been widely investigated to tailor the physicochemical properties of polymer-modified composites, such as surface wettability and pore structure, triggered by external stimuli, like pH, temperature, salt concentration, light, electronic and magnetic field.34-38 By far, the modification of stimuli-responsive polymers onto host materials mainly use a “grafting from” approach via living radical polymerization, plasma polymerization, radiation polymerization, etc.39-43 Different from common used “grafting from” approach where initiator is usually introduced onto the surface via chemical treatment in a harsh condition,44-46 in this work, the PEG analogue thermo-responsive copolymer, 4 ACS Paragon Plus Environment

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pyrene-terminated poly(MEO2MA-co-OEGMA), is grafted onto an ultrathin SWCNTs membrane by simple direct adsorption via π-π interaction as schematically depicted in Scheme 1a. As a mild and effective approach, the non-covalent grafting strategy could protect the inherent structure of carbon nanotube from destroying and simultaneously the π-π interaction is strong enough to immobilize the copolymer onto the surface of SWCNTs.31-33 The π-π interaction between grafted polymer and SWCNTs could be further proved by fluorescence spectra (see supporting information, Figure S1). The pure pyrene-terminated poly(MEO2MA-co-OEGMA) exhibits four distinct emission peaks at 376, 387, 397 and 417 nm which correspond to the characteristic fluorescence of pyrene.47 After mixing with SWCNTs to form poly(MEO2MA-co-OEGMA)/SWCNTs hybrid, the four emission peaks are still observed but their emission intensities are reduced strongly. The decrease of fluorescent emission is attributed to the formation of π-π interaction between pyrene and SWCNTs. The PEG analogue thermo-responsive copolymer is composed of two blocks, oligo

(ethylene

glycol)

methacrylate

macromonomer

(OEGMA)

and

2-(2’-methoxyethoxy) ethyl methacrylate (MEO2MA). In comparison with the mostly used thermo-responsive polymer, poly (N-isopropylacrylamide) (PNiPAm), the LCST of poly(MEO2MA-co-OEGMA) could be precisely tailored by varying the composition of two monomers and less affected by other parameters, such as ionic strength in solution, polymer concentration and polymerization degree.48,49 The thermo-responsive property of poly(MEO2MA-co-OEGMA) is used to tune the pore 5 ACS Paragon Plus Environment


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size of the membrane between 12~14 nm (Scheme 1b). Meanwhile, the inherent high porosity, ultrathin thickness and interconnected pore structure of ultrathin SWCNTs membrane are well maintained and ultrahigh water flux is achieved. Thus,

to

accomplish

π-π

grafting,

pyrene-terminated

poly(MEO2MA-co-OEGMA) was first synthesized via copper-mediated atom transfer radical polymerization (ATRP) with an amine-modified initiator, where the amine group was protected by tert-butoxycarbonyl (t-BOC) to avoid the interference with ATRP catalytic system. A typical “living” characterization of ATRP process, as evidenced by the linearly kinetic plot with respect to polymerization time and the molecular weight with conversion, has been observed in our experiment by using amine-modified initiator and PMDETA/CuBr catalyst (see supporting information, Figure S2). This approach is facile and the length of the grafted polymer chain could be well controlled just by adjusting the polymerization time as prescribed. The introduction of pyrene group onto the copolymer could be confirmed by UV-vis absorption spectrum (see supporting information, Figure S3). Then, a typical SWCNTs membrane with thickness of about 60 nm was immersed into the pyrene-terminated poly(MEO2MA-co-OEGMA) solution for prescribed time. Due to inherent hydrophobic property of SWCNTs membrane, water/ethanol co-solvents are used to enhance the compatibility between the polymer solution and the SWCNTs membrane. As shown in Figure 1a, SWCNTs membrane with dense and uniform network structure interconnected by SWCNT nanowires is clearly observed. The grafting of pyrene-terminated poly(MEO2MA-co-OEGMA) onto the surface of 6 ACS Paragon Plus Environment

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pristine SWCNTs membrane is confirmed by transmission electron microscopy (TEM) observation and X-ray photoelectron microscopy (XPS) spectra as shown in Figure 1b-1e. In comparison with the TEM image of pristine SWCNTs membrane with smooth surface (Figure 1b), a thin layer of

polymer that makes the SWCNT

surface rougher could be clearly seen after grafting (Figure 1c). The C1s XPS spectra of pristine SWCNTs membrane and polymer grafted SWCNTs membrane is used to probe the surface components (Figure 1d and 1e). An asymmetric peak centered at 284.48 eV with an extending tail in the high energy region is observed in the case of pristine SWCNTs membrane, which is ascribed to sp2 hybridized graphite-like carbon.50 By using the sp2 graphite-like carbon as a reference, three new peaks ascribed to O-C=O (288.93 eV), C-O-C (286.43 eV), and sp3 C-C (285.13 eV), respectively, are fitted in the case of polymer grafted SWCNTs membrane. These three

peaks

are

associated

poly(MEO2MA-co-OEGMA).

with

the

chemical

structure

of

Additionally, the wide scan XPS spectrum of pristine

SWCNTs membrane displays a weak O1s peak (centered at 532.5 eV), probably due to the inherent defects in SWCNTs (inset in Figure 1c). In contrast, the intensity of O1s peak in the wide scan XPS spectrum of polymer grafted SWCNTs membrane increases obviously, referenced by the intensity of C1s peak (inset in Figure 1e). It is attributed

to

the

numerous

oxygen

atoms

of

pyrene-terminated

poly(MEO2MA-co-OEGMA) immobilized on SWCNTs membrane. These results demonstrate that the pyrene-terminated poly(MEO2MA-co-OEGMA) has been

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successfully grafted onto the surface of SWCNTs membrane via simple π-π interaction. With

the

aid

of

pyrene

group

at

the

end

of

polymer

chain,

poly(MEO2MA-co-OEGMA) would be effectively immobilized onto the surface of pristine SWCNTs membrane. SEM images of pristine SWNCTs membrane and polymer grafted SWNCTs membrane show that the membrane structure and morphology has almost no change before and after grafting (Figure 2a and 2b). However, the existence of thin polymer layer grafted on SWNCTs membrane extensively improves the surface wettability. As shown in the inset of Figure 2a and 2b, the water contact angle (CA) of SWCNTs membrane decreases from 115° to 78° after polymer grafting, indicating the enhancement of hydrophilicity. As a typical thermo-responsive polymer, the thermo-responsive behavior of poly(MEO2MA-co-OEGMA) has been well explored. It is recognized that its LCST could be easily influenced by the component of the polymer chain. During the synthesis of pyrene-terminated poly(MEO2MA-co-OEGMA) in our experiment, there are two copolymer intermediates, t-BOC-terminated poly(MEO2MA-co-OEGMA) and amine-terminated poly(MEO2MA-co-OEGMA). To investigate the effect of pyrene on the thermo-responsive behavior of poly(MEO2MA-co-OEGMA), the plots of transmittance vs temperature of the three copolymers are tested as shown in Figure 3a. The t-BOC-terminated poly(MEO2MA-co-OEGMA) shows a typical sharp transition with phase transition temperature localized at 32°C, which is in good agreement

with

the

literatures.48,49

As


for

amine-terminated

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poly(MEO2MA-co-OEGMA), a broad transition region from 31 to 34°C is observed in the transmittance plot. It may be because the random cleavage of ethylene oxide unit in the side chain of poly(MEO2MA-co-OEGMA) in chloroform containing trifluoroacetic

acid

(TFA).

The

transition

of

pyrene-terminated

poly(MEO2MA-co-OEGMA) becomes quite sharp and its LCST is localized at 31°C. The decrease of LCST is attributed to the introduction of hydrophobic moiety of pyrene group.51,52 These results indicate that the deprotection of t-BOC and the introduction of pyrene group have little effect on the thermo-responsive behavior of poly(MEO2MA-co-OEGMA). To investigate the influence of thermo-responsive behavior after grafting, the changes of water contact angle with time and pure water permeation fluxes of pristine SWCNTs membrane and polymer grafted SWCNTs membrane were measured at 25°C and 40°C, respectively. As shown in Figure 3b, increasing the temperature above the LCST of the polymer, the water CA of the membrane increases only 8°. However, the effect of the temperature transition on the permeability of membrane is obvious, as confirmed by the results of changes of water contact angle with time under 25°C and 40°C, respectively. The time for the water droplet to permeate the membrane is just 60s under 40°C, shortened 25% compared to the time under 25°C. Pure water permeation fluxes of pristine SWCNTs membrane and polymer grafted SWCNTs membrane were measured at 25°C and 40°C, respectively, to investigate the influence of thermo-responsive behavior. As shown in Figure 3c, the water flux of pristine SWCNTs membrane increases from 4630 to 6050 Lm-2h-1 with 9 ACS Paragon Plus Environment


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the temperature increasing from 25 to 40°C. The increase of flux is probably due to the decrease of the viscosity of water with the increase of temperature. According to Poiseille’s law,53 J = (∆pNπd4) / (128µl) (where J is flux, N is the number of pores per square centimeter of membrane, ∆p is the pressure across the membrane, µ is the liquid viscosity, d is the diameter of pore and l is the length of pore), flux is reversely proportional to liquid viscosity. The water viscosity is 0.8937 and 0.6560 at 25 and 40°C, respectively. As calculated from Poiseille’s law equation, the water flux at 40°C is ~1.36 times to that of at 25 °C. This result is agreement with the experimental data (~1.3 times). While, the increase of water flux of polymer grafted SWCNTs membrane is much larger than pristine SWCNTs membrane, from 3730s to 6430 Lm-2h-1 corresponding to 25°C and 40°C, respectively. As for polymer grafted SWCNTs membrane, the increase of flux corresponding temperature is attributed to two aspects: one is caused by the decrease of water viscosity; another is due to the thermo-responsive behavior of grafted poly(MEO2MA-co-OEGMA) at different temperature

below

and

above

LCST.

Above

LCST,

the

chain

of

poly(MEO2MA-co-OEGMA) shrinks and results in the expanding of membrane pores. By using the flux variation of pristine SWCNTs membrane as reference, the change of pore size ascribed to grafted polymer corresponding to the two temperatures could be estimated. Our result based on calculation demonstrates that the pore size of polymer grafted SWCNT membrane is increased by over 7.2% when the external temperature turns to be above LCST.


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To further demonstrate the stability of π-π interaction during separation process, the temperature-dependent pure water flux variation of polymer grafted SWCNTs membrane is measured at 25°C and 40°C (Figure 4). During ten cycles of permeation at different temperature, the membrane exhibits a stable flux variation at the two temperatures. It indicates that pyrene-terminated poly(MEO2MA-co-OEGMA) is immobilized on SWCNTs membrane firmly and there is no polymer falling off the membrane during permeation process although water is a good solvent to poly(MEO2MA-co-OEGMA). The stability of the thermos-responsive copolymer poly (MEO2MA-co-OEGMA) under different pH condition is also examined. As shown in Figure S4, there is no change of the water CA on the surface of membrane after immersed in water with changing pH for 24 h. The variation of effective pore size of polymer grafted SWCNTs membrane below and above LCST was further determined by using 12 nm Au nanoparticles (see supporting information, Figure S5) and Ferritin to permeate through the membrane. Their hydrated diameters are around 13.4 nm and 12 nm as examined by dynamic light scattering (DLS) measurement (Figure 5a). When permeating the two particles through the polymer grafted SWCNTs membrane, they behave quite different rejection behavior as shown in Figure 5b and 5c. The rejection rate of Au nanoparticle for pristine SWCNTs membrane is as low as 8.2%, as calculated from absorption intensity at 520 nm of Au nanoparticle solution before and after permeation (Figure 5b). It means that almost all the Au nanoparticles pass through the membrane. In sharp contrast, the rejection rate of Au nanoparticle for polymer grafted SWCNTs 11 ACS Paragon Plus Environment


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membrane is as high as 97.4% when the filtration is operated at 25°C, indicating that almost all Au nanoparticles are rejected by the membrane. Four times continuous filtration experiments further reveal that the rejection behavior is caused by size sieving rather than adsorption (see supporting information, Figure S6). When the operation temperature increases to 40°C, above the LCST of pyrene-terminated poly(MEO2MA-co-OEGMA), the rejection rate of the membrane decreases to 23%. As for Ferritin, the rejection rates when permeating through the polymer grafted SWCNTs membrane at 25°C and 40°C are almost same as shown in Figure 5c. 66% and 69% Ferritin could pass through the membrane corresponding to the two temperatures, respectively. It indicates that the polymer grafted SWCNTs membrane could not reject Ferritin effectively due to the larger pore size of the membrane than Ferritin. The little retentate of Ferritin is mainly attributed to the adsorption of Ferritin on membrane surface driven by hydrophobic interaction between protein and grafted copolymer. As the temperature above LCST, the hydrophobic property of poly (MEO2MA-co-OEGMA) is strengthened, and thus the hydrophobic interaction between protein and the grafted copolymer is enhanced. As a result, a slightly increased rejection for Ferritin at 40°C is therefore observed. Analyzing the rejection results of Au nanoparticles and Ferritin, we conclude that the effective pore size of polymer grafted SWCNTs membrane is about 12~13 nm when the operated temperature is below the LCST of poly(MEO2MA-co-OEGMA). The result of filtration experiment as using Au nanoparticles and Ferritin mixed solution as feed solution further confirmed this conclusion (Figure 6). As observed 12 ACS Paragon Plus Environment

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from the Figure S5, obvious absorption peak at 276 nm attribute to Ferritin could be seen in the UV-vis absorption spectra of Au nanoparticles and Ferritin after permeating through the polymer grafted SWCNTs at 25°C. In comparison with the absorption intensity of feed solution, there are about 60% Ferritin have passed through the membrane as calculation. In contrast, there is no obvious absorption peak at 520 nm, which is ascribed to Au nanoparticles, in the curve of filtrate could be seen in the magnification UV-vis absorption spectra. As calculated, the rejection rate of the Au

nanoparticles

is

as

high

as

92%.

Above

the

LCST

of

poly(MEO2MA-co-OEGMA), the effective pore size of the membrane would extend by over 7.2% according to the aforementioned calculation to be about 13~14 nm. Therefore, by simply controlling the extending and shrinkage of thermo-responsive polymer chain, the effective pore size of the membrane is precisely tuned at a few nanometers. This thermo-responsive copolymer poly(MEO2MA-co-OEGMA) grafted SWCNTs membrane could be used to finely separate the matters with very small size difference. 3. Conclusion In

conclusion,

we

reported

a

pyrene-terminated

PEG

analogue

thermo-responsive copolymer, poly (MEO2MA-co-OEGMA), which could effectively immobilized on ultrathin SWCNTs membrane via π-π interaction between pyrene and SWCNTs. Due to the hydrophilic and thermo-responsive property of grafted polymer, the poly(MEO2MA-co-OEGMA)/SWCNTs hybrid membrane exhibited large improvement in hydrophilicity as well as the anti-protein adsorption in comparison to 13 ACS Paragon Plus Environment


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the

pristine

SWCNTs

membrane.

The

thermo-responsive

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property

of

poly(MEO2MA-co-OEGMA) was used to precisely tune the effective pore size of the membrane at a few nanometers scale. It opens a new way to achieve advanced membranes with high selectivity, high permeation flux and precisely controlled nanopores. 4. Experimental Section Chemicals and materials: SWCNTs powder was purchased from XFNANO (Nanjing, China) and dispersed into water involving SDS (Sinopharm, Beijing, China).

The

monomers MEO2MA and OEGMA were both received from Sigma and passed through a basic alumina column to remove the inhibitor prior to use. N,N,N’,N’,N”-penta methyldiethylenetriamine (PMDETA, 98%), 1-pyrenebutyric acid (97%), N-hydroxysuccinimide (NHS, 98%) were received from J&K (Beijing, China) without any purification. CuBr (Aladdin, Shanghai, China) was firstly washed with acetic acid, acetone and diethyl ether sequentially, and then dried in under vacuum. Initiator [2-[(2-bromo-propionylamino)-ethyl]-carbamic acid tert-butyl ester and pyrenebutyric acid N-hydroxysuccinimide ester (PyHSE) were both synthesized according to the literatures (see details in supporting information).47,

54

Other

chemicals and solvents were all purchased from Sinopharm without purification. 0.22 micron mixed cellulose ester (MCE) filter membranes (Beijing Shenghe Company, Beijing, China) were employed as support membranes in this work. The pure water flux of the MCE filter membrane is ~14000 Lm-2h-1 at 40°C with a pressure difference of 0.1 bar. It has no influence on the flux of ultrathin SWCNTs membranes. The 14 ACS Paragon Plus Environment

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detailed characterizations about this support membrane were shown in supplementary materials. The Ferritin cationized from horse spleen was purchased from Sigma-Aldrich and used as received. The 12 nm gold nanoparticle was synthesized as follows: 100 mL HAuCl4 solution (1 mM) was heated up to 100 °C with vigorous stirring. After 20 min, 15 ml sodium citrate (1wt %) was quickly injected into the HAuCl4 solution. The solution was allowed to stir for 15 min and then cooled with cold water. The obtained Au nanoparticles were stored at 4°C. Surface grafting: Pristine ultrathin SWCNTs membrane was fabricated by wet process as we previously reported.55 To graft copolymer onto SWCNTs membrane, pyrene-terminated poly(MEO2MA-co-OEGMA) was first dissolved in water/ethanol (with volume ratio 1/1) mixture with concentration of 10 mg/ml. Then, a piece of pristine SWCNTs membrane was placed into a wild-mouth bottle containing 15 ml 10 mg/ml pyrene-terminated poly(MEO2MA-co-OEGMA) solution, which was placed in a shaking incubator at 25°C with a shaking speed of 150 rpm. After 24 h, the membrane was taken out of the solution and washed with ultrapure water/ethanol mixed solvent and stored in water before use. Filtration process: All filtration processes were carried out by using a dead-end filtration system with effective filtration area of 2.54 cm2 at a pressure difference of 0.1 bar produced by vacuum. The water flux of polymer grafted SWCNTs membrane at 25°C and 40°C were measured by filtering 10 ml of water across the membrane. In a typical separation process, the temperature of the feed solution was preheated to 25 ºC or 40 ºC, respectively, to permeate through the SWCNTs film. To assure the 15 ACS Paragon Plus Environment


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temperature of the whole separation equipment similar to the feed solution, pure water with the same temperature as feed solution was firstly passed through the film in order to warm up the separation equipment. 5 ml 0.05 mg/ml Au nanoparticles aqueous solution (pH: 6.7) and 5 ml 0.1 mg/ml Ferritin solution (pH: 7.2~7.4 buffered with PBS) were used, respectively, to permeate through the membrane under 25°C and 40°C respectively to determine the rejection property of polymer grafted SWCNTs membrane. The rejection rate was calculated by R= (C0-Cp)/C0, where C0 is the concentration of feed solution and Cp is the concentration of collected filtrate. Both C0 and Cp were determined by UV-vis absorption spectra. For cycle experiment, the water fluxes of the membrane at 25°C and 40°C were first examined and then the membrane was placed in 25°C water for 1 h to recover the original state of the membrane. This process was repeated to proceed the next cycle.

ASSOCIATED CONTENT

Supporting

Information.

The

procedure

of

synthesis

of

initiator

[2-[(2-bromo-propionylamino)-ethyl]-carbamic acid tert-butyl ester, pyrenebutyric acid

N-hydroxysuccinimide

poly(MEO2MA-co-OEGMA);

ester

schematic

and

illustration

pyrene-terminated poly(MEO2MA-co-OEMGA);

of

pyrene-terminated synthesis

route

of

kinetic plots with respect to

polymerization time; evolution of molecular weight versus monomer conversion; UV-vis absorption spectra of as-prepared copolymer with and without pyrene group at the end of chain; TEM image of gold nanoparticles; fluorescence spectra of 16 ACS Paragon Plus Environment

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pyrene-terminated poly(MEO2MA-co-OEGMA) dispersion in water and the mixture of pyrene-terminated poly(MEO2MA-co-OEMGA) and SWCNTs in water. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest. Acknowledgment We acknowledge the funding support from the Key Project of National Natural Science Foundation of China (21433012), the National Basic Research Program of China (2013CB933000), and the Natural Science Foundation of Jiangsu Province (BK20130007).

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Scheme

1.

a)

Schematic

illustrations

of

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pyrene-terminated

poly(MEO2MA-co-OEGMA) grafting onto SWCNTs membrane via π-π interaction and b) Tuning the effective pore size of the membrane by utilizing the thermo-responsive property of poly(MEO2MA-co-OEGMA).


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Figure 1. Characterizations of SWCNTs membrane before and after surface grafting. a) AFM image of pristine SWCNTs membrane with thickness of around 60 nm. The inset is height profile of the membrane along its edge. TEM images of SWCNTs membrane before (b) and after (c) surface grafting. C1s XPS spectra of SWCNTs membrane before (d) and after (e) surface grafting. The insets are their XPS survey spectra correspondingly.



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Figure 2. a) and b) SEM images of SWCNTs membrane before and after surface grafting. The insets are water contact angles of the two membranes correspondingly. The inset images are optical images of water contact angle on the membrane surface.


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Figure 3. Thermo-responsive property of SWCNTs membrane before and after surface grafting. a) Transmittance vs temperature curves of t-BOC-terminated, amine-terminated, and pyrene-terminated poly(MEO2MA-co-OEGMA), respectively. b) Variation of water contact angle with time of pristine SWCNTs membrane and polymer grafted SWCNTs membrane measured at 25°C and 40°C, respectively. c) Water fluxes of pristine SWCNTs membrane and polymer grafted SWCNTs



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membrane at 25°C and 40°C. Filtration measurements were conducted at a pressure difference of 0.1 bar.

Figure 4. Flux variation of polymer grafted SWCNTs membrane at 25°C and 40°C during several cycles. Filtration measurements were conducted at a pressure difference of 0.1 bar.


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Figure 5. a) DLS data of ferritin and Au nanoparticle solutions. b) UV-vis absorption spectra of Au nanoparticle solution after permeated through pristine SWCNTs membrane and polymer grafted SWCNTs membrane at 25ºC and 40ºC. c) UV-vis absorption spectra of ferritin solution after permeated through pristine SWCNTs membrane and polymer grafted SWCNTs membrane at 25ºC and 40ºC.



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Figure 6. UV-vis absorption spectra of Au nanoparticles and Ferritin mixed solution after permeating through polymer grafted SWCNTs membrane at 25°C.


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Table of Contents Graphic


Thermoresponsive Ultrathin Membranes with Precisely Tuned

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