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CO2-Switchable Membranes Prepared by Immobilization of CO2-Breathing Microgels Qi Zhang, Zhenwu Wang, Lei Lei, Jun Tang, Jianli Wang, and Shiping Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15639 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on December 2, 2017

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ACS Applied Materials & Interfaces

CO2-Switchable

Membranes

Prepared

by

Immobilization

of

CO2-Breathing Microgels Qi Zhang, 1 Zhenwu Wang, 1 Lei Lei, 2 Jun Tang, 1 Jianli Wang,1,* Shiping Zhu 2,* 1 College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China 2 Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada. L8S 4L7 * Email: [email protected] (J. W.); [email protected] (S. Z.)

ABSTRACT: Herein, we report the development of a novel CO2-responsive membrane system through immobilization of CO2-responsive microgels into commercially available microfiltration membranes using a method of dynamic adsorption.

The microgels,

prepared from soap-free emulsion polymerization of CO2-responsive monomer 2-(diethylamino)ethyl methacrylate (DEA), can be reversibly expanded and shrunken upon CO2/N2 alternation. When incorporated into the membranes, this switching behavior was preserved and further led to transformation between microfiltration and ultrafiltration membranes, as indicated from the dramatic changes on water flux and BSA rejection results.

This CO2-regulated performance switching of membranes

was caused by the changes of water transportation channel, as revealed from the dynamic water contact angle tests and SEM observation.

This work represents a

simple yet versatile strategy for making CO2-responsive membranes. Keywords: CO2-responsive, microfiltration, microgel, stimuli-responsive membrane,

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ultrafiltration


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ACS Applied Materials & Interfaces 3

Introduction As a selective barrier that allows some matter (e.g., gas, liquid, small particles, ions or other molecules) to pass through but not others, porous polymeric membranes have received growing interest over the past several decades.1,2 The unique properties of membranes have not only raised plenty of opportunities for fundamental research, but also enabled a wide range of applications in many fields of practical relevance, such as water purification,3-6 tissue engineering scaffolds,7,8 controlled release,9,10 catalysis,11,12 sensing.13–15 Recently, the rapid progress and wide interest in the areas of stimuli-responsive polymers have inspired significant efforts in developing smart interactive membrane systems, for which mass transfer and interfacial properties can be regulated by external stimuli.16–18 Both physical and chemical stimuli, including light,19,20 electric and magnetic ﬁelds,21 temperature,22–24 pH,25–29 and ionic strength30 have been applied to trigger the reversible switching of physicochemical properties of stimuli-responsive membranes, enabling improved performance.

However, the current triggers have

some critical issues in practical applications.

For example, ultraviolet light causes

certain extent of damage to the system, not to mention the limited penetration depth. With pH, the repeated addition of acids and bases inevitably accumulates salts, which weaken switchability of the membrane system and give rise to complexity in the cycling process.

With thermo-responsive membranes, high energy and time

consumptions are acquired, if operated in a large volume.


It still remains


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challenging in developing novel stimuli-responsive membranes that can be triggered by environmentally friendly and cost-effective stimuli. Fortunately, the recent advent of gas stimuli has provided great opportunity for the development of smart materials and systems.31–33

Gas triggers have clear advantages

over other stimuli in industrial applications, since they can be added and removed easily in a large volume operation.

As the most studied gaseous trigger, CO2 has

been popular over the past ten years, and lots of CO2-responsive systems have been developed,

including

nanoaggregates,37–39

re-dispersible breathing

latexes,34–36

morphology

microgels.40,41

transformable

However,

introducing

CO2-responsiveness to polymeric membranes to fabricate CO2-responsive membranes remains to be explored.

Very recently, Yuan et al. reported nanostructured

electrospun polymer membranes with their surface wettability switchable via CO2/N2 alternation.42

It was demonstrated that such CO2-switchable nanofibrous membranes

could be very useful for highly controlled oil/water separation.

The success in

preparing CO2-reponsive membranes via electrospinning technique has encouraged us to search for other facial and versatile preparation methods. Generally,

there

are

several

stimuli-responsive membranes.16

major

approaches

for

the

preparation

of

The first approach is to use stimuli-responsive

polymer solely or as a component of blend in membrane formation. 43

An alternative

approach involves modification of existing membranes by various chemical/physical processes to incorporate stimuli-responsive polymers.44,45

In this modification

approach, useful properties of the base membrane are reserved, while responsive


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properties are introduced to the membrane surface.

Besides, Wessling’s group

recently reported a simple yet very effective method for preparation of thermo-responsive hollow-fiber membranes through

dynamic

adsorption

of

thermo-responsive poly(N-vinylcaprolactam) (PVCL) microgels, allowing tunable flow profile and resistance.46

In contrast to other studies on hydrogel/microgel

membranes, the simplicity of membrane modification is believed to be the major advantage.

Such easy method should also be applicable to the preparation of

CO2-responsive membranes. Herein, we report the development of a novel CO2-responsive membrane system through a simple and versatile method.

CO2-responsive poly(2-(diethylamino)ethyl

methacrylate) (PDEA) microgels were first synthesized via soap-free emulsion polymerization.

The microgels were then immobilized into commercially available

microfiltration membranes via dynamic adsorption.

The switching performance of

the prepared hybrid membranes towards CO2 was examined by flux and BSA rejection tests.

Dynamic water contact angle tests and scanning electron microscope

(SEM) observation were carried out to reveal the mechanisms of the switching membranes.



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Scheme 1. Schematic representation of CO2-switchable membranes through immobilization of CO2-breathing microgels into microporous membranes via dynamic adsorption.

Experimental Section Materials:

2-(Diethylamino)ethyl methacrylate (DEA, 99%) was purchased from

Jiangsu Yonghua Fine Chemicals Co. Ltd, and was passed through an inhibitor remover column and stored under freeze prior to use.

N, N'-Methylenebisacrylamide

(BisAM, 99%, Alading) and 2,2’-azobis(2-methylpropionamidine) dihydrochloride (V-50, 99%, Alading) were used as received.

Other chemicals were of analytical

grade and were used as received without further purification. were prepared with deionized (DI) water.

All aqueous solutions

Carbon dioxide (dry ice grade) was


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purchased from Jingong Air, and was controlled by flow meters to maintain constant gas flow.

The membranes used in this study were all commercially available

microfiltration membranes: M-1 (Merck Milipore, HPWP04700) was made from poly(ethersulfone) (PES), with 0.45 µm pore size; H1 (0.45 µm) and H2 (2.0 µm) were both PES microfiltration membranes from Hangzhou Han Membrane Materials; A1 (Shanghai Yaxing Purification Materials, Q/IEFJ01-1997) was made from mixed cellulose esters (MCE), with a pore size of 0.45 µm; P1 (0.45 µm) was made from polypropene (PP), and was received from Haining Guodian Taoyuan Medicinal Chemical Instruments Factory. Preparation of PDEA microgels: PDEA microgels were prepared via an aqueous soap-free emulsion polymerization.

2.0 g of DEA, 40 mg of BisAM and 45 mL of

DI water were charged into a 100 mL round-flask. magnetically stirred during N2 purging for 30 min.

The reaction mixture was

A pre-degassed initiator aqueous

solution (40 mg of V-50 in 5 mL of water) was then quickly injected.

After 6 h of

polymerization at 70 °C, the reaction was stopped and the microgel dispersion was purified through dialysis against DI water in order to remove residual monomers and oligomers.

The dialysis water was changed twice every day for 3 days.

Preparation of the hybrid membranes: The membrane modification was performed by dynamic adsorption. Hereby the diluted microgel dispersion (8 mg/L) was directly filtered through the membrane under a constant vacuum pressure of 0.09 MPa.

The deposition of the microgels was

stopped when 500 mL of microgel dispersion was consumed.


It was then


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backwashed with fresh DI water (250 mL) to remove the microgels that were not steadily absorbed. The final loading of microgels in membrane was around 4 wt%, as measured by the increase in membrane weight after freeze-drying. Membrane switching procedures via gas aeration: The hybrid membranes were immersed in DI water, through which CO2 was bubbled at 10 mL/min and room temperature for about 2 h (this time has been optimized to reach a stable water flux).

To recover the membrane flux, N2 treatment was

performed at 10 mL/min and room temperature for 3 days.

The following cycles of

CO2/N2 aeration were conducted under the same conditions as the first one, respectively. Characterization: The particle size of PDEA microgels in water was measured by dynamic light scattering (DLS) (Zetasizer Nano ZS90, Malven Instruments Ltd., Malvern, UK) at 25 °C.

The water flux through membranes was measured under a vacuum pressure of

0.09 MPa.

Prior to test, the membranes were pre-pressurized at a negative pressure

of 0.09 MPa for 30 min until the water flow was stable. water was recorded every 2 min at 0.09 MPa.

The volume of permeated

The average water flux was calculated

based on Equation (1) from four measurements for each sample. J=

V A×t

(1)

where J (L·m-2·h-1) is the water flux, V (L) is the volume of permeated water, A (m2) is the effective membrane area (11.15 cm2), t (h) is the running time. Bovine serum albumin (BSA) rejection test was also performed at a constant vacuum


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pressure of 0.09 MPa. BSA aqueous solution (1 g/L) was filtered through the test membrane, and the permeated liquid was consecutively collected and detected with a UV–vis spectrometer (3300 pro, Amersham, Sweden) at wavelength of 280 nm and the concentration was obtained based on a standard curve.

BSA rejection ratio (Ri)

was calculated by Equation (2). Ri = (1 −

Cp Cf

(2)

) ×100%

where Cp (g/L) is BSA concentration of the permeated solution, Cf (g/L) is BSA concentration of the original solution. Dynamic contact angle of the membranes was measured by optical contact angle meter system (OCA 20, Dataphysics, Germany). Scanning electron microscopy (SEM, S-4700, Hitachi, Japan) was used to visualize surface and cross-section morphology of the membranes before and after modification.



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Results and Discussion PDEA microgels were prepared via aqueous soap-free emulsion polymerization, with BisAM as cross-linker and V-50 as initiator, respectively.

The resulted microgel

dispersion was creamy-white, which became almost totally transparent after purging with CO2 for 30 min. DLS measurement was applied to characterize the changes of microgel diameter.

The microgels had a Z-average particle diameter (Dz) of 213 nm,

with a relatively narrow size distribution, while after CO2 treatment, Dz of the microgels increased to 1268 nm, and the distribution also broadened (PDI =0.422). This huge volume expansion of the microgel ((1268/213)3 = 211 times) was caused by the individual microgel swelling, and was in good agreement with the previous reports.47

The following alternative gas aeration clearly demonstrated repeatable the

switchability of PDEA microgels by CO2/N2 bubbling (Figure 1). The PDEA microgels were then used to modify commercially available microporous membranes via dynamic adsorption, whereby the microgel dispersion was filtered through the membrane under vacuum.

PES-based microfiltration membrane M1

from Millipore was first chosen as a model membrane to perform dynamic adsorption at room temperature.

The average pore size of the membrane's active layer is 450

nm while the pores on the upper surface are in a few micrometers.

Therefore, the

microgels can easily transport into the membranes, most of which could be retained in the active layer.

It was observed that the flux decreased dramatically during the

adsorption at a constant vacuum pressure of 0.09 MPa, indicating additional filtration resistance was built up.

The deposition of microgels was stopped when 500 mL of


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diluted microgel dispersion (8 mg/L) was consumed.

Microgel-embedded

membrane (M1-G) was obtained after backwashing with fresh DI water, to remove the microgels that were not steadily absorbed.

The final loading of microgels in

membrane was around 4 wt%, as measured by the increase in membrane weight. When tested under a constant vacuum pressure of 0.09 MPa, the original water flux of M1 membrane was at a quite high level, approximately 33840 L·m-2·h-1 (LMH), which dropped to 620 LMH after microgel modification (M1-G).

This obvious

decrease on flux also confirmed the success of membrane modification, since the trapped microgels would block some of the channels for water transportation. membrane M1-G was then treated with CO2, followed by flux test.

The

Surprisingly, the

flux dramatically decreased by an order of magnitude to around 71 LMH, as shown in Figure 1.

It was thus concluded that CO2-responsiveness of the microgels was

preserved even though the microgels were confined within the pores of the membrane. It should be mentioned that it took around 2 h of CO2 bubbling for the microgel-embedded membranes to reach a stable flux, whereas less than 10 min was needed for free microgels to reach an equilibrium swollen particle size.47 This could be attributed to the slower diffusion of protons in the confined space of membrane pores. We then examined if the flux of M1-G could be switched back via N2 treatment.

The

flux test was performed after treating with N2 for a certain period of time at room temperature.

The flux did not change too much (remained to be < 100 LMH) for the

first 24 h, indicating relative stability of the microgel-embedded membrane under



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mild conditions.

However, as the N2 treatment was going on, it started to increase

slowly, and levelled off at around 580 LMH after 3 days, which was very close to that of the as-modified membrane M1-G before CO2 treatment.

This reversible shift of

flux from 530~631 LMH to around 70 LMH could be repeated for many times upon alternative treatment of CO2 and N2, which was in good agreement with the particle size switching of the embedded PEDA microgels (Figure 1).

Figure 1. Water flux of the modified M1 membrane (circle), as well as the Z-average particle size of PEA microgels (triangle) after each cycle of CO2/N2 alternative aeration.

Furthermore, BSA rejection test was conducted to investigate the change of permeability performance with CO2/N2 switching.

The rejection ratio of untreated

microfiltration membrane M1 was around 7 %, which slightly increased to 11 % after microgel modification.

This modification therefore had very little effect on the


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permeability performance of membrane. around 93 % after CO2 treatment.

However, BSA rejection increased to This dramatic change again confirmed

CO2-responsive performance of the microgel-embedded membrane.

It thus can be

concluded that CO2/N2 could trigger the membrane switching between microfiltration and ultrafiltration, from the view point of BSA rejection changes.

To the best of our

knowledge, this is the first report on switchable membranes with tunable flux and permeability, as manipulated by CO2/N2 alternation. It is well known that the hydrophilicity of PDEA microgels in aqueous dispersion could be highly enhanced upon CO2 treatment, due to protonation of the tertiary amine groups, resulting in dramatic expansion of the particle size.47 Would this change of microgels be preserved within the confined space of membrane pores? How did the change of microgels affect the properties of membranes upon gas switching?

In this regard, the dynamic contact angle tests of M1-G membrane were

performed to investigate the change in hydrophilicity before and after CO2 treatment. Both membrane samples were freeze-dried prior to the test.

As shown in Figure 2,

the membrane sample after CO2 treatment had a lower water contact angle (103 º) at the very beginning (t = 0 s), suggesting enhanced hydrophilicity of the membrane surface.

Furthermore, the contact angle seemed to decrease much faster for the one

after CO2 treatment (a steeper slope in Figure 2a), which was in good agreement with the much higher swelling ratio of CO2-treated microgels.

TEM observation was

applied to visualize the microstructure change of the modified membranes.

As seen

from the on-top view (Figure 3a), microgels were stuck within the dense porous



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structure of the inner side, but could not be found on the surface.

The spherical

microgel particles had a diameter of 200 nm approximately, which was in good agreement with the DLS measurement.

The cross section view indicated that those

microgels were mostly transported several micrometers deep into the active layer of porous structure from the top surface and distributed randomly within the membrane matrix.

However, after CO2 treatment, most spherical microgels disappeared, while

some threadlike filaments were generated across the pores (see arrows in Figure 3c & 3d).

These filaments were of several micrometers in length and tens of nanometers

in diameter.

Due to the low Tg of PDEA (around 16 ~ 24 ºC),48 the highly swollen

microgels would deform to fit and fill in the membrane pores.

The irregular-shaped

swollen microgels would shrink upon freeze-drying, resulting in the formation of filaments, since similar filaments were observed by directly freeze-drying the CO2-treated PEA microgel dispersion.

Note that although the membrane after CO2

treatment had better surface hydrophilicity (dynamic contact angle tests), it had a much lower flux and higher BSA rejection, suggesting higher membrane resistance. This could be attributed to the change of water transportation channels.

As shown in

Scheme 2, before CO2 treatment, water molecules mainly went freely through the gaps between the membrane pores and microgels.

However, such channels were

effectively blocked during the microgel expansion.

Water molecules thus had to go

through the cross-linked microgels, which would be much more difficult.

For the

changes on BSA rejections, it should be mentioned that such distinct increase after CO2 aeration may be caused from both the membrane channel size reduction and BSA


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adsorption.

Under our experimental condition of BSA rejection tests, BSA carried

some negative charges while the membrane would generate positive charges upon CO2 treatment, leading to the BSA adsorption on the membrane surface, which was beneficial to the result of BSA rejection.

Figure 2. a) Time-dependent change of the water contact angle for the microgel-modified M1 membrane before (black square) and after (red circle) CO2 treatment; b) snapshots of the water droplets during the dynamic contact angle test.

Figure 3. SEM images of the microgel-modified M1 membrane: a) on-top view onto



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the membrane surface, b) and c) cross section view; and after CO2 treatment c) on-top view onto the membrane surface d) cross section view.

Scheme 2. Schematic representation of the change of water transportation channels before and after CO2 treatment.

We further explored the applicability of this microgel modification strategy to other commercially available membranes.

Two PES microfiltration membranes of

different pore size (H1: 0.45 µm, H2: 2.0 µm), as well as other membranes that were made from MCE (A1: 0.45 µm), PP (P1: 0.45 µm) were selected as representative samples.

Same modification procedures were conducted and water fluxes were

tested for the membranes before modification, after modification and after CO2 treatment.

The results are shown in Figure 4.

The original membranes had water

flux at a level of 104 LMH, which dropped to several hundred LMH for the microgel modified membranes.

After treated with CO2, most of fluxes decreased to below

180 LMH, except that of H2, which had an average pore size of 2 µm.

Since the

average particle size of microgels after CO2 treatment was around 1.3 µm, there


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would be some gaps left between microgels and pore inner wall of the membrane matrix to allow water molecules transport through freely.

This result again

confirmed the mechanism proposed above.

Figure 4. Water fluxes for original membranes, modified membranes before and after CO2 treatment.

Conclusions To conclude, we have successfully developed an easy, robust and scalable method for the preparation of CO2-responsive hybrid membrane whose flux and permeability can be reversibly switched between the level of microfiltration membrane and ultrafiltration membranes, simply by altering gas aeration.

The switchable

membrane was prepared by immobilizing CO2-responsive PDEA microgels into



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commercially available microfiltration membranes via dynamic adsorption.

Upon

CO2/N2 alternation, the microgels within the membrane pores could reversibly expand and shrink, leading to reversible membrane performance switching.

Dynamic water

contact angle tests and SEM observation have demonstrated that such switching was caused by the changes of water transportation channel.

Our work represents a novel

yet versatile method for the preparation of intelligent and high-performance membrane, which would have bright potential for the separation of complex matters in the area of membrane technology.


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Acknowledgement The authors sincerely acknowledge the National Natural Science Foundation of China (Grant No. 21606197 & 21374103) and MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Zhejiang University (2016MSF005) for supporting this fundamental research.

S. Z. thanks the Natural Science and Engineering

Research Council (NSERC) (RGPIN-2015-05841) of Canada and the Canada Research Chair (CRC) (950-229035) program for supporting his research at McMaster University.



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Breathing Microgels - ACS Publications - American Chemical Society

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