Preparation thin film nanocomposite membrane incorporating PMMA modified MWNT for nanofiltration

Chang-chao Yu, Hong-wei Yu, Yue-xia Chu, Hui-min Ruan, Jiang-nan Shen1,a （College of Chemical Engineering and Materials Science, Zhejiang University of Technology,

Hangzhou 310014, China）

* Corresponding Author：Jiang-nan Shen, Associate Professor, College of Chemical Engineering

and Materials Science, Zhejiang University of Technology, Tel: 86-571-88320711,

FAX: 86-571-88320711, email: shenjn@zjut.edu.cn

Keywords: Polyamide thin-film nanocomposite (TFN) membranes; Multiwalled carbon nanotubes; Interfacial polymerization.

Abstract. Multiwalled carbon nanotubes (MWNTs) grafted by Poly(methyl methacrylate) (PMMA)

can well disperse in organic solutions like toluene，which were synthesized via a microemulsion

polymerization of methyl methacrylate (MMA) in the presence of acid-modified multiwalled

carbon nanotubes (c-MWNTs). The polyamide thin-film nanocomposite (TFN) membranes with

embedded PMMA-MWNTs were prepared using piperazine (PIP) and trimesoyl chloride (TMC) on

a polyacrylonitrile (PAN) porous substrate via interfacial polymerization for selective permeability.

PMMA-MWNTs’ structure was analyzed by Fourier transform infrared spectrophotometry (FTIR),

Raman spectrophotmetry (RAM), Scanning electron microscope (SEM) and thermo gravimetric

analysis (TGA). Orthogonal experiment was used to study the effect of PIP concentration, TMC

concentration and concentration of PMMA-MWNTs in organic phase. The results showed that the

membrane performances is good, Na2SO4 rejection is above 98% and water flux is up to 150%

improvement over the TFC membrane as PIP in aqueous phase was 2g/L, TMC and

PMMA-MWNTs in organic phase were 4g/L and 0.67g/L, respectively . Demonstrated

PMMA-MWNTs in the nanofiltration membrane can improve selective permeability.

Introduction

Recent studies have demonstrated that membranes formed by embedding inorganic materials like

pure metal, metal oxide, silicon nanoparticles, carbon nanoparticles, etc into a matrix layer may

significantly improve membrane properties such as permeability, selectivity, stability, surface area,

or catalytic activity in various membrane separation processes [1-3].

In this work, novel thin film nanocomposite (TFN) membranes composed of polyamide and PMMA

functionalized multiwalled carbon nanotubes for nanofiltration application were investigated.

Functionalized MWNTs was first synthesized via a microemulsion polymerization of methyl

methacrylate in the presence of acid-modified multiwalled carbon nanotubes, then was dispersed

into toluene with TMC as a mixed organic phase solution by sonication. The nanofiltration

performance membranes were tested by rejection of Na2SO4 salts.

Experimental

Materials. The MWNTs were synthesized by chemical vapor deposition (CVD) and was

obtained from Timesnano, Inc., Chengdu, China. The average diameter of the nanotubes was

20-30nm with several micrometers in length. And the pure MWNTs content was more than 95wt%.

Analytical grade chemicals were used for synthesized PMMA-MWNTs: methyl methacrylate

(MMA), azo-bis-isobutryonitrile (AIBN), sodium dodecyl sulfate (SDS), HNO3 (67%), H2SO4

(98%), methanol. The support membranes used were polyacrylonitrile (PAN) porous membrane

(with a molecular weight cutoff (MWCO) of 30,000), supplied by National Engineering Research

Center for Liquid Separation Membrane, China. They were further hydrophilic treated with 1wt%

Key Engineering Materials Vols. 562-565 (2013) pp 882-886Online available since 2013/Jul/15 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.562-565.882

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SDS aqueous solution for 24h before used. Analytical grade chemicals were used for preparation of

membranes: piperazine (PIP), trimesoyl chloride (TMC), triethylamine. Deionized water was

generated by electrodialysis in the laboratory itself.

Synthesis of PMMA-MWNTs. The MWNTs were treated in concentrated HNO3 and H2SO4

(1:3 in vol.%) at 80°C for 6h. This acid treatment is known to introduce carboxyl and hydroxyl

functional groups on MWNTs, which render MWNT more compatible with common organic

solvents. PMMA-MWNTs composites were produced by typical microemulsion polymerization.

Acidified MWNTs (mass of MWNTs/MMA: 5/100, 8/100, 10/100, 12/100, 15/100, w/w) was added

to a sodium dodecyl sulfate (SDS) in deionized water in a three-neck 50ml round-bottom flask and

sonicated for 15min until it became homogeneous before equipped with a condenser, a mechanical

stirrer, and a nitrogen inlet, then placed in a 70◦C oil bath. The air in the flask was replaced by a

stream of nitrogen, and the mixture was stirred under nitrogen. A different mixture of AIBN in

MMA was added into the homogeneous solution. The solution was stirred for hours (6, 12, 18, 24h)

in the oil bath at 70◦C to produce gray solution. The sample had no odor, which indicates no

monomer remained. The latex dispersion was added to methanol, and the mixture was allowed to

stand overnight. The precipitation was filtered in the vacuum, washed successively with large

amount of methanol and deionized water, and dried in a vacuum at ambient temperature.

Characterization of PMMA-MWNTs. The weight fraction of PMMA grafted onto the MWNTs

was determined using thermo-gravimetric analysis (TGA). The thermal analysis was conducted at a

heating rate of 20oC/min in a nitrogen atmosphere. FT-IR and Raman spectroscopy was used to

characterize the chemical structure of pristine MWNTs，acidified MWNTs，PMMA-MWNTs，acyl

cholride-PMMA-MWNTs. And SEM was taking to observe the morphology of acidified MWNTs，

PMMA-MWNTs.

Preparation of thin-film nanocomposite (TFN) membrane. The polyamide thin-film

nanocomposite (TFN) membranes with embedded PMMA-MWNTs in organic phase (toluene) were

prepared using piperazine (PIP) and trimesoyl chloride (TMC) on a polyacrylonitrile (PAN) porous

substrate via interfacial polymerization (Fig. 1).

Fig. 1 Schematic diagram of synthesis TFN membrane

Characterization of TFN membranes. Pure water permeability, salt rejection, and salt

permeability of all membranes were determined in a membrane performance testing equipment

using an applied pressure of 1.0 MPa, and feed with 2000ppm Na2SO4 aqueous solution.

Results and Discussions

Characterization of PMMA-MWNTs. Thermo-gravimetric analysis (TGA) curves of

PMMA-MWNTs are presented in Fig. 2. Due to thermal stability of MWNTs, the descend stood for

the decomposition of PMMA. A minimum of 85-95% weight loss is observed up to 450℃ for the

PMMA-MWNTs composites. And summarized from the two TGA pictures, the highest mass

fraction of MWNTs in the PMMA-MWNTs is about 15%, which preferred to use for synthesis of

TFN membranes.

Key Engineering Materials Vols. 562-565 883

(a) (b)

Fig. 2 TGA curves of different mass fraction of MWNTs to MMA at a reaction time of 24h (a) and

different reaction times at mass fraction of 15/100 (b).

Fig. 3 shows the Raman (a) and FT-IR (b) spectrum of pristine MWNTs, acidified MWNTs,

PMMA-MWNTs, acyl chloride PMMA-MWNTs. The Raman spectrum shows characteristic

tangential-mode peaks (G-band) at 1566cm-1

and a disorder-band peak (D-band) at 1342cm-1

[3],

and the ratio of ID/IG is weakly changed after functionalization of MWNTs, which could

quantitatively describe the disorder and order structure of graphite. ID/IG of the acidified MWNTs

(1.28) decreased with acid treatment of pristine MWNTs (1.77), and ID/IG of the acyl cholride

PMMA-MWNTs (1.19) decreased with acylation of PMMA-MWNTs (1.38). This indicated that

acid treatment and the acylation of the MWNTs lead to the formation of sp3 hybridized carbon

defect sites. The FT-IR spectrum shows a new shift of C=O peak at 1799cm-1

of PMMA-MWNTs

dissolved in TMC/toluene organic phase via a ultrasonication, different from the C=O peak

1761cm-1

of pure TMC and the C=O peak 1731cm-1

of PMMA-MWNTs and pure PMMA, which

confirmed that the acyl-chloride groups on TMC had a esterification with the hydroxyl groups on

the MWNTs.

(a) (b)

Fig. 3 Raman (a) and FT-IR (b) spectrum of Pristine MWNTs，Acidified MWNTs，PMMA-MWNTs，

Acyl cholride-PMMA-MWNTs.

Fig. 4 shows the SEM pictures of acidified MWNTs (a) and PMMA-MWNTs (b). In picture (a),

the MWNTs were cut shorter by the mixed acid, and then were covered by PMMA which presented

in picture (b).

(a) (b)

Fig. 4 SEM pictures of acidified MWNTs (a) and PMMA-MWNTs (b)

884 Micro-Nano Technology XIV

Characterization of membranes. To optimize the conditions of the TFN membrane, an

orthogonal experimental design of setting PIP, TMC, PMMA-MWNTs as factors was obtained.

Table 1 shows the factors and level of experiment, and table 2 shows the orthogonal list of

experiment. By fuzzying these parameters and calculating the membership degree, the optimum

reaction conditions of monomers were showed as following: PIP in aqueous phase was 2g/L, TMC

and PMMA-MWNTs in organic phase were 4g/L and 0.67g/L, respectively.

Table 1 Factors and level of experiment

Level conc. of PIP(A) in aqueous phase /g·L-1 conc. of TMC(B) in organic phase /g·L-1 conc. of PMMA-MWNTs(C) in

organic phase /g·L-1

1 2 2 0.67

2 5 4 1.33

3 10 6 2.00

Table 2 Orthogonal list of experiment

No. A B C D

(Black) Rejection of Na2SO4(%)

Pure Water Flux(L/m2h)

Membership

degree of

Rejection

Membership degree of Flux

Sum of

Membership

degree

1 1 1 1 1 97.3 65.22 0.89 0.842 1.732

2 1 2 2 2 97.4 61.14 0.896 0.737 1.633

3 1 3 3 3 96.3 44.84 0.829 0.316 1.145

4 2 1 2 3 99.1 40.76 1 0.21 1.21

5 2 2 3 1 97.4 32.62 0.896 0 0.896

6 2 3 1 2 98.8 69.3 0.982 0.947 1.929

7 3 1 3 2 91.9 40.76 0.561 0.21 0.771

8 3 2 1 3 97.9 71.34 0.927 1 1.927

9 3 3 2 1 82.7 42.8 0 0.263 0.263

No. A B C D

Sum of membership

degree as an evaluation

criterion

K1 4.51 3.719 5.588 Black

K2 4.035 4.456 3.106 Black

K3 2.967 3.337 2.818 Black

R 1.543 1.119 2.77 Black

Order of factors C>A>B

Optimization A1B2C1

Next, the PMMA-MWNT embedded TFN membrane with optimum monomer conditions was

prepared, and compared with the pure TFC membrane and the commercial hydrophilic

nanofiltration membranes (N30F, NTR7450 [4]), which presented in table 3. The contact angle (o)

value of TFN membrane was similar to the commercial membranes, while was increased

significantly to the pure TFC membrane, due to the hydrophobic of PMMA. Whereas, the water

permeability (L/m2.h.bar) was somewhat increased which compared to both the commercial

membranes and pure TFC membrane.

Key Engineering Materials Vols. 562-565 885

Table 3 Comparison between self-made membrane and commercial nanofiltration membranes

membrane PMMA-MWNT TFN membrane pure TFC membrane N30F NTR7450

Manufacturer Self-made Self-made Nadir Nitto-Denko

composition top layer PMMA-MWNT/Polypiperazine

trimesoyl amide

Polypiperazine trimesoyl

amide

Permanently hydrophilic

polyether-sulfone

Sulfonated

polyether-sulfone

Water permeability (L/m2.h.bar) 6.97 4.28 3.8 5.7

Rejection of 2000ppm Na2SO4 (%) 99.0 98.2 99.5 99.4

Zeta potential pH7 (mV) -6.04 -4.99 -15 -17

Contact angle (o) 72.3 48.5 88 70

Conclusion

Multiwalled carbon nanotubes (MWNTs) grafted by Poly(methyl methacrylate) (PMMA) were

synthesized via a microemulsion polymerization of methyl methacrylate (MMA) in the presence of

acid-modified multiwalled carbon nanotubes (c-MWNTs). The polyamide thin-film nanocomposite

(TFN) membranes with embedded PMMA-MWNTs were prepared using piperazine (PIP) and

trimesoyl chloride (TMC) on a polyacrylonitrile (PAN) porous substrate via interfacial

polymerization for selective permeability. The membrane performances is good, Na2SO4 rejection is

above 98% and water flux is up to 150% improvement over the TFC membrane as PIP in aqueous

phase was 2g/L, TMC and PMMA-MWNTs in organic phase were 4g/L and 0.67g/L, respectively .

Demonstrated PMMA-MWNTs in the nanofiltration membrane can improve selective permeability.

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