DESALINATION

Desalination 146 (2002) 49-55 www.elsevier.com/locate/desal

Study on polyimide/TiO, nanocomposite membranes for gas separation

Ying Kong*, Hongwei Du, Jinrong Yang, Deqing Shi, Yunfmg Wang, Yuanyuan Zhang, Wei Xin

Department of Chemical Engineering, University of Petroleum (East China), Dongying, Shandong, 257061, P.R. China Tel. +86 (546) 839-1029; Fax: +86 (546) 839-1029; e-mail: kon&hdpu.edu.cn

Received 30 January 2002; accepted 21 March 2002

Abstract

In this paper, nanocomposite materials based on TiO, and polyimide derived from HQDPA and DMMDA were prepared by blending of TiO, sol and polyimide solution. Because of the improvement of the TiO, sol preparation process and blending methods, the TiO, content of the obtained TiOjpolyimide composite materials can reach about 4Owt%. The structures and properties of the obtained TiOdpolyimide composite materials were characterized in this study. The results showed that the obtained PI/TiO, composite membranes with TiO, content up to 30°wt were nanocomposite membranes. There existed strong interaction between TiO, phase and PI phase of PI/TiO, composite membranes. The high TiO, content in TiO,/polyimide nanocomposite membranes resulted in the great enhancement of gas separation performance of the TiOJpolyimide nanocomposite materials compared with polyimide. The H, and 0, permeability of TiO,/polyimide nanocomposite membrane with TiO, content of 25wt% was 14.1 and 0.718 barrer, respectively, which was 3.7 times and 4.3 times higher than that of pure polyimide (Pm = 3.809 barrer, P, = 0.166 barrer). The am2 and aomZ of TiO,/polyimide nanocomposite material containing 25wW0 TiO, is 187.5 and 9.5, respectively, which is higher than that of pure polyimide (cx-* = 166.9, aom2 = 9.3).

Keyword: TiO,; Polyimide; Nanocomposite membrane; Gas separation; Solution blending

1. Introduction

Polyimide is one of the most suitable membrane materials for gas separation, because

*Corresponding author.

it exhibits extraordinary high gas selectivity as well as excellent thermal, mechanical and chem- ical stability. Many research efforts have been concentrated on the improvement of the select- ivity and permeability of polyimide since

Presented at the International Congress on Membranes and Membrane Processes (ICON), Toulouse, France, July 7-12, 2002.

001 l-9164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved HI: SO0 1 I-9 I64(02)00476-9

50 Y. Kong et al. /Desalination 146 (2002) 49-55

polyimide hollow-fiber membranes were success- fully applied in the industry for gas separation by

the Ube Company in 1985. Most of these studies were based on the modification of the chemical structure of polyimide and had made a lot of progress in the improvement of gas separation performance of polyimide. However, according to the investigation of Robeson et al., the increase in permeability corresponds to a decrease in permselectivity for polyimide [l-3]. How to enhance both of the permeability and perm- selectivity of polyimide became the focal point of polyimide membrane materials for gas separation in recent years.

Polymer/inorganic nanocomposite materials have been recently developed to improve the physical properties of polymers. The polymer/ inorganic nanocomposite constitutes of two matrices, i.e., polymer and inorganic. In these kinds of materials, the inorganic phase is dis- persed at nanoscale level in the polymer phase. Due to the special structural characteristics of polymer/inorganic nanocomposites, they exhi- bited the high-order characteristics such as optical transparency, dielectric properties, electrical conductivity, nonlinear optical effects, quantum confinement effects, biological com- patibility and biological activity [4]. This offers new possibilities to improve the gas separation properties of pure polymers. The earliest gas permeability studies of polymer/inorganic nano- composites reported in the literature appeared in 1996 [5]. This study on gas permeability of polymer/inorganic materials, prepared by co- hydrolysis of phenyltrimethoxysilane or di- phenyldimethoxysilane with tetramethoxysilane, showed encouraging selectivity for CO,/N, and He/N,. The gas permeability investigations of polyimide-silica and poly (amide-imide)/TiO, nanocomposites were also reported in succession [6-lo].

The polyimide/silica nanocomposite mem- brane materials, investigated by Schrotter [6] and Katsuki Kusakabe [8], exhibited superior gas

permeability and selectivity especially at high silica content, compared to pure polyimide. However, the poly (amide-imide)/TiO, nano- composite material studied by Hu [lo] has demonstrated slight increase of selectivity and decrease of permeability compared with pure poly (amide-imide). This might be due to the low TiO, proportion in the used poly (amide- imide)/TiO, nanocomposite membranes. The used poly (amide-imide)/TiO, nanocomposite materials containing high TiO, content were too brittle to be applied in gas permeability charac- teristics. It can be supposed that the poly- imide/TiO, nanocomposites with high TiOz might exhibit much higher gas permeability and selectivity than that of pure polyimide, just as the same as that of polyimide/silica nanocomposite. In order to develop the membrane material with both high permselectivity and permeability, this work concerns the gas separation properties of polyimide/TiO, with high TiO, content. As a result, the supple polyimide derived from HQDPA and DMMDA were used to prepare

polyimide/TiO, nanocomposites in this paper. And the preparation process of TiO, sol was also improved. The structure and properties, such as thermal resistance, chemical stability and gas separation performance, of the obtained poly- imide/TiO, nanocomposite membranes were characterized.

Z.Experimental

2. I. Materials

The molecular structure of the polyimide derived from HQDPA and DMMDA used in this work is showed in Fig. 1. Tetrabutyl titanate were supplied by Shanghai No. 3 Chemical Reagent Factory.

Fig. 1. The molecular structure of polyimide.

Y. Kong et al. /Desalination I46 (2002) 49-55 51

Fig. 2. The preparation process of PI/TiO, nanocomposite membrane.

2.2. Preparation of PI/TiO, nanocomposite membrane

Fig. 2 illustrated the preparation process of PUTiO, nanocomposite membrane. At first, the polyimide was dissolved in NMP to get lOwt% polyimide solution. The TiO, sol was obtained by the hydrolysis of tetrabutyl titanate (TBT) in NMP with the H,O/TBT mole ratio of 4, the TBT/NMP volume ratio of 3 and a pH value of 4. Secondly, the obtained PI solution and TiO, sol were blended under stirring for 24 h. The pro- portion of PI solution to TiO, depended on the actual demand. Finally, the obtained mixtures were filtered and cast on a glass plate to a pre- determined thickness. After being dried at 40°C the PI/TiO, composite membranes were formed.

2.3. Characterization of structure andproperties of membrane

A JEOL JAX-840 transmission electronic microscope was used to observe the phase

structure of membrane. DSC was carried out to determine the Tg of PI/TiO, nanocomposites on a Perkin-Elmer DSC7 thermal analysis apparatus. Gas permeation experiments were realized by using the constant volume and variable pressure technique. The RSK Model K-3 15N-01 gas

transmission rate measurement apparatus was manufactured by RSK Rikaseiki Kogyo Co., Ltd.

3. Results and discussion

3. I. Membrane structure andphysicalproperties

The relationship between TiO, content and

some physical properties of the obtained poly- imide/TiO, composite membranes are given in Table 1. Optical appearance is a convenient method to judge the dispersing status of inorganic phase in polymer matrix for polymer/inorganic composite. Ifthe polymer/inorganic composite is transparent, the inorganic phase is then dispersed at nanoscale level in polymer matrix. As showed in Table 1, the polyimide/TiO, composite mem- branes are transparent while TiO, content is up to 3Owt%. The polyimide/TiO, composite mem- branes with TiO, content higher than 3Owt% become semi-transparent. Therefore, the poly- imide/TiO, composite membranes with TiO, content up to 3Owt% are nanocomposite mem- branes. This conclusion was confirmed by the TEM analysis of cross-section of the poly- imide/TiO, composite membrane containing 3Owt% TiO,. As shown in Fig. 3, the TiO, phase size is about 10 nm. The results in Table 1 also

52

Table 1

Y. Kong et al. /Desalination 146 (2002) 49-55

The optical appearance and toughness of polyimide/TiO, composite membranes with different TiO, content

Membrane FH-1 FH-2 FH-3 FH-4 FH-5 FH-6 FH-7 FH-8

TiO, contenta (wt%) 5 10 15 20 25 30 35 40

Optical appearance T T T T T T S s

Toughness Tough Tough Tough Tough Tough Brittle Brittle Brittle

“Calculated by assuming 100% of TBT to the TiO,, T: transparent; S: semitransparent

la,--

95- 93- ,,”

,*’

z f$j-

5 El-

,,J’ ,,”

,” ,,,..,,” 2 , 75-

8 iv- *,/” ,,,W”

2 z @-

,/’ ,/’

J @) _ ,,,,,/#”

55-

50 ” ” ” ” ” ’ ” 0 5 lo 15 a0 25 30

Tii*contentIwto/ol

Fig. 4. Variation of residue content with TiO, content of PI/TiO, composite membrane.

Fig. 3. TEM of cross-section of PI/TiO, com- posite membrane (TiO, content: 3Owt%; magnification x 130,000).

showed that the toughness of polyimide/TiO, composite membranes was greatly enhanced, compared with that reported in the literature [lo]. As reported by Hu et al. [lo], the polyimide/TiO, composite membranes were too brittle to be used for gas permeability measurement while the TiO, content was higher than 7.3wt%. In this study, the polyimide/TiO, composite membranes can be used to carry out gas permeability even if the TiO, content in composite membrane reaches 25wt%.

The thermal stability and chemical resistivity of the PI membrane could be both enhanced while it was developed into a nanocomposite membrane. The chemical resistance of polyimidel TiO, composite membranes was compared with PI in Table 2. It can be observed that the chemical resistance of the polyimide/TiO, com- posite membrane was better than that of PI. The polyimide/TiO, composite membrane is not able to fully dissolve in NMP, DMF, THF and CHCl,, in which PI dissolves very well. The poly- imide/TiO, composite membranes were soaked in NMP for 72 h in order to extract the soluble component ofthe PI/TiO, composite membranes.

Y Kong et al. / Desalination I46 (2002) 49-55 53

Table 2 Table 3 Solubility of TiO,/polyimide nanocomposite and pure polyimide in some solvents

Thermal resistance of polyimide and TiOdpolyimide nanocomposite

Solvent PI

NMP +

DMF +

THF +

CHCl, -

Acetone -

Ethanol -

Toluene -

Hexane Petroleum ether -

Composite

P P P - - - - - -

Sample PI FH-1 FH-2 FH-3

TiO, content (wt%) 0 5 10 15

Tg (“C) 248.8 256.7 259.1 261.0

PI/TiO, composite: containing 25wt%TiO,. +, soluble; -, insoluble; P, partially soluble.

membranes was very strong. The strong inter-

action between PI and TiO, in polyimide/TiO,

composite membranes resulted in the restriction of molecular chain motion. The Tg of PI then increased and solubility of polyimide/TiO, composite membranes decreased.

The insoluble component of PUTiO, composite membranes was called residue. The residue content of all PI/TiO* composite membranes was measured and the results are shown in Fig. 4. All of the residue content was much higher than the

TiO, content in the composite membrane. The residue content increased with the increasing of TiO, content. There was a linear relationship between residue content and TiO, content. The linear regression fit was carried out and obtained the following regression equation: residue % = 1.13 xTiO*% + 56.65. The correlation coefficient was 0.998. According to the above regression equation, the residue content of the PI/TiO, composite membrane should be 56.65wt% when the TiO, content was extrapolated to zero. In other words, the insoluble PI component in the PI/TiO, composite membrane was 56.65wt%. Table 3 reveals the glass transition temperature measured by DSC of PI in polyimide/TiO, composite membranes. The Tg of PI in PUTiO, nanocomposites increased with the increasing of TiO, content.

3.2. Permeation studies

Based on the above experimental results, it could be known that the interaction between PI and TiO, in polyimide/TiO, composite

The results of gas permeation properties of TiO,/polyimide nanocomposite membranes are illustrated in Table 4. Because the composite membranes with TiO, content higher than 25wt% were too brittle to be used for gas permeation measurement, Table 5 only lists the permeation properties of PI/TiO, composite membranes with TiO, content up to 25wt%. With the exception of PI/TiO, composite membranes with TiO, content of 5%, the H,, 0,, N,, and CH, permeability of PI/TiO, composite membranes are higher than that of pure PI and increased with the increasing of TiO, content. The H, and 0, permeability of TiOJpolyimide nanocomposite membranes with TiO, content of 25wt% is 14.1 barrer and 0.718 barrer, respectively, which is 3.7 times and 4.3 times than that of pure polyimide. The selectivity of PI/TiO, composite membranes revealed a com- plicated variation with the increasing of TiO, content. With the increasing of TiO, content, the a H2Ri2 and aom2 decreased at first and then increased. While the TiO, content reached 25wt%, the aHm2 and aom2 of polyimide/TiO, nanocomposite membranes was 187.5 and 9.5, respectively, both of which were higher than that

54 Y. Kong et al. /Desalination 146 (2002) 49-55

Table 4 Permeation properties of polyimide and TiO,/polyimide nanocomposites with different TiO, content

Membrane TiO, content (wt%) Pm P 01 P N2 P CH4 %zm %ZCH4 aom2

PI 0 3.809 0.166 0.023 0.018 166.9 214.0 9.3

FH-1 5 3.773 0.155 0.033 0.018 115.0 222.0 4.7

FH-2 10 4.696 0.199 0.045 0.030 104.3 157.6 4.4

FH-3 15 5.523 0.273 0.053 0.039 104.6 142.2 5.2

FH-4 20 6.686 0.290 0.037 0.041 180.7 163.4 7.8

FH-5 25 14.143 0.718 0.075 0.099 187.5 143.2 9.5

Table 5 Permeation properties of TiO,/polyimide nanocomposite with TiO, content of 25wt% under different temperatures

Temperature (“C) Pm P 02 P N2 P CH4 aHzM2 awCH4 ao2/N2

25 14.1 0.718 0.075 0.099 187.5 143.2 9.5

40 15.2 0.913 0.147 0.176 103.2 86.1 6.2

50 17.8 1.158 0.212 0.200 84.0 89.0 5.5

60 20.9 1.390 0.265 0.496 78.7 42.0 5.2

70 23.8 1.650 0.362 0.361 65.9 66.0 4.6

of pure polyimide (aHm2 = 166.9, aom2 = 9.3). With the exception of PI/TiO, composite mem- branes with TiO, content of 5%, the aH2,cH4 of the other PI/TiO, composite membranes was lower than that of pure PI and varied from 140 to 165 with the increasing of TiO, content. The above results suggested that the low TiO, content could not greatly enhance the permeation properties of PI/TiO, composite membranes. When the TiO, content in PI/TiO, composite membranes was above 2Owt%, the permeability of the PI/TiO, composite membranes was remarkably enhanced, and the selectivity of PI/TiO, composite mem- branes was still kept at a high level. This might be caused by the specific interaction between gases and the TiO, component in PI/TiO, composite membranes. These results were very encouraging because both ofthe permeability and selectivity of PI membrane could be enhanced and at the same time blended with TiO,.

Table 5 shows the effect of temperature on the permeation properties of PI/TiO, composite membranes. With the increasing of temperature, the permeability increased and the selectivity decreased for the PI/TiO, composite membranes. This result was just as the same as that observed in pure PI. The reason was that the interaction between PI phase and TiO, phase of PI/TiO, composite membranes was located at the inter- facial regions between two components. Most of the molecular chain segment was in PI matrix, and this part of the PI molecular chain exhibited the same molecular relaxation properties as pure PI. Furthermore, the interaction between the PI phase and the TiO, phase might be hydrogen- bonding interaction. The increasing of temp- erature would decrease the hydrogen-bonding interaction and cause the increasing of perme- ability and the decreasing of selectivity.

Y. Kong et al. /Desalination 146 (2002) 49-55 55

4. Conclusions

This work concerns the study ofPI/TiO, nano- composite membranes with high TiO, content for gas separation. The PI/TiO, composite mem- branes with a TiO, content up to 4Owt% could be prepared by the blending of PI solution and TiO, sol that was synthesized by the hydrolysis of TBT in NMP. The obtained PI/TiO, composite membranes with TiO, content up to 3Owt% were nanocomposite membranes and there was strong interaction between two components in PI/TiO,

composite membranes. The results of the permeation investigation of

PI/TiO, composite membranes were very encouraging. When the TiO, content in PI/TiO, composite membranes was above 2Owt%, the permeability of the PI/TiO, composite mem- branes was remarkably enhanced and the selectivity of PI/TiO, composite membranes remained at a high level. For the PI/TiO, nano- composite membranes with TiO, content of 25wt%, the H, and 0, permeability was 14.1 and 0.718 barrer, respectively, which was 3.7 times and 4.3 times higher than that of pure polyimide (P, = 3.809 barrer, P,, = 0.166 barrer). The

aHZM2andao21?i2 was 187.5 and 9.5, respectively, which is higher than that of pure polyimide

(a H2/N2= l@je9, a02iN2 = 9.3). Therefore, prepara- tion of nanocomposite membranes was an effectual method to enhance the permeation performance of gas separation membranes. A membrane with both high permeability and selectivity could then be obtained.

Acknowledgements

This work was supported by the national key developing project on basal research (“973” Project, Grant No. G2000026407), the Founda-

tion of Zhong Qing Nian Chuang Xing of the CNPC and the Shandong Natural Science Foundation.

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