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PAM composites by CO2 in water emulsion templating method
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EUROPEAN POLYMER JOURNAL
European Polymer Journal 43 (2007) 4814–4820
www.elsevier.com/locate/europolj
Preparation of porous CaCO3/PAM composites by CO2 in water emulsion templating method Zhou Bing 1, Jun Young Lee 1, Sung Wook Choi 1, Jung Hyun Kim
*
Nanosphere Process and Technology Laboratory, Department of Chemical Engineering, Yonsei University, 134 Shinchon-dong, Sudaemoon-ku, Seoul 120-749, Republic of Korea Received 12 December 2006; received in revised form 17 July 2007; accepted 28 August 2007 Available online 2 September 2007
Abstract Emulsion templating is an effective method to prepare well-defined porous polymeric materials. In this paper, porous CaCO3/polyacrylamide (PAM) composites were prepared by emulsion templating polymerization in supercritical CO2(scCO2) by using a commercial grade surfactant (FC4430), therefore, the amount of the fillers and the pore size distribution of the composites can be modulated based on the demands of those potential applications as biomaterials. Calcium carbonate crystals can be in situ synthesized in the porous PAM matrix, and the morphology of CaCO3 varied with the conditions of the reaction, the results indicated that three kinds of crystals were observed in the porous matrix. The results of scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) showed that the macropores in PAM were interconnected and with narrow pore size distributions. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: C/W emulsion; Porous hydrogels; Particulate stabilizer; CaCO3
1. Introduction The rapid development of porous materials used as scaffolds for sustained three-dimensional growth of tissue is a growing field. The porous materials (beads, membranes, hydrogels) are widely used in the biomedical field as drug-delivery devices, wound dressings, and scaffolds for tissue-engineering [1]. They are especially appealing for tissue-engineering
*
Corresponding author. E-mail address: [email protected] (J.H. Kim). 1 Current address: College of Chemistry, Jinlin University, 2519 Jie Fang Road, ChangChun, People’s Republic of China.
applications as they are analogous to the natural extracellular matrix of tissues [2,3]. In these materials, hydrogels are not only of threedimensional polymeric networks, but also capable of absorbing large quantities of water, and by the removal of the water in the hydrogels, we can get porous materials; however, the pore size is hard to control. Partap et al. reported a novel method to prepare porous calcium alginate hydrogels by CO2 in water emulsion templating, and in this system, CO2 had a dual role including increasing the acidity of the aqueous phase and being a templating agent [4]. There are many methods to produce porous hydrogels, such like freeze-drying, particulate leaching, gas-foaming methods (using supercritical
0014-3057/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2007.08.014
Z. Bing et al. / European Polymer Journal 43 (2007) 4814–4820
fluids), and the use of gas-foaming agents [5–11]. These methods often yield hydrogels with broad macropore-size distributions and with a mixture of open and closed pores, whilst emulsion templating methods can sometimes give polymers with a more defined and controllable porous structure. ‘‘CO2 in water’’ emulsion templating method has been successfully utilized by Butler et al. to synthesize porous crosslinked polyacrylamide with PFPE surfactants, with pore diameters of 4.3–11.7 lm and pore volumes of 1.18–4.68 cm3 g1, and in their work, crosslinked PVA was used as a co-surfactant to stabilize the emulsion [12]. The emulsion templating technique involves forming a high internal phase emulsion (HIPE) (>74.05% v/v internal phase) and locking in the structure of the external phase, usually by reaction induced phase separation (e.g., sol–gel chemistry, free-radical polymerization). By the removal of the emulsion droplets, skeletal replicas of the emulsion can be obtained. However, HIPE techniques are extremely solvent intensive because the internal oil phase (often an organic solvent) can constitute between 75% and 90% of the total reaction volume. Furthermore, it may be very difficult to remove the template from the material at the end of the reaction. For inorganic materials, purification tends to involve calcining the sample at high temperatures (>600 °C), thus completely removing any organic residues [13]. Supercritical carbon dioxide (scCO2) is a sustainable solvent because it is non-toxic, nonflammable, and naturally abundant [14]. And carbon dioxide is suitable for the preparation of porous polymers and inorganic/polymer composites. In particular, scCO2 has been shown to be a versatile solvent for polymer synthesis and processing [15]. A number of research groups have exploited carbon dioxide for the preparation of porous polymers and inorganic/ polymer composites. Most of these supercritical fluid (SCF) techniques involve either a foaming mechanism, gelation of the SCF medium (i.e., an organic equivalent of sol–gel chemistry), or a combination of both. This limits the range of porous materials which can be accessed via the scCO2 route because many materials cannot be foamed or derived from CO2-soluble precursors. For example, scCO2 has been used for the production of microcellular polymer foams [16], biodegradable composite materials [17], macroporous polyacrylates [18], fluorinated microcellular materials [19], zeolite/polymer composites [20], and thin CaCO3/biopolymer films [21].
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In this paper, HIPE method and gas foaming method by scCO2 were combined to prepare porous PAM hydrogels by a commercial surfactant FC4430, and non-crosslinked PVA as a co-surfactant, and that is nominally based on carbon dioxide fixation and biomass utilization, it could play an important general role in the fabrication of multifunctional inorganic–organic composites, therefore, such techniques can be widely applied to produce a mass of bio-inspired materials. And at the same time, CaCO3 crystals of various morphology can be in situ embedded in the porous PAM by the carbonation of Ca(OH)2 with scCO2. The technique has wide appeal because it allows the synthesis of materials with well-defined porous structures without the use of any volatile organic solvents, just water and CO2. Entrapment of the template phase is completely avoided because CO2 reverts to the gaseous state upon depressurization. 2. Experimental 2.1. Materials Acrylamide (AM) (Aldrich, 98%), N,N-methylene bisacrylamide (MBAM) (Aldrich, 99%), PVA (Aldrich, Mw = 30,000–70,000), CaO (Samchun Chemicals, 98%), FC4430 (3M Company), and potassium persulfate (K2S2O8, KPS) (Aldrich) were used as supplied. Highly purified water was used in all the processes. 2.2. Digestion of CaO by H2O A certain amount of CaO powder was dissolved in 50 ml 80 °C water and stirred for 4 h, then filtered by a 200 mesh sieve. The concentration of the Ca(OH)2 solution was determined by the titration of 0.5 M HCl. 2.3. Polymerization procedure in supercritical system On the first step, surfactant FC4430 was introduce into the 2 ml 15% PVA solution in a 30 ml glass vial, and 3 g 40% monomer (AM/MBAM = 4/1) solution was mixed with the above solution for 20 min. One milliliter 1.5% Ca(OH)2 solution and KPS (2% weight percent in the total water used above) powder were introduced into the surfactants and monomer solution before the emulsion polymerization in the stainless steel autoclave, and the reactor was equipped with a thermo-couple, a
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pressure sensor connected to a pressure protector, and a view window. Secondly, the temperature and the press of CO2 of the reactor were set at 25 °C and 80 bar (CO2/H2O = 8/2, v/v), and after being magnetic stirred for 30 min, a white and milky emulsion was obtained. To analyze the stability of the emulsion, the stirring will be stopped, and phase separation occurred in those unstable emulsions leading to two phases, as had been described in Cooper’s paper [1]. Then the emulsion was heated to 60 °C in 30 min, and the final pressure was 280 bar, the polymerization occurred and the gelation of the aqueous phase can be observed. The reaction time is 12 h. When the temperature of the reactor decreased to room temperature, CO2 was vented out in 1 h. The CaCO3/PAM hydrogel monolith can be obtained. The sample was dried in vacuum oven or by freeze-drying method, fractured into small pieces, and analyzed by SEM and mercury intrusion porosimetry (MIP). The morphology of the porous composites was investigated using a JEOL SEM instrument (JSM6701F, Japan) operating at 5 kV. Fractured segments were mounted on carbon fiber pads and attached to aluminium stubs and were Pt coated using Cressing Sputter coater. MIP (Auto Pore IV 9500, USA) was applied to collect the information of the porous structure of the samples. 3. Results and discussion 3.1. Effect of the amount of surfactant FC4430 on the stability of the emulsion Fluorosurfactant FC4430 is a non-ionic polymeric fluorochemical surfactant belonging to a class of coating additives which provide low surface tensions in the emulsions. In this work, it was used as CO2-philic surfactant to obtain stable CO2 in water (C/W) emulsion. Before the polymerization of AM and the cross-linker (MBAM), the stability of the CO2 in water emulsion without Ca(OH)2 was very poor; and when the content of FC4430 was lower than 5% (w/v to water) at room temperature, due to the insufficient dispersion of liquid CO2 in the aqueous phase, the polymerization occurred in the bottom of the reactor of the aqueous layer. The degree of porosity was very low. However, the stability of the emulsions was improved by the addition of Ca(OH)2 solution at the same content of surfactant FC4430, because after the carbonation of Ca(OH)2 by CO2, the presence of CaCO3 particles
can stabilize the emulsion by an interlayer of CaCO3 particles located between CO2 droplets and water at high pressure (more than 70 bar) [22], the formation of CaCO3 by the contact of Ca(OH)2 slurry with scCO2 is an in situ process during the polymerization of the monomer, and the size of the particles was about 1–3 lm, the loading is about 4%, which had been proved to be efficient to stabilize the C/W emulsion [22]. We can get the CaCO3/PAM hydrogel monolith when the content of surfactant FC4430 was at 10%, 12% and 15% (w/v to water) together with 3% Ca(OH)2 (w/v to water). 3.2. Morphology of the dual porous CaCO3/PAM composites The Ca(OH)2 in the aqueous phase can transfer to CaCO3 particles in the CO2 in water emulsion by the reaction as described below: Ca(OH)2 + CO2 ! CaCO3 (s) + H2 O In the work of Golomb et al., they had studied the influence on the stability of CO2 in water emulsion with several kinds of inorganic materials as stabilizer, and among those materials, they found the sheath of CaCO3 particles prevents coalescence of the CO2 droplets into a bulk phase [22]. Gu et al. prepared CaCO3 particles by carbonation of Ca(OH)2 solution and Ca(OH)2 powder with scCO2, and the carbonation process is carried out by a solid-SCF (supercritical fluid) method [23]. Fig. 1 shows the SEM pictures of the fractured surface of the composites, which were from the unstable CO2 in water emulsion. The content of surfactant FC4430 was set at 5% (w/v to water), the loading of Ca(OH)2 was 3% (w/v to water). As can be seen in Fig. 1a and b, the PAM matrix was like to be connected by small particles after freezedrying, and the degree of porosity of the samples was very low, and the median pore diameter was about 2.5 lm. However, CaCO3 particles can be found on the fracture surface in three types of morphology: cubic, sphere-like, and needle-like, as shown in Fig. 1c–f. The particles were embedded in the polymers, and the variety of the morphology is due to the crystalline structure of CaCO3 particles in different chemical environments, accordingly, the mechanism of the growth behavior of CaCO3 crystalline in the aqueous phase and the stabilization effect of CaCO3 particles cooperated with surfactant should be discussed in the future.
Z. Bing et al. / European Polymer Journal 43 (2007) 4814–4820
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Fig. 1. SEM images of fracture surface of the CO2 templated porous CaCO3/PAM composites produced from CO2 in water emulsion: (a) content of surfactant FC4430 was 5% (w/v to water), the loading of Ca(OH)2 was 3% (w/v to water); (b) magnification of the pores structure and (c)–(f) the morphology of the CaCO3 crystals in the composites.
In the aqueous phase, there are three components which can stabilize the CO2 in water emulsion, one is the surfactant FC4430, another one is PVA, and the other one is CaCO3 particles. Among these stabilizers, the presence of PVA can sufficiently improve the stability of the emulsion as a co-surfactant [1]. And the inorganic particles can also stabilize the CO2 droplets in the aqueous phase at high pressure. In this work, liquid CO2 and supercritical CO2 was well dispersed in the aqueous phase with Ca(OH)2 solution due to the formation of CaCO3 during the contact of CO2 and Ca(OH)2 solution before the
polymerization of acrylamide and the cross-linker. The SEM pictures of porous CaCO3/PAM composites with different content of surfactant FC4430 are shown in Fig. 2. As can be seen in all the images, emulsion templating method is effective to prepare porous materials, and in the C/W emulsion system, the role of the surfactant is the most important factor to obtain stable emulsion, so many researchers had paid a lot of efforts to synthesize and select special surfactants for C/W and W/C emulsion system [24]. FC4430 is a commercial grade surfactant, and in this work, it was proved that such non-ionic
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Fig. 2. SEM images of fracture surface of the CO2 templated porous CaCO3/PAM composites produced from CO2 in water emulsion with the loading of Ca(OH)2 was 3% (w/v to water): (a) and (b) content of surfactant FC4430 was 10% (w/v to water); (c) and (d) content of surfactant FC4430 was 12% (w/v to water); (e) content of surfactant FC4430 was 15% (w/v to water) and (f) image of the morphology of the CaCO3 crystals in the composites.
surfactant is suitable in this system. However, the content of FC4430 is much higher than those special surfactants [1], so it is necessary to find out the suitable mass ratio of inorganic particles and the surfactant to decrease the loading of FC4430. In Fig. 2, it was clear that the CO2 droplets were not monodisperse, which indicated that this method was not like the other HIPE methods to produce porous materials well-defined pore size distribution
[4]. And the pores of the materials were open and interconnected, and with the increase of the content of FC4430, the surfactant can stabilize an increasingly large interfacial area and the average CO2 droplet size became correspondingly bigger, as can be seen in Fig. 2c–e. Fig. 2f shows that some CaCO3 particles existed in the interface of CO2 droplets and PAM matrix, and after the removal of the CO2 phase, they were embedded in the polymers.
Z. Bing et al. / European Polymer Journal 43 (2007) 4814–4820
The dried CaCO3/PAM composites hydrogels was examined by mercury intrusion porosimetry (MIP), and the porosimetry data of the samples with different concentrations of surfactant FC4430 are shown in Fig. 3 and Table 1. The results showed that the total pore volume and mean diameter of the samples are lower than those of the porous PAM reported in the paper of Cooper et al. [1]. In this work, it was necessary to increase the concentration of the surfactant FC4430 more than 10% to obtain stable CO2 in water emulsion. The total pore volume decreased with the increase of concentration of the surfactant, which indicated more CO2 droplets were dispersed into water, and the mean pore diameter of the droplets increased from 4.7 lm to 14.9 lm. However, the total porosity of the samples increased from 47% to 73%, which indicated that the volume ratio of CO2 phase in the emulsion increased, and the degree of the interconnection of the pore was enhanced, at the same time, the thickness of the PAM pores became much denser after the polymerization. Among the three kinds of stabi-
10% FC4430 12% FC4430 15% FC4430
Log Differential Intrusion (mL/g)
4
3
2
1
0 1
10
100
Pore Diameter (μm)
Fig. 3. Size distribution of the supercritical CO2 templated porous CaCO3/PAM composites with different concentrations of surfactant FC4430.
Table 1 Analysis of porous CaCO3/PAM composites made via CO2 in water emulsion templating method Sample
Concentration of Mean pore Total pore Total FC4430 (w/v, %, diameter volume porosity based on water) (lm) (cm3/g) (%)
Sample 1 10 Sample 2 12 Sample 3 15
4.7 10.3 14.9
2.01 1.44 0.76
47 61 73
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lizers, FC4430 is the most effective surfactant than CaCO3 particles and PVA. 4. Conclusion Porous CaCO3/PAM composites were successfully prepared by the CO2 in water emulsion templating method. It is a new generic method of producing porous inorganic/polymer composites hydrogels by combining emulsion templating with an internal reaction. The carbonation reaction of Ca(OH)2 with CO2 occurred on the interface of the CO2 droplets and the aqueous phase, and the CaCO3 particles can stabilize the emulsion together with commercial surfactant FC4430 and PVA. The hydrogels exhibit an open, well interconnected pore network with a narrow pore-size distribution which may be suitable for potential bioengineering applications. Future work will focus on the mechanism of the stabilization effect of inorganic particles to obtain highly dispersed CO2 in water emulsion, and the preparation of uniform macroporous inorganic/polymer composites. References [1] Butler R, Hopkinson I, Cooper AI. J Am Chem Soc 2003;125:14473–81. [2] Zhang H, Cooper AI. Soft Matter 2005;1:107. [3] Cameron NR, Sherrington DC. Adv Polym Sci 1996;126:163. [4] Partap Sonia, Rehman Ihtesham, Jones Julian R, Darr Jawwad A. Adv Mater 2006;18:501–4. [5] Whang K, Thomas CH, Healy KE, Nuber G. Polymer 1995;36:837. [6] Zmora S, Glicklis R, Cohen S. Biomaterials 2002;23:4087. [7] Chen GP, Ushida T, Tateishi T. Biomaterials 2001;22:2563. [8] Sheridan MH, Shea LD, Peters MC, Mooney DJ. J Control Release 2000;64:91. [9] Mooney DJ, Baldwin DF, Suh NP, Vacanti LP, Langer R. Biomaterials 1996;17:1417. [10] Eiselt P, Yeh J, Latvala RK, Shea LD, Mooney DJ. Biomaterials 2000;21:1921. [11] Choi BY, Park HJ, Hwang SJ, Park JB. Int J Pharm 2002;239:81. [12] Butler Rachel, Davies Cait M, Cooper Andrew I. Adv Mater 2001;13:1459. [13] Manoharan VN, Imhof A, Thorne JD, Pine DJ. Adv Mater 2001;13:447. [14] Jessop PG, Leitner W, editors. Chemical synthesis using supercritical fluids. Weinheim: WILEY-VCH; 1999. [15] Cooper AI. J Mater Chem 2000;10:207. [16] Arora KA, Lesser AJ, McCarthy TJ. Polym Eng Sci 1998;38:2055. [17] Howdle SM, Watson MS, Whitaker MJ, Popov VK, Davies MC, Mandel FS, et al. Chem Commun 2001;109. [18] Wood CD, Cooper AI. Macromolecules 2001;34:5.
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[19] Shi C, Huang Z, Kilic S, Xu J, Enick RM, Beckman EJ, et al. Science 1999;286:1540. [20] Wang Yong, Liu Zhimin, Han Buxing, Li Junchun, Gao Haixiang, Wang Jiaqiu, et al. J Phys Chem B 2005;109: 2605–9. [21] Wakayama Hiroaki, Hall Simon R, Fukushima Yoshiaki, Mann Stephen. Ind Eng Chem Res 2006;45:3332–4.
[22] Golomb D, Barry E, Ryan D, Swett P, Duan H. Ind Eng Chem Res 2006;45:2728–33. [23] Gu Wei, Bousfield Douglas W, Tripp Carl P. J Mater Chem 2006;16:3312–7. [24] Consani KA, Smith RD. J Supercrit Fluid 1990;3:51–5.
EUROPEAN POLYMER JOURNAL
European Polymer Journal 43 (2007) 4814–4820
www.elsevier.com/locate/europolj
Preparation of porous CaCO3/PAM composites by CO2 in water emulsion templating method Zhou Bing 1, Jun Young Lee 1, Sung Wook Choi 1, Jung Hyun Kim
*
Nanosphere Process and Technology Laboratory, Department of Chemical Engineering, Yonsei University, 134 Shinchon-dong, Sudaemoon-ku, Seoul 120-749, Republic of Korea Received 12 December 2006; received in revised form 17 July 2007; accepted 28 August 2007 Available online 2 September 2007
Abstract Emulsion templating is an effective method to prepare well-defined porous polymeric materials. In this paper, porous CaCO3/polyacrylamide (PAM) composites were prepared by emulsion templating polymerization in supercritical CO2(scCO2) by using a commercial grade surfactant (FC4430), therefore, the amount of the fillers and the pore size distribution of the composites can be modulated based on the demands of those potential applications as biomaterials. Calcium carbonate crystals can be in situ synthesized in the porous PAM matrix, and the morphology of CaCO3 varied with the conditions of the reaction, the results indicated that three kinds of crystals were observed in the porous matrix. The results of scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) showed that the macropores in PAM were interconnected and with narrow pore size distributions. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: C/W emulsion; Porous hydrogels; Particulate stabilizer; CaCO3
1. Introduction The rapid development of porous materials used as scaffolds for sustained three-dimensional growth of tissue is a growing field. The porous materials (beads, membranes, hydrogels) are widely used in the biomedical field as drug-delivery devices, wound dressings, and scaffolds for tissue-engineering [1]. They are especially appealing for tissue-engineering
*
Corresponding author. E-mail address: [email protected] (J.H. Kim). 1 Current address: College of Chemistry, Jinlin University, 2519 Jie Fang Road, ChangChun, People’s Republic of China.
applications as they are analogous to the natural extracellular matrix of tissues [2,3]. In these materials, hydrogels are not only of threedimensional polymeric networks, but also capable of absorbing large quantities of water, and by the removal of the water in the hydrogels, we can get porous materials; however, the pore size is hard to control. Partap et al. reported a novel method to prepare porous calcium alginate hydrogels by CO2 in water emulsion templating, and in this system, CO2 had a dual role including increasing the acidity of the aqueous phase and being a templating agent [4]. There are many methods to produce porous hydrogels, such like freeze-drying, particulate leaching, gas-foaming methods (using supercritical
0014-3057/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2007.08.014
Z. Bing et al. / European Polymer Journal 43 (2007) 4814–4820
fluids), and the use of gas-foaming agents [5–11]. These methods often yield hydrogels with broad macropore-size distributions and with a mixture of open and closed pores, whilst emulsion templating methods can sometimes give polymers with a more defined and controllable porous structure. ‘‘CO2 in water’’ emulsion templating method has been successfully utilized by Butler et al. to synthesize porous crosslinked polyacrylamide with PFPE surfactants, with pore diameters of 4.3–11.7 lm and pore volumes of 1.18–4.68 cm3 g1, and in their work, crosslinked PVA was used as a co-surfactant to stabilize the emulsion [12]. The emulsion templating technique involves forming a high internal phase emulsion (HIPE) (>74.05% v/v internal phase) and locking in the structure of the external phase, usually by reaction induced phase separation (e.g., sol–gel chemistry, free-radical polymerization). By the removal of the emulsion droplets, skeletal replicas of the emulsion can be obtained. However, HIPE techniques are extremely solvent intensive because the internal oil phase (often an organic solvent) can constitute between 75% and 90% of the total reaction volume. Furthermore, it may be very difficult to remove the template from the material at the end of the reaction. For inorganic materials, purification tends to involve calcining the sample at high temperatures (>600 °C), thus completely removing any organic residues [13]. Supercritical carbon dioxide (scCO2) is a sustainable solvent because it is non-toxic, nonflammable, and naturally abundant [14]. And carbon dioxide is suitable for the preparation of porous polymers and inorganic/polymer composites. In particular, scCO2 has been shown to be a versatile solvent for polymer synthesis and processing [15]. A number of research groups have exploited carbon dioxide for the preparation of porous polymers and inorganic/ polymer composites. Most of these supercritical fluid (SCF) techniques involve either a foaming mechanism, gelation of the SCF medium (i.e., an organic equivalent of sol–gel chemistry), or a combination of both. This limits the range of porous materials which can be accessed via the scCO2 route because many materials cannot be foamed or derived from CO2-soluble precursors. For example, scCO2 has been used for the production of microcellular polymer foams [16], biodegradable composite materials [17], macroporous polyacrylates [18], fluorinated microcellular materials [19], zeolite/polymer composites [20], and thin CaCO3/biopolymer films [21].
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In this paper, HIPE method and gas foaming method by scCO2 were combined to prepare porous PAM hydrogels by a commercial surfactant FC4430, and non-crosslinked PVA as a co-surfactant, and that is nominally based on carbon dioxide fixation and biomass utilization, it could play an important general role in the fabrication of multifunctional inorganic–organic composites, therefore, such techniques can be widely applied to produce a mass of bio-inspired materials. And at the same time, CaCO3 crystals of various morphology can be in situ embedded in the porous PAM by the carbonation of Ca(OH)2 with scCO2. The technique has wide appeal because it allows the synthesis of materials with well-defined porous structures without the use of any volatile organic solvents, just water and CO2. Entrapment of the template phase is completely avoided because CO2 reverts to the gaseous state upon depressurization. 2. Experimental 2.1. Materials Acrylamide (AM) (Aldrich, 98%), N,N-methylene bisacrylamide (MBAM) (Aldrich, 99%), PVA (Aldrich, Mw = 30,000–70,000), CaO (Samchun Chemicals, 98%), FC4430 (3M Company), and potassium persulfate (K2S2O8, KPS) (Aldrich) were used as supplied. Highly purified water was used in all the processes. 2.2. Digestion of CaO by H2O A certain amount of CaO powder was dissolved in 50 ml 80 °C water and stirred for 4 h, then filtered by a 200 mesh sieve. The concentration of the Ca(OH)2 solution was determined by the titration of 0.5 M HCl. 2.3. Polymerization procedure in supercritical system On the first step, surfactant FC4430 was introduce into the 2 ml 15% PVA solution in a 30 ml glass vial, and 3 g 40% monomer (AM/MBAM = 4/1) solution was mixed with the above solution for 20 min. One milliliter 1.5% Ca(OH)2 solution and KPS (2% weight percent in the total water used above) powder were introduced into the surfactants and monomer solution before the emulsion polymerization in the stainless steel autoclave, and the reactor was equipped with a thermo-couple, a
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pressure sensor connected to a pressure protector, and a view window. Secondly, the temperature and the press of CO2 of the reactor were set at 25 °C and 80 bar (CO2/H2O = 8/2, v/v), and after being magnetic stirred for 30 min, a white and milky emulsion was obtained. To analyze the stability of the emulsion, the stirring will be stopped, and phase separation occurred in those unstable emulsions leading to two phases, as had been described in Cooper’s paper [1]. Then the emulsion was heated to 60 °C in 30 min, and the final pressure was 280 bar, the polymerization occurred and the gelation of the aqueous phase can be observed. The reaction time is 12 h. When the temperature of the reactor decreased to room temperature, CO2 was vented out in 1 h. The CaCO3/PAM hydrogel monolith can be obtained. The sample was dried in vacuum oven or by freeze-drying method, fractured into small pieces, and analyzed by SEM and mercury intrusion porosimetry (MIP). The morphology of the porous composites was investigated using a JEOL SEM instrument (JSM6701F, Japan) operating at 5 kV. Fractured segments were mounted on carbon fiber pads and attached to aluminium stubs and were Pt coated using Cressing Sputter coater. MIP (Auto Pore IV 9500, USA) was applied to collect the information of the porous structure of the samples. 3. Results and discussion 3.1. Effect of the amount of surfactant FC4430 on the stability of the emulsion Fluorosurfactant FC4430 is a non-ionic polymeric fluorochemical surfactant belonging to a class of coating additives which provide low surface tensions in the emulsions. In this work, it was used as CO2-philic surfactant to obtain stable CO2 in water (C/W) emulsion. Before the polymerization of AM and the cross-linker (MBAM), the stability of the CO2 in water emulsion without Ca(OH)2 was very poor; and when the content of FC4430 was lower than 5% (w/v to water) at room temperature, due to the insufficient dispersion of liquid CO2 in the aqueous phase, the polymerization occurred in the bottom of the reactor of the aqueous layer. The degree of porosity was very low. However, the stability of the emulsions was improved by the addition of Ca(OH)2 solution at the same content of surfactant FC4430, because after the carbonation of Ca(OH)2 by CO2, the presence of CaCO3 particles
can stabilize the emulsion by an interlayer of CaCO3 particles located between CO2 droplets and water at high pressure (more than 70 bar) [22], the formation of CaCO3 by the contact of Ca(OH)2 slurry with scCO2 is an in situ process during the polymerization of the monomer, and the size of the particles was about 1–3 lm, the loading is about 4%, which had been proved to be efficient to stabilize the C/W emulsion [22]. We can get the CaCO3/PAM hydrogel monolith when the content of surfactant FC4430 was at 10%, 12% and 15% (w/v to water) together with 3% Ca(OH)2 (w/v to water). 3.2. Morphology of the dual porous CaCO3/PAM composites The Ca(OH)2 in the aqueous phase can transfer to CaCO3 particles in the CO2 in water emulsion by the reaction as described below: Ca(OH)2 + CO2 ! CaCO3 (s) + H2 O In the work of Golomb et al., they had studied the influence on the stability of CO2 in water emulsion with several kinds of inorganic materials as stabilizer, and among those materials, they found the sheath of CaCO3 particles prevents coalescence of the CO2 droplets into a bulk phase [22]. Gu et al. prepared CaCO3 particles by carbonation of Ca(OH)2 solution and Ca(OH)2 powder with scCO2, and the carbonation process is carried out by a solid-SCF (supercritical fluid) method [23]. Fig. 1 shows the SEM pictures of the fractured surface of the composites, which were from the unstable CO2 in water emulsion. The content of surfactant FC4430 was set at 5% (w/v to water), the loading of Ca(OH)2 was 3% (w/v to water). As can be seen in Fig. 1a and b, the PAM matrix was like to be connected by small particles after freezedrying, and the degree of porosity of the samples was very low, and the median pore diameter was about 2.5 lm. However, CaCO3 particles can be found on the fracture surface in three types of morphology: cubic, sphere-like, and needle-like, as shown in Fig. 1c–f. The particles were embedded in the polymers, and the variety of the morphology is due to the crystalline structure of CaCO3 particles in different chemical environments, accordingly, the mechanism of the growth behavior of CaCO3 crystalline in the aqueous phase and the stabilization effect of CaCO3 particles cooperated with surfactant should be discussed in the future.
Z. Bing et al. / European Polymer Journal 43 (2007) 4814–4820
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Fig. 1. SEM images of fracture surface of the CO2 templated porous CaCO3/PAM composites produced from CO2 in water emulsion: (a) content of surfactant FC4430 was 5% (w/v to water), the loading of Ca(OH)2 was 3% (w/v to water); (b) magnification of the pores structure and (c)–(f) the morphology of the CaCO3 crystals in the composites.
In the aqueous phase, there are three components which can stabilize the CO2 in water emulsion, one is the surfactant FC4430, another one is PVA, and the other one is CaCO3 particles. Among these stabilizers, the presence of PVA can sufficiently improve the stability of the emulsion as a co-surfactant [1]. And the inorganic particles can also stabilize the CO2 droplets in the aqueous phase at high pressure. In this work, liquid CO2 and supercritical CO2 was well dispersed in the aqueous phase with Ca(OH)2 solution due to the formation of CaCO3 during the contact of CO2 and Ca(OH)2 solution before the
polymerization of acrylamide and the cross-linker. The SEM pictures of porous CaCO3/PAM composites with different content of surfactant FC4430 are shown in Fig. 2. As can be seen in all the images, emulsion templating method is effective to prepare porous materials, and in the C/W emulsion system, the role of the surfactant is the most important factor to obtain stable emulsion, so many researchers had paid a lot of efforts to synthesize and select special surfactants for C/W and W/C emulsion system [24]. FC4430 is a commercial grade surfactant, and in this work, it was proved that such non-ionic
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Fig. 2. SEM images of fracture surface of the CO2 templated porous CaCO3/PAM composites produced from CO2 in water emulsion with the loading of Ca(OH)2 was 3% (w/v to water): (a) and (b) content of surfactant FC4430 was 10% (w/v to water); (c) and (d) content of surfactant FC4430 was 12% (w/v to water); (e) content of surfactant FC4430 was 15% (w/v to water) and (f) image of the morphology of the CaCO3 crystals in the composites.
surfactant is suitable in this system. However, the content of FC4430 is much higher than those special surfactants [1], so it is necessary to find out the suitable mass ratio of inorganic particles and the surfactant to decrease the loading of FC4430. In Fig. 2, it was clear that the CO2 droplets were not monodisperse, which indicated that this method was not like the other HIPE methods to produce porous materials well-defined pore size distribution
[4]. And the pores of the materials were open and interconnected, and with the increase of the content of FC4430, the surfactant can stabilize an increasingly large interfacial area and the average CO2 droplet size became correspondingly bigger, as can be seen in Fig. 2c–e. Fig. 2f shows that some CaCO3 particles existed in the interface of CO2 droplets and PAM matrix, and after the removal of the CO2 phase, they were embedded in the polymers.
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The dried CaCO3/PAM composites hydrogels was examined by mercury intrusion porosimetry (MIP), and the porosimetry data of the samples with different concentrations of surfactant FC4430 are shown in Fig. 3 and Table 1. The results showed that the total pore volume and mean diameter of the samples are lower than those of the porous PAM reported in the paper of Cooper et al. [1]. In this work, it was necessary to increase the concentration of the surfactant FC4430 more than 10% to obtain stable CO2 in water emulsion. The total pore volume decreased with the increase of concentration of the surfactant, which indicated more CO2 droplets were dispersed into water, and the mean pore diameter of the droplets increased from 4.7 lm to 14.9 lm. However, the total porosity of the samples increased from 47% to 73%, which indicated that the volume ratio of CO2 phase in the emulsion increased, and the degree of the interconnection of the pore was enhanced, at the same time, the thickness of the PAM pores became much denser after the polymerization. Among the three kinds of stabi-
10% FC4430 12% FC4430 15% FC4430
Log Differential Intrusion (mL/g)
4
3
2
1
0 1
10
100
Pore Diameter (μm)
Fig. 3. Size distribution of the supercritical CO2 templated porous CaCO3/PAM composites with different concentrations of surfactant FC4430.
Table 1 Analysis of porous CaCO3/PAM composites made via CO2 in water emulsion templating method Sample
Concentration of Mean pore Total pore Total FC4430 (w/v, %, diameter volume porosity based on water) (lm) (cm3/g) (%)
Sample 1 10 Sample 2 12 Sample 3 15
4.7 10.3 14.9
2.01 1.44 0.76
47 61 73
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lizers, FC4430 is the most effective surfactant than CaCO3 particles and PVA. 4. Conclusion Porous CaCO3/PAM composites were successfully prepared by the CO2 in water emulsion templating method. It is a new generic method of producing porous inorganic/polymer composites hydrogels by combining emulsion templating with an internal reaction. The carbonation reaction of Ca(OH)2 with CO2 occurred on the interface of the CO2 droplets and the aqueous phase, and the CaCO3 particles can stabilize the emulsion together with commercial surfactant FC4430 and PVA. The hydrogels exhibit an open, well interconnected pore network with a narrow pore-size distribution which may be suitable for potential bioengineering applications. Future work will focus on the mechanism of the stabilization effect of inorganic particles to obtain highly dispersed CO2 in water emulsion, and the preparation of uniform macroporous inorganic/polymer composites. References [1] Butler R, Hopkinson I, Cooper AI. J Am Chem Soc 2003;125:14473–81. [2] Zhang H, Cooper AI. Soft Matter 2005;1:107. [3] Cameron NR, Sherrington DC. Adv Polym Sci 1996;126:163. [4] Partap Sonia, Rehman Ihtesham, Jones Julian R, Darr Jawwad A. Adv Mater 2006;18:501–4. [5] Whang K, Thomas CH, Healy KE, Nuber G. Polymer 1995;36:837. [6] Zmora S, Glicklis R, Cohen S. Biomaterials 2002;23:4087. [7] Chen GP, Ushida T, Tateishi T. Biomaterials 2001;22:2563. [8] Sheridan MH, Shea LD, Peters MC, Mooney DJ. J Control Release 2000;64:91. [9] Mooney DJ, Baldwin DF, Suh NP, Vacanti LP, Langer R. Biomaterials 1996;17:1417. [10] Eiselt P, Yeh J, Latvala RK, Shea LD, Mooney DJ. Biomaterials 2000;21:1921. [11] Choi BY, Park HJ, Hwang SJ, Park JB. Int J Pharm 2002;239:81. [12] Butler Rachel, Davies Cait M, Cooper Andrew I. Adv Mater 2001;13:1459. [13] Manoharan VN, Imhof A, Thorne JD, Pine DJ. Adv Mater 2001;13:447. [14] Jessop PG, Leitner W, editors. Chemical synthesis using supercritical fluids. Weinheim: WILEY-VCH; 1999. [15] Cooper AI. J Mater Chem 2000;10:207. [16] Arora KA, Lesser AJ, McCarthy TJ. Polym Eng Sci 1998;38:2055. [17] Howdle SM, Watson MS, Whitaker MJ, Popov VK, Davies MC, Mandel FS, et al. Chem Commun 2001;109. [18] Wood CD, Cooper AI. Macromolecules 2001;34:5.
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