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BY-NC-ND 3.0 license Open Access Published by De Gruyter January 8, 2014

Porous functional poly(unsaturated polyester-co-glycidyl methacrylate-co-divinylbenzene) polyHIPE beads through w/o/w multiple emulsions: preparation, characterization and application

Emine Hilal Mert and Hüseyin Yıldırım
From the journal e-Polymers

Abstract

Poly(unsaturated polyester-co-glycidyl methacrylate-co-divinylbenzene) poly high internal phase emulsion (HIPE) beads were synthesized via water-in-oil-in-water (w/o/w) multiple emulsions. HIPEs were prepared by using a commercial unsaturated polyester resin (UPR) and a mixture of glycidyl methacrylate (GMA) and divinylbenzene (DVB) as the cross-linker. The external surfactant was found to be a strong influence on the morphology of the beads. The porosity and the pore morphology of the resulting polyHIPE beads were investigated by scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) molecular adsorption method, respectively. Post-functionalization of the beads was carried out with multifunctional amines such as 1,4-ethylenediamine (EDA), 1,6-hexamethylenediamine (HMDA) and 4-aminosalicylic acid (ASA). Elemental analysis was used to confirm the functionalization. Resulting functional beads were tested on the adsorption of Ag(I), Cu(II), and Cr(III) under non-competitive conditions and atomic absorption spectroscopy (AAS) was used to calculate the adsorption capacities. The maximum adsorption capacities of the functional beads were found to be decreasing in the order of Ag(I)>Cu(II)>Cr(III).

1 Introduction

Emulsion templating, which is based on the formation and polymerization of high internal phase emulsions (HIPEs), is a versatile method for the preparation of porous polymer supports that have been used extensively for a variety of applications, such as chromatographic separation (1, 2), membrane filtration (3), ion-exchange systems (4), scavengers in batch and flow-through processes (5–8), scaffolds for tissue engineering (9) and supports for organic synthesis (10–12).

A HIPE is usually described as a concentrated emulsion system, consisting of a high ratio of internal or dispersed phase. The volume fraction (Ø) of a HIPE is usually >0.74 and can reach levels of 0.99 (13). Preparation of well-defined cellular polymers from emulsion templates were pioneered by Bartl and Bonin (14, 15) and Lissant and Mayhan (16). Later, a new class of macroporous polymers was invented by polymerizing the continuous phase of HIPEs, by Unilever researchers Barby and Haq (17). In general, the continuous phase of a HIPE consists of monomers, a cross-linker, and a surfactant whereas the aqueous phase usually contains an electrolyte. Polymerization of HIPEs enables formation of highly cross-linked, low-density polymers – polyHIPEs – with interconnected pores accompanied with an open chemical structure for post-functionalization (13, 17). However, the preparation of polyHIPE polymers with reactive surface groups is limited, because of the polar character of monomers (18). In this respect, there have been two conventional methods used for the post-functionalization of polyHIPE polymers: (i) the co-polymerization of a functional monomer with styrene (St) and (ii) the grafting of polymer chains onto the surface (3, 4, 18–20).

In their recent study, Krajnc et al. prepared amine functional monolithic scavenger resins by introducing 4-vinylbenzyl chloride in the continuous phase (21). In addition, Stefanec and Krajnc (6) prepared beads of polyHIPEs with high levels of 4-vinylbenzyl chloride and described the functionalization of the polyHIPE beads with piperidine, piperazine, tris(2-aminoethyl) amine and tris(hydroxymethyl) methylamine. The same conceptual approach has also been used for the preparation of 2-hydroxyethyl methacrylate based polyHIPEs by Kulygin and Silverstein (22) and Kovacic et al. (23). Based on the reactive epoxy group of glycidyl methacrylate (GMA), which enables post-functionalization, preparation of the GMA grafted polyHIPE polymers in monolithic form was previously described by Krajnc et al. (1) and Barbetta et al. (18).

In this study, a synthetic approach was developed to prepare functional polyHIPE polymers in bead form. As distinct from the previous studies, a commercial resin (unsaturated polyester resin, UPR) was used instead of a simple monomeric unit. As is commonly known, UPRs are viscous materials of a low degree of polymerization and molecular weight. In most cases, the viscous polyesters are dissolved in a vinyl type monomer (usually styrene) to reduce the viscosity. By contrast, the unsaturated acid residues in the initial polyester chains provide a site for cross-linking. Addition of a suitable initiator enables the formation of cross-linked polymers consisting of interconnected polyester chains in which the saturated acids reduce the brittleness of the final cross-linked product by reducing the frequency of cross-links (24). Within this context, we used a mixture of GMA and divinylbenzene (DVB) to reduce the viscosity, achieve the curing step and create suitable sites for functionalization.

By contrast, polyHIPEs are cellular polymers. Under the correct conditions, small interconnecting windows are formed between adjacent emulsion droplets, allowing the droplet phase to be removed by drying. This produces a highly porous and permeable material (13). Thus, the mechanical strengths of these polymers are relatively low compared with other kinds of porous polymers. At this point, applications of these polymers are restricted, because of their poor mechanical properties. To overcome this issue, several approaches have been used such as increasing the foam density (which is not desirable for applications), changing monomers or cross-linker and introducing a suitable reinforcement into the structure (25). In this respect, changing the monomer composition is a convenient way to improve the mechanical properties and the issue of using a UPR is addressed.

In our previous study, such polymers in monolithic form have already been prepared and their functionalization and application have been described (26). In this paper, the preparation of the polyHIPE beads via water-in-oil-in-water (w/o/w) multiple emulsions and the post-functionalization of such beads with chelating agents, such as 1,4-ethylenediamine (EDA), 1,6-hexamethylenediamine (HMDA) and 4-aminosalicylic acid (ASA) through the reaction between the epoxy group of GMA and the multifunctional amine molecules were carried out. In addition, the heavy metal adsorption capacities of the post-functionalized polyHIPE beads under non-competitive conditions were investigated.

2 Experimental

2.1 Materials

Erco E-81 coded commercial UPR (solid resin, does not contain styrene and additives, melts at 80oC) was supplied by Ece Boya Kimya (Istanbul, Turkey) and used as received without further treatment. GMA (97%, Fluka), DVB (technical grade, 80% mixture of isomers, Aldrich), triethanolamine (TEA, 99%, Merck), EDA (99.5%, Fluka), HMDA (98%, Fluka), ASA (95%, Fluka), tetrahydrofuran (THF, 99%, Merck), toluene (99.7%, Riedel-de Haën), N,N′-dimethylformamide (DMF, 99%, Fluka), chlorobenzene (CB, 99%, Aldrich), hydroxyethyl cellulose (HEC, Fluka) and hydroxypropyl cellulose (HPC, Mw~80,000, Fluka) were used without any further purification. CertiPUR standard copper solution [Cu(NO3)2 in 0.5 M HNO3, 1000 mg/l Cu, Merck], CertiPUR standard silver solution (AgNO3 in 0.5 M HNO3, 1000 mg/l Ag, Merck), CertiPUR standard chrome solution [Cr2(NO3)3 in 0.5 M HNO3, 1000 mg/l Cr, Merck] were used as received. Azobisisobutyronitrile (analytical grade, Merck) was re-crystallized from ethanol, and potassium persulfate (Merck) and potassium phosphate (Merck) were used as received.

2.2 Methods

The average molecular weight and the heterogeneity index of the UPR were determined by gel permeation chromatography (GPC). GPC measurements were carried out with THF by using three high resolution columns (5 μm 8×300 mm SDV columns with 106, 105 and 103 Å), a P1000 pump, refractive index detector and UV detector (UV1000, λ=260 nm) at 30°C under 1 ml min-1 flow rate. The average molecular weight of the resin was calculated from the calibration curves of polystyrene standards (Table 1).

Table 1

Physicochemical properties of the commercial unsaturated polyester resin (UPR).

Average molecular weightDetector
UV (260 nm)RI
1.71×1031.86×103
5.60×1035.57×103
1.79×1041.72×104
HI3.282.99

Q, adsorption capacity; RI, refractive index.

Bead morphologies were investigated using scanning electron microscopy (SEM) with an LEO 1550 scanning electron microscope, 1.0 nm at 20 kV, 2.0 nm at 1 kV. The particle size distribution of the obtained microbeads was determined from the SEM micrographs by taking greater than 100 measurements from each SEM image. Specific surface areas and pore sizes of beads were measured using a Quantachrome’s Quadrasorb SI surface area and pore size analyzer using the Brunauer-Emmett-Teller (BET) molecular adsorption method. Elemental analysis data were obtained with a Thermo Finnigan Flash EA 1112. Atomic absorption data were collected by Perkin Elmer Analyst 200 Flame Atomic Absorption Spectrophotometer (AAS).

2.2.1 HIPE preparation

A total of 5.25 g (0.94 mmol) of UPR, 1.35 g (9.49 mmol) of GMA, 0.9 g (6.91 mmol) of DVB, 4 ml of CB, 2.25 g (15.01 mmol) of TEA (30% in regard to total monomer weight) and 0.15 g (0.91 mmol) of azobisisobutyronitrile (2% in regard to total monomer weight) were placed in a reaction vessel and the mixture was mixed with an overhead stirrer at 300 rpm. The aqueous phase (42.5 ml) was added drop by drop under constant stirring at 300 rpm. Once all the aqueous phase had been added, stirring was continued for another 10 min, to produce a uniform emulsion.

2.2.2 Preparation of polyHIPE beads

Once HIPE was prepared, as described above, it was placed in a polyethylene syringe and added to the pre-heated (100oC) inert aqueous suspension medium. The aqueous suspension consists of 1% (w/w) polymeric stabilizer (HEC or HPC), 0.1% (w/w) potassium phosphate as protective colloid and 0.1% (w/w) potassium persulfate as the initiator. HIPE was added directly to the suspension medium under a constant stirring rate of 400 rpm. The polymerization was carried out for 24 h at 100oC under constant stirring at 400 rpm. After 24 h, the reaction vessel was cooled and the polyHIPE beads were filtered and extracted with methanol in a Soxhlet apparatus for 10 h; the resulting beads were dried in a vacuum oven at 40oC.

2.2.3 Post-functionalization of beads

The post-functionalization of beads was based on a reaction between the epoxy group of GMA and multifunctional amine molecules (EDA, HMDA, and ASA). A total of 0.1 g of beads was placed in a three-necked reaction flask and 10 ml of DMF and 10 ml of multifunctional amine solution in DMF (0.1 g amine /10 ml DMF) was added into the mixture. The mixture was placed in an oil bath at 100oC for 24 h, and stirred under argon atmosphere. The resulting product was removed by filtration, extracted with methanol in a Soxhlet apparatus for 10 h, and then dried in a vacuum oven at 40oC.

2.2.4 Batch metal-uptake with functional beads

Adsorption of metal ions from aqueous solutions in batch experiments under non-competitive conditions was investigated. The metal adsorption capacities of functional polyHIPE beads were determined for Ag(I), Cu(II) and Cr(III), by contacting 0.1 g of functional polyHIPE beads with the aqueous metal solutions. At appropriate times, polyHIPE beads were filtered from the solutions and the metal ion concentrations of the remaining solutions were measured by AAS.

3 Results and discussion

3.1 Preparation of polyHIPE beads

The simplest multiple emulsions are ternary systems, having either a w/o/w or an o/w/o structure, whereby the dispersed droplets contain smaller droplets of a different phase. These emulsions are complex systems where both w/o and o/w emulsion types exist simultaneously. In the case of w/o/w emulsions, the oil droplets have smaller water droplets within them, and the oil droplets themselves are dispersed in a continuous water phase. Multiple emulsions typically require two or more emulsifiers, one that is predominantly hydrophobic that stabilizes the primary w/o emulsion and one that is predominantly hydrophilic that stabilizes the secondary o/w emulsion. The hydrophobic and hydrophilic emulsifiers are added to the oil and continuous aqueous phases, respectively (27).

The preparation of unsaturated polyester based HIPEs has been described by the same authors in a previous work (26). In the case of unsaturated polyester based polyHIPE beads, a HIPE from UPR, GMA and DVB was prepared by stabilizing the emulsion with TEA. Once a stable HIPE is obtained, it can be suspended in a second aqueous phase, which comprises an external stabilizer for stabilizing the HIPE droplets. The external surfactant was found to have a great influence on stabilizing HIPE droplets and on the morphology of the polyHIPE beads.

HEC and HPC were used as the external surfactant and protective colloid, respectively. The concentrations of HEC and HPC were changed between 0.5% and 2%. We found that HEC is more effective than HPC in providing stability of HIPE droplets in the suspension medium. While lower amounts of the external surfactant led to phase inversion, higher amounts broke down the polymer particles. Depending on phase inversion, irregularly shaped particles were obtained. The conversion amount of HIPEs into polyHIPE microbeads was calculated from the ratio of total monomer weight of the emulsions to the dry weight of the obtained microbeads. The conversion of emulsion prepared with 1% of HEC was estimated as 82%, while it was estimated as 65% for the emulsion prepared with 1% of HEC. The morphology of the microbeads was mainly investigated by SEM (Figures 1 and 2) and particle size distribution of the resulting polymers was determined from the SEM micrographs (Figure 3). It was revealed that the particle size of the microbeads prepared with 1% of HEC was changed between 0.0 µm and 5.5 µm, while in the case of microbeads prepared with 1% of HPC the particle size distribution was between 0.0 µm and 10.5 µm. According to the particle size analysis given in Figure 3, approximately 56% of all particles were between 1.5 µm and 2.5 µm, 40% of all particles were changed equally between 2.5 µm and 3.5 µm and 0.5 µm and 1.5 µm. By contrast, microbeads prepared with 1% of HPC showed a broad distribution. In this case, 60% of all particles were between 0.5 µm and 1.5 µm, 17% of all particles were between 2.5 µm and 1.5 µm, and 14% of all particles were changed equally between 3.5 µm and 2.5 µm and 0.0 µm and 0.5 µm. The average surface area and pore size of the microbeads prepared with 1% of HEC was determined by the BET adsorption method. According to the BET measurements, the average surface area and pore size of the obtained beads were found to be 10.01 m2g-1 and 4.5oA, respectively.

Figure 1 Scanning electron microscopy (SEM) micrographs of polyHIPE beads obtained from the suspension system containing 1% of hydroxypropyl cellulose (HPC): (A) at 2 μm, (B) at 3 μm.

Figure 1

Scanning electron microscopy (SEM) micrographs of polyHIPE beads obtained from the suspension system containing 1% of hydroxypropyl cellulose (HPC): (A) at 2 μm, (B) at 3 μm.

Figure 2 Scanning electron microscopy (SEM) micrograph of a polyHIPE bead obtained from the suspension system containing 1% of hydroxyethyl cellulose (HEC): (A) inside the bead, (B) surface of the bead.

Figure 2

Scanning electron microscopy (SEM) micrograph of a polyHIPE bead obtained from the suspension system containing 1% of hydroxyethyl cellulose (HEC): (A) inside the bead, (B) surface of the bead.

Figure 3 Particle size distributions of the resulting microbeads.

Figure 3

Particle size distributions of the resulting microbeads.

An emulsion can be destabilized by two mechanisms: coalescence and Ostwald ripening. Coalescence, the contact merging of emulsion droplets to form larger droplets, is an irreversible process. It occurs when a thin film of continuous phase between two closely approaching droplets breaks. In most cases, coalescence occurs only when the droplets are close to each other for a prolonged time. Eventually coalescence leads to the separation of emulsions into two layers; oil and aqueous phase. Ostwald ripening, by contrast, is the result that comes from the difference in solubility. It occurs when the molecules of the dispersed phase have a relatively high solubility in the continuous phase and consists of a diffusive transfer of the dispersed phase from small to large drops. The rate at which it occurs depends on the solubility of the disperse phase in the dispersion medium (28, 29). The pore dimensions and pore distribution are affected by coalescence, while Ostwald ripening affects the surface area. Thus, choosing an appropriate composition can essentially eliminate both destabilization mechanisms for emulsions and makes it possible to control the morphology. In this respect, Barbetta et al. suggested that, in case of polar functional monomers, emulsion stability could be provided by decreasing the influence of coalescence and Ostwald ripening [18]. As polarity is the common effect that causes both coalescence and Ostwald ripening, reducing the polarity of the continuous phase is the first way to eliminate the destabilizing effects. The first choice for reducing the polarity is to use a relatively more hydrophobic cross-linker or co-monomer to functional monomer, while the second choice is to use a porogenic solvent. The second way to eliminate the destabilizing effects is through the correct choice of surfactant. Although using an inert porogenic solvent in such systems enables formation of regular pores and increases the surface area, porogens are also needed to increase the solubility of the UPR in the continuous phase.

Formation of discrete droplets mostly depends on the viscosity of the HIPE. When the viscosity is too high, discrete droplets cannot be formed in the suspension medium and consequently, irregular shaped polymer particles will be obtained only after the polymerization. The HIPE should be more flowing while added from the needle thoroughly in order to polymerize emulsion droplets in spherical forms.

With several solvents such as toluene, THF and CB, we attempted to decrease the polarity of the continuous phase, to increase the solubility of the UPR and to decrease the viscosity of the HIPE at the same time. However, due to the miscibility of toluene and THF with the aqueous phase, phase inversion occurred during polymerization and led to the formation of o/w type emulsions. By contrast, in the case of CB, the miscibility of CB with the aqueous phase was negligible and it did not diffuse into the aqueous suspension medium.

The polarity of the monomer mixture was the main factor affecting the stability of HIPEs, but the ratio of unsaturated polyester and GMA and cross-linker (DVB) were also very important for the preparation of polyHIPE beads. We changed the weight percentage of the unsaturated polyester from 30% to 70% in the total continuous phase, while the GMA:DVB ratio was constant at 1:1. After several experimentations, we found that HIPEs prepared with lower amounts of unsaturated polyester were not stable enough during polymerization in the suspension medium, where the phase inversion occurred. Eventually, coalescence occurred with the increase of the polyester concentration in the continuous phase. The results were consistent and optimized by using 70% of unsaturated polyester in the continuous phase. We studied the effect of the GMA:DVB ratio. The optimum GMA:DVB ratio was found to be 3:2 in 70% of polyester containing HIPEs. When the DVB content of the HIPEs was increased, phase separation was facilitated because of the increase of the hydrophobic character.

3.2 Post-functionalization of polyHIPE beads

The post-functionalization of the epoxy group of GMA with multifunctional amines is a facile route to obtain functional polymers with ion-exchange groups on the surface of pores for various applications. Poly(unsaturated polyester-co-glycidyl methacrylate-co-divinylbenzene) polyHIPE beads were functionalized by using three different multifunctional amines; EDA, HMDA, and ASA (Figures 4 and 5). In the case of functionalization of a GMA based polymer with multifunctional amines, possible side reactions should be considered. For many applications, free amino groups are needed for further applications. However, as a consequence of side reactions, a considerable amount of additional cross-linking may occur over the free amino groups, depending on the chemical structure of the multifunctional amine molecule and the reaction conditions (temperature, and the polarity of the used solvent) (21, 30–33). In such cases, especially when the polymer and the amine have complex structures, it is not easy to determine the degree of functionalization by basic spectroscopic methods, i.e., infrared because of overlapping of the peaks. For this reason, we used elemental analysis to calculate the functionalization degree. The degrees of conversion with multifunctional amines were found to be between 21.93% and 83.52%, as calculated from the nitrogen percentage by elemental analysis data (Table 2). The steric groups of the ASA molecule restricted the functionalization of polyHIPE beads and decreased the yield of functionalization. According to the results given in Table 2, the highest degree of conversion was achieved by EDA.

Figure 4 Chemical structures of the multifunctional amines: 1,4-ethylenediamine (EDA) (i), 1,6-hexamethylenediamine (HMDA) (ii), 4-aminosalicylic acid (ASA) (iii).

Figure 4

Chemical structures of the multifunctional amines: 1,4-ethylenediamine (EDA) (i), 1,6-hexamethylenediamine (HMDA) (ii), 4-aminosalicylic acid (ASA) (iii).

Figure 5 Post-functionalization of polyHIPE beads.

Figure 5

Post-functionalization of polyHIPE beads.

Table 2

Post-functionalization results for polyHIPE beads.

polyHIPE beadsNtheo (%)Nexp(%)Functionalization (%)
EDA functionalized3.52.982.9
HMDA functionalized3.52.880.0
ASA functionalized1.70.423.5

Nexp%, Experimental N%; determined by elemental analysis.

Ntheo%, Theoretical N%.

Functionalization%, Calculated degree of functionalization.

ASA, 4-aminosalicylic acid; EDA, 1,4-ethylenediamine; HMDA, 1,6-hexamethylenediamine.

3.3 Metal removal using the functional polyHIPE beads

The metal ion uptake capacities of functionalized polyHIPE beads were determined under non-competitive conditions at inherent pHs of the metal ion solutions measured to be 3.0. The metal ion capacities of the beads for Ag(I), Cu(II) and Cr(III) are presented in Figure 6. Table 3 shows the maximum capacities of functionalized beads for Ag(I), Cu(II) and Cr(III) ions. The metal ions can be adsorbed non-specifically on the polyHIPE beads by physical adsorption, associated with the weak Van der Waals forces. In addition, due to the porous structure, heavy metal ions may diffuse and absorb within the pores of the beads. Chemical adsorption, associated with the exchange of electrons and the formation of a chemical bond between the metal ions and the carbonyl groups of the polymer chain, may also occur. The presence of chelating amine groups also contributes to the adsorption of the metal ions (Figure 7). The specificity of the metal-chelating ligand (multifunctional amine groups) also increased the adsorption capacity. As can be seen from Table 3, the adsorption capacities Q (mmol/g) (amount of metal ions adsorbed per unit mass of bead) on EDA functionalized, HMDA functionalized, and ASA functionalized beads decreased in the order of: Ag(I)>Cu(II)>Cr(III). PolyHIPE beads with EDA groups were the most effective adsorbents for Ag(I) and Cu(II) heavy metal, and for Cr(III) ions, polyHIPE beads with ASA groups were the most effective adsorbents.

Figure 6 Capacities of functional polyHIPE beads for metal ions under non-competitive conditions.

Figure 6

Capacities of functional polyHIPE beads for metal ions under non-competitive conditions.

Table 3

Maximum capacities of functional polyHIPE beads for Ag(I), Cu(II) and Cr(III) ions under non-competitive conditions (adsorption time 6 h).

polyHIPE beadsQ (mmol/g)
Ag(I)Cu(II)Cr(III)
EDA functionalized9.998.253.24
HMDA functionalized9.485.454.07
ASA functionalized9.977.973.79

ASA, 4-aminosalicylic acid; EDA, 1,4-ethylenediamine; HMDA, 1,6-hexamethylenediamine.

Figure 7 Metal removal with functional polyHIPE beads.

Figure 7

Metal removal with functional polyHIPE beads.

4 Conclusion

The preparation of poly(unsaturated polyester-co-glycidyl methacrylate-co-divinylbenzene) polyHIPE microbeads was carried out with the aim of producing functionalized microbeads. In this respect, a stable HIPE of a mixture of unsaturated polyester, GMA and DVB, was prepared without using an emulsifying agent. Later, the obtained emulsion was dispersed in a second suspension medium in order to achieve cross-linking by droplets and produce microbeads. For the functionalization, the reaction between the epoxy groups of GMA and multifunctional amines was carried out. It was shown that polyHIPE microbeads can be functionalized with multifunctional amines to yield polymer supports for ion exchange processes. For the determination of actual applicability, the maximum ion exchange capacity of the resulting microbeads for Ag(I), Cu(II) and Cr(III) was also investigated under non-competitive conditions.


Corresponding author: Emine Hilal Mert, Faculty of Engineering, Polymer Engineering Department, Yalova University, 77100 Yalova, Turkey, e-mail:

The authors thank the Scientific and Technological Research Council of Turkey (TUBITAK, Project Number: 107T843) for funding this research and Ece Boya Kimya (Istanbul, Turkey) for the supply of unsaturated polyester resin.

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Received: 2013-11-21
Accepted: 2013-11-22
Published Online: 2014-01-08
Published in Print: 2014-01-01

©2014 by Walter de Gruyter Berlin Boston

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