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Methacrylic acid microcellular highly porous monoliths: Preparation and functionalisation
Reactive & Functional Polymers 72 (2012) 221–226
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Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Methacrylic acid microcellular highly porous monoliths: Preparation and functionalisation Urška Sevšek a, Peter Krajnc a,b,⇑ a b
University of Maribor, Faculty of Chemistry and Chemical Engineering, Smetanova 17, SI-2000 Maribor, Slovenia Centre of Excellence PoliMaT, Tehnološki park 24, SI-1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 25 November 2011 Received in revised form 24 January 2012 Accepted 14 February 2012 Available online 23 February 2012 Keywords: Poly(methacrylic acid) PolyHIPE Porous polymers Emulsions Functional polymers
a b s t r a c t By polymerisation of high internal phase emulsions (HIPEs), containing styrene (STY), divinylbenzene (DVB) and methacrylic acid (MAA) in the continuous phase, highly porous polymers including carboxylic functional groups were prepared. The ratio of methacrylic acid to divinylbenzene was varied in order to obtain polyHIPEs with a different degree of crosslinking which influenced a surface area of the polymers, being substantially higher (185 m2/g) with a higher degree of crosslinking (51% DVB) than with a lower degree of crosslinking (24% DVB, 46 m2/g). Up to 90% porous samples were prepared and the optimum hidrophilicity-lipophilicity balance (HLB) of the surfactant was found to be around 4.8–4.9. Both thermal and photo initiation were used to induce polymerisation. The resulting polymers had an open cellular morphology with cavity diameters between 21.8 lm and 44.2 lm and with interconnecting pores between 2.2 lm and 5.0 lm. Monolithic supports were used for further functionalisation with thionyl chloride and multifunctional amines, namely 1,4-diaminobutane and 1,12-diaminododecane. The functionalisation degree with thionyl chloride was 76%. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Emulsion templating using high internal phase emulsions is now an established technique for the preparation of microcellular highly porous monoliths with an interconnected porous structure [1]. According to most definitions, high internal phase emulsions (HIPEs) have a volume fraction of internal phase higher than 74% which represents a maximum space occupied by uniform spheres [2]. Polymerisation of the continuous phase of HIPE results in a solid polymer which is a replica of the precursor emulsion, having large pores (cavities, sometimes referred to as voids) in place of the droplets of internal phase which normally does not include monomers. These pores are connected with a number of smaller interconnecting pores (sometimes referred to as windows). With the use of porogenic solvent a third degree of pores at a nanometre scale can be formed, significantly increasing the surface area of polyHIPE. Generally rather low surface area can be increased by a higher degree of crosslinking [3] or by post polymerisation functionalisation inducing additional crosslinking [4]. By tuning the preparation conditions and monomer content, polyHIPE material with both high surface area and open interconnecting structure ⇑ Corresponding author at: University of Maribor, Faculty of Chemistry and Chemical Engineering, Smetanova 17, SI-2000 Maribor, Slovenia. Tel.: +386 2 2294422; fax: +386 2 2527774. E-mail address: [email protected] (P. Krajnc). 1381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2012.02.007
with larger pores can be obtained, giving the material functional properties like low density, high permeability, appropriate surface area, etc. Another advantageous property of the HIPE emulsion is that it can be moulded into various shapes, e.g. tubes, discs, cones. Furthermore, a HIPE emulsion can be cast onto a flat surface and form a porous membrane [5] or can even be suspended into another phase to prepare porous beads with polyHIPE structure [6]. Various functionalities in a polyHIPE matrix can be used to support reactive species like reagents, catalysts or scavengers. Vinylbenzyl chloride based polyHIPEs have been prepared and chloromethyl groups functionalised to amine and hydroxyl groups and applied as solid phase synthesis support [7]. Such a polyHIPE has also been used in flow-through setups for the facilitation of organic synthesis [8]. Another possibility demonstrated was the use of the remaining double bonds in the polyHIPE matrix [9], or using electrophilic aromatic substitution in the case of polystyrene polyHIPEs [10]. Hydroxyl-functionalised polyHIPEs have proven to be efficient scavengers for acid chlorides and can serve as a linker for attachment via esterification [11]. Piperazine-functionalised polyHIPEs were found to be useful for removal of atrazine from drinking water [12]. PolyHIPE material can also be used for the incorporation of metal catalysts [13]. On the other hand, relatively little is known of polyHIPEs with carboxylic acid functionalities. Acidic moieties can be introduced into polyHIPE material using acrylic acid [14], however oil in water emulsions have to be applied which tend to be more difficult to
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stabilise than water in oil emulsions. Methacrylic acid, on the other hand, is significantly less polar and can be introduced into an oil phase of HIPE. In this paper, we are reporting the use of water-in-oil high internal phase emulsions for the preparation of microcellular, open porous crosslinked poly(methacrylic acid).
2. Experimental 2.1. Materials Styrene (STY, Sigma Aldrich), divinylbenzene (DVB, Aldrich, 80%, tech.) and methacrylic acid (MAA, Merck) were purified by passing through basic alumina (Al2O3, Fluka) to remove the inhibitors. Toluene (TOL, Aldrich), potassium persulfate (KPS, Fluka), azobisisobutyronitrile (AIBN, Fluka), Irgacure 819 (I819, Ciba), N,N,N’,N’-tetramethyl-ethane-1,2-diamine (TEMED, Fluka), calcium chloride hexahydrate (CaCl2 6H2O, Sigma–Aldrich), sorbitan monooleate (Span 80, HLB (Hidrophilicity-Lipophilicity Balance) = 4.3, Fluka), sorbitan trioleate (Span 85, HLB = 1.8, Aldrich), polyoxyethylene sorbitan trioleate (Tween 85, HLB = 11.0, Aldrich), polyethylene/polypropylene glycol (1/9; Pluronic L121, HLB = 0.5, Gerbu), ethanol (Merck), acetone (Sigma–Aldrich), thionyl chloride (SOCl2, Merck), pyridine (Kemika Zagreb), N,N-dimethylformamide (DMF, Carlo Erba Reagenti), nitric acid (Kemika Zagreb), silver nitrate (AgNO3, Kemika Zagreb), 1,12-diaminododecane (Sigma Aldrich) and 1,4-diaminobutane (Acros Organics) were all used as received. Dichloromethane (DCM, Carlo Erba Reagenti) was distilled and stored over molecular sieves. 2.2. Physico-chemical characterisation FTIR spectra were recorded on a Shimadzu IRAffinity-1 spectrometer (KBr pellets), SEM pictures were taken on a Quanta 200 3D (FEI Company) and CHN elemental analyses were done on a Perkin Elmer CHN 2400 analyser. Potentiometric measurements were performed on a Hanna 8818 Microprocessor pH/mV/°C Metre. Nitrogen adsorption/desorption measurements were done on a Micromeritics TriStar II 3020 porosimeter using a BET model for surface area evaluation. 2.3. Preparation of MAA/STY/DVB polyHIPEs Oil phase, consisting of monomers (methacrylic acid, styrene, divinylbenzene), surfactant (sorbitan monooleate, sorbitan trioleate, Pluronic L121 or polyoxyethylene sorbitan trioleate), toluene and initiator (AIBN; in the case of photo polymerised samples I819E) was placed in a reactor (darkened for photo polymerisation). The aqueous phase was prepared separately by dissolving calcium chloride hexahydrate (1.78 g) and initiator (KPS or APS, 0.11 g) in deionised and degassed water (100 mL). An appropriate amount of aqueous phase (to produce emulsions with 90 vol.% of aqueous phase) was added drop wise and under constant stirring at 300 rpm with an overhead stirrer to the organic solution. Once all the aqueous phase had been added, stirring was continued for a further 60 min, to produce a uniform W/O emulsion. The emulsion was transferred into a polypropylene tube or onto a petri dish, covered with a transparent polyethylene (PE) film and exposed to 60 °C for 24 h or UV light for 2 min (Uvitron IntelliRay 600 was used for curing) in the case of photo polymerised samples. The resulting polyHIPE was purified via Soxhlet extraction with ethanol for 24 h and acetone for a further 24 h. The monolith was dried in vacuo at 30 °C for 72 h. A series of eleven formulations were prepared varying the emulsion components according to Table 1.
Nitrogen adsorption/desorption measurements were done using a BET model for surface area evaluation. 2.4. Functionalisation of methacrylic acid polyHIPE (B) The powdered A10 sample (518 mg containing 0.967 mmol of carboxyl groups) was placed in a 50 mL flask fitted with a reflux condenser. 10.0 mL of anhydrous dichloromethane and 2.26 g (19 mmol) of thionyl chloride were added. The mixture was stirred and heated under reflux for 24 h. Polymer was filtered and washed fivefold with a 10 mL portion of anhydrous DCM. The material was dried in vacuo at ambient temperature for 30 min to give acid chloride modified polyHIPE. FTIR spectra were recorded and the amount of chlorine determined (see Table 3). Nitrogen adsorption/desorption measurements were done using a BET model for surface area evaluation. 2.5. Nucleophilic substitution with multifunctional amines Different amounts of polyHIPE sample B (0.460 g for C1; 0.265 g for C2; 0.186 g for C3; 0.385 g for D1; 0.319 g for D2; 0.170 g for D3) were placed into a 50 mL flask. 10 mL of anhydrous dichloromethane and various amounts of diamine (0.431 mmol (0.038 g) of 1,4-diaminobutane for C1; 0.473 mmol (0.0417 g) of 1,4diaminobutane for C2; 0.942 mmol (0.083 g) of 1,4-diaminobutane for C3; 0.339 mmol (0.068 g) of 1,12-diaminododecane for D1; 0.684 mmol (0.137 g) of 1,12-diaminododecane for D2; 0.913 mmol (0.183 g) of 1,12-diaminododecane for D3) were added. The mixture was stirred and mixed for 3.5 h at ambient temperature. The resulting polymer was filtered and washed with anhydrous DCM. The polymer was dried in vacuo at ambient temperature for 30 min. FTIR spectra were recorded, CHN analyses were performed and nitrogen adsorption/desorption measurements were done. 2.6. Chlorine determination Chlorine was determined by potentiometric titration with AgNO3. 42 mg of sample B was placed in a flask and 10.0 mL of N,N-dimethlyformamide and 5.0 mL of pyridine were added. The mixture was stirred and heated for 2 h at 100 °C. After cooling, 7.0 mL of HNO3 was added and the mixture diluted to 100 mL. 20 mL of this mixture was titrated with 0.01 M AgNO3. The amount of chlorine in the sample was calculated from the volumes of AgNO3 used by titration. 2.7. Morphological features of the samples Average pore diameter and interconnecting pore diameter were measured from SEM images. Over 50 measurements of pore diameters were taken from each SEM image and the average value was corrected with a correction factor (2/31/2) to account for irregular cutting of the samples (correction factor was not applied in the case of interconnecting pores). 3. Results and discussion 3.1. Preparation of methacrylic acid based PolyHIPEs Polymer supported acid chloride can serve as a very versatile precursor for functionalisations and thus preparing various functional polymers [15]. It can be prepared by the hydrolysis of polymer supported esters and subsequent reaction with tionyl chloride [16] or by the polymerisation of acrylic acid in aqueous medium and appropriate functionalisation. In the latter case, the procedure
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U. Sevšek, P. Krajnc / Reactive & Functional Polymers 72 (2012) 221–226 Table 1 Compositions of emulsions. Sample A1 A2 A3h A4 A5 A6 A7 A8 A9 A10 A11 a b c d e f g h i j
mol% MAAa
mol% DVBb
mol% STYc
vol.% TOLd
HLBe
INIT.f
24 16 21 12 8 15 20 20 20 20 20
71 79 64 83 46 20 45 30 30 30 30
5 5 5 5 46 65 35 50 50 50 50
15 15 15 15 15 15 15 8 8 8 8
4.30 4.30 4.30 4.30 4.30 4.30 4.30 4.98 4.88 4.86 4.79
AIBN AIBN AIBN AIBN AIBN AIBN AIBN KPS KPS I819E j
mMAA (g)
mDVB (g)
mSTY (g)
mTOL (g)
VAPg mL
mINIT. (g)
Emulsion stability
0.981 0.654 0.878 0.491 0.262 0.452 0.602 0.602 0.602 0.602 0.602
4.452 4.947 3.984 5.195 2.276 0.911 2.050 1.367 1.367 1.376 1.376
0.250 0.250 0.250 0.250 1.821 2.369 1.276 1.823 1.823 1.823 1.823
0.795 0.823 0.845 0.838 0.618 0.562 0.551 0.284 0.284 0.284 0.284
55.02 56.99 58.46 57.97 42.75 36.44 38.16 36.85 36.85 36.85 36.85
0.058 0.059 0.060 0.059 0.044 0.037 0.039 0.110 0.110 0.038
Stable Stable Stable Stable Stable PSi during stirring PS during stirring Stable PS during curing Stable PS during stirring
j
Methacrylic acid. Divinylbenzene. Styrene. Toluene. Hydrophilicity-lipophilicity balance value. Initiator. Aqueous phase. Added 10% EHA (m (EHA) = 0.885 g). Phase separation. Redox initiating (APS (0.11 g) + TEMED (0.037 mg)).
is shortened by two synthetic steps as no ester (monomer) synthesis and hydrolysis are needed. In the case of a polyHIPE preparation based on acrylic acid, oil in water HIPE emulsions have to be used which tend to be more difficult to stabilise than water in oil HIPE emulsions [14]. Efforts of functionalising this kind of material to supported acid chloride have resulted in difficulties with efficiency which were most probably due to site-site interactions within the polymer matrix and the formation of anhydride functionalities. In order to use a more controllable way of polyHIPE preparation, namely water in oil emulsions, methacrylic acid could be used as a reactive monomer. In comparison with acrylic acid, methacrylic acid is significantly less polar and will partition mostly in the organic phase (octanol/water partition coefficient (log P) for acrylic acid is 0.28 while the value for methacrylic acid is 1.33 [17]). For STY/DVB polyHIPEs, usually sorbitan monooleate (Span 80) is used as a stabilising agent. In our case different amounts of methacrylic acid were added to the organic phase, replacing part of styrene, to produce polyHIPEs with carboxylic acid functionalities (see Table 1, Scheme 1). Span 80 sufficiently stabilised the emulsions in the case of samples A1-A5, when up to 24% of methacrylic acid with regards to monomer content was used. With these samples, a high degree of crosslinking (between 37% and 67%, Table 1) was used in order to obtain polyHIPE samples with high rigidity and surface area. However, when reducing the amount of crosslinker (to produce samples with a higher loading of acid groups), Span 80 failed to stabilise the emulsion and phase separation of the emulsion occurred before obtaining the solid polymer (sample A6, 20% DVB, Table 1). Increasing the share of methacrylic acid in the continuous phase most probably caused some transfer of methacrylic acid into the aqueous phase. A combination of surfactants was therefore investigated to obtain a kinetically stable emulsion for polymerisations. For stabilising HIPEs with a more polar oil phase (compared to STY/DVB HIPEs) usually surfactant with a lower HLB value is applied (Span 80 has HLB value of 4.3). For instance, glycidyl methacrylate based HIPEs require a surfactant with a HLB value around 1 [18]. In our study, combinations of Span 85 (HLB = 1.8) and Pluronic L121 (HLB = 0.5) were used to decrease the HLB value. It was found that using a combination of surfactants with HLB value lower than 4.3 did not produce stable enough emulsions for polymerisations (HLB values between 0.5 and 4.3 were tested). On the other hand a slight increase of HLB value by using a combination of Span 80 and Tween 85 (HLB values between 4.85 and 5.01) sufficiently
stabilised emulsions of samples A8, A10 and monolithic samples were obtained, with the amount of methacrylic acid 20 mol%. Another very important factor determining the form of the final product was found to be the initiation of the polymerisation. When using thermal initiators, namely KPS or APS, (samples A9, A11, initiator in aqueous phase) or AIBN (sample A7, initiator in organic phase) emulsion was prone to phase separation during the stirring or curing stage. Thus, either partly nonporous material or partly material with agglomerated beads of polymeric domains inside the cavities (Fig. 1) resulted. It was therefore postulated that a faster curing method would be beneficial for obtaining polymers with polyHIPE type morphology. In the case of sample A10 (Table 1) photo initiator Irgacure 819E in combination with UV light (wavelength 315–400 nm) was used for curing and monolith with typical polyHIPE morphology was obtained (Fig. 2). The average diameter of cavities in the sample A10 is 31.6 lm and average diameter of interconnecting pores is 3.95 lm. Nitrogen adsorption/desorption was used to determine the dry surface area of the product. Use of BET model resulted in the surface area of 46 m2/g, which is relatively high compared to styrene based polyHIPEs. This is most likely due to a high degree of crosslinking (24% DVB), creating pores in the meso range. Samples with a higher degree of crosslinking (63–37%) exhibited an even substantially higher surface area, namely between 186 m2/g and 82 m2/g (samples A3 and A5). The results confirmed the influence of the crosslinking degree on the dry surface area (Table 2). 3.2. Functionalisation of the samples For obtaining polymer supported acid chloride, sample A10 was chosen for functionalisation with thionyl chloride. Most common problems with functionalisation of polymer supports reducing the efficiency of functionalisation are reactive site inaccessibility and site-site interactions forming an undesired product [19]. Solvent for functionalisation of carboxylic acid functionalities to acid chloride must be aprotic and on the other hand compatible with the crosslinked polymer to enable the contact with reactive sites. Dichloromethane proved to be an appropriate solvent and sample B was obtained with 6.0% Cl (determined with potentiometric titration). This corresponds to approximately 76% of functionalised carboxylic groups as 7.9% of chloride would be found for complete conversion. Reactions were monitored by FTIR spectroscopy
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Fig. 1. SEM image of sample A8.
Fig. 2. SEM image of sample A10. Scheme 1. Preparation of a polyHIPE polymer and its functionalisation. Table 2 Influence of crosslinking degree on morphology and surface area.
(Fig. 3). Spectrum of unfunctionalised sample (Fig. 3, spectrum a) revealed a carbonyl peak at 1695 cm 1 which confirms incorporation of methacrylic acid into the polymer chain. After functionalisation the presence of acid chloride was confirmed by absorption peak around 1800 cm 1 (Fig. 3, spectrum b). Reactivity of B to nucleophiles was tested by using multifunctional amines. It had been shown that multifunctional amines can react with more than one amine group and therefore cause additional crosslinking [20]. Since the length of multifunctional amine chain and its concentration in the reaction medium can influence the structure of functionalised product (additional crosslinking), two different chain lengths of diamine, namely 1,4-diaminobutane and 1,12diaminododecane, and three different molar ratios of amine to acid chloride (1:1, 1:3, 1:0.5; Table 3) were used. In all cases transformations to amides took place. FTIR spectroscopy showed the complete disappearance of acid chloride carbonyl signal at 1800 cm 1 and on the other hand a signal for amide carbonyl at 1656 cm 1 appeared (Fig. 3, spectrum c). The amount of amide or amine groups in the polymer matrix was determined from elemental
a b
Sample
Degree of crosslinking (mol% DVB)
Da (lm)
db (lm)
d/D
BET (m2/g)
A2 A3 A5 A10
63 51 37 24
26.6 9.0 37.9 31.6
4.6 1.2 6.5 4.0
0.17 0.14 0.17 0.11
100.4 185.9 82.0 46.4
Cavity diameter. Interconnecting pore diameter.
analyses and lower percentages of nitrogen than calculated were found for all samples. Since no acid chloride was present after the functionalisation, lower amounts of nitrogen than calculated suggest that, to some extent, both amine groups of diamine reacted with acid chloride groups in the polymer matrix. This has been demonstrated already with some other polymers [20]. Concentration of the reagent within the reaction mixture can also influence site-site interactions. In the case of 1,12-diaminododecane, lowering the concentration of amine resulted in a higher degree of
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Fig. 3. FTIR spectra of polymerised and functionalised samples: (a) A10, (b) B, (c) C1.
Table 3 Elemental analyses data and loading of reactive groups. Samplea
mol%
BET
Chlorine
Nitrogen
Foundb
B1 C1 C2 C3 D1 D2 D3 a b c
0:0.5 1:1 1:3 0:0.5 1:1 1:3
Loading of amine gr.
Foundc
Calculated
m2/g
%
mmol/g
%
mmol/g
46.4 46.3 31.9 32.0 39.9 25.3 25.7
6.0
0.071
7.9
0.094
% 2.94 2.68 2.84 2.38 2.78 3.10
Calculated mmol/g 0.264 0.277 0.206 0.212 0.411 0.732
% 4.57 4.57 4.57 3.86 3.86 3.86
mmol/g
mmol/g
0.411 0.473 0.332 0.344 0.570 0.911
/ 2.1 1.9 2.0 1.7 2.0 2.2
B denotes thionyl chloride functionalised sample, C denotes 1,4-diaminobutane functionalised samples, D denotes 1,12-diaminododecane functionalised samples. By potentiometric titration. From CHN analyses.
additional crosslinking (Table 3), while in the case of a shorter chain length (1,4-diaminobutane) the concentration did not have a profound effect on the structure of the product. In the case of a shorter amine chain, the reaction of the second amine group with an adjacent polymer chain or site is obviously too hindered to take place. 4. Conclusions We have demonstrated that methacrylic acid can be successfully incorporated into polymer matrix of a highly porous open cellular polyHIPE material. Emulsion templating technique with the reactive monomer in the organic phase can be applied to yield polyHIPEs with accessible acid functionalities and varying degree of crosslinking which influences the surface area of the material. Acid groups can be easily functionalised into a very reactive acid chloride functionalities and further with nucleophiles. This opens opportunities for applications of such polyHIPE material. Acknowledgments Financial support of the Slovenian Research Agency (scholarship to Urška Sevšek, Contract No. 1000-08-310094) is gratefully acknowledged. The study was partly financed by Project RRP6 in
the framework of Centre of Excellence PoliMaT (Contract No. 3211-10-000057). References [1] (a) S.D. Kimmins, N.R. Cameron, Adv. Funct. Mater. 21 (2011) 211–225; (b) N.R. Cameron, P. Krajnc, M.S. Silverstein, Colloidal Templating, in: M.S. Silverstein, N.R. Cameron, M.A. Hillmyer (Eds.), Porous Polymers, John Wiley & Sons, Inc., Hoboken, New Jersey, 2011, pp. 119–173. [2] K.J. Lissant (Ed.), Emulsions and Emulsion Technology, Part 1, New York, 1974, (Chap. 1). [3] N.R. Cameron, A. Barbetta, J. Mater. Chem. 10 (2000) 2466–2471. [4] I. Pulko, J. Wall, P. Krajnc, N.R. Cameron, Chem. Eur. J. 16 (2010) 2350–2354. [5] (a) C. Zhao, E. Danish, N.R. Cameron, R. Kataky, J. Mater. Chem. 17 (2007) 2446– 2453; (b) I. Pulko, P. Krajnc, Chem. Commun. 37 (2008) 4481–4483; (c) U. Sevsek, S. Seifried, C. Stropnik, I. Pulko, P. Krajnc, Materiali in Tehnologije 45 (2011) 247–251; (d) I. Pulko, V. Smrekar, A. Podgornik, P. Krajnc, J. Chromatogr. A 1218 (2011) 2396–2401. [6] (a) A. Desforges, M. Arpontet, H. Deleuze, O. Mondain-Monval, React. Funct. Polym. 53 (2002) 183–192; (b) D. Stefanec, P. Krajnc, React. Funct. Polym. 65 (2005) 37–45; (c) D. Stefanec, P. Krajnc, Polym. Int. 56 (2007) 1313–1319. [7] (a) A. Barbetta, N.R. Cameron, S.J. Cooper, Chem. Commun. (2000) 221–222; (b) J. F Brown, P. Krajnc, N.R. Cameron, Ind. Eng. Chem. Res. 44 (2005) 8565– 8572. [8] (a) A. Mercier, H. Deleuze, O. Mondain-Monval, React. Funct. Polym. 46 (2000) 67–79; (b) P. Krajnc, J.F. Brown, N.R. Cameron, Org. Lett. 4 (2002) 2497–2500; (c) S. Kovacic, P. Krajnc, J. Polym, Part A: Polym. Chem. 47 (2009) 6726–6734.
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Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Methacrylic acid microcellular highly porous monoliths: Preparation and functionalisation Urška Sevšek a, Peter Krajnc a,b,⇑ a b
University of Maribor, Faculty of Chemistry and Chemical Engineering, Smetanova 17, SI-2000 Maribor, Slovenia Centre of Excellence PoliMaT, Tehnološki park 24, SI-1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 25 November 2011 Received in revised form 24 January 2012 Accepted 14 February 2012 Available online 23 February 2012 Keywords: Poly(methacrylic acid) PolyHIPE Porous polymers Emulsions Functional polymers
a b s t r a c t By polymerisation of high internal phase emulsions (HIPEs), containing styrene (STY), divinylbenzene (DVB) and methacrylic acid (MAA) in the continuous phase, highly porous polymers including carboxylic functional groups were prepared. The ratio of methacrylic acid to divinylbenzene was varied in order to obtain polyHIPEs with a different degree of crosslinking which influenced a surface area of the polymers, being substantially higher (185 m2/g) with a higher degree of crosslinking (51% DVB) than with a lower degree of crosslinking (24% DVB, 46 m2/g). Up to 90% porous samples were prepared and the optimum hidrophilicity-lipophilicity balance (HLB) of the surfactant was found to be around 4.8–4.9. Both thermal and photo initiation were used to induce polymerisation. The resulting polymers had an open cellular morphology with cavity diameters between 21.8 lm and 44.2 lm and with interconnecting pores between 2.2 lm and 5.0 lm. Monolithic supports were used for further functionalisation with thionyl chloride and multifunctional amines, namely 1,4-diaminobutane and 1,12-diaminododecane. The functionalisation degree with thionyl chloride was 76%. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Emulsion templating using high internal phase emulsions is now an established technique for the preparation of microcellular highly porous monoliths with an interconnected porous structure [1]. According to most definitions, high internal phase emulsions (HIPEs) have a volume fraction of internal phase higher than 74% which represents a maximum space occupied by uniform spheres [2]. Polymerisation of the continuous phase of HIPE results in a solid polymer which is a replica of the precursor emulsion, having large pores (cavities, sometimes referred to as voids) in place of the droplets of internal phase which normally does not include monomers. These pores are connected with a number of smaller interconnecting pores (sometimes referred to as windows). With the use of porogenic solvent a third degree of pores at a nanometre scale can be formed, significantly increasing the surface area of polyHIPE. Generally rather low surface area can be increased by a higher degree of crosslinking [3] or by post polymerisation functionalisation inducing additional crosslinking [4]. By tuning the preparation conditions and monomer content, polyHIPE material with both high surface area and open interconnecting structure ⇑ Corresponding author at: University of Maribor, Faculty of Chemistry and Chemical Engineering, Smetanova 17, SI-2000 Maribor, Slovenia. Tel.: +386 2 2294422; fax: +386 2 2527774. E-mail address: [email protected] (P. Krajnc). 1381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2012.02.007
with larger pores can be obtained, giving the material functional properties like low density, high permeability, appropriate surface area, etc. Another advantageous property of the HIPE emulsion is that it can be moulded into various shapes, e.g. tubes, discs, cones. Furthermore, a HIPE emulsion can be cast onto a flat surface and form a porous membrane [5] or can even be suspended into another phase to prepare porous beads with polyHIPE structure [6]. Various functionalities in a polyHIPE matrix can be used to support reactive species like reagents, catalysts or scavengers. Vinylbenzyl chloride based polyHIPEs have been prepared and chloromethyl groups functionalised to amine and hydroxyl groups and applied as solid phase synthesis support [7]. Such a polyHIPE has also been used in flow-through setups for the facilitation of organic synthesis [8]. Another possibility demonstrated was the use of the remaining double bonds in the polyHIPE matrix [9], or using electrophilic aromatic substitution in the case of polystyrene polyHIPEs [10]. Hydroxyl-functionalised polyHIPEs have proven to be efficient scavengers for acid chlorides and can serve as a linker for attachment via esterification [11]. Piperazine-functionalised polyHIPEs were found to be useful for removal of atrazine from drinking water [12]. PolyHIPE material can also be used for the incorporation of metal catalysts [13]. On the other hand, relatively little is known of polyHIPEs with carboxylic acid functionalities. Acidic moieties can be introduced into polyHIPE material using acrylic acid [14], however oil in water emulsions have to be applied which tend to be more difficult to
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stabilise than water in oil emulsions. Methacrylic acid, on the other hand, is significantly less polar and can be introduced into an oil phase of HIPE. In this paper, we are reporting the use of water-in-oil high internal phase emulsions for the preparation of microcellular, open porous crosslinked poly(methacrylic acid).
2. Experimental 2.1. Materials Styrene (STY, Sigma Aldrich), divinylbenzene (DVB, Aldrich, 80%, tech.) and methacrylic acid (MAA, Merck) were purified by passing through basic alumina (Al2O3, Fluka) to remove the inhibitors. Toluene (TOL, Aldrich), potassium persulfate (KPS, Fluka), azobisisobutyronitrile (AIBN, Fluka), Irgacure 819 (I819, Ciba), N,N,N’,N’-tetramethyl-ethane-1,2-diamine (TEMED, Fluka), calcium chloride hexahydrate (CaCl2 6H2O, Sigma–Aldrich), sorbitan monooleate (Span 80, HLB (Hidrophilicity-Lipophilicity Balance) = 4.3, Fluka), sorbitan trioleate (Span 85, HLB = 1.8, Aldrich), polyoxyethylene sorbitan trioleate (Tween 85, HLB = 11.0, Aldrich), polyethylene/polypropylene glycol (1/9; Pluronic L121, HLB = 0.5, Gerbu), ethanol (Merck), acetone (Sigma–Aldrich), thionyl chloride (SOCl2, Merck), pyridine (Kemika Zagreb), N,N-dimethylformamide (DMF, Carlo Erba Reagenti), nitric acid (Kemika Zagreb), silver nitrate (AgNO3, Kemika Zagreb), 1,12-diaminododecane (Sigma Aldrich) and 1,4-diaminobutane (Acros Organics) were all used as received. Dichloromethane (DCM, Carlo Erba Reagenti) was distilled and stored over molecular sieves. 2.2. Physico-chemical characterisation FTIR spectra were recorded on a Shimadzu IRAffinity-1 spectrometer (KBr pellets), SEM pictures were taken on a Quanta 200 3D (FEI Company) and CHN elemental analyses were done on a Perkin Elmer CHN 2400 analyser. Potentiometric measurements were performed on a Hanna 8818 Microprocessor pH/mV/°C Metre. Nitrogen adsorption/desorption measurements were done on a Micromeritics TriStar II 3020 porosimeter using a BET model for surface area evaluation. 2.3. Preparation of MAA/STY/DVB polyHIPEs Oil phase, consisting of monomers (methacrylic acid, styrene, divinylbenzene), surfactant (sorbitan monooleate, sorbitan trioleate, Pluronic L121 or polyoxyethylene sorbitan trioleate), toluene and initiator (AIBN; in the case of photo polymerised samples I819E) was placed in a reactor (darkened for photo polymerisation). The aqueous phase was prepared separately by dissolving calcium chloride hexahydrate (1.78 g) and initiator (KPS or APS, 0.11 g) in deionised and degassed water (100 mL). An appropriate amount of aqueous phase (to produce emulsions with 90 vol.% of aqueous phase) was added drop wise and under constant stirring at 300 rpm with an overhead stirrer to the organic solution. Once all the aqueous phase had been added, stirring was continued for a further 60 min, to produce a uniform W/O emulsion. The emulsion was transferred into a polypropylene tube or onto a petri dish, covered with a transparent polyethylene (PE) film and exposed to 60 °C for 24 h or UV light for 2 min (Uvitron IntelliRay 600 was used for curing) in the case of photo polymerised samples. The resulting polyHIPE was purified via Soxhlet extraction with ethanol for 24 h and acetone for a further 24 h. The monolith was dried in vacuo at 30 °C for 72 h. A series of eleven formulations were prepared varying the emulsion components according to Table 1.
Nitrogen adsorption/desorption measurements were done using a BET model for surface area evaluation. 2.4. Functionalisation of methacrylic acid polyHIPE (B) The powdered A10 sample (518 mg containing 0.967 mmol of carboxyl groups) was placed in a 50 mL flask fitted with a reflux condenser. 10.0 mL of anhydrous dichloromethane and 2.26 g (19 mmol) of thionyl chloride were added. The mixture was stirred and heated under reflux for 24 h. Polymer was filtered and washed fivefold with a 10 mL portion of anhydrous DCM. The material was dried in vacuo at ambient temperature for 30 min to give acid chloride modified polyHIPE. FTIR spectra were recorded and the amount of chlorine determined (see Table 3). Nitrogen adsorption/desorption measurements were done using a BET model for surface area evaluation. 2.5. Nucleophilic substitution with multifunctional amines Different amounts of polyHIPE sample B (0.460 g for C1; 0.265 g for C2; 0.186 g for C3; 0.385 g for D1; 0.319 g for D2; 0.170 g for D3) were placed into a 50 mL flask. 10 mL of anhydrous dichloromethane and various amounts of diamine (0.431 mmol (0.038 g) of 1,4-diaminobutane for C1; 0.473 mmol (0.0417 g) of 1,4diaminobutane for C2; 0.942 mmol (0.083 g) of 1,4-diaminobutane for C3; 0.339 mmol (0.068 g) of 1,12-diaminododecane for D1; 0.684 mmol (0.137 g) of 1,12-diaminododecane for D2; 0.913 mmol (0.183 g) of 1,12-diaminododecane for D3) were added. The mixture was stirred and mixed for 3.5 h at ambient temperature. The resulting polymer was filtered and washed with anhydrous DCM. The polymer was dried in vacuo at ambient temperature for 30 min. FTIR spectra were recorded, CHN analyses were performed and nitrogen adsorption/desorption measurements were done. 2.6. Chlorine determination Chlorine was determined by potentiometric titration with AgNO3. 42 mg of sample B was placed in a flask and 10.0 mL of N,N-dimethlyformamide and 5.0 mL of pyridine were added. The mixture was stirred and heated for 2 h at 100 °C. After cooling, 7.0 mL of HNO3 was added and the mixture diluted to 100 mL. 20 mL of this mixture was titrated with 0.01 M AgNO3. The amount of chlorine in the sample was calculated from the volumes of AgNO3 used by titration. 2.7. Morphological features of the samples Average pore diameter and interconnecting pore diameter were measured from SEM images. Over 50 measurements of pore diameters were taken from each SEM image and the average value was corrected with a correction factor (2/31/2) to account for irregular cutting of the samples (correction factor was not applied in the case of interconnecting pores). 3. Results and discussion 3.1. Preparation of methacrylic acid based PolyHIPEs Polymer supported acid chloride can serve as a very versatile precursor for functionalisations and thus preparing various functional polymers [15]. It can be prepared by the hydrolysis of polymer supported esters and subsequent reaction with tionyl chloride [16] or by the polymerisation of acrylic acid in aqueous medium and appropriate functionalisation. In the latter case, the procedure
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U. Sevšek, P. Krajnc / Reactive & Functional Polymers 72 (2012) 221–226 Table 1 Compositions of emulsions. Sample A1 A2 A3h A4 A5 A6 A7 A8 A9 A10 A11 a b c d e f g h i j
mol% MAAa
mol% DVBb
mol% STYc
vol.% TOLd
HLBe
INIT.f
24 16 21 12 8 15 20 20 20 20 20
71 79 64 83 46 20 45 30 30 30 30
5 5 5 5 46 65 35 50 50 50 50
15 15 15 15 15 15 15 8 8 8 8
4.30 4.30 4.30 4.30 4.30 4.30 4.30 4.98 4.88 4.86 4.79
AIBN AIBN AIBN AIBN AIBN AIBN AIBN KPS KPS I819E j
mMAA (g)
mDVB (g)
mSTY (g)
mTOL (g)
VAPg mL
mINIT. (g)
Emulsion stability
0.981 0.654 0.878 0.491 0.262 0.452 0.602 0.602 0.602 0.602 0.602
4.452 4.947 3.984 5.195 2.276 0.911 2.050 1.367 1.367 1.376 1.376
0.250 0.250 0.250 0.250 1.821 2.369 1.276 1.823 1.823 1.823 1.823
0.795 0.823 0.845 0.838 0.618 0.562 0.551 0.284 0.284 0.284 0.284
55.02 56.99 58.46 57.97 42.75 36.44 38.16 36.85 36.85 36.85 36.85
0.058 0.059 0.060 0.059 0.044 0.037 0.039 0.110 0.110 0.038
Stable Stable Stable Stable Stable PSi during stirring PS during stirring Stable PS during curing Stable PS during stirring
j
Methacrylic acid. Divinylbenzene. Styrene. Toluene. Hydrophilicity-lipophilicity balance value. Initiator. Aqueous phase. Added 10% EHA (m (EHA) = 0.885 g). Phase separation. Redox initiating (APS (0.11 g) + TEMED (0.037 mg)).
is shortened by two synthetic steps as no ester (monomer) synthesis and hydrolysis are needed. In the case of a polyHIPE preparation based on acrylic acid, oil in water HIPE emulsions have to be used which tend to be more difficult to stabilise than water in oil HIPE emulsions [14]. Efforts of functionalising this kind of material to supported acid chloride have resulted in difficulties with efficiency which were most probably due to site-site interactions within the polymer matrix and the formation of anhydride functionalities. In order to use a more controllable way of polyHIPE preparation, namely water in oil emulsions, methacrylic acid could be used as a reactive monomer. In comparison with acrylic acid, methacrylic acid is significantly less polar and will partition mostly in the organic phase (octanol/water partition coefficient (log P) for acrylic acid is 0.28 while the value for methacrylic acid is 1.33 [17]). For STY/DVB polyHIPEs, usually sorbitan monooleate (Span 80) is used as a stabilising agent. In our case different amounts of methacrylic acid were added to the organic phase, replacing part of styrene, to produce polyHIPEs with carboxylic acid functionalities (see Table 1, Scheme 1). Span 80 sufficiently stabilised the emulsions in the case of samples A1-A5, when up to 24% of methacrylic acid with regards to monomer content was used. With these samples, a high degree of crosslinking (between 37% and 67%, Table 1) was used in order to obtain polyHIPE samples with high rigidity and surface area. However, when reducing the amount of crosslinker (to produce samples with a higher loading of acid groups), Span 80 failed to stabilise the emulsion and phase separation of the emulsion occurred before obtaining the solid polymer (sample A6, 20% DVB, Table 1). Increasing the share of methacrylic acid in the continuous phase most probably caused some transfer of methacrylic acid into the aqueous phase. A combination of surfactants was therefore investigated to obtain a kinetically stable emulsion for polymerisations. For stabilising HIPEs with a more polar oil phase (compared to STY/DVB HIPEs) usually surfactant with a lower HLB value is applied (Span 80 has HLB value of 4.3). For instance, glycidyl methacrylate based HIPEs require a surfactant with a HLB value around 1 [18]. In our study, combinations of Span 85 (HLB = 1.8) and Pluronic L121 (HLB = 0.5) were used to decrease the HLB value. It was found that using a combination of surfactants with HLB value lower than 4.3 did not produce stable enough emulsions for polymerisations (HLB values between 0.5 and 4.3 were tested). On the other hand a slight increase of HLB value by using a combination of Span 80 and Tween 85 (HLB values between 4.85 and 5.01) sufficiently
stabilised emulsions of samples A8, A10 and monolithic samples were obtained, with the amount of methacrylic acid 20 mol%. Another very important factor determining the form of the final product was found to be the initiation of the polymerisation. When using thermal initiators, namely KPS or APS, (samples A9, A11, initiator in aqueous phase) or AIBN (sample A7, initiator in organic phase) emulsion was prone to phase separation during the stirring or curing stage. Thus, either partly nonporous material or partly material with agglomerated beads of polymeric domains inside the cavities (Fig. 1) resulted. It was therefore postulated that a faster curing method would be beneficial for obtaining polymers with polyHIPE type morphology. In the case of sample A10 (Table 1) photo initiator Irgacure 819E in combination with UV light (wavelength 315–400 nm) was used for curing and monolith with typical polyHIPE morphology was obtained (Fig. 2). The average diameter of cavities in the sample A10 is 31.6 lm and average diameter of interconnecting pores is 3.95 lm. Nitrogen adsorption/desorption was used to determine the dry surface area of the product. Use of BET model resulted in the surface area of 46 m2/g, which is relatively high compared to styrene based polyHIPEs. This is most likely due to a high degree of crosslinking (24% DVB), creating pores in the meso range. Samples with a higher degree of crosslinking (63–37%) exhibited an even substantially higher surface area, namely between 186 m2/g and 82 m2/g (samples A3 and A5). The results confirmed the influence of the crosslinking degree on the dry surface area (Table 2). 3.2. Functionalisation of the samples For obtaining polymer supported acid chloride, sample A10 was chosen for functionalisation with thionyl chloride. Most common problems with functionalisation of polymer supports reducing the efficiency of functionalisation are reactive site inaccessibility and site-site interactions forming an undesired product [19]. Solvent for functionalisation of carboxylic acid functionalities to acid chloride must be aprotic and on the other hand compatible with the crosslinked polymer to enable the contact with reactive sites. Dichloromethane proved to be an appropriate solvent and sample B was obtained with 6.0% Cl (determined with potentiometric titration). This corresponds to approximately 76% of functionalised carboxylic groups as 7.9% of chloride would be found for complete conversion. Reactions were monitored by FTIR spectroscopy
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Fig. 1. SEM image of sample A8.
Fig. 2. SEM image of sample A10. Scheme 1. Preparation of a polyHIPE polymer and its functionalisation. Table 2 Influence of crosslinking degree on morphology and surface area.
(Fig. 3). Spectrum of unfunctionalised sample (Fig. 3, spectrum a) revealed a carbonyl peak at 1695 cm 1 which confirms incorporation of methacrylic acid into the polymer chain. After functionalisation the presence of acid chloride was confirmed by absorption peak around 1800 cm 1 (Fig. 3, spectrum b). Reactivity of B to nucleophiles was tested by using multifunctional amines. It had been shown that multifunctional amines can react with more than one amine group and therefore cause additional crosslinking [20]. Since the length of multifunctional amine chain and its concentration in the reaction medium can influence the structure of functionalised product (additional crosslinking), two different chain lengths of diamine, namely 1,4-diaminobutane and 1,12diaminododecane, and three different molar ratios of amine to acid chloride (1:1, 1:3, 1:0.5; Table 3) were used. In all cases transformations to amides took place. FTIR spectroscopy showed the complete disappearance of acid chloride carbonyl signal at 1800 cm 1 and on the other hand a signal for amide carbonyl at 1656 cm 1 appeared (Fig. 3, spectrum c). The amount of amide or amine groups in the polymer matrix was determined from elemental
a b
Sample
Degree of crosslinking (mol% DVB)
Da (lm)
db (lm)
d/D
BET (m2/g)
A2 A3 A5 A10
63 51 37 24
26.6 9.0 37.9 31.6
4.6 1.2 6.5 4.0
0.17 0.14 0.17 0.11
100.4 185.9 82.0 46.4
Cavity diameter. Interconnecting pore diameter.
analyses and lower percentages of nitrogen than calculated were found for all samples. Since no acid chloride was present after the functionalisation, lower amounts of nitrogen than calculated suggest that, to some extent, both amine groups of diamine reacted with acid chloride groups in the polymer matrix. This has been demonstrated already with some other polymers [20]. Concentration of the reagent within the reaction mixture can also influence site-site interactions. In the case of 1,12-diaminododecane, lowering the concentration of amine resulted in a higher degree of
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Fig. 3. FTIR spectra of polymerised and functionalised samples: (a) A10, (b) B, (c) C1.
Table 3 Elemental analyses data and loading of reactive groups. Samplea
mol%
BET
Chlorine
Nitrogen
Foundb
B1 C1 C2 C3 D1 D2 D3 a b c
0:0.5 1:1 1:3 0:0.5 1:1 1:3
Loading of amine gr.
Foundc
Calculated
m2/g
%
mmol/g
%
mmol/g
46.4 46.3 31.9 32.0 39.9 25.3 25.7
6.0
0.071
7.9
0.094
% 2.94 2.68 2.84 2.38 2.78 3.10
Calculated mmol/g 0.264 0.277 0.206 0.212 0.411 0.732
% 4.57 4.57 4.57 3.86 3.86 3.86
mmol/g
mmol/g
0.411 0.473 0.332 0.344 0.570 0.911
/ 2.1 1.9 2.0 1.7 2.0 2.2
B denotes thionyl chloride functionalised sample, C denotes 1,4-diaminobutane functionalised samples, D denotes 1,12-diaminododecane functionalised samples. By potentiometric titration. From CHN analyses.
additional crosslinking (Table 3), while in the case of a shorter chain length (1,4-diaminobutane) the concentration did not have a profound effect on the structure of the product. In the case of a shorter amine chain, the reaction of the second amine group with an adjacent polymer chain or site is obviously too hindered to take place. 4. Conclusions We have demonstrated that methacrylic acid can be successfully incorporated into polymer matrix of a highly porous open cellular polyHIPE material. Emulsion templating technique with the reactive monomer in the organic phase can be applied to yield polyHIPEs with accessible acid functionalities and varying degree of crosslinking which influences the surface area of the material. Acid groups can be easily functionalised into a very reactive acid chloride functionalities and further with nucleophiles. This opens opportunities for applications of such polyHIPE material. Acknowledgments Financial support of the Slovenian Research Agency (scholarship to Urška Sevšek, Contract No. 1000-08-310094) is gratefully acknowledged. The study was partly financed by Project RRP6 in
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