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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
Highly Porous Cationic Polyelectrolytes via Oil-in-Water Concentrated Emulsions: Synthesis and Adsorption Kinetic Study Sebastijan Kovacic, Nina Drasinac, Albin Pintar, and Ema Žagar Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01645 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018
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Langmuir
Highly Porous Cationic Polyelectrolytes via Oil-in-Water Concentrated Emulsions: Synthesis and Adsorption Kinetic Study Sebastijan Kovačič,*a,b Nina Drašinac,c Albin Pintar,c Ema Žagara
a
Department of Polymer Chemistry and Technology, National Institute of Chemistry, Hajdrihova 19, 1000
Ljubljana, Slovenia b
Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, SI-2000 Maribor,
Slovenia c
Department of Environmental Sciences and Engineering, National Institute of Chemistry, Hajdrihova 19, 1000
Ljubljana, Slovenia
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Abstract This work merges the fields of highly porous polymers (PolyHIPEs) and synthetic cationic polyelectrolytes, and introduces a new approach toward the synthesis of highly porous cationic polyelectrolytes. Cationic polyelectrolytes based on (3-acrylamidopropyl)-trimethylammonium chloride (AMPTMA) were synthesized directly through the oil-in-water (O/W) high internal phase emulsions (HIPEs). The resulting polyelectrolyte-based polyHIPEs are distinguished by the highly porous morphology as well as high concentration and accessibility of the cationic Nquaternized functional groups. The most efficient AMPTMA-based polyelectrolyte polyHIPE exhibits the total ion exchange capacity of 3.53 mmol of AgNO3 per gram of dry resin and water uptake of up to 95 g·g-1, which is a great improvement as compared to the state-of-the-art of polyHIPE absorbents bearing cationic moieties. Results of erythrosine dye adsorption show that chemisorption is a rate-determining step since adsorption follows the pseudo-second-order kinetic model. Multi-linearity of the Weber and Morris plots assumes that more than one regime is involved in the diffusion of the erythrosine dye molecules into the polyHIPE structure with the diffusion in-between the swollen polymer chains as a rate limiting step. Keywords: high internal phase emulsion (HIPE), cationic polyelectrolyte, adsorption kinetics, (3-acrylamidopropyl)-trimethylammonium chloride (AMPTMA)
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Introduction Polyelectrolyte hydrogels are crosslinked macromolecules composed of the electrolyte repeating units. They have vast applications ranging from coatings to biomedical materials.1 Among them, cationic polyelectrolytes are widely used in water treatment purposes since a great part of water contaminants (e.g. clays, metal ions/oxides, detergents, dyes, greases) are negatively charged.2 Moreover, cationic-based polyelectrolyte hydrogels are essential in the separation processes as well as in biomedical applications.3 Commonly used synthetic cationic polyelectrolytes include polymers with ammonium,4 phosphonium,5 and imidazolium6 moieties. Those based on the quaternary nitrogen structure can be easily synthesized and exhibit high hydrophilicity as well as adequate chemical and thermal stability for aqueous applications.7 They are mostly synthesized through the post-polymerization modification approaches, whereby the tertiary amine-based polymeric precursors are reacted with the alkyl halides in the so-called quaternization reaction, creating the quaternized nitrogen atom (−NR$# ) where R is usually an alkyl group.7 However, the quaternization reaction demands a solvent with a high dielectric constant in order to be able to dissolve all the reaction components, i.e. the initial reactive polymer, alkylating agent, and modified polymer.8 These conditions are often difficult to fulfill simultaneously. Hence, direct synthesis via polymerization of the vinyl monomers bearing quaternary nitrogen, either on aromatic- / heterocyclic- or acrylic- / methacrylic- moiety, is a way to avoid the post-polymerization functionalization and to attain 100 % functionality. Different direct synthetic routes to synthesize well-defined cationic polyelectrolyte hydrogels with the high charge density have been described in the literature such as the solution polymerization, precipitation copolymerization, suspension polymerization or emulsion polymerization.7,8 Herein, we disclose a high internal phase emulsion (HIPE) polymerization aiming at the preparation of highly porous N-quaternized polyelectrolytes as a new format of cationic 3 ACS Paragon Plus Environment
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polyelectrolytes. Polymerized HIPEs,9 the so-called polyHIPEs, are distinguished by the highly interconnected macroporous structure, thus allowing for the convective mass transfer through the polyHIPE structure and large fluid-to-surface contact area, respectively.10 Moreover, polyHIPEs are characterized by a high pore volume (up to 10 cm3·g−1),11 permitting polyHIPEs to rapidly absorb large quantities of liquid through a capillary action. Nevertheless, polyHIPE foams prepared through emulsion templating possess relatively low dry specific surface area, typically between 5−20 m2·g−1. Reason lies in low amount of micro/mesopores, however, to increase the volume of sub-microcellular pores additional treatments such as solid-state12 or solution-based13 hypercrosslinking, and addition of inert porogens14 or microporous nanoparticles15,16 have been performed. Hydrophilic polyHIPE absorbents have received increasing attention in recent years as the potential ion-exchange materials for the removal of water contaminants such as heavy metals.17,18,19 However, significant improvement in this respect represents the so-called polyelectrolyte polyHIPE absorbents composed of the backbone based on the ionically charged repeating units synthesized through the oil-in-water (O/W) HIPEs. Due to a synergy between the electrostatic interactions and inherent hydrophilic properties, the polyelectrolyte-based polyHIPE absorbents are distinguished by the high dye contaminant removal efficiency and high adsorption kinetics in water.20 Moreover, electrostatic interactions offer a variety of ways to improve the mechanical properties of basically fragile monolithic polyelectrolyte-based polyHIPEs.21,22 Using oil-in-water (O/W) HIPEs as the templates, also other hydrophilic polyHIPEs were prepared, e.g. based on acrylamide,23-26 acrylic acid,27 2-hydroxyethyl methacrylate,28,29 gelatin,30-32 1-vinyl-5-aminotetrazole,33,34 Nisopropyl acrylamide,35,36 styrene sulfonate,37 poly(vinyl alcohol),38 alginate39 chitosan,40 glycidyl methacrylate,41 and
the copolymers containing methacrylic acid,42 N-isopropyl
acrylamide,43 or acrylamide44.
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In this paper, we report on a direct synthetic approach to prepare the highly porous cationic polyelectrolyte-based absorbents via O/W HIPE polymerization. So far, polyHIPE absorbents bearing cationic moieties have been produced post-synthetically via a conventional chemical functionalization of the originally non-ionic and inherently hydrophobic precursors. Such prepared polyHIPE absorbents exhibit only moderate contaminant and water adsorption capacities.45,46,47 Direct synthesis via polymerization of the N-quaternized acrylate monomers as will be disclosed herein is a step forward since it is expected that due to a combination of inherently hydrophilic properties and ionically charged pendant groups in a single piece material such cationic polyelectrolyte-based polyHIPEs will show significantly higher ion exchange capacity and water-uptake kinetics. The effect of direct synthetic approach on the HIPE stability as well as the porous structure, ion exchange properties, water absorption, and organic dye adsorption kinetics of the (3-acrylamidopropyl)-trimethylammonium chloride (AMPTMA)-based polyHIPEs will be discussed. Experimental Materials (3-acrylamidopropyl)-trimethylammonium chloride (AMPTMA; 75 wt. % in H2O; SigmaAldrich), methylene bis-acrylamide (MBAAm; Sigma-Aldrich), poly(ethylene oxide)-blockpoly(propylene oxide)-block-poly(ethylene oxide); MW = 12,600 g·mol-1; the so-called Pluronic F-127; Sigma-Aldrich), poly(ethylene glycol)-block-poly(propylene glycol)-blockpoly(ethylene glycol); MW = 14,600 g·mol-1; the so-called Pluronic F-108; Sigma-Aldrich), poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol); MW = 8,400 g mol-1; the so-called Pluronic F-68; Sigma-Aldrich), ammonium persulfate (APS, Fluka), N,N,N',N'-tetramethylethylenediamine (TMEDA; Sigma-Aldrich), ethanol (Sigma-Aldrich), diethyl ether (Merck), toluene (Merck), disodium 2-(2,4,5,7-tetraiodo-3-oxido-6-oxoxanthen9-yl) benzoate monohydrate (erythrosine dye; Sigma-Aldrich) were all used as received. 5 ACS Paragon Plus Environment
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Preparation of (3-acrylamidopropyl)-trimethylammonium chloride based polyHIPE from O/W HIPE. The polyHIPE samples based on AMPTMA are described by denotation x-y, where x indicates the surfactant type used to stabilize HIPE (x is either 127 for the Pluronic F-127, 108 for the Pluronic F-108, or 68 for the Pluronic F-68), whereas y indicates the MBAAm share in the total monomers (5 or 10 mol %). The bulk-hydrogel samples based on AMPTMA are described using the denotation BHG.
Figure 1. Schematic illustration of the AMPTMA-based polyHIPE synthesis.
Water (5 mL), AMPTMA (2.51 g), MBAAm (0.08 g or 0.15 g), Pluronic F-108 (0.4 g) and APS (0.1 g) were placed in a three necked 250 mL flask and the mixture was stirred with an overhead stirrer at 400 rpm (Table 1). Then, the corresponding amount of toluene (Table 1) was added drop-wise under constant stirring and once all toluene had been added, stirring was continued for further 10 min to produce the uniform O/W emulsion as depicted in Figure 1. Afterward, stirring was reduced to 20 rpm and the TMEDA reducing agent (80 µL) was added. After 3 min of additional stirring at 20 rpm, the emulsion was transferred to the mold and cured for 24 h at 40 °C. The resulting polyHIPE was purified via Soxhlet extraction with ethanol and ether, each for 24 h, and then it was vacuum dried until the constant weight.
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The AMPTMA-based bulk hydrogels (BHGs) were polymerized using the same recipe as described above, but without the Pluronic F127 and toluene. The monomer aqueous solutions were polymerized at 40 °C over the night. The resulting hydrogels were purified via Soxhlet extraction with ethanol and vacuum dried. Table 1. HIPE recipes
Sample 108-5 108-10 Aqueous external phase, wt % H2O 13.64 13.62 AMPTMA 4.56 4.55 MBAAm 0.19 0.36 F108 0.97 0.97 APS 0.24 0.24 TEMED 0.15 0.15 Total 19.75 19.89 Organic internal phase, wt % Toluene 80.25 80.11 Total 80.25 80.11
Characterization Structure and density Chemical structure was characterized by Fourier transform infrared spectroscopy (FTIR). FTIR spectra were recorded on a Perkin-Elmer Spectrum One instrument (Perkin-Elmer, Inc., Waltham, MA, USA). Dried samples were ground with KBr and pressed to pellets for the FTIR measurements. FTIR spectra were recorded in the 450 – 4000 cm-1 range at a resolution of 4 cm-1. Elemental analyses were performed to determine the nitrogen content in the resulting polyHIPES (Flash 2000 CHNS Analyzer, Thermo Scientific). EY concentration and its uptake by polyHIPEs were followed by an UV/Vis spectrophotometer (Perkin Elmer, model Lambda 35) in the range of 200-800 nm with a scanning speed of 240 nm/min and a slit set to 2 nm. Emulsion droplets were observed using an optical microscope Leica DM2500 (Leica Microsystems) equipped with a camera (Tucsen, model IS500). Average droplet size of
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emulsions was determined by an optical microscope using SemAfore software (version 4.01). Analysis was repeated three times; in each analysis, at least 100 droplets were counted to calculate the average droplet size. The measurement of average droplet size by optical microcopy might be subject to large errors, mainly because small droplets cannot be sized appropriately. Hence, the size distributions obtained be image analysis are only for approximate comparisons. Porous structure of the dry polyHIPEs was studied by a scanning electron microscopy (SEM) (Carl Zeiss, SUPRA 35 VP microscope). A piece of each sample was cryogenically fractured and mounted on a carbon tab for better conductivity. A thin layer of gold was sputtered on the sample’s surface prior to SEM analysis. The average void (dV) was determined by manually measuring 50 to 100 voids in the SEM micrographs, calculating an average, and applying a correction (multiplying by 2/31/2) to account for the random nature of the fracture plane through the void. The polyHIPE and bulk-hydrogel densities (ρpolyHIPE and ρBHG, respectively) were determined gravimetrically. The polyHIPE skeletal (polymer wall) densities (ρP) (an average of ten consecutive measurements) were evaluated using fully automated, highly-precision helium pycnometry (Micromeritics AccuPyc II 1340). The polyHIPEs were thoroughly dried and purged with nitrogen to exclude the influence of moisture and adsorbed impurities on the measured data. Calculation of nitrogen loading and theoretical ion exchange capacity (IEC) value (IECT) First, from the nitrogen content determined by elemental analysis (Nfound) we calculated the amount of nitrogen per gram of polyHIPE. The following example is for the polyHIPE 108-10. N(found) =
%.''() ·
, (.) , (01234567)
'8.%' 9/;
