Synthesis of amphiphilic block copolyamines via click reaction Vanga Devendar Goud 1 , Roshan Dsouza 1 , Suresh Valiyaveettil ⇑ Department of Chemistry, 3 Science Drive 3, National University of Singapore, Singapore 117543, Singapore article info Article history: Received 27 May 2015 Received in revised form 14 July 2015 Accepted 15 July 2015 Available online 29 July 2015 Keywords: RAFT polymerization Amphiphilic block copolymers Click reaction Nanoparticles Extraction kinetics abstract Amine functional polymers have been used in many applications owing to their interesting properties, but are unstable under ambient conditions. Here we report the synthesis of four diblock polyamines using a reversible addition–fragmentation chain-transfer (RAFT) poly- merization followed by click chemistry. Four amphiphilic block copolyamines P1–P4 were synthesized and fully characterized using different spectroscopic and physicochemical techniques. Self-assembly of the polymers shows porous films and spherical particles under different conditions. The polymers were used for the liquid–liquid extraction and solid–liquid extraction of pollutants such as metal nanoparticles, organic dyes and heavy metal ions from water. All polymers showed good extraction efficiency towards nanopar- ticles and anionic dyes. Such stable and processable polyamines can be explored for poten- tial applications in many areas. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction The reversible addition–fragmentation chain transfer (RAFT) polymerization is widely used for the synthesis of block copolymers with a specific architecture and a narrow distribution of molecular weights [1–4]. Owing to the presence of hydrophobic and hydrophilic blocks, amphiphilic block copolymers display interesting organizations and properties in solid state and in solution [5]. In addition, amphiphilic polymers have attracted much attention in material science and biomedical applications [6–9]. Polyamines or aminogels are used to extract heavy metal ions, nanoparticles and anionic metal species through electrostatic interactions [10–17]. Polymers such as polyethylenimine (PEI) and polyallylamines with large number of primary and secondary amino groups exhibit good extraction ability for heavy metals [18–20]. Water contamination has been a severe problem around the globe and many countries develop innovative solutions to obtain potable water supply at affordable cost. In addition to heavy metal ions, volatile organic compounds, organic dyes, microorganisms and other industrial pollutants, emerging toxic contaminants such as metal nanoparticles, which are tiny and reactive, warrant development of new materials for water purification. Recently, we have reported the synthesis and characterization of polyamines through functional group modifications on the polymer backbone and used them for remov- ing nanoparticles from water [21]. In general, polymers with amine functionalities are challenging to prepare owing to poor stability, high reactivity, low solubility in common solvents and processing difficulties under ambient conditions. Additionally, free amine groups are prone to oxidation by atmospheric oxygen, triggering degradation of the active amine species. Introduction of amine http://dx.doi.org/10.1016/j.eurpolymj.2015.07.027 0014-3057/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: [email protected](S. Valiyaveettil). 1 Both authors equally contributed to this work. European Polymer Journal 71 (2015) 114–125 Contents lists available at ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Amine functional polymers have been used in many applications owing to their interestingproperties, but are unstable under ambient conditions. Here we report the synthesis of fourdiblock polyamines using a reversible addition–fragmentation chain-transfer (RAFT) poly-merization followed by click chemistry. Four amphiphilic block copolyamines P1–P4 weresynthesized and fully characterized using different spectroscopic and physicochemicaltechniques. Self-assembly of the polymers shows porous films and spherical particlesunder different conditions. The polymers were used for the liquid–liquid extraction andsolid–liquid extraction of pollutants such as metal nanoparticles, organic dyes and heavymetal ions from water. All polymers showed good extraction efficiency towards nanopar-ticles and anionic dyes. Such stable and processable polyamines can be explored for poten-tial applications in many areas.
� 2015 Elsevier Ltd. All rights reserved.
1. Introduction
The reversible addition–fragmentation chain transfer (RAFT) polymerization is widely used for the synthesis of blockcopolymers with a specific architecture and a narrow distribution of molecular weights [1–4]. Owing to the presence ofhydrophobic and hydrophilic blocks, amphiphilic block copolymers display interesting organizations and properties in solidstate and in solution [5]. In addition, amphiphilic polymers have attracted much attention in material science and biomedicalapplications [6–9]. Polyamines or aminogels are used to extract heavy metal ions, nanoparticles and anionic metal speciesthrough electrostatic interactions [10–17]. Polymers such as polyethylenimine (PEI) and polyallylamines with large numberof primary and secondary amino groups exhibit good extraction ability for heavy metals [18–20].
Water contamination has been a severe problem around the globe and many countries develop innovative solutions toobtain potable water supply at affordable cost. In addition to heavy metal ions, volatile organic compounds, organic dyes,microorganisms and other industrial pollutants, emerging toxic contaminants such as metal nanoparticles, which are tinyand reactive, warrant development of new materials for water purification. Recently, we have reported the synthesis andcharacterization of polyamines through functional group modifications on the polymer backbone and used them for remov-ing nanoparticles from water [21].
In general, polymers with amine functionalities are challenging to prepare owing to poor stability, high reactivity, lowsolubility in common solvents and processing difficulties under ambient conditions. Additionally, free amine groups areprone to oxidation by atmospheric oxygen, triggering degradation of the active amine species. Introduction of amine
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protecting groups during synthesis introduces additional synthetic steps that need to be mitigated in order to increase thereaction yield. Also, the pH dependence of amines make the synthetic conditions rather selective.
Here, we report a strategy to obtain diblock polyamines using a combination of RAFT and click reactions. This simplestrategy is versatile and can be used for a convenient synthesis of amphiphilic block polymers. 1,2,3-Triazole group, a ver-satile functional heterocycle unit formed via click chemistry is known for its coordination with anionic, neutral and cationicguest molecules [22,23]. The concept and the structures of target polymers, PES-b-PS (5), PAPTPA-b-PS (P-1), PAPTOA-b-PS(P-2), PAPTXA-b-PS (P-3), and PAPTPP-b-PS (P-4) are shown in Fig. 1. The target molecules are designed to incorporate aminefunctionalities from anilines and pyridines linked to the polymer backbone by flexible spacer alkyl or aryl chains. Presence ofthe hydrophobic styrene block is crucial towards increasing the solubility in common organic solvents, air-stability andself-organization polymers under ambient conditions and the spacer alkyl chains provide flexibility to the amine groupsto coordinate to metals and organic dyes. The synthesized amphiphilic polymers were used to extract metal nanoparticles,metal ions and organic dyes from aqueous solutions.
2. Experimental
2.1. Materials
Styrene (99%), 4-bromostyrene (98%), silver nitrate (AgNO3), sodium citrate tribasic, hydrogen tetrachloroaurate trihy-drate (HAuCl4�3H2O), azobisisobutyronitrile (AIBN), diisopropylethylamine (DIPEA), bromotris(triphenylphosphine)copper(I) (CuBr(PPh3)3), cyanomethyl dodecyl trithiocarbonate, tetrabutylammonium fluoride (TBAF, 1.0 M in THF) andsodium borohydride (NaBH4) were purchased from commercial sources and used without further purification. Chloroform(CHCl3), tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) were used for dissolving block copolymers.Polyvinylpyrrolidone (PVP) capped water soluble Au and Ag nanoparticles were prepared according to reported procedure[24]. Deionised water was used for preparing stock solutions of nanoparticles.
2.2. Measurements
1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on Bruker Avance AV300 (300 MHz) NMR instru-ment with CDCl3 solvent. FT-IR analyses were performed using Bruker ALPHA FT-IR spectrophotometer by preparing KBr pel-lets with polymers. Gel permeation chromatography (GPC) was used to determine the average molecular weight of polymerson Waters e2695 alliance system equipped with Waters 2414 refractive index detector using THF as the eluent at a flow rateof 0.3 mL/min at 30 �C. Polystyrene was used as standard for calibration. Thermal degradation studies (TGA) were studiedwith a SDT 2960 TA Instrument. Samples were heated from 25 to 800 �C at a heating rate of 10 �C/min under N2 atmosphere.Differential scanning calorimetry (DSC) measurements were done using a Mettler Toledo DSC1 STARe System at 10 �C/minunder nitrogen atmosphere. Surface morphology of the dropcasted film was studied using JEOL JSM-6701F field emissionscanning electron microscope (FESEM). The size and surface charges of nanoparticles were determined using transmissionelectron microscope (TEM, JEOL JEM-3011), dynamic light scattering (DLS) and Malvern Zetasizer Nano-ZS analyser. Thequantitative determinants of extraction efficiency of nanoparticles were carried out using a UV–Vis spectrophotometer(Shimadzu-1601 PC spectrophotometer) and the concentrations of metal ions were quantitatively measured withDual-view Optima 5300 DV Inductively coupled plasma-optical emission spectroscopy (ICP-OES) system.
2.2.1. General procedure for click reactionA solution of the prepolymer 5 (1.0 eq., based on (trimethylsilyl)ethynylstyrene units) in THF (20 mL) was treated with
azide derivatives (1.0 eq.), CuBr(PPh3)3 (0.01 eq.) and DIPEA (0.5 mL, excess) and the mixture was allowed to stir at roomtemperature for 24 h. After removing the solvent under reduced pressure, the crude product was precipitated from excessof methanol. The solid was filtered, washed repeatedly with methanol and dried under high vacuum at 40 �C for 16 h.
2.2.2. Synthesis of poly[(trimethylsilyl)ethynylstyrene] 2To a solution of (trimethylsilyl)ethynylstyrene 1 (2.0 g, 9.98 mmol) in dry THF (5 mL) was added RAFT reagent cyano-
methyl dodecyl trithiocarbonate (0.03 g, 0.099 mmol), AIBN (0.01 g, 0.06 mmol), and the mixture was degassed by repeatedfreeze–pump–thaw cycles, sealed under vacuum and heated at 70 �C for 12 h. The viscous reaction mixture was dissolved inTHF (5 mL) and precipitated from excess methanol to give polymer 2 as a white powder (1.6 g, 80%, Mn = 8600 g/mol,PDI = 1.5); 1H NMR (300 MHz, CDCl3): d 7.25–6.95 (m, ArAH), 6.75–6.22 (m, ArAH), 1.95–1.25 (m, ACH2ACHA),0.37–0.25 (s, Si(CH3)3); IR (KBr, cm�1): 3023 (ArAH), 2925 (CAH), 2140 (AC„CH), 1610 (AC@CA).
2.2.3. Synthesis of poly[styrene-b-(trimethylsilyl)ethynylstyrene] 4A mixture of styrene 3 (2.5 g, 24.9 mmol, 5 eq.) and poly[(trimethylsilyl)ethynylstyrene] 2 (1.0 g, 4.9 mmol, 1 eq.) in dry
THF (5 mL) was added AIBN (0.01 g, 0.01 eq.) and the mixture was degassed by repeated freeze–pump–thaw cycles, sealedunder vacuum and heated at 70 �C for 6 h. The viscous reaction mixture was dissolved in THF (5 mL) and precipitated fromexcess methanol to give polymer 4 as a white powder (2.9 g, 82%, Mn = 44400 g/mol, PDI = 1.4); 1H NMR (300 MHz, CDCl3): d
Click reactionwith azidereagent
Acetylene incorporatedprepolymer
Amine functionalizedpolymer
Nanoparticles
Polymer – nanoparticle complex
Fig. 1. Schematic representation of the binding of nanoparticles with amine functionalized polymer and the molecular structure of amine block copolymersP1–P4.
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2.2.4. Synthesis of poly[styrene-b-4-ethynylstyrene] 5To a solution of poly(styrene-b-(trimethylsilyl)ethynylstyrene) 4 (5.0 g, 0.81 mmol based on (trimethylsilyl)ethynylstyr
ene units) in dry THF (100 mL), was added tetrabutylammonium fluoride (1.6 mL, 1 M in THF, 2 eq.) at 0 �C. After additionto the solution, the mixture was stirred for additional two hours at 25 �C. The solution was then concentrated under reducedpressure and the polymer was precipitated three times from methanol to give 5 as a white powder (Yield: 95%,Mn = 33,800 g/mol, PDI = 1.6); 1H NMR (300 MHz, CDCl3): d 7.25–6.94 (m, ArAH), 6.75–6.32 (m, ArAH), 3.14–3.04 (s,AC„CH), 1.96–1.27 (m, ACH2ACHA); IR (KBr, cm�1): 3021 (ArAH), 2910 (aliphatic CAH), 2106 (AC„CH), 1605 (AC@CA).
2.2.5. PAPTPA-b-PS (P1)Following the general procedure for click reaction, starting from poly[styrene-b-4-ethynylstyrene] 5 (1.0 g), azide deriva-
Diblock polyamines were dissolved in chloroform (5 mg/4 mL) and a small amount (400 lL) was added to a 2 mL cen-trifuge tube containing aqueous solution of metal nanoparticles (1.5 mL, 2.5 � 10�4 M) or aqueous solutions of dyes or metalions (1.5 mL, 50–100 ppm) and placed on a mechanical shaker (200 rpm) for 6 h. All extraction experiments were performedat room temperature (25 �C) and neutral pH. During this period, the residual concentration of nanoparticles and dyes in theaqueous solution was analysed using UV–Vis spectroscopy at different time-intervals until the system reached equilibrium.For quantification calibration graphs were plotted to estimate the amount of nanoparticles or dyes remained in the aqueoussolution after extraction. The concentrations of metal ions were determined periodically using inductively coupledplasma-optical emission spectroscopy (ICP-OES) system. The extraction capacity Qe (mg/g) of the polymers was calculatedusing the following equation [25].
Q e ¼ ðCo � CeÞV=M ð1Þ
where Co and Ce (mg/L) are initial and equilibrium concentrations of pollutants, V is the volume of the adsorbate solutionand M is mass of the adsorbent used.
2.4. Solid–liquid extraction of nanoparticles
For solid–liquid extraction, polymer solution in CHCl3 (1 mg/mL) was drop casted on a glass slide to form a porous filmthrough slow evaporation of the solvent. After drying the film under ambient conditions, morphology of the film was char-acterized using SEM. The glass slide with the polymer film was then dipped in aqueous solution of nanoparticles (2 mL,50 ppm) and allowed to stand at room temperature for 3 h, removed, washed with water, dried and analysed using SEMto observe the changes in surface characteristics before and after extraction studies. Since the objective of these experimentsis to show the affinity of diblock polyamines towards nanoparticles, no quantitative studies were carried out using solid–liquid extraction method. We will be investigating this in detail in the near future.
3. Results and discussion
As depicted in Scheme 1, RAFT polymerization was used to synthesize the living polymer poly[(trimethylsilyl)ethynylstyrene], 2 (Mn = 8600 amu; PDI = 1.5) from compound 1 [26–30]. Polymerization of styrene with the macroinitiator 2 affordedthe required block copolymer 4 in good yield (Mn = 44,400 amu; PDI = 1.4). A long styrene block was incorporated to bringthe desired solubility and processability in nonpolar solvents. RAFT polymerization yielded a polymer with a relatively nar-row polydispersity (Fig. S4, ESI). The average molecular weights determined by GPC correspond to about 45 and 340 units offunctionalized styrene and styrene units, respectively, suggesting a monomer ratio of �1:7. Subsequent deprotection oftrimethylsilyl groups was achieved using tetrabutylammonium fluoride (TBAF) to give the block copolymer 5(Mn = 33,800 amu; PDI = 1.6) in quantitative yield. From the 1H NMR spectrum of pre-polymer 5, the peak integral ratioof the styrene units and acetylene groups is 7:1, which shows an approximately 300 styrene units and 43 units of p-(acet-ylene)styrene on the polymer backbone. The observations from NMR studies comply with the values obtained from GPCanalysis and thus rule out possibility of styrene being homopolymerized during RAFT synthesis. The target polymers weresynthesized from prepolymer 5 in single steps using click chemistry. Molecular weight distribution and composition of thepolymers were determined using GPC and 1H NMR spectroscopy, respectively. The acetylene groups present on polymer 5were used for click reaction with various azide derivatives under copper catalysed condition to synthesize target blockcopolymers P1–P4 in quantitative yields [31–35].
The molecular structures of all amine block copolymers were established using 1H NMR analyses (Fig. 2). Typical 1H NMRspectrum showed a broad peak (a) at d = 4.60–4.40 ppm for P1, P2 and P4 and 5.4 ppm for P3, which is attributed to the peakdue to CH2ANtriaz. Another broad peak (b) observed at d = 3.7–3.5 ppm for P1–P3 and 2.3 ppm for P4 could be assigned toACH2A group attached to the dimethylaniline or pyridine moiety, respectively. A sharp intense broad singlet (c) centeredat d = 2.96–2.80 ppm could be assigned to the methyl protons of AN(CH3)2. The signal due to aliphatic protons appearedin the range from 1.2 to 2.7 ppm, whereas, the alkyne proton of the precursor polymer, PES-b-PS, showed a broad signal cen-tered at 3.07 ppm (s, AC„CAH). The appearance of three new peaks in between d 5.6–2.8 ppm (a, b, c) after click reactionwhich are completely absent in the NMR of 5, implies a quantitative conversion of alkyne groups into amine groups throughtriazole ring formation. The formation of triazole ring is further confirmed by the disappearance of the acetylenic proton at d3.07 ppm and appearance of a peak in the region of d 7.76–7.96 ppm in polymers P1–P4.
Scheme 1. Synthesis of amine functionalized block copolymers PES-b-PS (5), PAPTPA-b-PS (P1), PAPTOA-b-PS (P2), PAPTXA-b-PS (P3), and PAPTPP-b-PS(P4).
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In Fig. S1 (ESI), the characteristic alkyne (AC„CAH) stretching was observed at 2106 cm�1 for pre-polymer 5. The forma-tion of triazole rings after click reaction was identified using the appearance of a broad ANAH stretching absorption peak at3500–3100 cm�1 and disappearance of acetylene (AC„CA) peak at 2106 cm�1 and the sharp AC„CAH peak at 3290 cm�1.The absorption peaks observed in the range of 3100–2900 cm�1 (ACAH), and 1600 cm�1 (C@C groups) are also assignedaccordingly.
The thermal stability and weight loss percentage were obtained from thermogravimetric analyses (TGA) under nitrogenatmosphere. TGA curves of all block copolymers are shown in Fig. S2 (ESI), which suggest comparable thermal stabilities forall four block copolymers. This is expected considering the common polymer backbone for all polymers and close similarityin their overall architecture. The data reveals that the polymers P1�P3 start to degrade at 220–240 �C, whereas P4 is stableup to 350 �C and all polymers show a 50% weight loss at around 425 �C. The early weight loss in the case of P1–P3 is due tothe presence of pendant groups on the polymer backbone, which are not present in P4. All polymers showed single stagedecomposition and char yield of 10–15% above 450 �C. The glass transition temperatures (Tg) were determined using differ-ential scanning calorimetry (DSC) under nitrogen atmosphere, which showed increase in chain mobility owing to the intro-duction of pendant alkyl groups, as compared to the PES-b-PS (5) prepolymer. While 5 showed a Tg of 105.1 �C comparable topure polystyrene backbone, the Tg
’s of block copolymers P1–P4 were observed at 96.3 �C, 100.5 �C, 100 �C and 103.2 �C. AllDSC traces are shown in Fig. S3.
3.1. Morphologies of block polymers
Self-assembly of block copolymers depends on multiple factors such as structure of polymers, presence of polar or non-polar functional groups, interchain interactions and nature of the solvents used [36,37]. In the case of P1–P4, the presence of
Fig. 2. 1H NMR spectra of copolymers PES-b-PS (5), PAPTPA-b-PS (P1), PAPTOA-b-PS (P2), PAPTXA-b-PS (P3) and PAPTPP-b-PS (P4) in CDCl3 under ambientconditions.
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styrene block and different polyamine blocks determine the morphology. In the synthesized polymers the hydrophobic styr-ene block was kept constant while changing the nature of amine functional groups. Polymer–solvent interactions and prop-erties of solvents are known to control the structure and packing [38,39].
Thin films with porous morphologies were formed by drop-casting CHCl3 solutions of polymer (1 mg mL�1) on a glasssubstrate under ambient condition. Self-assembly of amphiphilic polymers into honey comb structures are influenced bymany factors which include the effect of solvents, moisture and polymer concentrations [40]. Based on the reported mech-anism of breath figure formation, [21,41–43] it is expected that the polar amine functional groups organize on the surface ofthe films and are used for the extraction of nanoparticles from aqueous solution. In contrast to this, the polymer solutions inpolar, high-boiling solvents such as DMF (0.1 mg mL�1) gave spherical particles upon evaporation of solvents under ambientconditions. FESEM was used to characterize the morphology of the films formed from different diblock polyamines (Fig. 3).
3.2. Liquid–liquid extraction of pollutants from aqueous solution
Water soluble Au- and Ag-nanoparticles were synthesized using a reported procedure [24]. TEM measurements showedthe formation of monodispersed spherical Au NPs (15–20 nm) and Ag NPs (20–40 nm) (Fig. S9). Absorbance spectra of theparticle showed an absorption maxima (kmax) of 390, 406, 526 and 520 nm for Ag-Cit, Ag-PVP, Au-Cit and Au-PVP NP’s,respectively (Fig. S8). Extraction experiments were done using initial nanoparticle concentrations of, 2.5 � 10�4 M(1.5 mL) at room temperature and neutral pH. The chloroform solution of polymers P1–P4 (0.5 mL, 1 mg/mL) was addedto a 2 mL centrifuge tube containing aqueous nanoparticle solutions and shaken on a mechanical stirrer for 6 h. The residualnanoparticle concentration was determined by drawing out small volumes of samples at pre-determined time intervals andmeasuring the absorption using UV–Vis spectroscopy. The concentration of nanoparticles was measured by the reduction inabsorbance intensity at kmax of NP solution. The extraction efficiency of polymers P1–P4 was calculated by Eq. (1) mentionedin the experimental section.
3.3. Effect of contact time on extraction of NPs
The extraction of nanoparticles by polymers attained steady state condition within a short period of 15–20 min, afterwhich it showed no increase in the adsorption. Block polymers P1–P3 showed similar trends in extraction efficiencies(Qe P 15 mg/g), while P4 showed lower Qe values (<15 mg/g). Polymers P1–P3 have two amine groups each per repeatingunit for coordination, while coordination of P4 is limited to terminal pyridyl groups. In addition, orientation, basicity and
Fig. 3. SEM micrographs of the drop-casted film from polymer P-1 on glass plates from CHCl3 (a, 1 mg/mL) and DMF (b, 0.1 mg/mL) solutions.
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positions of these amine groups in the polymer chain also influence the extraction efficiencies. Higher extraction efficiencieswere observed with polymer P1 in case of Ag-Cit (34 mg/g), Au-Cit (25 mg/g) and Au-PVP NPs (13 mg/g), which is attributedto the shorter alkyl chain and high hydrophilicity of amine groups (Fig. 4).
In order to determine the kinetics of the extraction process, two widely used kinetic models – pseudo-first order andpseudo-second order kinetic models were evaluated. The linear form of pseudo-first order proposed by Lagergren is givenby linear equation:
LogðQ e � Q tÞ ¼ LogQ e � ðK1=2:303Þt ð2Þ
where Qe and Qt are the amount of nanoparticles adsorbed at equilibrium and at a given time ‘t’, respectively. K1 (min�1) isthe rate constant for pseudo-first order extraction. The plots of log(Qe � Qt) versus t were used to determine the correspond-ing rate constants (Fig. S5). The correlation coefficients for the pseudo-first order kinetics were shifted significantly from unitvalue and therefore pseudo-second order kinetic equation was used for the analysis of data.
The pseudo-second order equation is given in the linear form:
t=Q t ¼ t=Q e þ 1=ðK2 � Q 2eÞ ð3Þ
where K2 (g/mg min) is the pseudo-second order rate constant. The values of K2 and Qe were calculated experimentally fromthe slope and intercept of the t/Qt versus t plots, which are shown in Fig. 5. The calculated rate constants and Qe values frompseudo-first and second order kinetic equations are given in Table S1. The correlation coefficients corresponding topseudo-second order are closer to unity and also the calculated Qe values are closer to those determined experimentallyimplying that the extraction follows a pseudo-second order kinetic model.
3.4. Extraction of metal ions
Separation of metal ions from water by liquid–liquid extraction is applied in various industries. Some of the extractantsused for the removal metal ions from aqueous solutions include ionic liquid, [44,45] small organic molecules, either in freeform [46,47] or immobilized on polymeric surfaces [48,49] and functional polymers bearing ligands for metal complexation[50]. In comparison to the solid–liquid extractions this method has advantage of utilizing larger surface of the adsorbents forextractions and the process is proven to be useful in extracting several metal ions. In order to examine the extraction effi-ciencies of Pb(II) and Cr(VI) metal ions by polymer P1–P4, we carried out liquid–liquid extractions, by dissolving polymers inchloroform (2 mg/ml) and shaking with aqueous solutions of metal ions (1.5 mL, 50 ppm). The contact time for the extractionof the metal ions by the polyamines is a crucial parameter affecting the extraction efficiency of contaminants from wastewater. In order to study the effect of time on the extractions, samples were drawn from the aqueous solutions at differenttime intervals (10, 30, 60, 120, 240 and 600 min) and were analysed by ICP to determine the residual metal ion concentra-tion. The plots of equilibrium concentration Qe (calculated from Eq. (1)) versus time are shown in Fig. 6. The extraction pro-cess reached steady state within 2 h in case of chromium ions, whereas it required more than 3 h for lead ions. Similar to theextractions of nanoparticles, the maximum extraction efficiency was shown by P1 (5 and 8 mg/g), while P4 showed the leastefficiencies.
3.5. Extraction kinetics with metal ions
Both pseudo-first order and pseudo-second order kinetics (Fig. S6, Fig. 7) were studied for the extraction of metal ions andit was found that the extraction follows pseudo-second order kinetics (Eq. (3)). The equilibrium extraction amount, Qe andthe rate constants are calculated from the slope and intercept of the plots of t/Qt versus time. The fitting results and the cal-culated values of Qe, K and R2 are given in Table S2. All correlation coefficient values for metal ions were close to unity,
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Fig. 4. Time dependent extraction of nanoparticles, (A) Ag-Cit (B) Ag-PVP (C) Au-Cit and (D) Au-PVP NP’s, using polyamines, P1–P4 from water to organicphase.
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Fig. 5. Pseudo-second order kinetic data for the extraction of Ag-Cit (A), Ag-PVP (B) and Au-Cit (C) and Au-PVP (D) NP’s using polyamines, P1–P4.
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Fig. 7. Pseudo-second order kinetics for the extraction of (A) Lead and (B) Chromium by polyamines, P1–P4.
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suggesting that the adsorption of pollutants was the rate limiting step involving complexation of metal ions and the coor-dinating sites on the polymer backbone.
3.6. Extraction of dyes
The extraction of anionic and neutral dyes by the polyamines is discussed below. All polyamines showed similar extrac-tion efficiencies for both groups of dyes. The presence of protonated amine groups under neutral or slightly acidic conditionshelps the extraction of anionic dyes efficiently than the neutral ones (Fig. 8). Cationic dyes such as alcian blue were notextracted by the polyamines under neutral conditions. The kinetics studies indicated that all four polyamines followedpseudo-second order kinetics, as seen in the linear plots (Fig. 9). The experimentally determined values of Qe match withthose determined by kinetic model (Table S3).
3.7. Solid–liquid extraction of pollutants by polymer porous films
The self-assembled porous films prepared by drop casting the chloroform solution of polymer on a glass plate were usedfor extracting the nanoparticles from water. In this solid–liquid extraction, the polymer film was placed in aqueous solutionof nanoparticles (2 mL) and allowed to stand at room temperature for 3 h. After extraction, the polymer films were imagedusing SEM and the micrographs are given in Fig. 10.
It is clear that the small white particles observed on the surface are metal nanoparticles extracted from solution.However, since only the surface functional groups interact with the hydrophilic nanoparticles in solution rather than thebulk polymer, the extraction efficiencies of solid films are expected to be lower. The extractions under solid–liquid condi-tions are usually slow and efficiency is low owing to the fact that only surface functional groups are involved in the extrac-tion process. In order to highlight that amine functional groups are on the surface and the nanoparticles are extracted as it isand not in any other form, we have performed thin film – liquid extraction and imaged the surface of the film using SEM.Porous nature of the films and ability to control the concentration of amine groups on the polymer backbone will helptowards designing new polymers that would establish higher efficiency and selectivity. We are in the process of optimizingthe procedure to improve the efficiency by changing the polymer backbone.
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Time (min)
P1 P2 P3 P4
A B
Fig. 9. Pseudo-second order kinetics for the extraction of (A) neutral red and (B) brilliant blue dyes by polyamines, P1–P4.
Fig. 10. SEM images of porous film prepared by drop-casting chloroform solution of PAPTPA-b-PS (P1) and extracting Ag-Cit (A and B) and Au-Cit NPs(C and D) from water.
V.D. Goud et al. / European Polymer Journal 71 (2015) 114–125 123
124 V.D. Goud et al. / European Polymer Journal 71 (2015) 114–125
4. Conclusions
In summary, four amine functionalized block copolymers, P1–P4 were synthesized by using RAFT polymerization fol-lowed by click reaction. The synthetic strategy employed here is versatile and can be used to prepare different polyaminesin multigram scale quantities. All polyamines are stable under ambient conditions, soluble in common organic solvents andform porous films or spherical particles via drop casting on a glass slide. Full details of extraction of metal nanoparticles,heavy metal ions and dyes from aqueous medium at moderate pH were demonstrated. The results from extractions studiesrevealed that polyamines, P1–P4, were highly efficient in extracting emerging pollutants such as metal nanoparticles – AgNPs, Au NPs, common dyes – brilliant blue, neutral red dyes and heavy metal ions like lead and chromium ions from water.Easy synthetic steps and good processability of these water insoluble polyamines makes them interesting candidates forother applications in the future.
Acknowledgements
The authors gratefully acknowledge the financial support from the Environment and Water Industry Programme Office(EWI) under the National Research Foundation of Singapore (PUBPP 21100/36/2, NUS WBS R-706-002-013-290,R-143-000-458-750, R-143-000-458-731). They also thank the Department of Chemistry, National University of Singaporeand NUS Environmental Research Institute (NERI) for all technical support.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.eurpolymj.2015.07.027.
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