Temperature-Responsive Hyperbranched Amine-Based Polymers for

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Temperature-Responsive Hyperbranched Amine-Based Polymers for Solid−Liquid Separation Yinan Wang,†,‡ Yohei Kotsuchibashi,†,§ Yang Liu,*,‡ and Ravin Narain*,† †

Department of Chemical and Materials Engineering, University of Alberta, 116 Street and 85 Avenue, Edmonton, AB T6G 2G6, Canada ‡ Department of Civil and Environmental Engineering, University of Alberta, 116 Street and 85 Avenue, Edmonton, AB T6G 2G6, Canada § International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan S Supporting Information *

ABSTRACT: Temperature-responsive hyperbranched polymers containing primary amines as pendent groups have been synthesized for solid− liquid separation of kaolinite clay suspension. The effects of temperature, polymer charge density, and polymer architecture on particle flocculation have been investigated. Suspensions treated with the temperatureresponsive amine-based hyperbranched polymers showed remarkable separation of the fine particles at a low polymer dosage of 10 ppm and at testing temperatures of 40 °C. In comparison to other polymers studied (linear and hyperbranched homopolymers and copolymers), the temperature-responsive amine-based hyperbranched copolymers showed better particle flocculation at 40 °C, as evidenced by the formation of a thinner sediment bed without compromising the amount of clay particles being flocculated. This superior solid−liquid separation performance can be explained by the hydrophobic interaction of PNIPAM segments on particle surfaces or the capture of additional free particles or small floc due to the exposure of buried positive charges (because of the phase separation of the hydrophilic amines and hydrophobic PNIPAM part) at temperatures above the lower critical solution temperature (LCST).

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INTRODUCTION Solid−liquid separation is necessary for the recovery of water from tailings streams. Increasing the efficiency of separation can contribute to reductions in industry operating costs and waste volume production.1−3 The application of poly(N-isopropylacrylamide) (PNIPAM) as a thermally responsive flocculant has been proposed as an effective rapid settling method.2,4,5 PNIPAM is well-known to have a sharp phase transition from a hydrophilic to a hydrophobic state above the lower critical solution temperature (LCST).6−8 The hydrophobic state favors surface polymer−particle interactions that result in aggregation and rapid settling. 2,5 However, the use of PNIPAM homopolymers as flocculating agents is limited, as the nonionic PNIPAM insufficiently adsorbs onto the charged particle surfaces7,9−13 resulting in limited interparticle hydrophobic interactions at a temperatures over the polymer’s LCST.13 The low flocculation efficiency of the nonionic PNIPAM is also likely due to the failure of neutralization of the surface charge of the particles.14 To control particle−particle interactions and aid enhanced consolidation, several groups have investigated the introduction of cationic groups,7,12,14,15 anionic groups,7,11 dual ionics,16 or different polymer architectures such as linear random,7 linear block,10 and organic−inorganic hybrid groups12 to PNIPAM to enhance the polymer’s flocculation ability. For © 2014 American Chemical Society

example, random copolymers with opposite charge to the particle charge adsorbed strongly to mineral surfaces and flocculated particles rapidly at temperatures both above and below the LCST.7 Although using PNIPAM-based random cationic copolymers could increase the efficiency of solid−liquid separations, polymers are usually required to have relatively high molecular weight (in most cases larger than 106 Da) for sufficient polymer initial adsorption on particle surfaces.2,5,12 Also, as hydrophilic cationic monomers are randomly distributed in the copolymer backbone, their charge densities have to be low enough to result in an LCST value low enough17 to be acceptable to industry. An alternative way to incorporate charges into a thermally responsive polymer is by using a block copolymer, that enables higher charge densities on polymer backbones without significantly increasing the LCST.18,19 At temperatures below the LCST, clay particles treated by block polymers showed no flocculation or sedimentation because all charges were located at one end of the polymer chain; thus, the Received: December 16, 2013 Revised: February 13, 2014 Published: February 17, 2014 2360

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Scheme 1. Synthesis of (a) Linear and (b) Hyperbranched P(AEMA-b-NIPAM)

0.036 mmol) and ACVA (thermal initiator, 5 mg, 0.018 mmol) 1,4dioxane stock solution (1 mL). After degassing under a nitrogen atmosphere for 30 min, the flask was placed in a preheated oil bath for 5 h. PAEMA macroCTP was obtained by precipitating the polymer solution in acetone; it was extensively washed with methanol to remove any residual monomers. The dried macroCTP (0.177 g) was dissolved in 1 mL of distilled water and mixed with 7 mL of ACVA (2.5 mg, 0.009 mmol), NIPAM monomer (1.09 g, 0.96 mol), and N,N′-methylenebisacrylamide (83 mg, 0.54 mmol) dimethylformamide (DMF) stock solution in a sealed flask. After degassing with nitrogen for 30 min, the flask was placed in a preheated oil bath for 24 h. The polymer was purified by repeated precipitation in diethyl ether and subsequent drying under vacuum. Linear P(AEMA-st-NIPAM) was also synthesized by conventional free radical polymerization. AEMA (0.353 g, 2.16 mmol) was dissolved in 2 mL of double distilled deionized water in a 10 mL Schlenk tube. After the addition of 10 mg of ACVA (0.036 mg) and 1.09 g of NIPAM (0.96 mol) DMF stock solution (12 mL), the flask was degassed under nitrogen for 30 min and then placed in a 70 °C oil bath for 24 h. The polymer was purified by dialysis against double distilled deionized water for 3 days and freeze-dried. Synthesis of Linear and Hyperbranched PAEMA and PNIPAM Homopolymers by Conventional Free Radical Polymerization. To synthesize linear and hyperbranched PAEMA homopolymers, 1.25 g of AEMA monomers was dissolved in 5 mL of double distilled deionized water followed by the addition of 2 mL of ACVA (5 mg, 0.018 mmol) DMF stock solution. For the hyperbranched homopolymer, 58.77 mg (5 mol %) of N,N′methylenebisacrylamide was used. After degassing under nitrogen for 30 min, the flask was placed in a preheated oil bath for 24 h. The polymer was then purified by precipitation in acetone and repeated washing with methanol. The syntheses of linear and hyperbranched PNIPAMs were similar to those of the PAEMAs. Briefly, 1.25 g of NIPMA monomers, 5 mg of ACVA, and 85 mg of N,N′-methylenebisacrylamide (only for hyperbranched polymer synthesis) were mixed in 20 mL of DMF. The solution was degassed under nitrogen for 30 min, and the reaction was heated in a 70 °C oil bath for 24 h. The polymers were purified by repeated precipitation in diethyl ether. Characterizations of Polymers. 1H NMR spectra of the polymers were recorded on a Varian 500 MHz spectrometer using D2O as the solvent. Molecular weight and polydispersity (PDI) of the synthesized polymers were determined by gel permeation chromatography (GPC) at room temperature and a Viscotek model 250 dual detector (refractometer/viscometer in aqueous eluents (0.5 M sodium

polymer failed to bridge individual particles with opposite charges.10 Recent research used a starlike organic−inorganic hybrid polymer (Al-PAM) to improve the flocculation of mature fine tailings (MFT) that resulted from bitumen extraction from oil sands.12 We applied here low molecular weight, thermally responsive amine-based hyperbranched copolymers as novel flocculants for solid−liquid separation of stable clay particles. These copolymers were synthesized by reversible additionfragmentation chain transfer (RAFT) polymerization. Light transmittance of released water, final solid volume, and temperature-responsive sediment consolidation were investigated in clay suspensions treated with different polymers. To the best of our knowledge, this is the first report of temperature-responsive cationic copolymers with hyperbranched structure being used in particle flocculation and solid−liquid separation. The flocculation performance of the hyperbranched cationic copolymers were compared with neutral linear and hyperbranched PNIPAM homopolymers, linear and hyperbranched cationic homopolymers, and linear cationic copolymers with approximately similar molecular weights to understand the role of compositions and morphological factors (e.g., charge density, molecular architecture) in solid−liquid separations.

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MATERIALS AND METHODS

Materials. Chemicals were purchased from Sigma-Aldrich Chemicals (Oakville, ON, Canada) and organic solvents were from Caledon Laboratories Ltd. (Georgetown, ON, Canada). The chain transfer agent 4-cyanopentanoic acid dithiobenzoate (CTP) and monomers were synthesized as previously described20−23 (Scheme 1). N-Isopropylacrylamide (NIPAM) was purified in benzene, and recrystallization was performed in hexane. Synthesis of Hyperbranched Block Copolymers of NIPAM Using RAFT Polymerization. The hyperbranched poly(2-aminoethyl methacrylamide hydrochloride block N-isopropylacrylamide) (P(AEMA-b-NIPAM)) used in this study was prepared using reversible-addition fragmentation chain transfer (RAFT) polymerization techniques as described in Scheme 1. In a 10-mL Schlenk tube, AEMA (0.353 g, 2.16 mmol) was dissolved in 2.3 mL of double distilled deionized water followed by the addition of CTP (10 mg, 2361

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acetate and 0.5 M acetic acid for linear and hyperbranched PAEMA homopolymers) or DMF containing 0.01% LiBr for PNIPAM-based polymers) with a flow rate of 1.0 mL/min. Polymer LCST was determined at 50% transmittance of the solution at 500 nm when an aqueous solution of 0.1 wt % synthesized PNIPAM-based polymer was continuously recorded by a UV−vis spectrometer at a heating rate of 0.5 °C/min. Settling Tests. Clay suspensions were prepared by dispersing kaolinite clays (Wards Natural Science Ltd., Ontario, Canada) in deionized water to 5 wt % of solid. Different polymer concentrations in aqueous solution were prepared at room temperature. Settling tests were conducted in 20 mL glass vials. The following two settling procedures were used.

tion of a range of linear and hyperbranched polymers. The polymers were synthesized either by conventional free radical polymerization or the reversible addition−fragmentation chain transfer (RAFT) process as shown in Scheme 1. The hyperbranched copolymer was synthesized by RAFT chain extension of NIPAM form PAEMA macro-CTP. The details of polymer syntheses are shown in Table 1. As expected, GPC Table 1. Synthetic Parameters, Compositions, and Structure of Polymers polymer architectures

• Procedure A: Settling at room temperature A 10 mL clay suspension was shaken vigorously for 10 s to ensure complete initial suspension, followed by addition of a predetermined volume of polymer solution. The mixture was then shaken vigorously for another 10 s and allowed to stand still. The settling was recorded by observing the position of the interface (mud line) between the liquid supernatant and the solid suspension as a function of time. • Procedure B: Settling at 40 °C The polymer was mixed with a clay suspension at room temperature following the same procedure described in A. The glass vial with the suspension of clay plus polymer was placed in a 40 °C water bath and left to stand still while settling was recorded as in A. The initial settling rate was considered to be equal to the initial linear part of a plot of interface height versus time. Released water clarity at the end of a 1 h settling period was determined by measuring the released water transmittance with a UV−vis spectrometer at 500 nm. Solid content in the sediment was determined by drying the concentrated suspension layer at the bottom of the vial at 70 °C for 24 h, weighing the dried sediment, and dividing the weight of the dried sediment by the initial weight of the clay plus the polymer. The mudline position (%) was measured at the interface between liquid and settled solid and determined as % mudline position = (hs/H) × 100%, hs (height of the sediment, cm), H (height of the column, cm). Because the kaolinite clays would self-aggregate and precipitate (particles in the sediment bed that precipitated by gravity (without the assistance of the polymer flocculants)), we normalized the solid content in the sediment bed by dividing the mass of the flocculated solids with the one that precipitated without polymer treatments and calculated as:

%solid content =

linear hyperbranched linear hyperbranched linear hyperbrancheda

polymer compositions PAEMA92 PAEMA94 PNIPAM97 PNIPAM100 P(AEMA8-stNIPAM75) P(AEMA12-bNIPAM90)

cross-linking density (mol %)

Mn (Da)

Mw/ Mn

0 5 0 5 0

15 147 15 468 11 006 11 364 9924

1.64 2.84 1.66 2.10 1.99

5

12 114

−

a GPC data for hyperbranched P(AEMA-b-NIPAM) is unavailable as the polymer is not very soluble in DMF. The Mn value was calculated based on the NMR results.

results showed that polymers synthesized by conventional free radical polymerization possessed relatively large molecular weight distributions (PDI values (Mw/Mn) were larger than 1.5). Molecular weight distributions for the hyperbranched polymers were higher (>2). The compositions of the copolymers were determined by 1H NMR (Figure S1, Supporting Information). A narrow range of cationic charge density (10−12 mol %) was determined in linear P(NIPAM-stAEMA) and hyperbranched P(AEMA-b-NIPAM). Lower critical solution temperatures (LCSTs) of the polymers were determined by using the UV−vis spectrometer at a wavelength of 500 nm and at a heating rate of 0.5 °C/min (Figure 1). The LCST was recorded at 50% light transmittance.

ms − m Tϕ × 100% m T(1 − ϕ)

where mT is the total mineral mass (g), ms is the mass of clay in the sediment bed (g), and ϕ is the mass ratio between the self-aggregated particles and total solids. The sediment solid volume fraction (φ) is also determined in this study and calculated as φ = ms/ρmVf with ρm (mineral density, g/cm3), Vf (final sediment bed volume, mL). Statistical Analysis. The settling tests were repeated at least three times. The data are reported as the average ± SD of the results of three independent experiments. Comparison of the values between groups was performed by analysis of variance (ANOVA) with p < 0.05 being considered significant.

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Figure 1. Lower critical solution temperatures (LCSTs) of PNIPAMbased polymers.

RESULTS AND DISCUSSION Synthesis of Linear and Hyperbranched Polymers. Linear and hyperbranched polymers were synthesized for solid−liquid separation. The polymers are designed with different architectures, molecular weights, compositions, and charge densities to evaluate the flocculation performances with respect to those parameters. For this study, we chose 2aminoethyl methacrylamide (AEMA) hydrochloride and Nisopropylacrylamide (NIPAM) as monomers for the prepara-

As expected, the LCST of NIPAM homopolymer was found to be around 32 °C, which agrees with results reported elsewhere.6,8,17,24 The LCSTs of the hyperbranched NIPAM homopolymer and block copolymer were found to be 36 °C and 37 °C, respectively. The higher LCST can be either due to the hydrophilic cross-linker (N,N′-methylenebisacrylamide (MBAm))17 or to the reduced areas of interaction of 2362

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Figure 2. The impact of hyperbranched P(AEMA12-b-NIPAM90) on particle flocculation (a) initial settling rate, (b) liquid−solid interface height, (c) light transmittance of the released water, (d) solid content in the sediment bed, and (e) sediment solid volume fraction. Data are presented as the mean ± SD (n = 3). (f) Schematic illustration of initial adsorption of hyperbranched P(AEMA12-b-NIPAM90) on clay particle surfaces at 25 °C and exposure of buried cationic charges to the polymer surface (red dots) at 40 °C.

hyperbranched PNIPAM segments.25 The LCST of linear P(AEMA8-st-NIPAM75) was found to be 40 °C, probably due to the hydrophilic AEMA residues which can promote polymer−solvent interactions,.17−19 Impact of Temperature-Responsive Hyperbranched Block Copolymers Dosage on Clay Particle Flocculation. The impact of hyperbranched P(AEMA12-b-NIPAM90) dosage on the initial settling rate, clarity of the release water, and liquid−solid interface height of kaolinite clay suspensions was studied. Figure 2a shows changes in the initial settling rate with polymer dosage and temperature. A significantly higher initial settling rate (p < 0.05) was observed when the clay suspensions were treated with 500 ppm hyperbranched P(AEMA12-bNIPAM90) at 25 °C (Figure 2a, blank bars), whereas no significant difference in initial settling rate (p > 0.07) was observed between the negative control group (suspensions without polymer treatment) and the groups treated with lower dosages (1−100 ppm). At 40 °C, the initial settling rates of the clay suspensions treated with hyperbranched P(AEMA12-bNIPAM90) were observed to increase significantly (p < 0.05) in all polymer dosages compared to the tests performed at 25 °C and showed values much higher than that of the negative control group (Figure 2a, patterned bars). We hypothesize that these observations are due to the introduction of hydrophobic interactions at temperatures above the polymer’s LCST.2,11,26,27

On the other hand, some hydrophilic amines that are buried by the random PNIPAM coil at 25 °C might phase separate with the hydrophobic PNIPAM at 40 °C and migrate to the surface of the particles10,19,28,29 (see Figure 2e). We speculate that the exposure of more cationic charges to the particle surface would also assist the flocculation process by capturing additional free particles. Figure 2b shows the stabilized liquid−solid interface height observed after 60 min of suspension settling. Although clay particles were flocculated by hyperbranched P(AEMA12-bNIPAM90) at dosages of 1−500 ppm at 25 and 40 °C, thicker solid sediment beds were observed at higher polymer dosages (100 and 500 ppm) (Figure 2b). At polymer dosages of 10− 100 ppm, a significantly lower final liquid−solid interface height at 40 °C (compared to 25 °C) was observed (p < 0.05) (Figure 2b, patterned bars). These observations indicate that flocculation of the negatively charged clay particles were dependent on the number of cations in the suspension and the test temperature. At 25 and 40 °C, cationic charges on the polymer chains neutralized anionic surface charges on the clay particles, allowing them to aggregate.7,12 However, when the polymer dosage exceeded 100 ppm, the surface potential of the polymer-coated particles became positive, resulting in a higher liquid−solid interface height (Figure 2b, blank bars).7,12,27 At 40 °C, the final liquid−solid interface height decreased (Figure 2363

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Figure 3. The impact of charge densities on hyperbranched PAEMA94, PNIPAM100, and P(AEMA12-b-NIPAM90) on particle flocculation: (a) liquid−solid interface height, (b) released water light transmittance, (c) solid content in the sediment bed, (d) sediment solids volume fraction. The polymer dosage is 10 ppm in all cases, and data are presented as the mean ± SD (n = 3).

P(NIPAM12-b-AEMA90), the locally ultralow polymer concentration would make the polymer fail to fully dehydrate at 40 °C,17,32 and charges left on the clay particle surfaces would bind water. The bound water would prevent the clay particles from approaching each other and would stabilize the clay suspension. Figure 2d shows the percentage of solid content of the sediment beds after 60 min settling of hyperbranched P(AEMA12-b-NIPAM90)−clay suspension mixtures. At 25 °C, the amount of flocs in the sediment beds decreased as polymer dosage decreased from 500 to 1 ppm (Figure 2d, blank bars), whereas at 40 °C, only suspensions treated with 1 ppm hyperbranched P(AEMA12-b-NIPAM90) showed significantly less solid content (p = 0.015) (Figure 2d, patterned bars). These results are in good agreement with the results of released water clarity; that is, clay suspensions with more solids in the sediment beds (Figure 2d) corresponded to higher light transmittance values and thus more released water (Figure 2c). Suspensions treated with 10 ppm hyperbranched P(AEMA12-bNIPAM90) showed significantly higher solid content in the sediment bed at 40 °C (p < 0.05) (Figure 3c, patterned bar) compared to 25 °C (Figure 3c, blank bar), possibly due to thermally responsive polymer-induced multiple flocculation processes.7,10,29 At 25 °C, the relatively poor flocculation performance in the suspension treated with 10 ppm hyperbranched P(AEMA12-b-NIPAM90) (Figure 2, parts c and d, blank bars) might reflect the failure of cationic polymers to neutralize some of the negatively charged clay particles.

2b, patterned bars) because of the water released from the interparticle space when polymers collapse at a temperature above the LCST.3,4,7,30 The light transmittances of the released water and solid content in the sediment beds (Figure 2, parts c and d, respectively) are important parameters for evaluating the dewatering efficiency of polymer flocculants. As shown in Figure 2c, the light transmittance of the released water increased to above 80% at 25 °C (p = 0.0065) if the polymer concentration was higher than 100 ppm. The light transmittance values of the released water were relatively higher in the suspensions treated with hyperbranched P(AEMA12-bNIPAM90) in this study than those treated with other counterionic polymer flocculants under the same conditions in other studies.2,7,12,15 At a temperature above the polymer’s LCST (40 °C), the released water light transmittances in clay suspensions with 1 ppm polymer dosage were reduced dramatically (Figure 2c, patterned bars), indicating a poor flocculating performance of the hyperbranched thermally responsive cationic copolymer. At 1 ppm polymer dosage, some surfaces of the clay particles might not be completely covered by polymer at 25 °C, and the surface potential of the clay surfaces might remain negatively charged. When the test temperature was increased to 40 °C, the negative potential values should increase, resulting in a more stable clay suspension.31 On the other hand, if the clay particle surfaces were not completely covered by 1 ppm hyperbranched 2364

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However, when the tests were performed at 40 °C (above the polymer’s LCST), polymers on the particle surfaces were expected to undergo coil-to-globule transitions, and some free particles or small flocs would interact with each other and be flocculated by the hydrophobic interactions.2,5,10 In addition, the unique architecture of hyperbranched P(AEMA12-bNIPAM90) (hyperbranched block) would possibly expose more cationic charges to surface at 40 °C,10,19,28,29 increasing neutralization of the negatively charged clay particles and enhancing flocculation. Figure 2e gives information on the sediment solid volume fraction of the particles flocculated by hyperbranched P(AEMA12-b-NIPAM90) at 25 and 40 °C. Because the higher sediment solid volume fraction is usually related to a densely packed sediment bed by higher compressive yield stress,7,10,33 we believe those results could be used to further support our argument on the mechanism of the particles flocculated by hyperbranched P(AEMA12-b-NIPAM90). For example, the insignificant changes in the relatively low sediment solid volume fraction values at 25 °C (Figure 2e, blank bars) might indicate loosely packed flocculated particles in the sediment bed due to electrostatic repulsion force or water captured in the hydrated PNIPAM network.7,14 The significant increase in the sediment solids volume fraction for particles flocculated by 10 ppm polymer at 40 °C indicates a much denser packed sediment bed by either hydrophobic interactions from PNIPAM segments on particle surfaces or because of the exposure of cationic charges from the polymer coil which interact with some bare particle surface area through electrostatic attraction force (Figure 2f). Although the dehydration of the PNIPAM at 40 °C is also supposed to lead to further consolidations for particles dosed with 50−500 ppm polymer, electrostatic repulsion force induced by the excess cations present at the interparticle space during polymer phase separation might balance the hydrophobic interactions and result in a loosely packed sediment bed. Our results showed that hyperbranched P(AEMA12-bNIPAM90) worked well for flocculating clay particles at a dosage as low as 10 ppm (Figure 2), which is lower than the dosage of most thermally responsive polymer-based flocculants reported previously (20−500 ppm).2−4,7,10,15 Therefore, the effects of counterionic charges and polymer architecture on dewatering efficiency were evaluated by comparing the particle flocculation performance of different polymers at 10 ppm. Impact of Polymer Charge Density on Clay Flocculation. The effect of polymer cationic charge density on particle flocculation was tested with three polymers of different charge density: hyperbranched PNIPAM100 (neutral), hyperbranched PAEMA94 (100 mol % cationic charges), and hyperbranched P(AEMA12-b-NIPAM90) (12 mol % cationic charges) (Table 1). Results of experiments in which 10 ppm polymer was added to a clay suspension and the mixture was allowed to settle at 25 or 40 °C for 1 h are shown in Figure 3. The significantly higher released water clarity (higher than 90% transmittance) (p < 0.05) (Figure 3b) and solid content (p < 0.05) (Figure 3c) show that most of the clay particles were flocculated by 10 ppm hyperbranched PAEMA94. However, results for the sediment bed height (Figure 3a) and sediment solid volume fraction (Figure 3d) suggested that a lot of water was retained at the interparticle spaces because of the interparticle repulsion force introduced from the exceeding cationic charges presented on the hyperbranched PAEMA94 bridged particles.12The sediment bed height was significantly

reduced after the suspension was treated with polymers with fewer cationic charges (hyperbranched P(AEMA12-b-NIPAM90) and PNIPAM100) (Figure 3a blank bars), as was the observed released water clarity (Figure 3b) and the solid content (Figure 3c) because the counterion densities (12 mol % and 0 mol % for hyperbranched P(AEMA12-b-NIPAM90) and PNIPAM100, respectively (Table 1)) were too low to flocculate all the clay particles efficiently. Therefore, at 25 °C, the dewatering efficiency of the polymer flocculants was significantly affected by the counterion density in individual polymer chains, and the low molecular weight hyperbranched P(AEMA12-b-NIPAM90) was considered to be a poor candidate for particle flocculation under the conditions of these experiments. When tested at 40 °C, no significant changes in solid−liquid interface height (Figure 3a), released water clarity (Figure 3b), solid content (Figure 3c), and sediment solids volume fraction (Figure 3d) were observed on clay suspensions treated with 10 ppm hyperbranched PAEMA94 compared to the same tests performed at 25 °C (Figure 3, blank bars). These observations suggest that temperature exerted a subtle effect when anionic charged clay particles were bridged by the cationic charged homopolymer. Compared with the hyperbranched PAEMA94, clay suspensions that were treated with 10 ppm low molecular weight neutral hyperbranched PNIPAM100 and tested for flocculation at 40 °C showed significantly lower values on released water light transmittance (Figure 3b, patterned bars), solid content in the sediment bed (Figure 3c, patterned bars), and sediment solids volume fraction (Figure 3d, patterned bars) (p < 0.05), which indicated that only a small amount of particles could be flocculated and packed loosely in the sediment bed. These results contradict reports that the anionic clay suspensions can be flocculated by hydrophobic interactions with PNIPAM homopolymers at temperatures above the LCST of the polymers.2,5 However, other researchers have noted that PNIPAM homopolymer-based flocculants require a relatively high molecular weight (Mw > 106 Da) to effectively flocculate particles at temperatures above the LCST.4,5 On the basis of the work presented by O’shea and co-workers,5 the amount of PNIPAM deposited on particle surface at 25 °C was reduced with the decrease in the molecular weight.5,10,27 In this study, the molecular weight of PNIPAM (11 kDa) is at least 20 times lower than the lowest one studied in O’shea’s works (230 kDa); therefore, we speculate that an even lower amount of polymer was deposited on the particle surface at 25 °C. On the other hand, on the basis of statements that (1) particle surfaces with insufficient amounts of deposited PNIPAM are less hydrophobic, resulting in weaker particle−particle bonding force at a temperature over the LCST,13 and that (2) nonionic PNIPAMbased flocculant is unable to neutralize the charges on particle surfaces,14 we believe that the lower initial adsorption of our low molecular weight hyperbranched PNIPAM100 on clay particle surfaces might result in less water molecule replacement or greater electrostatic repulsion force among particle surfaces. The particle aggregation was therefore inhibited. Interestingly, although the cationic charges on the low molecular weight hyperbranched P(AEMA12-b-NIPAM90) were lower than that of the hyperbranched PAEMA94 (Table 1), the former was a better flocculant at 40 °C, in terms of solid content (Figure 3c) and solid volume fraction (Figure 3d) in the respective sediment beds. It is possible that the exposure of buried positive charges at a temperature above the polymer’s LCST has caused additional bridging of suspended par2365

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Figure 4. The impact of polymer architecture on particle flocculation: (a) liquid−solid interface height, (b) released water light transmittance, (c) solid content in the sediment bed, (d) sediment solid volume fraction. The polymer dosage is 10 pm in all cases, and data are presented as the mean ± SD (n = 3).

significantly better flocculation in terms of lower turbidity in released water (p < 0.05) (Figure 4b) and higher solid content and solid volume fraction (p < 0.05) in the sediment bed (Figure 4, parts c and d, respectively). These observations support our conclusion that cationic primary amine groups on polymer chains play an important role in initially bridging suspended negatively charged particles.5,7,10 Interestingly, polymer architecture did play an important role when cationic thermally responsive polymers were used as flocculating agents. When clay suspensions were treated with 10 ppm hyperbranched P(AEMA12-b-NIPAM90) at 25 °C, the sediment bed height (Figure 4a, blank bars), released water light transmittance (Figure 4b, blank bars), and solid content in the sediment bed (Figure 4c, blank bars) were higher than similar results for suspensions treated with 10 ppm linear P(AEMA8-st-NIPAM75). At 40 °C, clay suspensions treated with P(AEMA12-b-NIPAM90) and P(AEMA8-st-NIPAM75) showed even larger differences in flocculation. For example, significantly higher values for the released water light transmittance (p < 0.05), sediment bed solid content (p = 0.0016), and sediment solid volume fraction (p < 0.05) were observed for clay suspensions treated with 10 ppm hyperbranched P(AEMA12-b-NIPAM90) at 40 °C than for clay suspensions treated with 10 ppm P(AEMA8-st-NIPAM75) (Figure 4, parts b−d, patterned bars). These results suggest

ticles10,19,28 or further consolidation among the bridged particles. As the flocculation efficiency of the polymer improved with an increase in cationic charges on the polymer surface (Figure 2f), it is likely that the charge densities play an important role in particle flocculation. Effect of Polymer Architecture on Clay Particle Flocculation. We compared the flocculation ability of hyperbranched polymers to that of linear polymers to investigate how polymer architecture affects flocculation. To explore the effect of polymer architecture on clay particle flocculation, the settling of clay suspensions treated with 10 ppm linear PAEMA92, PNIPAM97, and P(NIPAM8-st-AEMA75) were compared with the settling of suspensions treated with their hyperbranched derivatives (Figure 4). Table 1 shows that linear and hyperbranched PAEMA have similar molecular weights and numbers for the cationic primary amine groups. In terms of the liquid−solid interface height (Figure 4a), released water clarity (Figure 4b), solid content (Figure 4c), and sediment solid volume fraction (Figure 4d), the flocculation efficiency of the linear and hyperbranched PAEMA were independent of polymer architecture at 25 °C and 40 °C. On the other hand, compared to the clay suspensions treated with cationic charged and thermally responsive homopolymers (linear PAEMA92 and PNIPAM97, respectively), the suspensions treated with the cationic homopolymers showed 2366

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that more particles could be flocculated at 40 °C and form a densely packed sediment bed when dosed with 10 ppm P(AEMA12-b-NIPAM90). Possibly, 10 ppm linear P(AEMA8-st-NIPAM75) was too low a concentration to obtain results similar to those in other reports.7,11 Furthermore, because the clay particle surfaces might not be fully covered by linear P(AEMA8-st-NIPAM75) at 40 °C, the negative potential values on particle surfaces increased with increasing temperature,31 resulting in a flocculation performance (Figure 4, patterned bars) poorer than that of suspensions treated with linear P(AEMA8-stNIPAM75) at 25 °C (Figure 4, blank bars). In contrast, clay suspensions treated with 10 ppm hyperbranched P(AEMA12-bNIPAM90) showed relatively better flocculation performance at 25 °C (Figure 4, blank bars), which can be attributed to the exposure of more cationic charges on the hyperbranched block structured polymer surface.3,7,26,28 The thicker sediment beds observed under such conditions (Figure 4a, blank bars) were probably due to a larger quantity of water being retained in the polymer’s hyperbranched structure. At 40 °C, the improved flocculation performance of hyperbranched P(AEMA12-bNIPAM90) (reduced sediment bed height (Figure 4a, patterned bar), increased solid content (Figure 4c, patterned bar) and more densely packed (Figure 4d, patterned bar) particle layers in the sediment bed) can be explained by its coil-to-globule transition that exposes more buried cationic charges that can capture additional suspended free particles or consolidate the bridged particles.18,28 Although results in Figure 4b indicated that the released water light transmittance of the hyperbranched P(AEMA12-b-NIPAM90) treated clay suspension was significantly reduced compared to the one with cationic homopolymers (linear and hyperbranched) treatment, the released water turbidity value (NTU = 173.48, NTU ≈ 0.191 + 926.1942 × [−log(%T/100)], where %T is light transmittance measured from UV−vis spectrometer) is comparable to the values obtained from commercial Magnafloc and cationic AlPAM treated clay suspension.12

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by research grants from the Natural Sciences and Engineering Research Council (NSERC) of Canada and the Canada Foundation for Innovation (CFI).

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CONCLUSION This study presents the first example of clay suspensions flocculated by low molecular weight thermally responsive amine-based hyperbranched copolymers. The influence of polymer charge density and architecture on particle flocculation, sedimentation, and consolidation were evaluated. Liquid−solid separations were successful at a polymer dosage as low as 10 ppm. The hyperbranched copolymer produced enhanced secondary consolidation at temperatures above its LCST. Moreover, the exposure of buried positive charges during the polymer’s coil-to-globule transition has the potential to provide additional electrostatic attractions that can capture more clay particles or small flocs.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectra of hyperbranched P(NIPAM12-b-AEMA90), PNIPAM100, and PAEMA94. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 2367

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