Xanthate-Functional Temperature-Responsive Polymers: Effect on

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Xanthate-Functional Temperature-Responsive Polymers: Effect on Lower Critical Solution Temperature Behavior and Affinity toward Sulfide Surfaces Wei Sung Ng,†,‡ Elizaveta Forbes,‡ George V. Franks,*,† and Luke A. Connal*,† †

Chemical and Biomolecular Engineering, University of Melbourne, Parkville, VIC 3010, Australia CSIRO Mineral Resources Flagship, Clayton, VIC 3168, Australia

‡

S Supporting Information *

ABSTRACT: Xanthate-functional polymers represent an exciting opportunity to provide temperature-responsive materials with the ability to selectively attach to specific metals, while also modifying the lower critical solution temperature (LCST) behavior. To investigate this, random copolymers of poly(N-isopropylacrylamide) (PNIPAM) with xanthate incorporations ranging from 2 to 32% were prepared via free radical polymerization. Functionalization with 2% xanthate increased the LCST by 5 °C relative to the same polymer without xanthate. With increasing xanthate composition, the transition temperature increased and the transition range broadened until a critical composition of the hydrophilic xanthate groups (≥18%) where the transition disappeared completely. The adsorption of the polymers at room temperature onto chalcopyrite (CuFeS2) surfaces increased with xanthate composition, while adsorption onto quartz (SiO2) was negligible. These findings demonstrate the affinity of these functional smart polymers toward copper iron sulfide relative to quartz surfaces, presumably due to the interactions between xanthate and specific metal centers.

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water purification and solid−liquid separation processes.23−25 One such example is P(NIPAM-co-acrylic acid), which we have previously demonstrated to be capable of providing advantages to the selective recovery of iron ore in the froth flotation process.10 Xanthates are functional groups bearing a R−OCSS− group, with several interesting properties that are potentially relevant for the tailoring of additional functions to temperatureresponsive polymers. As a charged polar group, xanthates are highly hydrophilic and have been used for over a century in the rayon industry to induce solubility of the rayon fiber in aqueous solutions.26,27 Xanthates are also known to have a strong affinity for particular metal surfaces and ions, especially copper.26,29 This property has been applied extensively in the minerals processing industry through the use of xanthatefunctional small molecules acting as surfactants to recover specific particulate minerals from solution, typically in the selective recovery of copper-containing chalcopyrite and chalcocite from quartz in froth flotation.29−31 Recently, xanthate esters have been utilized for the attachment of temperature-responsive polymers to gold nanoparticles.32 In this study, we report a simple and versatile synthesis of PNIPAM copolymers with xanthate functionality. A series of

INTRODUCTION Stimuli-responsive polymers offer unique and tunable properties that can be controlled by an external switch. For example, reversible hydrophilic−hydrophobic transitions can be triggered by a change in environmental conditions, such as temperature, redox potential, pH, or light.1−3 Recent progress in stimuli-responsive polymers has revealed a host of potential applications in drug delivery systems,2,4−6 wastewater treatment,7,8 minerals processing,9−11 and gel actuator systems.12,13 The bulk of these examples are centered on temperatureresponsive polymers, typically those exhibiting lower critical solution temperatures (LCST), whereby a dissolved polymer rapidly turns insoluble upon heating above the LCST. The most common example is poly(N-isopropylacrylamide) (PNIPAM), which is hydrophilic below an LCST of 32 °C and hydrophobic above.14 The engineering of specific properties of interest into temperature-responsive materials often involves copolymerization with suitable comonomers to impart the desired function. These additional functions include biocompatibility,15 catalytic activity,16,17 diagnostic capabilities,18 metal chelation,19,20 responsiveness to multiple stimuli,4,21 and/or modifications of the LCST.2,22 Of these applications, the study of metalchelating temperature-responsive materials that can selectively adsorb onto or collect metals or minerals is a promising area that could prove valuable to a number of industries, ranging from the concentration of valuable minerals in mining10 to © XXXX American Chemical Society

Received: January 20, 2016 Revised: March 17, 2016

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DOI: 10.1021/acs.langmuir.6b00211 Langmuir XXXX, XXX, XXX−XXX

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backfilling with nitrogen gas. The solution was allowed to react at 60 °C for 24 h before the vessel was quenched in cold water. A detailed procedure for the synthesis of P(NIPAM560-co-VAc84) is provided. NIPAM (3.38 g, 29.3 mmol) was dissolved in 20 mL of THF, VAc (1.62 g, 19.5 mmol) was added to make up 5 g of feed monomer, and AIBN (0.41 g, 5.0 μmol) was then added to initiate free radical polymerization. The reaction was conducted at 60 °C for 24 h followed by precipitation of the product into DEE. The recovered polymer was then dried under vacuum and further purified through repeated dissolution in THF and precipitation into DEE, with a resulting yield of 73%. 1H NMR (CDCl3, 400 MHz): δ 1.10 (s, 600H, −NH−CH(CH3)−CH3), 1.59 (br, 200H, −CH(CO−NH−CH(CH 3)CH 3)−CH2−CH(CO−NH−CH(CH3)CH3)−), 1.81 (m, 30H, −CH(O−CO−CH3)−CH2−CH(O−CO−CH3)), 1.98 (s, 45H, −CH(O−CO−CH3)), 2.09 (br, 100H, −CH2−CH(CO− NH−CH(CH3)CH3)−CH2), 3.96 (s, 100H, −NH−CH(CH3)− CH3)), 4.82 (br, 15H, −CH2−CH(O−CO−CH3)−CH2−), and 6.45 ppm (br, 100H, −CH2−CH(CO−NH−CH(CH3)CH3)−CH2). Mn (DMF GPC) = 70.6 kDa; Đ (DMF GPC) = 1.82. General Procedure for the Synthesis of Random P(NIPAM-co-VA) Polymers. P(NIPAM-co-VAc) obtained from the previous step was subjected to base-catalyzed hydrolysis in ethanol. Potassium carbonate was added as a base catalyst in stoichiometric equivalents to the acetate groups in the polymer. The reaction was allowed to proceed for 48 h at 60 °C. A detailed procedure for the synthesis of P(NIPAM560-co-VA84) is provided. P(NIPAM560-co-VAc84) (2.5 g, 3.0 mmol of VAc units) was dissolved in 20 mL of ethanol at room temperature (25 °C), after which potassium carbonate (1.3 g, 9.0 mmol) was added. The reaction was done at 60 °C for 48 h, after which the polymer was precipitated in DEE and filtered. The recovered polymer was then dried under vacuum and further purified through repeated dissolution in ethanol and precipitation into DEE to remove the residue salt, with a resulting yield of 95%. 1H NMR (D2O, 400 MHz): δ 1.15 (s, 600H, −NH− CH(CH3)−CH3), 1.59 (br, 200H, −CH(CO−NH−CH(CH3)CH3)− CH2−CH(CO−NH−CH(CH3)CH3)−), 1.72 (br, 30H, −CH(OH)− CH2−CH(OH)−), 2.03 (br, 100H, −CH2−CH(CO−NH−CH(CH3)CH3)−CH2), 3.91(s, 100H, −NH−CH(CH3)−CH3)), and 4.03 ppm (br, 15H, −CH2−CH(OH)−CH2−). General Procedure for the Synthesis of Random P(NIPAM-co-VAco-VX) Polymers. P(NIPAM-co-VA) obtained from the previous step was reacted in DMSO through xanthation with carbon disulfide (30 times the VA groups present) in the presence of KOH (1.5 times the VA groups present). The reaction was allowed to proceed for 24 h at room temperature, after which the solution was purified through dialysis. Only a fraction of the VA proceeded to form the VX groups, resulting in P(NIPAM-co-VA-co-VX). A detailed procedure for the synthesis of P(NIPAM560-co-VA33-coVX51) is provided. P(NIPAM560-co-VA84) (0.5 g, 0.6 mmol of VA units) was first dissolved in 15 mL of DMSO at 60 °C. KOH(aq) (1.0 mL, 1.0 M) was then added at room temperature, followed by carbon disulfide (1.4 g, 18.0 mmol). The reaction was conducted at room temperature for 24 h. The reaction product was then transferred into a dialysis tubing (3500 Da cutoff point, SnakeSkin) and dialyzed for 48 h in distilled water, replaced as required. The dialysis product was frozen at −40 °C and freeze-dried for 48 h to obtain the P(NIPAM-co-VA-coVX) polymers at a yield of 52%. 1H NMR (D2O, 400 MHz): δ 1.15 (s, 600H, −NH−CH(CH3)−CH3), 1.59 (br, 200H, −CH(CO−NH− CH(CH3)CH3)−CH2−CH(CO−NH−CH(CH3)CH3)−), 1.72 (br, 12H, −CH(OH)−CH2−CH(OH)−), 1.86 (br, 18H, −CH(O− C(S−)S)−CH2−CH(OC(S−)S)−), 2.03 (br, 100H, −CH2− CH(CO−NH−CH(CH3)CH3)−CH2), 3.91(s, 100H, −NH−CH(CH3)−CH3)), and 4.03 (br, 6H, −CH2−CH(OH)−CH2−), and 5.37 ppm (m, 9H, −CH2−CH(O(S−)S)−CH2−). LCST Determination. To examine the LCST of the synthesized polymers, polymer solutions were made up in distilled water at concentrations of 10 mg/mL with no pH modification. The measured pH of the polymer solutions can be found in the Supporting Information, with all samples displaying a pH above 7. The addition of KCl was also performed at this step for tests requiring the presence of

polymers were prepared by postfunctionalization of PNIPAM copolymers with vinyl alcohol groups, whereby the properties could be tuned based on the incorporation of xanthate groups. These materials displayed interesting temperature-responsive behaviors, with the LCST tunable through engineering of the xanthate composition in the final copolymer. We also examined the potential of applying these polymers to mineral processing applications by investigating the interaction of these materials with two different mineral surfaces, with adsorption isotherms indicating a strong affinity for chalcopyrite (copper iron sulfide) surfaces relative to quartz (silica), presumably due to the preference of xanthate for particular metal centers.

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EXPERIMENTAL SECTION

Materials. N-Isopropylacrylamide (NIPAM) monomer (98%, Tokyo Chemical Industry) was recrystallized twice using a 40:60 mixture of toluene:n-hexane. Vinyl acetate (VAc, 99.9%, Aldrich) was filtered and passed through a column of basic aluminum oxide (Scharlau) to remove the hydroquinone inhibitors. 2,2′-Azobis(isobutyronitrile) initiator (AIBN, Aldrich) was recrystallized in methanol and stored in a refrigerator at 5 °C. Potassium carbonate (Chem-Supply), potassium chloride (KCl, Chem-Supply), carbon disulfide (Aldrich), and potassium hydroxide (KOH, VWR) were used without additional purification. Deuterated solvents chloroform-d (CDCl3) and deuterium oxide (D2O) were procured from Cambridge Isotopes and used as received. Pure PNIPAM (32 kDa, Aldrich) was used as received following 1H NMR and GPC characterization. Tetrahydrofuran (THF, VWR), diethyl ether (DEE, Chem-Supply), ethanol (Merck), dimethyl sulfoxide (DMSO, Univar), toluene (Scharlau), n-hexane (VWR), and dimethylformamide (DMF, Aldrich) were used as received. Potassium amyl xanthate (PAX, Tall-Bennett)) was used as received. Chalcopyrite (CuFeS2, assayed at 90% purity) and pure quartz (SiO2) samples were provided by CSIRO, with a specific surface area of 0.09 and 0.55 m2/g, respectively. Instrumentation. 1H NMR spectroscopy was conducted with a Varian Unity 400 spectrometer operating at 400 MHz. CDCl3 was used as the solvent for the P(NIPAM-co-VAc) polymers, and D2O for the P(NIPAM-co-vinyl alcohol (VA)) and P(NIPAM-co-vinyl xanthate (VX)) polymers, at concentrations of 20 mg/mL. UV−vis spectroscopy for characterization purposes was performed in a Shimadzu UV1800 spectrophotometer, while LCST studies were carried out in a Shimadzu UV Mini 1240 spectrophotometer with Peltier temperature control. Distilled water was used as the solvent along with 1 cm path length quartz cuvettes in all UV−vis tests. Fourier transform infrared (FTIR) spectroscopy was performed on the dry samples using a PerkinElmer Frontier FTIR spectrometer. The samples were pulverized prior to analysis to ensure sufficient contact and coverage over the FTIR detector. Characterization of the sample molecular weights was conducted via GPC using DMF with 0.05 M LiBr as the mobile phase. All samples were filtered through 0.45 μm nylon filters prior to analysis. The GPC study was performed using a Shimadzu liquid chromatography system featuring a PostNova PN3621 MALLS detector (λ = 532 nm), a Shimadzu RID-10 refractometer (λ = 633 nm), and a Shimadzu SPD-20A UV−vis detector with three Jordi columns (5 μm bead size, Jordi Gel Fluorinated DVB Mixed Bed) in series at 70 °C. The molecular weight characteristics of the samples were examined with NovaMALLS (PostNova Analytics) and determined through comparison with polystyrene standards. A Telstar Lyoquest freeze-drier was used for the freeze-drying of dialyzed polymers. Total organic carbon (TOC) analysis was performed using a Shimadzu TOC-VCSH analyzer, while particle size distributions were measured with a Malvern Mastersizer 2000. A Hach PH3 benchtop pH meter was used for pH measurements. Preparation of Polymers. General Procedure for the Synthesis of Random P(NIPAM-co-VAc) Polymers. Free radical polymerization was carried out on a Schlenk line with the NIPAM and VAc monomers and AIBN initiator in THF solution. This was preceded by five freeze− pump−thaw cycles in a 27 Pa (abs) vacuum line followed by B

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Langmuir salt. The samples were then passed through a 0.45 μm nylon membrane filter, before being added to a 4 mL quartz cuvette with a 1 cm path length. The transmittance of light through the samples was measured by UV−vis at 540 nm, with reference to a sample of distilled water. A Peltier temperature controller was used to heat the samples at an approximate heating rate of 1 °C/min, while the temperature inside the cuvette was continuously monitored with a thermistor probe. The transmittance was recorded at 0.2 °C increments between 30 and 60 °C. The set-point temperatures were increased in 3 °C intervals to reduce the variation in the heating rate, whereby each change in the set-point was conducted only after the transmittance had reached a steady value for 5 s. This was done to account for the use of the proportional controller for the Peltier, whereby the heating rate is typically larger at temperatures further from the set point. The LCST was approximated as the temperature whereby the transmittance had decreased by 5% from the initial baseline reading at 30 °C. Adsorption Isotherms. Adsorption studies of the polymers and potassium amyl xanthate (PAX) on chalcopyrite and silica surfaces were performed at room temperature (∼25 °C) using distilled water with 0.01 M KCl as electrolyte. Stock solutions of each polymer and PAX at concentrations of 2 mg/mL were prepared on the day prior to each experiment. To refresh the surface of the chalcopyrite samples, the particles were ground in a 10 cm diameter glass jar with 20 ceramic beads for 20 min at 60 rpm in representative lots of 1.5 g in 20 mL of water. Following grinding, the slurry was washed out into a vial with minimal water. Each sample was then topped up with the desired amount of polymer solution and sufficient water to make up a 30 mL solution containing 5 mass % solids. For the silica samples, 1.5 g of particles was added to each vial, which was subsequently topped up to 30 mL with the desired amount of polymer solution and water. The samples were sealed and shaken over a 6 h period and then allowed to settle for 20 min. Subsequently, the supernatant was extracted and filtered through 0.45 μm nylon filters to remove all solids prior to total organic carbon (TOC) analysis. The measured amount of organic carbon content in the supernatant was compared to a calibration curve of known polymer concentrations in solution to determine the equilibrium concentration of polymer remaining in solution (Ceq) for the tested samples. The quantity of adsorbed polymer was then found by subtracting Ceq from the initial dosage concentration. The amount of surface area present on the chalcopyrite and silica particles were determined using the measured size distributions and assuming spherical particles. Adsorption isotherms were then constructed using the known Ceq and calculated specific surface adsorption. Isotherms were fitted to a finite-layer Brunauer− Emmett−Teller (BET) multilayer isotherm model modified for liquid phase adsorption,33 with the following equation: Y = Ym

Scheme 1. Pathway for Synthesizing P(NIPAM-co-VA-coVX) Temperature-Responsive Copolymers with Xanthate Functionality

P(NIPAM-co-VAc) copolymers were prepared via free radical polymerization, with feed quantities designed to incorporate different amounts of VAc into the resulting copolymer. The amount of acetate incorporation was estimated by 1H NMR based on the ratio of the −CH− (peak g, δ = 4.81 ppm) present in the VAc backbone relative to the −CH− (peak e, δ = 3.97 ppm) present in the NIPAM isopropyl group, illustrated in Figure 1A. From Table 1, the resulting ratio of NIPAM to

Figure 1. 1H NMR spectra of (A) P(NIPAM-co-VAc) in CDCl3, (B) P(NIPAM-co-VA) in D2O, and (C) P(NIPAM-co-VA-co-VX) in D2O. The NIPAM isopropyl peak d, the acetate peak h, and the H2O peak are not shown in entirety to preserve clarity. All spectra were normalized to the NIPAM isopropyl peak d. The progression from (A) to (C) shows the effective synthesis, hydrolysis, and functionalization of PNIPAM with a xanthate moiety.

KSCeq[1 − (n + 1)(KLCeq)n + n(KLCeq)n + 1] ⎡ ⎤ K K (1 − KLCeq)⎢1 + KS − 1 KLCeq − KS (KLCeq)n + 1⎥ ⎣ ⎦ L L

(

acetate units in the polymer appears to differ significantly from the initial feed ratios. This can be explained by the difference in polymerization rates between the two monomers, with NIPAM being a known “more-activated” monomer (MAM) carrying a radical-stabilizing substituent in the form of an acrylamide electron-withdrawing group, while VAc is understood as a “lessactivated” monomer (LAM).34,35 Using the Kelen−Tudos least-squares technique,36 the reactivity ratios for NIPAM and VAc in this study have been calculated as 5.9 and 0.2, respectively, which supports the notion of a difference in polymer reactivity. The result is a discrepancy between the feed and resulting comonomer ratios as the MAM is incorporated into the growing copolymer chain by preference, yielding a gradient structure rather than a random copolymer. Nevertheless, increasing the feed quantities of VAc to NIPAM increased the acetate incorporation into the copolymers. The polymer was then hydrolyzed under mild basic conditions to yield P(NIPAM-co-VA). Near-complete hydrolysis of the acetate group was confirmed by NMR in Figure 1B with the disappearance of the VAc acetate peak (peak h, δ = 1.98 ppm) and the movement of the backbone −CH− from

)

where Ceq is the residual concentration of polymer in solution (mg/ mL), Y is the amount of adsorbed polymer per unit of surface, Ym is the saturated amount of monolayer adsorption per unit of surface, KS is the equilibrium constant of adsorption for the first layer, KL is the equilibrium constant of adsorption for the second layer and above, and n is the number of layers. Values for the Ym, KL, and KS parameters were first obtained using the well-established method of the n = ∞ approximation at the low concentration region and then solving for the best average value of n.

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RESULTS AND DISCUSSION Polymer Synthesis. Temperature-responsive, xanthatefunctionalized polymers were prepared in a three-step procedure shown in Scheme 1. Initially, NIPAM was copolymerised with VAc to yield P(NIPAM-co-VAc). This was followed by hydrolysis of the acetate groups to produce P(NIPAM-co-VA), which was finally reacted with carbon disulfide to form P(NIPAM-co-VA-co-VX). A library of C

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Langmuir Table 1. Main Characteristics of the Synthesised P(NIPAM-co-VX-co-VA) Polymers NIPAM:VAc (mol) polymer precursor

feed

polymer

Mna (kDa)

Đa

xanthate product

% VXb

P(NIPAM493-co-VAc21) P(NIPAM471-co-VAc35) P(NIPAM560-co-VAc84) P(NIPAM436-co-VAc109) P(NIPAM223-co-VAc363)

80:20 70:30 60:40 40:60 20:80

96:4 93:7 87:13 80:20 38:62

57.6 56.3 70.6 58.7 56.5

1.81 1.82 1.82 1.82 1.80

P(NIPAM493-co-VA10-co-VX11) P(NIPAM471-co-VA10-co-VX25) P(NIPAM560-co-VA33-co-VX51) P(NIPAM436-co-VA11-co-VX98) P(NIPAM223-co-VA180-co-VX183)

2 5 8 18 32

a Calculated from GPC in DMF, based on linear polystyrene standards. b% VX composition in final polymer as calculated by NMR. As the pKa of xanthic acid is between pH 1.5 and 337 the apparent charge density at neutral pHs and above is expected to equal the % VX.

peak g (δ = 4.81 ppm) to peak j (δ = 4.00 ppm). Peak e shifts slightly to the right due to the change in NMR solvent from CDCl3 to D2O and forms an overlap with peak j. Although the presence of the H2O peak obscures peak g and leaves some ambiguity as to the degree of hydrolysis, the FTIR results (discussed below), the high solubility in water, and integration of peak j indicate that the polymer is almost fully hydrolyzed into P(NIPAM-co-VA). Xanthation of the P(NIPAM-co-VA) was then performed with the aim to produce P(NIPAM-co-VX). Dialysis was used to purify the end product due to high polymer solubility in water. The conversion of VA units to VX was found to be incomplete as determined by NMR using the backbone −CH− present in VX (peak m, δ = 4.50 ppm), as shown in Figure 1C and Table 1. Other than P(NIPAM223-co-VA11-co-VX98), all samples displayed alcohol-to-xanthate conversions within the range of 50−70%. A slightly higher molecular weight was found with P(NIPAM560-co-VAc84) at 70.6 kDa compared to the other four samples at the 55−60 kDa range. The 1H NMR characterization is supported by UV−vis and FTIR measurements conducted at each step of the synthesis process, shown in Figures 2 and 3. The emergence of a clear,

Figure 3. FTIR spectra of dry samples of (A) P(NIPAM-co-VAc), (B) P(NIPAM-co-VA), and (C) P(NIPAM-co-VA-co-VX). The hydrolysis and xanthation of the PNIPAM copolymers are supported by the disappearance of the VAc CO stretch and the appearance of a strong a VX C−O−C stretch going from (A) to (C).

at 1000−1200 cm−1 and a very weak S−H peak at 2550 cm−1. The hydroxyl stretch at 3350 cm−1 does not fully disappear due to the residue VA groups. While all three copolymers contain alkoxy C−O groups, the alcohol C−O peak for VA is expected to be weaker than the ester C−O−C peak for VAc and the ethoxy C−O−C peak for VX, resulting in a dampening and subsequent strengthening of the alkoxy peak over the synthesis process. The presence of the NIPAM amide carbonyl stretch at 1650 cm−1 for all three samples also indicates that the NIPAM groups are stable throughout the synthesis and are not cleaved under basic hydrolysis. From the GPC results in Table 1, despite having varying ratios of NIPAM-to-VAc comonomers, the molecular weights (Mn) of the polymers are very similar, at around 57 kDa, with a single outlier at 70 kDa. The polydispersity (Đ) values are consistently around 1.8 for all the prepared polymers. The units of comonomer for each polymer in Table 1 were calculated based on the NMR compositions and the Mn. Polymer Properties in Aqueous Solution. Effect of VAc Incorporation on LCST. The copolymerization of PNIPAM with hydrophobic functional groups has the potential to reduce the LCST below that of pure PNIPAM, and vice versa for hydrophilic functional groups, due to the change in the overall hydrophilicity of the copolymer.39 In the absence of pH effects, the copolymer may even end up being fully soluble or insoluble, losing any LCST properties. Increasing the VAc content causes the polymer to become more hydrophobic, and hence the LCST is expected to decrease, as observed in Figure

Figure 2. UV−vis spectra showing the emergence of a xanthate peak at 300 nm for the P(NIPAM-co-VA-co-VX) end product at polymer solutions of 0.20 mg/mL.

strong UV−vis peak at 300 nm corresponds to the known absorbance of xanthate29,38 and supports the presence of xanthate in the final product. The FTIR spectra in Figures 3A and 3B confirm the near-complete hydrolysis of the P(NIPAMco-VAc) samples into P(NIPAM-co-VA) with the disappearance of the VAc ester carbonyl CO stretch at 1720 cm−1 and a reduction in the acyl and alkoxy stretches at 1250 and 1050 cm−1. Following xanthation, there is a strong alkoxy C−O peak D

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to play a role in affecting the LCST as the polymer approaches a statistically random conformation, with reduced NIPAM sequence lengths. Hence, the hydrophilic VA groups have a smaller impact on the LCST for the block copolymers, resulting in a smaller increase in LCST relative to our nonblock copolymers. Effect of VX Incorporation on LCST. While the presence of 4% VA in P(NIPAM493-co-VA21) is capable of raising the LCST from 33.5 ± 0.5 to 34.2 ± 0.5 °C, 2% incorporation of VX in P(NIPAM493-co-VA10-co-VX11) increases the LCST by a further 5.0 °C to 39.2 ± 0.5 °C, as illustrated in Figure 4C. This increase in LCST can be attributed to the significant hydrophilicity of the xanthate groups. While work with other hydrophilic groups such as N-hydroxymethylacrylamide,45 N,Ndimethylacrylamide,46 and 2-hydroxyisopropylacrylamide47 has yielded high LCSTs above 70 °C, these typically incorporate moderate to high degrees of comonomer. In terms of LCST change per unit of incorporation, some of the highest reported include acrylamide,48 with a 4 °C increase in LCST with 6% incorporation, and methacrylic acid,49 which provides an increase of 4.2 °C with 2.4% incorporation under particular pH conditions. This is still lower than the 5 °C increase in LCST observed with just the presence of 2% VX going from P(NIPAM 493 -co-VA 21 ) to P(NIPAM493 -co-VA 10 -co-VX 11 ). When the use of other non-PNIPAM temperature-responsive polymers with higher LCSTs is not a viable option, this technique represents a way to control the LCST of PNIPAM with minimal incorporation of additional comonomers. However, when the xanthate composition was increased further, a different transition behavior was observed for P(NIPAM 471 -co-VA 10 -co-VX 25 ) (5% mol VX) and P(NIPAM 560 -co-VA 33 -co-VX 51 ) (8% mol VX). While P(NIPAM493-co-VA10-co-VX11) displayed a characteristic sharp drop in transmittance at the LCST, the polymers with 5% and 8% VX incorporations showed a gradual drop in transmittance over a range of 10 °C. While large transition ranges are sometimes observed with sterically hindered temperatureresponsive polymers containing a methyl group in the backbone (such as poly(N-n-propylmethacrylamide)),50 such a methyl group is not present for P(NIPAM-co-VA-co-VX). When the amount of VX composition is increased even further above 8%, no LCST was observed below 60 °C, as observed with the P(NIPAM436-co-VA11-co-VX98) (18 mol % VX) and P(NIPAM223-co-VA180-co-VX183) (32 mol % VX) copolymers. To investigate further, both samples were heated in an oil bath up to 98 °C, and no visual obscuration was found. This is not unexpected, as the phase transition is known to disappear with an increase in charge density.51 The observed broadening of the LCST could be due to the fact that the VX copolymer is a polyelectrolyte, unlike VA and VAc. As xanthation was carried out with KOH, the xanthate groups were likely present as alkali metal salt instead of free xanthic acid. In either case, the VX comonomers will be charged, since the pKa of xanthic acid is within the range of pH 1.5−3.37 Therefore, the deswelling of the polymer chain during the LCST transition will be hindered by the charge repulsion between the xanthate groups, which need to be neutralized before the polymer can contract into a globular state. This will be dictated by the rate of counterion condensation, which in turn will depend on the ionic content, namely the availability and mobility of the K+ cations.52 Under this model, higher salt concentrations would be expected to produce narrow LCST transitions for the P(NIPAM-co-VA-co-VX) copolymers due to

4A. The hydrogen bonding between the amide group in NIPAM and the carbonyl group in VAc results in less NIPAM

Figure 4. UV−vis transmittance measurements of the LCST for the (A) P(NIPAM-co-VAc), (B) P(NIPAM-co-VA), and (C) P(NIPAMco-VA-co-VX) copolymers at sample concentrations of 10 mg/mL. Distilled water was used as a baseline for 100% transmittance. Pure PNIPAM is provided as a basis for comparison.

units being available for binding to water,40 eventually leading to insolubility, as seen with P(NIPAM223-co-VAc363) (62% mol VAc). This sample remained insoluble when cooled to 0 °C, confirming the loss of a LCST. As a baseline for comparing the LCSTs of the polymers in water, a pure PNIPAM sample with a Mn of 32 kDa was used as a reference point. The sample had an LCST of 33.5 ± 0.5 °C, which differs slightly from known literature values of 32 °C.41 This may be due to a combination of several effects, including the molecular weight of the polymer, LCST kinetics, pH, and salt content.40,42 Effect of VA Incorporation on LCST. In contrast to PVAc, PVA is hydrophilic and water-soluble, and the LCST of the resulting P(NIPAM-co-VA) copolymers is expected to increase with VA incorporation. This is demonstrated in Figure 4B, with the LCST increasing from 34.2 ± 0.5 to 42.8 ± 0.5 °C as more alcohol groups are present. The increase in overall hydrophilicity raises the temperature required before the transition of the polymer into a globular conformation occurs. The LCST range is slightly higher than that of other studies on similar polymers,43 which recorded a LCST of 35 °C for P(NIPAM85b-VA170). A potential explanation lies with the NIPAM sequence lengths: as the chain of temperature-responsive comonomers becomes longer in a copolymer, the copolymer is more likely to express a LCST closer to the pure comonomer.44 Conversely, the effect of copolymerization with hydrophilic/hydrophobic functional groups is more likely E

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Langmuir the charge screening effects of the salt as well as the increased likelihood of counterion condensation. This was indeed observed in Figure 5 for P(NIPAM560-co-VA33-co-VX51), as a

Figure 6. Quantity of polymer and PAX, a small molecule xanthate adsorbed onto chalcopyrite surfaces versus the residual solution concentration after 6 h. (B) displays a magnification of (A) in the region of 0−3 mg/m2 of polymer adsorption. The solid lines are isotherms constructed assuming a finite-layer BET adsorption model. Polymer adsorption onto chalcopyrite increases with xanthate composition, with multilayer formation for P(NIPAM436-co-VA11-coVX98), P(NIPAM223-co-VA180-co-VX183), and PAX.

Figure 5. Effect of salt concentrations on the LCST characteristics of P(NIPAM560-co-VA33-co-VX51) at sample concentrations of 10 mg/ mL. Distilled water was used as a baseline for 100% transmittance. Reduced LCSTs and a narrow transition range were observed at increasing salt concentrations.

atures above the LCST, whereby the hydrophobic polymer in solution continually deposits onto the polymer layer(s) present on the adsorption surface.24 Effectively, multilayering occurs because the amount of available surface does not deplete itself with increased polymer adsorption, as the newly attached polymer itself acts as a suitable interface for further polymer deposition through hydrophobic interaction. Since the adsorption isotherms in the current work were measured at a constant temperature (∼25 °C) that was below the LCST of the polymers, this was not expected to have a large effect. Nevertheless, a finite-layer BET model was used to account for the possible presence of multilayer adsorption, as the model reverts to the Langmuir monolayer isotherm if the fitted number of layers (n) is found to be one. The modeled parameters for the finite-layer BET model have been tabulated in the Supporting Information. The incorporation of a VX group has a significant effect on the adsorption onto chalcopyrite surfaces, with the surface adsorption increasing with increasing xanthate composition (Figure 6). Furthermore, the negligible adsorption of PNIPAM and the low adsorption of P(NIPAM471-co-VA35) suggest that it is the xanthate that is adsorbing onto the chalcopyrite surface, not the NIPAM or residue VA groups. The single outlier is a slight decrease in adsorption going from P(NIPAM471-co-VA10co-VX25) to P(NIPAM560-co-VA33-co-VX51), which may be due to the increased VA presence in the latter having an effect on the attachment of the VX species onto the mineral surface, but the difference is small enough to be an instrumental error. The significant jump in adsorption with P(NIPAM436-co-VA11-coVX98) (18 mol % VX) and P(NIPAM223-co-VA180-co-VX183) (32 mol % VX) may potentially be attributable to the polymer chain being forced to adsorb to the surface in a particular conformation following the increase in charge density,59 although the more likely explanation is the formation of multilayers with the increase in xanthate composition. Based on the fitted n parameters in the finite-layer BET isotherm, the polymer adsorbs as a monolayer at low VX incorporations, increasing to three layers for P(NIPAM436-co-VA11-co-VX98) and then to six for P(NIPAM223-co-VA180-co-VX183). Compared to the synthesized polymers, a higher amount of PAX adsorbs to the chalcopyrite surfaces, which is expected as PAX is a small molecule, free from the repulsive steric interactions typically

rapid drop in transmittance was achieved in the presence of 0.01 M KCl. The increase in salt concentration also lowers the LCST, which is a well-documented effect.53,54 Interestingly, the presence of a wide LCST range which narrows upon salt addition has also been observed with PNIPAM copolymers containing high-density, grafted, charged poly(2-vinylpyridine) side chains, whereby it was suggested that the broad LCST may be a result of the electrostatic forces of the side chains preventing a full collapse of the PNIPAM.55 Another factor that could have resulted in the large temperature range of the phase transition is the presence of a gradient structure in the copolymers, with different sections of the polymer chains transitioning at different temperatures due to variations in comonomer densities. Furthermore, at a Đ of 1.8, differences in Mn may have played a role in a similar fashion, as a higher chain length corresponds to an increased LCST.56 However, the influence of Mn on the LCST is typically minimal (±1 °C), and under both explanations, a change in salt content should have little effect on the width of the transition zone, although it is possible that the presence of salt may have overwhelmed the dispersity effect stemming from the gradient structure. Effect of Xanthate Incorporation on Surface Adsorption. Xanthate functional groups have an exceptional affinity and selectivity for particular metal ions and surfaces.26,29 To ascertain if this functionality is still present in the synthesized polymers, adsorption isotherms were obtained comparing the adsorption of the VX copolymers to surfaces composed of chalcopyrite (CuFeS2) and quartz (SiO2) at room temperature, as shown in Figure 6. Chalcopyrite and quartz were chosen for this work to simulate the well-known separation of chalcopyrite from quartz minerals with small molecule xanthate surfactants in the froth flotation process, a key operation in the production of copper.29,30 The adsorption isotherms of potassium amyl xanthate (PAX) are also shown in Figure 6 for comparison. PAX was chosen as it is one of the most widely used examples of the aforementioned xanthate surfactants in flotation. These isotherm results agree with previously reported data.57,58 Previous studies on temperature-responsive polymers have suggested that multilayer adsorption mainly occurs at temperF

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The adsorption of the VX copolymers appears to be hugely in favor of chalcopyrite over quartz, indicating that the VX retains its affinity and attachment properties in P(NIPAM-coVA-co-VX). Although the isotherms were conducted separately on single-mineral systems instead of mineral mixtures, and selectivity is partially dependent on adsorption kinetics in addition to isotherms, these results can be taken to suggest that the polymer can be expected to be selective toward chalcopyrite surfaces over quartz. Furthermore, high incorporations of VX are not required to impart this property, as this behavior is present even at low VX compositions between 2 and 8%, where the LCST of the copolymer remains in an accessible range. Hence, such copolymers are likely to be useful in mining applications such involving the selective recovery of chalcopyrite from quartz.

observed with adsorbed polymer layers, as well as possessing close to 100% xanthate functionality, assuming minimal degradation of the xanthate into alcohol. The multilayer formation of the high % VX polymers is consistent with the PAX isotherm, modeled at seven layers. This is potentially due to the formation of dixanthogen bonds between the first layer and subsequent layers occurring for samples above a sufficient level of xanthate incorporation. While the adsorption of P(NIPAM436-co-VA11-co-VX98) (18 mol % VX) and P(NIPAM223-co-VA180-co-VX183) (32 mol % VX) onto chalcopyrite surfaces is significantly higher than the other synthesized polymers, the use of these materials is less desirable due to the absence of an LCST in accessible ranges. Nevertheless, polymer adsorption above 1.5 mg per m2 of mineral surface can be imparted even with low incorporations of VX between 2 and 8%. While these levels of adsorption are significantly lower than those observed for PAX, it has been noted that monolayer coverage with surfactant is sufficient for flotation,60 and high VX compositions are unnecessary. If higher levels of polymer attachment are required, the addition of salt or a hydrophobic comonomer may be needed in conjunction with high VX compositions such that an LCST is present at reasonable temperatures, although it is unknown how the introduction of these factors will impact the adsorption isotherms. The adsorption of PAX and the P(NIPAM-co-VA-co-VX) copolymers onto chalcopyrite is contrasted with the adsorption of the same copolymers onto quartz (Figure 7). The difference

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CONCLUSIONS Temperature-responsive polymers containing varying degrees of xanthate functionality were synthesized in the form of P(NIPAM-co-VA-co-VX), through the hydrolysis of P(NIPAMco-VAc) into P(NIPAM-co-VA) followed by reaction with carbon disulfide. The addition of hydrophobic acetate groups in P(NIPAM-co-VAc) resulted in lower LCSTs compared to pure PNIPAM, while increasing the content of the hydrophilic alcohol group in P(NIPAM-co-VA) raised the LCST. P(NIPAM493-co-VA10-co-VX11) displayed a 5 °C increase in LCST over P(NIPAM493-co-VA21) following the incorporation of 2% VX, with a rapid change in conformation. At 5−8% incorporation of xanthate, the LCST remained relatively similar to the P(NIPAM493-co-VA10-co-VX11), rising only slightly with the increased xanthate content. However, this was accompanied by a wide temperature range for the phase transition due to the charge repulsion of the xanthate groups hindering the deswelling of the polymer, which can be suppressed with the addition of salt. An LCST was not observed up to 98 °C for P(NIPAM-co-VA-co-VX) with 18% and above xanthate functionality. Adsorption isotherms conducted with the P(NIPAM-co-VAco-VX) polymers on chalcopyrite (CuFeS2) and quartz (SiO2) surfaces indicate an overwhelming affinity for chalcopyrite. The adsorption of all polymer samples on quartz was found to be negligible, while an increase in xanthate incorporation increased the amount of polymer adsorbed on the chalcopyrite surface. The hydrophilic nature and surface affinity of the xanthate group appear to be intact in the copolymer. Copolymers with VX compositions between 2 and 8% content possess sufficient affinity for adsorption to chalcopyrite surfaces relative to quartz while displaying an LCST within an accessible range.

Figure 7. Quantity of adsorbed polymer versus the residual concentration in solution after 6 h for the P(NIPAM-co-VA-co-VX) samples on chalcopyrite (dotted lines) and quartz (solid line) surfaces. The lines are isotherms constructed assuming a finite-layer BET adsorption model. Adsorption onto quartz is uniformly poor at all xanthate incorporations.

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is clear: the adsorption of the xanthate-functional materials onto chalcopyrite surfaces is significantly higher than the adsorption onto quartz, even for the sample with the least VX incorporation. In fact, the adsorbed quantities onto the quartz surfaces were found to be so small that they were close to the measurement error of the TOC analysis. Furthermore, negative R2 values were obtained when modeling the adsorption onto quartz. While this may indicate that the finite-layer BET model is incorrect for the data, the more likely reason is that there is negligible adsorption. Additionally, an increase in the xanthate incorporation had no effect on the adsorption of polymer onto quartz. An enlarged plot showing the quartz isotherms can be found in the Supporting Information.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00211. Measured pH values for the polymer solutions during LCST determination; modeled finite-layer BET isotherm parameters for the adsorption of the synthesized polymers and potassium amyl xanthate onto quartz and chalcopyrite surfaces; expanded adsorption isotherm for the attachment of the synthesized polymers onto quartz surfaces (PDF) G

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(11) Ng, W.; Connal, L. A.; Forbes, E.; Franks, G. V. Xanthatefunctional temperature-responsive polymers as selective flocculants and collectors for fines recovery.. Miner. Eng. 2016, in press, DOI: 10.1016/j.mineng.2016.05.013. (12) Ionov, L. Hydrogel-based actuators: possibilities and limitations. Mater. Today 2014, 17 (10), 494−503. (13) Karg, M.; Pastoriza-Santos, I.; Rodriguez-González, B.; von Klitzing, R.; Wellert, S.; Hellweg, T. Temperature, pH, and Ionic Strength Induced Changes of the Swelling Behavior of PNIPAM− Poly(allylacetic acid) Copolymer Microgels. Langmuir 2008, 24 (12), 6300−6306. (14) Schild, H. G. Poly(N-isopropylacrylamide): experiment, theory and application. Prog. Polym. Sci. 1992, 17 (2), 163−249. (15) Li, C.; Buurma, N. J.; Haq, I.; Turner, C.; Armes, S. P.; Castelletto, V.; Hamley, I. W.; Lewis, A. L. Synthesis and characterization of biocompatible, thermoresponsive ABC and ABA triblock copolymer gelators. Langmuir 2005, 21 (24), 11026−33. (16) Wallyn, S.; Lammens, M.; O’Reilly, R. K.; Prez, F. D. Highly active, thermo-responsive polymeric catalytic system for reuse in aqueous and organic CuAAC reactions. J. Polym. Sci., Part A: Polym. Chem. 2011, 49 (13), 2878−2885. (17) Chen, Z.; Cui, Z.-M.; Cao, C.-Y.; He, W.-D.; Jiang, L.; Song, W.G. Temperature-Responsive Smart Nanoreactors: Poly(N-isopropylacrylamide)-Coated Au@Mesoporous-SiO2 Hollow Nanospheres. Langmuir 2012, 28 (37), 13452−13458. (18) Nash, M. A.; Yager, P.; Hoffman, A. S.; Stayton, P. S. Mixed Stimuli-Responsive Magnetic and Gold Nanoparticle System for Rapid Purification, Enrichment, and Detection of Biomarkers. Bioconjugate Chem. 2010, 21 (12), 2197−2204. (19) Tsai, H.-Y.; Lee, A.; Peng, W.; Yates, M. Z. Synthesis of poly(Nisopropylacrylamide) particles for metal affinity binding of peptides. Colloids Surf., B 2014, 114, 104. (20) Yin, J.; Guan, X.; Wang, D.; Liu, S. Metal-chelating and dansyllabeled poly(N-isopropylacrylamide) microgels as fluorescent Cu2+ sensors with thermo-enhanced detection sensitivity. Langmuir 2009, 25 (19), 11367−74. (21) Khine, Y. Y.; Jiang, Y.; Dag, A.; Lu, H.; Stenzel, M. H. DualResponsive pH and Temperature Sensitive Nanoparticles Based on Methacrylic Acid and Di(ethylene glycol) Methyl Ether Methacrylate for the Triggered Release of Drugs. Macromol. Biosci. 2015, 15 (8), 1091−1104. (22) Mi Kyong, Y.; Yong Kiel, S.; Chong, S. C.; Young, M. L. Effect of polymer complex formation on the cloud-point of poly(N-isopropyl acrylamide) (PNIPAAm) in the poly(NIPAAm-co-acrylic acid): polyelectrolyte complex between poly(acrylic acid) and poly(allylamine). Polymer 1997, 38 (11), 2759−2765. (23) Li, H.; O’Shea, J.-P.; Franks, G. V. Effect of molecular weight of poly(N-isopropyl acrylamide) temperature-sensitive flocculants on dewatering. AIChE J. 2009, 55 (8), 2070−2080. (24) O’Shea, J.-P.; Qiao, G. G.; Franks, G. V. Solid−liquid separations with a temperature-responsive polymeric flocculant: Effect of temperature and molecular weight on polymer adsorption and deposition. J. Colloid Interface Sci. 2010, 348 (1), 9−23. (25) Li, H.; Long, J.; Xu, Z.; Masliyah, J. H. Flocculation of kaolinite clay suspensions using a temperature-sensitive polymer. AIChE J. 2007, 53 (2), 479−488. (26) Rao, S. R. Xanthates and Related Compounds; M. Dekker: New York, 1971. (27) Cook, J. G. 1 - CELLULOSE FIBRES; RAYONS. In Handbook of Textile Fibres; Cook, J. G., Ed.; Woodhead Publishing: 2001; pp 9− 78. (28) Leffmann, H. The rayon industry: By Mois̈ H. Avram, B.Sc., M.E.Second Edition. 893 pages, illustrations, 8vo. New York, D. Van Nostrand Company, Inc., 1929. Price, $12.00. J. Franklin Inst. 1930, 209 (2), 277. (29) Fornasiero, D.; Montalti, M.; Ralston, J. Kinetics of Adsorption of Ethyl Xanthate on Pyrrhotite: In Situ UV and Infrared Spectroscopic Studies. J. Colloid Interface Sci. 1995, 172 (2), 467−478.

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (G.V.F.). *E-mail [email protected] (L.A.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Commonwealth Scientific and Industrial Research Organisation (CSIRO) as well as the Victorian Government Veski Innovation Fellowship for providing the financial support required for this work. We also thank Sioe See Volaric, Steve Peacock, Cheryl McHugh, the University of Melbourne Polymer Science Group (PSG), the Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), and the Monash Institute of Pharmaceutical Science (MIPS) for training and the use of equipment. Special thanks also to Angus Gray-Weale, David Danaci, Thomas McKenzie, Samuel Pinches, Emma Brisson, and our reviewers for their insightful recommendations and support at various stages of this study.

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ABBREVIATIONS AIBN, 2,2′-azobis(isobutyronitrile) initiator; DEE, diethyl ether; DMSO, dimethyl sulfoxide; FTIR, Fourier transform infrared spectroscopy; GPC, gel permeation chromatography; LAM, less-activated monomer; LCST, lower critical solution temperature; MAM, more-activated monomer; NIPAM, Nisopropylacrylamide; NMR, nuclear magnetic resonance; PAX, potassium amyl xanthate; PNIPAM, poly(N-isopropylacrylamide); THF, tetrahydrofuran; TOC, total organic carbon; VA, vinyl alcohol; VAc, vinyl acetate; VX, vinyl xanthate.

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