Triple Stimuli-Responsive N-Isopropylacrylamide Copolymer toward

Article pubs.acs.org/IECR

Triple Stimuli-Responsive N‑Isopropylacrylamide Copolymer toward Metal Ion Recognition and Adsorption via a Thermally Induced Sol− Gel Transition Jinjin Cheng, Guorong Shan,* and Pengju Pan State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Poly(N-isopropylacrylamide-co-maleic acid-co-1-vinylimidazole), P(NIPAM-MA-VI), has been prepared in water by free radical polymerization. The aqueous solution of low-molecular-weight P(NIPAM-MA-VI) has four distinct phases above a critical concentration. Effects of monomer ratio, copolymer concentration, and pH on the phase transition temperatures of P(NIPAM-MA-VI) were studied. The addition of divalent metal ions promoted the gelation process of P(NIPAM-MA-VI). Therefore, the adsorption behavior of P(NIPAM-MA-VI) to Cu2+ ions was further investigated; this adsorption behavior was pH-dependent, and the adsorption isotherm could be well fitted by the Freundlich model. The maximum adsorption capacity reaches 21.1 mg g−1 dried copolymer at 60 °C and pH 5. The adsorbed P(NIPAM-MA-VI) can be regenerated by the treatment of hydrochloric acid and reused in the following adsorption process. X-ray photoelectron spectroscopy (XPS) and synchrotron radiation small-angle X-ray scattering (SAXS) were used to investigate the effect of Cu2+ ions on the phase transition behavior of P(NIPAM-MA-VI). O 1s and N 1s XPS spectra and SAXS results analyzed by the Ornstein−Zernike and generalized Ornstein−Zernike models are in agreement with the proposed mechanisms and adsorption behavior. As a thermo-, pH-, and metal ion-responsive copolymer, P(NIPAM-MA-VI) could be used for the ion recognition, water purification, and enrichment of heavy metal ions.

1. INTRODUCTION The thermoresponsive polymers, whose physical configuration and chemical properties are responsive to the temperature, have been intensively studied for decades and widely used in drug delivery, sensors, tissue engineering, logic gates, and so on.1−5 In recent years, heavy metal ion pollution has gradually been a serious environmental problem due to its harm to the water ecosystem and human health. Therefore, the thermoresponsive polymers have been developed as the novel adsorbents or carriers of heavy metal ions in water purification. As one of the most studied thermoresponsive polymers, poly(N-isopropylacrylamide) (PNIPAM) changes from a hydrophilic random coil state to a hydrophobic globular state in aqueous solution upon heating to the lower critical solution temperature (LCST, about 32 °C).6,7 Therefore, PNIPAM has been widely studied as the thermosensitive polymer for potential applications in the temperature-controlled metal ion adsorption. At present, PNIPAM-based hydrogels have attracted much attention on the adsorption and detection of heavy metal ions such as copper(II) (Cu2+),8,9 nickel(II) (Ni2+),10 lead(II) (Pb2+) ions,11 and so on.12−14 Such kinds of chemical cross-linked gels have high water absorption, porous network structure, and high sensitivity to the temperature change.15,16 However, the diffusive resistivity of cross-linked gel networks may cause the slow adsorption/desorption rate of PNIPAM-based hydrogels to heavy metal ions.17 © XXXX American Chemical Society

In general, when the temperature is elevated to above LCST, the aqueous solution of PNIPAM precipitates at low concentration and exhibits a volume shrinking at high concentrations without turning into the physical cross-linked gel state. However, Han et al.18 found the aqueous solution (5 wt %) of high-molecular-weight (Mw ≈ 106) P(NIPAM-coacrylic acid) prepared in benzene experienced a reversible phase transition from a soluble solution to an insoluble gel state upon heating, while the low-molecular-weight (Mw ≈ 105) copolymer synthesized in 1,4-dioxane with the same feed ratio did not show the sol−gel transition and became shrunken at elevated temperatures. The reversible sol−gel transition was also found in the aqueous solutions of P(NIPAM-co-acrylamide) (Mw = 3.68 × 106, 0.2 g mL−1)19 and P(NIPAM-co-1-vinylimidazole) (Mw = 1.5 × 106, 5 wt %).20 Zhao et al.21 have copolymerized the zwitterionic monomers (e.g., carboxybetaine methacrylate and sulfobetaine methacrylate) with NIPAM. Even though these copolymers synthesized in deionized water had low molecular weights (Mw ≈ 104), they showed a clear sol−gel transition above LCST in solutions (8 wt %), due to the intermolecular association of zwitterionic groups. Therefore, for the NIPAM-based polymers, the formation of gel state is Received: September 19, 2016 Revised: December 19, 2016 Accepted: January 20, 2017

A

DOI: 10.1021/acs.iecr.6b03626 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

molecular-weight compounds. The polymer was finally isolated by freeze-drying. The copolymer compositions were measured by 1H nuclear magnetic resonance (1H NMR) spectroscopy. 1H NMR spectra were measured on a 500 MHz DMX-500 NMR spectrometer (BRULCER Co., Switzerland) with deuterium oxide (D2O) and dimethyl sulfoxide-d6 (DMSO-d6) as the solvent. Molecular weights of copolymers were measured by gel permeation chromatography (GPC, Waters 1515). The temperature of the column was 60 °C, and DMF was used as the mobile phase at a flow rate of 1.0 mL min−1. All of the GPC data were calibrated by the poly(methyl methacrylate) (PMMA) standards. 2.3. Measurement of Phase Transition Temperatures. The gelation process of copolymer was examined by a vial inversion method. Three milliliters of polymer solution in a glass vial was kept in a water bath with a constant temperature for 30 min. The temperature then was raised from 25 to 65 °C at 0.1 °C intervals. The sol−gel transition temperature (Tsol−gel) was determined visually when the polymer solution did not flow by inverting the vial. The gel-shrinking temperature (Tshrunken) was determined visually when the expelled droplet was observed by inverting the vial. After the gelation process, P(NIPAM-MA-VI) will transit from gel to clear solution upon cooling. The polymer solution then was added into a lidded glass cell and placed into the sample chamber of a UV−vis spectrophotometer (Shimadzu UV-1800) equipped with a temperature controller (Shimadzu S-1700). The temperature of the solution was raised from 15 to 55 °C at a rate of 1 °C min−1. The transmittance at 500 nm was recorded, and the LCST (or cloud point temperature) was defined as the temperature at which the transmittance decreased to 90%. 2.4. XPS Measurement. XPS spectra were collected on a VG Scientific ESCALAB MARK II spectrometer equipped with a Mg Kα X-ray source (1235.6 eV photons) and a hemispherical energy analyzer. P(NIPAM-MA-VI) shrunken gels (P-90, pH 5) before and after adsorption of Cu2+ ions were dried in a vacuum oven at 60 °C (−0.1 MPa); they were then added into a chamber and evacuated for 12 h with a base pressure less than 10−7 Pa. In the XPS data analysis, all of the core-level spectra were referenced to the C 1s neutral carbon peak at the binding energy (BE) of 284.6 eV. For fitting the spectral peaks, a Shirley-type background was subtracted from the signals. The recorded spectra were fitted using Gaussian−Lorentzian curves; the widths (fwhm) of the Gaussian peaks were maintained constant for all components. 2.5. Adsorption Experiment. To study the effect of pH on the Cu2+ adsorption of P(NIPAM-MA-VI), 157.9 mg of dried P-90 was added into a 30 mmol L−1 Cu(Ac)2 solutions (3 mL) with various pH values. The initial pH values of Cu(Ac)2 solutions were adjusted to 1.0−5.0 by hydrochloric acid (HCl). One milliliter of polymer solution (5 wt %) was transferred into a sealed plastic centrifuge tube. The centrifuge tube was placed in an air-circulating oven at 60 °C for 3 h. The concentration of Cu2+ ions in the expelled solution was determined with 2,9dimethyl-1,10-phenanthroline as complexing agent by a UV− vis spectrophotometer (Shimadzu UV-1800). The adsorption capacity q (mg g−1) of the polymer was expressed as follows:25

related to the various factors including molecular weight and chain entanglement. As the copolymers mentioned above are applied in the field of biomedical materials and electrochemical storage devices, we will apply the sol−gel transition behavior of NIPAM-based copolymer for the adsorption of heavy metal ions. In this work, we have copolymerized maleic acid (MA) and 1-vinylimidazole (VI) with NIPAM to prepare the zwitterionic copolymers. Although the P(NIPAM-MA-VI) copolymers synthesized in water had low molecular weight, they showed a clear sol−gel transition in the aqueous solution (5 wt %) at pH 5. This sol−gel transition did not exist for the P(NIPAMMA) and P(NIPAM-VI) solutions prepared by the same method. As the pH-sensitive units, MA and VI can bind with the divalent metal ions.22,23 It was found that the addition of metal ions promoted the gelation process and affected the phase transition temperatures of P(NIPAM-MA-VI).24 We chose Cu2+ ions as the model metal ion and investigated its effects on the thermo- and pH-dependent adsorption behavior of P(NIPAM-MA-VI). After the phase transition, P(NIPAMMA-VI) gel can release at least 85% of absorbed water that contains few heavy metal ions under the proper concentration of Cu2+ ions. The Cu2+-loaded P(NIPAM-MA-VI) can be regenerated and reused almost without the loss of Cu2+ uptake capacity. Therefore, this thermo-, pH-, and metal ionresponsive copolymer can be applied to the ion recognition, water purification, and enrichment of heavy metal ions. The mechanisms of adsorption behavior and phase transition were also investigated by X-ray photoelectron spectroscopy (XPS) and synchrotron radiation small-angle X-ray scattering (SAXS). This study has provided a simple method to prepare the novel and reusable multiresponsive polymer sensor and adsorbent of metal ions, and also has illustrated the mechanisms of phase transition and adsorption behavior of P(NIPAM-MA-VI) copolymers.

2. EXPERIMENTAL SECTION 2.1. Materials. MA (J&K Chemical), VI (Aladdin Reagent), sodium sulfite (Na2SO3, J&K Chemical), copper(II) acetate monohydrate [Cu(AC)2·H2O, J&K Chemical], nickel(II) acetate tetrahydrate [Ni(Ac)2·4H2O, J&K Chemical], lead(II) acetate trihydrate [Pb(AC)2·3H2O, J&K Chemical], and zinc acetate dihydrate [Zn(AC)2·2H2O, J&K Chemical] were used without further purification. Potassium persulfate (KPS, J&K Chemical) was recrystallized from water twice, and NIPAM (J&K Chemical) was recrystallized from hexane three times before use. All of the other chemical reagents used were of analytical grade. 2.2. Preparation and Characterization of P(NIPAMMA-VI). P(NIPAM-MA-VI) copolymers were prepared by free radical polymerization. Thirty millimoles of monomer mixtures and 0.625 mmol of KPS initiator were dissolved in 40 mL of deionized water. The mixed solution was added into a 100 mL of three-necked flask and thermostated in a water bath at 30 °C under continuous stirring. After the solution was purged with dried nitrogen for 30 min, 10 mL of sodium sulfite aqueous solution (6.25 × 10−2 mol L−1) was dripped slowly into the mixed solution to initiate the polymerization. The polymerization was performed at 30 °C for 12 h under the nitrogen atmosphere. The copolymer solution was further purified by dialysis (molecular weight cut off: 14 000) against deionized water for 3 days to remove the unreacted monomers and low-

q=

(C0V0 − CeVe) m

(1) −1

where C0 and Ce (mg L ) are the initial and final concentrations of Cu2+ ions in solution, respectively. V0 and B


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Industrial & Engineering Chemistry Research Table 1. Preparation, Composition, and Molecular Weight of Copolymers composition calculated by 1H NMR samples

feed molar ratio [NIPAM]/[MA]/[VI] (mol/mol/mol)

content of NIPAM (mol %)

content of MA (mol %)

content of VI (mol %)

Mn (×104)

P(NIPAM-MA) P(NIPAM-VI) P-80 P-85 P-90

90:5:0 90:0:5 80:10:10 85:7.5:7.5 90:5:5

98.0 95.2 90.8 92.9 94.7

2.0 0.0 2.3 1.9 1.5

0.0 4.8 6.9 5.2 3.8

16.8 7.6 4.3 4.5 5.5

Figure 1. Phase transitions of an aqueous solution of P(NIPAM-MA-VI) (P-90, 5 wt %) with increasing temperature.

Ve (L) are the initial and final volumes of solution, respectively. m (g) is the weight of dried P(NIPAM-MA-VI). The adsorption behavior of Cu2+ onto P(NIPAM-MA-VI) was measured in a similar way. The preweighed polymer (157.9 mg) was added into 3 mL of Cu(Ac)2 solutions with the Cu2+ concentrations ranging from 1 to 30 mmol L−1. The initial pH value of Cu(Ac)2 solutions was adjusted to 5 by hydrochloric acid. 2.6. Desorption and Recycling Experiment. After adsorption, the Cu2+-loaded shrunken gel was dissolved in 0.8 mL of HCl solution in the centrifuge tube. The polymer solution was then placed in an oven at 60 °C for 3 h. The final Cu2+ concentration in expelled solution was determined as described above. Desorption efficiency (%) of Cu2+-loaded polymer was expressed as follows: desorption efficiency (%) = −1

C′ × V / m × 100 q

distance from the sample to the detector was 1820 mm, and the X-ray wavelength was λ = 0.124 nm. Scattering patterns were collected by a Rayonix SX-165 CCD detector (Rayonix, IL), which had a resolution of 2048 × 2048 pixels and a pixel size of 80 × 80 μm2. Heating was provided by a Linkam hot stage, and measurements were carried out after 20 min equilibration time. 2D data were converted into 1D intensity I(q) as a function of the scattering vector q [q = (4π/λ) sin θ] by circularly averaging with Fit2D software, where 2θ was the scattering angle.26,27

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of P(NIPAMMA-VI). The preparation method of P(NIPAM-MA-VI) has been introduced in detail above. The monomer concentration was maintained as 0.6 mol L−1, and the feed molar ratios of NIPAM, MA, and VI were summarized in Table 1. GPC data (Table 1) showed that the copolymer containing a higher content of VI and MA had the lower molecular weight. Because P-80, P-85, and P-90 have the same chemical structures, we only show the 1H NMR spectra of P-80 in D2O (Figure S1a) and DMSO-d6 (Figure S1b) solvents. In Figure S1a, the methylene (peaks a and g) and methine (peaks b, e, f, and h) protons of the backbone show broad peaks at 1.22 and 1.66 ppm, respectively. The isopropyl methine proton (c) of the PNIPAM unit appears at δ = 3.54 ppm, while the methyl protons (d) appear at 0.80 ppm. The characteristic signals of imidazole ring appear at δ = 7.0−7.6 and 8.1−8.6 ppm. In Figure S1b, the peaks at 1.04, 1.42, and 1.95 ppm are attributed to the methyl (d), methylene (a and g), and methine (peaks b, e, f, and h) protons. The isopropyl methine and amide protons of PNIPAM appear at δ = 3.84 (peak c) and 7.21 ppm (peak m), respectively. The characteristic signal of MA carboxyl

(2)

where C′ (mg L ) is the final concentration of Cu ions in the expelled solution, V (L) is the volume of expelled solution, m (g) is the weight of dried polymer, and q (mg g−1) is the amount of Cu2+ ions adsorbed per unit mass of polymer. After the complete desorption of polymer by 0.5 mol L−1 HCl solution, the polymer was diluted to 10 mL, purified by dialysis, and isolated by freeze-drying. 157.9 mg of recovered polymer was added into 3 mL of Cu(Ac)2 solution (30 mmol L−1, pH 5), which was reused in the next adsorption− desorption cycle. To examine the reproducibility of P(NIPAMMA-VI), the adsorption−desorption cycle was repeated another three times. 2.7. Synchrotron Radiation SAXS Analysis. Synchrotron radiation SAXS experiments were performed on the BL16B1 beamline of Shanghai Synchrotron Radiation Facility. The 2+

C


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Industrial & Engineering Chemistry Research protons appears at δ = 11.0−14.0 ppm. Besides, 1H NMR spectra were measured to determine the composition of P(NIPAM-MA-VI) through comparing the intensities of peaks c, i, m, and n; the corresponding results are shown in Table 1.28−30 3.2. Thermally Induced Phase Transition. A typical thermally induced phase transition behavior of P-90 in aqueous solution (5 wt %) is shown in Figure 1. The corresponding phase transition temperatures are listed in Table 2. The clear

P(NIPAM-MA-VI), the presence of MA and VI leads to the formation of electrostatic attractions between carboxylate and imidazole ions, which promotes the chain aggregations and gelation. For the P(NIPAM-MA) and P(NIPAM-VI), the lack of electrostatic attraction makes it difficult for polymer chains to intertwine; thus the polymer chains precipitate directly without experiencing phases C and D. According to the composition and molecular weight in Table 1, it can be concluded that the amounts of carboxyl and imidazole groups in copolymer solutions are in the order of P-80 > P-85 > P-90. Therefore, P-80 and P-85 only form shrunken gels due to the stronger electrostatic attraction. The phase transition behavior of P(NIPAM-MA), P(NIPAM-VI), P-80, P-85, and P-90 in the phosphate buffered saline (PBS, pH 7.4, 0.2 mol L−1) with the concentration of 5 wt % was also investigated. All of the polymer solutions precipitated upon heating, and these phase transitions were irreversible. The effect of copolymer concentration (1, 3, 5, 7, and 9 wt %) on the phase transition temperatures was investigated, as shown in Figure S2. The gelation occurs when the concentration is higher than 5 wt %. With increasing concentration, more polymer chains change from the expended coils to collapsed globules upon heating, leading to a reduction of cloud point temperature. More globules would make the aggregation and gelation of polymer chains easier; therefore, both Tsol−gel and Tshrunken decrease as the concentration of P-90 solution increases from 5 to 9 wt %. 3.3. pH Dependence of Phase Transition Behavior. A certain amount of P-90 was added into hydrochloric acid solution, making it so the pH values varied from 1.0 to 6.0. The concentration of P-90 was kept as 5 wt %. The pH dependence of the phase transition behavior for P-90 solution is shown in Table S1. According to the gelation process mentioned above, the effect of pH on the phase transition can be ascribed to the ionization of carboxyl and imidazole groups. To explain the relationship between ionization and phase transition behavior, we list three possible neighbor interactions in Scheme 1. Equations 3 and 4 in Scheme 1 are assumed on the basis of a hydrogen-bonding mechanism. Equation 3 displays the possible formation of hydrogen bonds between the carboxyl and amide groups. Equation 4 predicts a water-mediated hydrogen bonding between the imidazole and amide groups. At the proper pH value, the imidazole rings can be protonated and attract carboxylate ions to form ion-pairs due to the electrostatic attraction, as shown in eq 5.31

Table 2. Phase Transition Temperatures of Copolymer Solutions (5 wt %) sample

cloud point temperature (°C)

Tsol−gel (°C)a

Tshrunken (°C)b

P(NIPAM-MA)-5 wt % P(NIPAM-VI)-5 wt % P-80-5 wt % P-85-5 wt % P-90-5 wt %

35.6 39.5 39.7 38.9 37.4

× × × × 39.1

− − 42.5 42.1 41.1

“×” represents no gel state found upon heating. b“−” represents no shrunken gel, but precipitate was found upon heating.

a

solution (phase A) became cloudy when the temperature reached 37.4 °C (defined as the LCST), but it was flowable (phase B) and even had reduced viscosity. As the temperature was further increased, the opaque polymer solution became nonflowable at 39.1 °C (defined as the sol−gel transition temperature, Tsol−gel). At 41.1 °C (defined as the gel-shrinking temperature, Tshrunken), the gel (phase C) started to shrink by expelling water (syneresis, phase D). Phase D turned into phase A directly in a few seconds upon cooling. It can be expected that the polymer chain remains in a fully expanded coil state at low temperature. A portion of the polymer chains collapses into compact globules upon heating. In the heating process, the uncollapsed polymer chains entangle with globules, followed by the chain aggregations due to the intermolecular interactions between hydrophobic groups and the attractions between carboxyl and imidazole groups. The gelation was induced by the formation of weak physical junctions.18 The phase transition behavior of P(NIPAM-MA), P(NIPAM-VI), P-80, and P-85 with the same concentration (5 wt %) was also studied. As shown in Table 2, P(NIPAM-MA) and P(NIPAM-VI) only show phases A and B, while P-80 and P-85 show phases A, B, and D upon heating. Phase C is only found in the aqueous solution of P-90. In the case of

Scheme 1. Available Neighbor Interactions between Amide, Carboxyl, and Imidazole Groups

D


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interactions between Cu2+ and carboxyl groups are unidentate and bidentate bindings, as shown in Scheme 2a and b. To clarify the adsorption behavior, the peaks at 531.2 eV are divided into two parts, which are assigned to the oxygen atoms in the −CONH2 and −COOH groups, respectively. After the adsorption of Cu2+ ions, the peaks at 531.2 (−CO in carboxyl groups) and 533.1 eV (−C−O−H in carboxyl groups) are not overlapped, and the binding energy of the −C−O−H peak (observed after Cu2+ adsorption) undergoes a significant increase, indicating that the carboxyl groups mainly exist in the form of unidentate carboxylates with Cu2+ ions, as shown in Scheme 2a.33 The N 1s spectra are also fitted by three Gaussian− Lorentzian peaks. From the N 1s XPS spectrum of P(NIPAMMA-VI) before the metal ion adsorption (Figure 3c), the three peaks at 399.4, 399.5, and 401.8 eV can be attributed to −NH2, CN−C (N(3)), and C−N−C (N(1)), respectively. Luo et al.34 have previously reported the binding energies of N(3) and N(1) of PVI at 398.6 and 400.6 eV, respectively. When the nitrogen atoms of pyridine and piperidine groups are protonated, the binding energy of N 1s electrons will increase by 2.0−2.4 eV. Thus, it is inferred that part of the protonated imidazole groups exist in P(NIPAM-MA-VI) gel at pH 5, whereas the content of imidazole ions is too low to display the corresponding peak in XPS spectrum. After the adsorption of Cu2+ ions, the binding energies of three peaks are 399.4, 400.7, and 401.8 eV, respectively (Figure 3d). As the lone pair electrons of N(3) are shared with Cu2+ ions, the binding energy of N(3) shifts from 399.5 to 400.7 eV. From the conductometric and viscometric measurements, it has been considered that every metal ion chelates four imidazole units in the Cu2+−PVI complex,35 as shown in Scheme 2c. 3.5. Effect of pH on Cu2+ Adsorption for P(NIPAM-MAVI) Solution. The Cu2+ adsorption of P-90 was investigated in Cu(Ac)2 solutions (0.03 mol L−1) with pH 1.0−5.0 at 60 °C (Figure 4). As seen from Figure 3 and Scheme 2, each Cu2+ ion attracts four imidazole groups or two negatively charged carboxyl groups through a chelate-connection, which can be used to explain the results shown in Figure 4. At pH 1, the imidazole groups are protonated and the carboxyl groups are uncharged; thus, P(NIPAM-MA-VI) does not adsorb Cu2+ ions. At pH 2, trace amounts of Cu2+ ions are adsorbed, and the polymer solution exhibits a sol−gel transition. However, P(NIPAM-MA-VI) in water cannot form a gel at the same concentration; this also illustrates that the chelation of Cu2+ ions with carboxylate/imidazole groups promotes the gelation process. With increasing the pH value, the imidazole ions are deprotonated, and the carboxyl groups are ionized into carboxylate ions; this significantly enhances the adsorption ability of polymer chains to Cu2+ ions. Therefore, a higher pH value is favorable to Cu2+ adsorption of P(NIPAM-MA-VI). As the Cu2+ uptake capacity reaches the highest at pH 5, the pH values of metal ion solutions used in the following experiments are all adjusted to 5. 3.6. Effect of Cu2+ Ions on Phase Transition Behavior for P(NIPAM-MA-VI) Solution. Figure 5 shows the phase transition temperatures of P-80, P-85, and P-90 aqueous solutions (5 wt %, pH 5) with different concentrations of Cu2+ ions. At pH 5, the carboxyl groups are ionized into carboxylate ions, and the imidazole ions are deprotonated. When the Cu2+ ions are added, the chelation of Cu2+ with carboxylate ions weakens the electrostatic repulsion. The Cu2+ ions are also coordinated to imidazole groups, which reduces the hydrogen

With the increase of pH, the ionization of carboxyl groups will be dominated by eq 3, resulting in the stronger electrostatic repulsion. This drives the polymer chains to extend and to be difficult to form a cluster, making the LCST shift toward the higher temperature.32 At the same time, the reverse reaction of eq 4 occurs; more hydrogen bonds are formed between polymer chain and water, which enhances the hydrogenbonding effects and also elevates the LCST. Therefore, LCST increases from 36.4 to 37.4 °C with the pH value increasing from 1.0 to 6.0. The change of pH has less effect on Tsol−gel, except for the case at pH 2. When the pH value is 2, the polymer chains precipitate directly without experiencing phases C and D. Most imidazole groups are protonated, and part of the carboxyl groups are ionized under this condition. It can be concluded that the interactions between imidazole and carboxylate ions in the solution make the polymer chains difficult to aggregate, exhibiting a wide range of phase transition temperatures in Figure 2. After the gel formation, the gel-

Figure 2. Temperature dependence of optical transmittance for P-90 aqueous solutions (5 wt %, pH 1.0−6.0).

shrinking temperatures at different pH values were also measured. At pH 1, the imidazole groups are fully ionized with few carboxylate ions, and the repulsive interactions among the imidazole cations hinder the collapse of polymer gel. The stable gel is maintained in a wide temperature range with Tshrunken up to 45.1 °C. When the pH value is above 3, the electrostatic repulsion, mainly caused by the carboxylate ions, is decreased because of the lower content of carboxyl groups and the formation of ion-pairs, leading to a reduction of Tshrunken as compared to the case of pH 1. As the pH value increases from 3 to 6, the amount of carboxylate ions increases with decreasing the amount of imidazole ions; the stronger repulsion results in a higher Tshrunken again. However, the change in Tshrunken is not obvious due to the low content of MA in copolymer. 3.4. XPS Measurement. The carboxyl and imidazole groups can bind with divalent metal ions; thus, the synthesized polymer can be used as the adsorbents for heavy metal ions. To further study the interactions between adsorbents and adsorbates, O 1s and N 1s XPS spectra of P(NIPAM-MA-VI) before and after the adsorption of Cu2+ ions were measured, as shown in Figure 3. The deconvolution of O 1s peak in Figure 3a and b yields two contributions. The peaks at 532.9 eV (observed before Cu2+ adsorption) and 533.1 eV (observed after Cu2+ adsorption) are attributed to the −C−O−H groups, whereas the peaks at 531.2 eV (before and after Cu2+ adsorption) are attributed to the −CO groups. Possible E


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Figure 3. O 1s and N 1s XPS spectra of P(NIPAM-MA-VI) before (a,c) and after (b,d) adsorption of Cu2+ ions.

one hand, the addition of Cu2+ ions increases the electrostatic screening effect and reduces the electrostatic interactions between polymer chains.36 On the other hand, the Cu2+ ions coordinated to imidazole groups and carboxylate ions make it easier for polymer chains to intertwine, aggregate, and form a gel. As shown in Figure 5c, at the low concentrations of Cu2+ ions, Tshrunken decreases with increasing Cu2+ concentration. It can be inferred that a spot of carboxylate ions still exists without chelating with Cu2+ ions. Under this condition, the electrostatic repulsion will be reduced as more Cu2+ ions are added, which impels the collapse of polymer gel. When the concentration of Cu2+ ions reaches 0.01 mol L−1, more imidazole groups are associated by Cu2+ ions, causing the formation of stronger physical junctions and the increase of Tshrunken. Tshrunken decreases again as the Cu2+ concentration exceeds 0.02 mol L−1, due to the charge shielding effect and/or specific interaction with imidazole rings.20 3.7. Metal Ion-Sensitive Phase Transition of P(NIPAMMA-VI) Solution. To determine the metal ion sensitivity of P(NIPAM-MA-VI), the solutions of Cu(Ac)2, Ni(Ac)2, Pb(Ac)2, and Zn(Ac)2 were prepared with the same molar concentration and pH value (0.03 mol L−1, pH 5). The pH values of solutions were adjusted by hydrochloric acid. A certain amount of P-90 then was added into the metal ion solution to ensure the copolymer concentration to be 5 wt %. The effect of diverse divalent metal ions on phase transition temperatures of P-90 solution is shown in Figure 6. When the various divalent metal ions (Cu2+, Ni2+, Pb2+, and Zn2+) with the same molar concentration are added into P-90, the LCST, Tsol−gel, and Tshrunken are obviously dependent on the metal ions, which may be attributed to the discrepancy of coordination ability between the carboxylate ions/imidazole units and divalent metal ions.24 Therefore, it is concluded that P(NIPAM-MA-VI) can recognize the metal ion species from phase transition temperatures, which can be used as the sensor of metal ions.

Scheme 2. Proposed Structures of Unidentate Carboxylate (a), Bidentate Carboxylate (b), and Imidazole Groups (c) for P(NIPAM-MA-VI)-Cu2+ Complexes

Figure 4. Effect of pH on Cu2+ adsorption of P(NIPAM-MA-VI) solution (5 wt %).

bonds between polymer chains and water. Thus, LCST decreases with increasing the Cu2+ concentration in all cases. In the case of P-80 and P-85, the gel forms when the concentration of Cu2+ ions is higher than 15 mmol L−1. Tsol−gel is gradually lowered with increasing Cu2+ concentration. On the F


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Figure 5. Phase transition temperatures of P-80 (a), P-85 (b), and P-90 (c) aqueous solutions (5 wt %, pH 5) with different concentrations of Cu2+ ions.

adsorbed per unit mass of polymer at equilibrium, qmax (mg g−1) is the maximum adsorption capacity of Cu2+ ions, b (L mg−1) is the Langmuir isotherm constant related to the free energy of adsorption, K is the Freundlich isotherm constant related to the adsorption capacity, and 1/n is the heterogeneity factor related to the adsorption intensity. The Langmuir model is based on the assumption of monolayer adsorption onto the homogeneous surface with equivalent binding sites, while the Freundlich model is applied to describe the surface heterogeneity or support binding sites of various affinities. Table 3 presents the corresponding Langmuir and Freundlich parameters obtained from Figure S3. According to

Figure 6. Phase transition temperatures of P-90 solutions (5 wt %) loaded with different divalent metal ions with the same molar concentration and pH value (0.03 mol L−1, pH 5).

Table 3. Langmuir and Freundlich Isotherm Constants for Cu2+ Adsorption at 60 °C

3.8. Adsorption of Cu2+ Ions. The Cu2+ adsorption of P(NIPAM-MA-VI) at 60 °C was further measured in the Cu(Ac)2 solution (pH 5), as shown in Figure 7. Within a

Langmuir model −1

Freundlich model −1

qmax,cal (mg g )

b × 10 (L mg )

R

21.1

3.63

0.977

3

2

1/n

K

R2

0.432

0.861

0.999

the correlation coefficient (R2), it is found that the adsorption of Cu2+ ions can be described by both Langmuir and Freundlich models in the concentration range of 5−30 mmol L−1; moreover, the Freundlich model fits better than the Langmuir model. On the basis of the constant b, the essential characteristic of Langmuir model can be expressed in terms of a dimensionless constant RL, which is defined as follows:33 1 RL = 1 + bC0 (8) where C0 (mg L−1) is the initial concentration of Cu2+ ions in solution. The calculated value of RL is greater than 0 and less than 1, indicating a favorable adsorption of Cu2+ ions onto P(NIPAM-MA-VI). In addition, the heterogeneity factor 1/n is less than 1, which also represents that the adsorption intensity is favorable over the entire range of concentrations studied. From the Langmuir model, the maximum adsorption capacity of Cu2+ ions onto P(NIPAM-MA-VI) reaches 21.1 mg g−1 dried polymer at 60 °C.39 Upon heating, P(NIPAM-MA-VI) solutions with Cu2+ ions will transform into a gel and shrunken gel phase. Part of the Cu2+ ions will be adsorbed by the collapsed chains, and more than 85% of the aqueous solution is expelled out of the gel. Figure 7 shows the adsorption ratio of Cu2+ ions onto P(NIPAM-MA-VI) at pH 5. At the lower Cu2+ concentration, the polymer chains offer a higher ratio of active sites to the total Cu2+ ions in the solution.40 Therefore, Cu2+ ions can be

Figure 7. Effects of initial Cu2+ concentration on Cu2+ uptake capacity and adsorption ratio (5 wt % P-90, pH 5).

certain range, the Cu2+ adsorption capacity onto P(NIPAMMA-VI) increases with increasing Cu(Ac)2 concentration. The experimental data are analyzed by the Langmuir (eq 6)37 and Freundlich (eq 7)38 models, as shown in Figure S3. Ce C 1 = + e qe qmax b qmax

log qe =

1 log Ce + log K n

(6) (7)

−1

where Ce (mg L ) is the equilibrium concentration of Cu2+ ions in solution, qe (mg g−1) is the amount of Cu2+ ions G


Article


desorption cycles. Therefore, P(NIPAM-MA-VI) is suitable for the ion recognition and water treatment due to its high recycling efficiency. 3.10. Synchrotron Radiation SAXS Analysis. We tried to use synchrotron radiation SAXS to investigate the impact of Cu2+ ions on the sol−gel transition of P(NIPAM-MA-VI).41,42 Figure 9 shows the double logarithmic plot of SAXS profiles for the P(NIPAM-MA-VI) solutions (5 wt %, pH 5) with and without Cu2+ ions at 30 (sol state) and 40 °C (gel state). There was no remarkable scattering peak found in these SAXS profiles. The generalized Ornstein−Zernike (GOZ) equation26,41 was used to analyze the SAXS curves for both sols and gels:

completely adsorbed by P(NIPAM-MA-VI) when the initial concentration of Cu(Ac)2 solution is as low as 1 mmol L−1. The percentage ratios then decrease from 83.2% to 49.8% with increasing Cu2+ concentration. It can be concluded that, at the proper concentration of Cu2+ ions, P(NIPAM-MA-VI) can adsorb total metal ions in the solution and release a large amount of clean water after the phase transition. By increasing the polymer concentration and introducing additional adsorption sites, the adsorption ratio can reach 100% at a higher metal ion concentration. 3.9. Desorption and Reusability. To ensure the recovery of metal ions and regeneration of polymer, desorption experiments were conducted on the Cu2+-loaded P(NIPAMMA-VI). The desorption data under 0.1 and 0.5 mol L−1 HCl solutions are shown in Figure 8. Obviously, Cu2+ ions loaded

I(q) =

IGOZ(0) ⎡ ⎣1 +

( D 3+ 1 )ξ2q2⎤⎦

D /2

(9)

where IGOZ(0) is the asymptotic value of the GOZ intensity at q → 0, ξ is the correlation length, and D is the fractal dimension. For the semidilute polymer solutions, ξ roughly translates to the average interchain polymer spacing.43 D is 1 for the elongated chains and 2 for the Gaussian chains in a theta solvent. For the nonuniform domains in heterogeneous gels, ξ is proportional to the average radius of the domains, and D is indicative of the “openness” of domains or aggregates. The D value is from 2 to 3 for the diffuse domains, 3 for the agglomerated domains with rough surface, and 4 for the compact domains with smooth surface.44 The fitted results by eq 9 are shown in Figure 9a and Table 4. The correlation Figure 8. Desorption efficiency of Cu2+ ions from P(NIPAM-MA-VI) by HCl solutions.

Table 4. Parameters Obtained by Fitting the SAXS Profiles of P(NIPAM-MA-VI) with Equations 9 and 10 at 30 and 40 °C

on the polymer can be desorbed by HCl solution utilizing phase transition behavior. The desorption efficiency decreases with the increasing concentration of Cu(Ac)2 when the HCl solution (0.1 mol L−1) is added, while the loaded Cu2+ ions can be completely removed from all gels by the use of 0.5 mol L−1 HCl. Another three cycles of adsorption and desorption then were carried out to examine the potential of P(NIPAM-MA-VI) in practical applications. According to Figure 8, the desorption processes were performed in 0.5 mol/L HCl solution. As shown in Table S2, the Cu2+ adsorption capacity of regenerated P(NIPAM-MA-VI) is hardly affected after three adsorption−

P(NIPAM-MA-VI) D ξ (nm) ξOZ (nm)

P(NIPAM-MA-VI)+Cu2+

30 °C

40 °C

30 °C

40 °C

1.17 8.15 1.45

4.04 15.4 0.96

1.31 19.0 1.45

3.97 24.8 0.317

coefficients of these fits are all larger than 0.998. As shown in Figure 9a, the GOZ model fits well in the entire region for sols but only in the low-q region for gels. The fractal dimension D of solutions is closer to 1, indicating the elongated polymer chains due to the repulsion between carboxylate ions. When the

Figure 9. SAXS profiles of P(NIPAM-MA-VI) with and without Cu2+ ions at 30 and 40 °C in the low-q (a) and high-q (b) regions. The solid red lines in (a) show the fits by generalized OZ model, and the solid red lines in (b) show the fits by OZ model. H



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Cu2+ ions are added into the P(NIPAM-MA-VI) solution at 30 °C, the divalent metal ions chelate with carboxylate ions and imidazole groups; thus, the electrostatic repulsion is reduced and polymer chains are less extended. Therefore, the D value increases from 1.17 to 1.31. Moreover, the addition of Cu2+ ions breaks the interchain hydrogen bonds, resulting in an increase of interchain polymer spacing, as shown in Table 4. As the temperature increases to 40 °C, the expanded chains collapse into compact globules and aggregate to form a gel with the weak physical junctions. The D value is equal to 4, indicating a smooth surface of compact domains. The addition of Cu2+ ions leads to an increase of ξ, indicating a larger correlation length of cross-linked domains after adsorption. This result resulted from the reduction of electrostatic repulsion. To estimate the scattering intensity of free polymer chains, the Ornstein−Zernike (OZ) equation26,45 was used to analyze the SAXS curves at the high-q region (q > 0.5 nm−1): I(q) =

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b03626. Partial experimental procedures, characterizations of copolymers, phase transition temperatures, recycling experiments, and scanning electron microscopy images of P(NIPAM-MA-VI) gels with and without Cu2+ ions (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-571-87951334. E-mail: [email protected]. ORCID

Guorong Shan: 0000-0001-5676-6310 Pengju Pan: 0000-0001-6924-5485 Notes

IOZ(0) 1 + ξOZ 2q2

Article

The authors declare no competing financial interest.

■

(10)

ACKNOWLEDGMENTS We acknowledge the financial support of the Outstanding Youth Foundation of Zhejiang Province (R4110199). We are grateful to BL16B1 beamline, Shanghai Synchrotron Radiation Facility, for SAXS measurements.

where IOZ(0) is the asymptotic value of the OZ intensity at q → 0, and ξOZ is the correlation length that indicates the size of free polymer chains.46 The fitted results by eq 10 are shown in Figure 9b and Table 4. For the sols, the addition of Cu2+ ions has no effect on the size of free polymer chains. For the gels, the added Cu2+ ions make more polymer chains aggregate and form a gel by interacting with the carboxylate ions and imidazole groups, which also decreases the size of the free polymer chains and the value of ξOZ. The SAXS results are in agreement with the mechanisms of phase transition and adsorption behavior proposed above.

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REFERENCES

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4. CONCLUSIONS P(NIPAM-MA-VI) exhibiting thermo- and pH-sensitive phase transition behavior has been synthesized, characterized, and applied on heavy metal ion adsorption. Upon heating, the aqueous solution of P(NIPAM-MA-VI) experiences four distinct phases (i.e., clear solution, opaque solution, gel, and shrunken gel) above a critical concentration. The addition of different divalent metal ions into the aqueous solution of copolymer can promote the gelation process, due to the discrepancy of coordination ability between carboxylate/ imidazole units and divalent metal ions. The phase transition of P(NIPAM-MA-VI) is tunable by adjusting the monomer ratio, polymer concentration, pH values, and concentration of metal ions. The adsorption of P(NIPAM-MA-VI) to Cu2+ ions is further investigated. At 60 °C, the adsorption capacity of P(NIPAM-MA-VI) increases as the pH value changes from 2.0 to 5.0. The maximum adsorption capacity of Cu2+ ions is 21.1 mg g−1 dried polymer in the Cu(Ac)2 solution at pH 5, and the adsorption isotherm can be well fitted by the Freundlich model. The mechanisms of adsorption behavior and phase transition are proposed and verified by XPS and SAXS measurements. When the temperature is above Tshrunken, P(NIPAM-MA-VI) gel will release at least 85% of absorbed water containing a few heavy metal ions under the proper concentration of Cu2+ ions. The Cu2+-loaded P(NIPAM-MA-VI) can be regenerated by hydrochloric acid. P(NIPAM-MA-VI) can be reused in the next three adsorption−desorption cycles. Therefore, the prepared triple-responsive copolymer can be used as a novel adsorbent and sensor of heavy metal ions. I


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Triple Stimuli-Responsive N-Isopropylacrylamide Copolymer toward

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