N2-Switchable Thermoresponsive Ionic Liquid Copolymer

Article Cite This: Macromolecules XXXX, XXX, XXX-XXX

pubs.acs.org/Macromolecules

CO2/N2‑Switchable Thermoresponsive Ionic Liquid Copolymer Yin-Ning Zhou,†,‡ Lei Lei,† Zheng-Hong Luo,*,‡ and Shiping Zhu*,† †

Department of Chemical Engineering, McMaster University, Hamilton, ON, Canada L8S 4L7 Department of Chemical Engineering, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, China 200240

‡

S Supporting Information *

ABSTRACT: Thermoresponsive random copolymers consisting of poly(N-isopropylacrylamide) (PNIPAM) and polymerized ionic liquid (IL) poly(1,1,3,3-tetramethylguanidine acrylate) (PTMGA) were synthesized via reversible addition−fragmentation chain transfer radical polymerization (RAFT). The reactivity ratios of NIPAM (rNIPAM = 2.11) and TMGA (rTMGA = 0.56) were determined by the extended Kelen−Tö düs method. Glass transition temperatures (Tg) of the copolymers were analyzed, which followed the Fox equation very well. The phase transition behaviors of the copolymers in aqueous solution were studied through UV−vis transmission measurements. Their lower critical solution temperature (LCST) ranged from 30.5 to 73.2 °C, depending on the hydrophilic IL content. The apparent pKa related to LCST was determined, and thus the protonation degree was calculated. The hydrophilicity of the copolymers could be regulated by gas treatments. Bubbling CO2 led to lowering the transition temperature while bubbling N2 resulted in its recovery. This CO2/N2 switchability became more profound with higher IL content. With the ability to undergo reversible protonation caused by the change of pH, the system showed good reversibility in LCST when bubbled with CO2 and N2. SO2 could also be used to lower LCST. However, a basic compound (e.g., NaOH) was required for its recovery. The pH-dependent solution phase transition behavior provided great insight into the LCST regulation mechanism. The widest LCST shifting window (∼12 °C) was found between pH 5.16 and 5.96, which could be fulfilled by the CO2 regulation approach. This work provides guidance for the design and synthesis of gas-switchable thermoresponsive polymers based on ionic liquids.

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INTRODUCTION Stimuli-responsive polymers as an emerging type of smart materials have received much attention over the past decade. Many interesting applications, such as smart coating, sensor, control release, and tunable catalysis, have been demonstrated.1−5 Among them, thermoresponsive polymers with lower critical solution temperature (LCST) have been well studied in terms of fundamental research and material design,6,7 with poly(N-isopropylacrylamide) (PNIPAM) having LCST of about 32 °C in water as the best known example.8 Over the decade, developing the PNIPAM-based polymers having a wide range of LCST’s represents a direction of intensive research efforts. The LCST of thermoresponsive polymers can be regulated using various approaches, which include addition of salt,9,10 control of polymer chain structure,11,12 supramolecular interaction,13−15 in situ degradation,16 design of new molecular structure,17,18 and incorporation of hydrophilic or hydrophobic comonomers.19−22 In the recent years, introduction of ionic liquid (IL) functionalities into PNIPAM-based polymer chains has attracted great interest because of the unique feature of polymerized IL (e.g., high charge density, high conductivity).23−33 Polymerized IL provides hydrophobicity or hydrophilicity, as well as abundant cationic or anionic charges, which © XXXX American Chemical Society

alter the physical properties of PNIPAM (e.g., LCST). Furthermore, thermoresponsive polymers with LCST tuned by external stimuli (e.g., pH) might suffer for the chemical contamination.34−39 Therefore, if LCST is modulated in situ with a trigger that could be fully removed, it would be of significant interest for the development of smart polymer systems. Recently, gas stimuli start to attract significant interest in developing smart materials and systems.40 There have been numerous investigations on smart polymers induced by various gases, including carbon dioxide (CO2),41,42 oxygen (O2),43−47 nitric oxide (NO),48,49 hydrogen sulfide (H2S),50,51 and sulfur oxide (SO2).52 CO2 as a benign and inexpensive gas was widely explored in the past few years.53−60 In CO2 responsive systems, the hydrophilicity and polarity of polymers can be varied by chemical reversible interactions between CO2 and specific functional groups on the polymer chains (e.g., tertiary amine, amidine, guanidine, and carboxylic groups), accompanying with the change pH of the aqueous solutions. Because of the reversible reactions, CO2 can be easily removed by washing Received: July 8, 2017 Revised: October 1, 2017

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DOI: 10.1021/acs.macromol.7b01456 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 1. Synthesis and Characterization of PNIPAM and Poly(NIPAM-co-TMGA) Copolymers conv (%)b sample

[NIPAM]:[TMGA]:[DMP]:[AIBN]

TMGA (mol %)

NIPAM

TMGA

Mn,theo (g/mol)c

Mn,GPC (g/mol)

Đ

PNIPAM-0 PNIPAM-PTMGA-1 PNIPAM-PTMGA-2 PNIPAM-PTMGA-3

100:0:1:0.4 90:10:1:0.4 85:15:1:0.4 80:20:1:0.4

0 7.62 11.51 14.04

83.18 89.78 90.05 89.13

N/A 65.14 66.03 57.88

9780 10730 10880 10600

6930 3880 5920 6950

1.35 1.23 1.35 1.39

a

a

Mole fraction of TMGA was determined by 1H NMR. bNIPAM conversion for PNIPAM was measured gravimetrically; NIPAM and TMGA conversions were calculated from total copolymer mass and weight fraction of NIPAM and TMGA units in the copolymer. cMn,theo was obtained from conversion (i.e., Mn,theo = MDMP + [NIPAM]0/[DMP]0 × conversion × MNIPAM + [TMGA]0/[DMP]0 × conversion × MTMGA). (1:2, v/v) and dried under vacuum prior to use. 2,2′-Azobis(2methylpropionitrile) (AIBN, 98%, Aldrich) was recrystallized from methanol and dried under vacuum prior to use. Milli-Q grade water prepared from Barnstead Nanopure Diamond system was used in aqueous solution preparation. CO2 and N2 gases were controlled by FMA-A2100 flow meters (Omega) to maintain a constant gas flow rate of 10 mL/min. SO2 was generated through dropping H2SO4 (70 wt %) into a flask charged with NaHSO3. The freshly generated SO2 was used immediately, and the residual SO2 was collected by NaOH (20 wt %) solution. Synthesis of Ionic Liquid Monomer 1,1,3,3-Tetramethylguanidine Acrylate (TMGA). TMGA was synthesized through direct neutralization of TMG and AA under a nitrogen atmosphere according to the literature work.61 Specifically, TMG (9.18 g, 79.7 mmol, 10 mL) and H2O (20 mL) were added into a dried 250 mL flask equipped with a magnetic stirrer. The flask was immersed in an ice water bath (4 °C) followed by dropwise addition of AA (6.31 g, 87.5 mmol, 6 mL) in 15 min. The reaction was kept at 4 °C for 4 h and carried out at room temperature for another 20 h. After that, the reaction mixture was distilled under vacuum (40 mbar) at 70 °C to remove water and unreacted TMG and AA. Finally, a colorless and transparent ionic liquid (TMGA) with high viscosity was obtained (14.70 g, yield 98.5%). Synthesis of Poly(N-isopropylacrylamide-co-1,1,3,3tetramethylguanidine acrylate) and Poly(N-isopropylacrylamide). The ionic liquid containing thermoresponsive copolymers were prepared through the RAFT polymerization. In a typical experiment (no. 1 in Table 1), NIPAM (1018.9 mg, 9.0 mmol), TMGA (191.5 mg, 1.02 mmol) DMP (36.46 mg, 0.1 mmol), AIBN (6.57 mg, 0.04 mmol), and 3 mL of DMF were charged into a 25 mL rubber septum sealed Schlenk flask equipped with a magnetic stirrer. The solution was thoroughly purged by vacuum and flushed with nitrogen for three times. After that, the polymerization was carried out in an oil bath of 80 °C for 24 h. The resulting copolymer was collected by twice repeated precipitation from cold diethyl ether and then dried under vacuum at 50 °C until constant weight. Further purification was done by dialyzing the resulting copolymers against deionized water. Finally, a white powder polymer product (poly(NIPAM-co-TMGA)) was obtained. Poly(N-isopropylacrylamide) homopolymer (PNIPAM) was prepared by the same procedure except for the addition of TMGA. Preparation of Polymer Solution and Gas Treatment. In a typical experiment, the solution was prepared by directly dissolving the polymer into DI water and stirred at room temperature. Note: pH of the DI water used in this work ranged from 5.7 to 6.0. The concentration of all the polymer solutions was fixed at 5 mg mL−1. Before characterization, the copolymer solution was transferred into two glass tubes: one was treated with N2, and the other was treated with CO2 or SO2. The copolymer solutions with different pH were prepared by adding suitable amount of HCl or NaOH aqueous solution. Characterization. 1H NMR and 13C NMR spectra of all the samples were recorded in D2O on Bruker AV200 MHz NMR spectrometer. FTIR measurement was performed on Thermal NICOLET 6700 spectrophotometer. The spectrum was recorded by scanning 64 times at a resolution of 2 cm−1. The number-average molecular weight (Mn) and dispersity (Đ) of PNIPAM homopolymers were analyzed by a Polymer Laboratories PL-GPC 50 Plus Integrated

with an inert gas (e.g., N2). The systems are thus free of chemical contamination and have a full recoverability. So far, the thermal responsive and/or CO2 responsive polymers are usually nonionic or weakly charged in aqueous medium.53−60 A few examples reported PNIPAM-based thermoresponsive ionic liquid copolymers;25−31 however, their thermal responsive properties could not be in situ tuned by external stimuli (e.g., light, electric, and gas). Hence, developing a new smart PIL system with unique physiochemical properties, which can be controlled by an environmental-friendly trigger, has a very good novelty and significance in expanding this class of smart polymers, which behaves very differently and is potentially better than the previous systems. In this work, a series of thermoresponsive ionic liquid copolymers were designed and synthesized by reversible addition−fragmentation chain transfer (RAFT) copolymerization of 1,1,3,3-tetramethylguanidine acrylate (TMGA) and Nisopropylacrylamide (NIPAM). The reactivity ratios of TMGA and NIPAM were determined for better understanding macromolecular structure of the copolymers. The effects of IL composition on LCST behaviors of the as-prepared PNIPAM-based copolymers were studied. The apparent pKa values were estimated for giving a quantitative relationship between protonation degree and LCST. Gas (i.e., CO2 or SO2)induced LCST regulation was investigated. The CO2-induced shift of LCST could be fully recovered to its initial state by bubbling N2, while base (e.g., NaOH) was required for its recovery with the SO2-treated system. The pH-sensitive thermoresponsive phase transition was studied and provided good insight into the LCST regulation mechanism. This contribution, for the first time, demonstrated the gas-triggered LCST behaviors of thermoresponsive ionic liquid copolymers. Notably, the present smart copolymer has three advantages: (1) the facile synthesis of IL-containing copolymers with unique characteristics of NIPAM and IL, (2) the switchable LCST regulated by CO2/N2 without the accumulation of impurities, and (3) the cations introduced via IL units being inert to acidic medium.

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EXPERIMENTAL SECTION

Materials. 1,1,3,3-Tetramethylguanidine (TMG, 99%, Aldrich), water (H2O, HPLC Plus, Aldrich), hexane (ACS reagent, Caledon Laboratory Chemicals), methanol (MeOH, ACS reagent, Caledon Laboratory Chemicals), sulfuric acid (H2SO4, ACS reagent, 95.0− 98.0%, Caledon Laboratory Chemicals), sodium bisulfite (NaHSO3, ACS reagent, Aldrich), 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DMP, 98%), and N,N-dimethylformamide (DMF, anhydrous, 99.8%, Aldrich) were used as received. Acrylic acid (AA, anhydrous, contains 200 ppm MEHQ as the inhibitor, 99%, Aldrich) was passed through a short column filled with inhibitor remover to remove inhibitors prior to use. N-Isopropylacrylamide (NIPAM, ≥99%, Aldrich) was recrystallized from a toluene/hexane solution B


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Macromolecules Scheme 1. Synthesis Routes of (A) TMGA and (B) Poly(NIPAM-co-TMGA) Copolymers

Figure 1. (A) Chemical structure, (B) FTIR spectrum, (C) 1H NMR spectrum, and (D) 13C NMR spectrum of TMGA. Laboratories PL-GPC 50 Plus Integrated System, comprising three Ultrahydrogel columns (Agilent tech 30, 40, and 50), and a differential refractive index detector using water (H2O) with 0.1 M of NaNO3 as an eluent at 30 °C with a flow rate of 0.80 mL min−1. A series of narrow molecular weight distribution poly(ethylene oxide) samples were used as standards.

System, comprising three Phenogel columns connected in series (guard, 104, 500, and 100 Å), and a differential refractive index detector using N,N-dimethylformamide (DMF) with 0.03 wt % LiBr as an eluent at 20 °C with a flow rate of 0.35 mL min−1. A series of narrow molecular weight distribution poly(methyl methacrylate) samples were used as standards. The Mn and Đ of ionic liquid containing thermoresponsive copolymers were analyzed by a Polymer C


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Figure 2. (A) 1H NMR spectra of PNIPAM and poly(NIPAM-co-TMGA) copolymers. (B) GPC trace of PNIPAM in DMF eluent. (C) GPC traces of poly(NIPAM-co-TMGA) copolymers in H2O eluent. The glass transition temperature (Tg) was measured by a DSC 2910 (TA Instruments) under a nitrogen atmosphere. Samples were first heated from room temperature to 200 °C at 50 °C/min and kept isothermally for 5 min to eliminate thermal history. The temperature was then cooled to 20 °C at a rate of 10 °C/min, maintained at 20 °C for 5 min, and heated to 160 °C at 10 °C/min again. The second heating curve was used to analyze Tg through Universal Analysis 2000 software. Phase transition properties of polymer aqueous solution (5.0 mg/mL) were monitored by a Variant Cary Bio 100 UV−vis spectrophotometer at 500 nm wavelength. Measurements were undertaken over a temperature ramp with a heating rate of 0.5 °C min−1. The pH value of polymer solution was measured by an Orion Star A215 pH/conductivity benchtop multiparameter meter (ThermoScientific). Three standard buffers with pH 4.01, 7.00, and 10.01 were employed for calibration prior to use. The conductometric titrations were performed with a PC Titrator (Mandel, Mantech Inc.), performing a base-into-acid titration and 10 min per unit pH as the addition rate. In a typical experiment, a certain amount of resulting copolymer was dissolved in 50 mL of 1 mM NaCl solution. The initial pH of the sample solution was adjusted to 2.5 with 1.0 M HCl, and then the sample solution was titrated with 0.1 M NaOH at room temperature until the pH reached 11.5. The apparent pKa of samples were determined by the conductometric titration curves.62

the characteristic peaks corresponding to the proton and carbon of related groups (Figure 1A) were analyzed by 1H NMR and 13C NMR spectra (Figure 1C,D). Specifically, the peak area integration ratio of the multiple peak a+a′, multiple peak b, and single peak c was 1.99:1.00:12.49, in good agreement with the number of assigned protons. Because of the rapid proton exchange in deuteroxide solution, the protons of NH2 were unable to be detected using 1H NMR. Five distinct peaks in the 13C NMR spectrum were all from the carbonbased groups. Synthesis of PNIPAM and Poly(NIPAM-co-TMGA). Well-defined PNIPAM and random copolymers containing NIPAM and TMGA units were synthesized by RAFT polymerization using DMP as chain transfer agent and AIBN as initiator in DMF (Scheme 1B). Table 1 summarizes the polymerization recipes and results of characterized polymers. All the polymerizations were performed with the [monomer]0: [RAFT agent]0 at 100:1 to obtain the polymers with similar theoretical molecular weights (Mn,theo). PNIPAM homopolymer was synthesized for the control experiments. The chemical structure of PNIPAM was identified through 1H NMR and 13C NMR, as shown in Figure 2A and Figure S1. The broad peaks (i.e., 1.56 ppm (peak d)/1.98 ppm (peak c), and 36.13 ppm (peak A)/43.63 ppm (peak B)) observed in NMR spectra were assigned to the protons and carbons of methylene bridge and methanetriyl group in the polymer backbone, respectively.63 Also, the other characteristic peaks in both NMR spectra were well assigned to the carbon and proton of corresponding groups. The Mn,theo of PNIPAM (9780 g/mol) was estimated from monomer conversion, further verified by the peak area integration ratio of the proton of methanetriyl group (a, 3.86 ppm, 1H) and the methylene protons of RAFT agent close to trithiocarbonate (f, 2.81 ppm, 2H). The integration ratio was 42.21:1.00, as shown in Figure 2A (PNIPAM-0). Therefore, the Mn calculated from 1H NMR was 9920 g/mol, agreeing well

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RESULTS AND DISCUSSION Synthesis and Characterization of Ionic Liquid Monomer TMGA. Neutralization of AA and TMG was carried out at room temperature to prepare polymerizable monomer TMGA, as shown in Scheme 1A. The functional groups and structure of TMGA were confirmed by FTIR, 1H NMR, and 13 C NMR spectra. As shown in Figure 1B, the absorbance of imine group (CN) in TMGA appeared at 1608 cm−1, indicating the unbroken double bonds. The bending frequencies of tertiary amide group at 1270 and 1350 cm−1 were detected as well. Also, CC with trans and cis hydrogen appeared at 650−1000 cm−1, and the carboxylate (−COO−) absorbance in AA at 1420 cm−1 was observed. Furthermore, all D


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Macromolecules with Mn,theo. GPC measurements with DMF as eluent (Figure 2B) gave Mn,GPC of PNIPAM (6930 g/mol) and Đ (1.35). The differences between GPC and NMR are common, which might be attributed to the use of the narrow PMMA standards or interactions of the PNIPAM and GPC columns. Three copolymers with different TMGA contents were prepared by varying the ratio of NIPAM and TMGA (Table 1). Compared to the 13C NMR spectrum of PNIPAM shown in Figure S1, a new signal located at 39.67 ppm (peak G) was observed in all spectra of copolymers, corresponding to the methyl group of the TMGA unit. The TMGA content in random copolymers (PNIPAM-PTMGA-1/-2/-3) was determined from 1H NMR spectra, as shown in Figure 2A. The characteristic peak at 3.86 ppm was assigned to the proton of methanetriyl group (peak a, 1H) in NIPAM units. The peak ascribed to the protons of methyl groups (peak b, 12H) in TMGA units located at 2.91 ppm was observed. The mole ratio of TMGA units in the copolymer poly(NIPAM-co-TMGA) was estimated through integration of the values marked under corresponding peaks in Figure 2A. Subsequently, the respective monomer conversions were calculated and used to estimate Mn,theo of the as-prepared copolymers. The results in Table 1 show that three different copolymers possessed similar molecular weights. GPC characterization of ionic liquid containing copolymers using a common eluent is known to be challenging.64 There was no signal found in GPC traces using DMF with LiBr as eluent. A similar phenomenon was observed in other types of poly(ionic liquid).64 GPC traces of the copolymers prepared in this work were gained using aqueous solution of NaNO3 as eluent and are presented in Figure 2C. However, the results for samples with lower ionic liquid contents were unsatisfactory. An increase in the copolymer molecular weight with the increase of ionic liquid content was observed. The discontinuous breakpoint of GPC trace of PNIPAM-PTMGA-1 might be caused by signal fluctuation during the measurement. Actually, the response signal of PNIPAM-PTMGA-1 is weakest compared to the other samples. Such abnormal response signals for IL-containing copolymers might be attributed to their different inherent water solubilities (probably in the presence of incompletely extended chain conformation in solution with a lower IL content) as well as the different charge interactions between ionic liquid unit and GPC column due to their different charge contents under the test conditions. The reactivity ratios of NIPAM (rNIPAM) and TMGA (rTMGA) were determined. Because of the slow propagation feature of reversible-deactivation radical polymerization, the data gained under high conversions were preferably used for the reactivity ratio calculation.34 Based on the extended Kelen-Tödüs method,65 which is particularly applicable for the case at high conversions, the reactivity ratios rNIPAM = 2.11 and rTMGA = 0.56 were obtained (see Table S1 as well as Figures S2 and S3 in the Supporting Information for details). Both NIPAM and TMGA radicals preferred to react with NIPAM monomer or, in other words, that NIPAM was randomly linked to itself or TMGA. Thus, in these copolymers, short TMGA segments or TMGA units were incorporated between the long NIPAM-containing blocks. The conversion of TMGA went to ∼60% when the conversion reached ∼90% for NIPAM. It was because of the reactivity ratios (rNIPAM = 2.11 and rTMGA = 0.56). The batch system experienced significant composition drifting. At high conversion, the system was in rich of TMGA, which was difficult to be polymerized due to the cationic repulsion. The

polymerization was stopped at a predetermined time; therefore, the conversion of TMGA was low. Furthermore, it was evident from the thermal analysis by DSC in Table 2 and Figure S4 that all the copolymers Table 2. Experimental and Theoretical Glass Transition Temperature of PNIPAM and Poly(NIPAM-co-TMGA) Copolymers sample PNIPAM-0 PTMGA-0 PNIPAMPTMGA-1 PNIPAMPTMGA-2 PNIPAMPTMGA-3

WNIPAM (wt %)a

WTMGA (wt %)a

Tg,DSC (°C)b

Tg,theo (°C)

100 0 88.00

0 100 12.00

137 N/A 127

N/A 83c N/A

82.29

17.71

122

123d

78.74

21.26

119

120d

a

Weight fraction of NIPAM and TMGA units in the copolymer. Determined by DSC. cDetermined by the Fox equation using the experimental data of PNIAPM-0 and PNIPAM-PTMGA-1. The Fox equation is given by 1/Tg,copolymer = WNIPAM/Tg,PNIPAM + WTMGA/ Tg,PTMGA, where Tg is the glass transition temperature (in K) of homopolymer.66 dPredicted by the Fox equation using the data of PNIAPM-0 and PTMGA-0. b

possessed a single Tg. Increasing TMGA content in the copolymer led to a decrease in Tg,DSC. The theoretical Tg of PTMGA (the homopolymer with low molecular weight) was calculated through the Fox equation, giving a value of 83 °C (Tg,theo). Subsequently, both Tg,theo values of PNIPAMPTMGA-2 and -3 were estimated, which showed good agreement with experimental data. These results suggested that the copolymers were homogeneous in the bulk state, providing further evidence to support their random monomer composition. CO2-Induced Solution Phase Transition Behavior. PNIPAM has a LCST of approximate 32 °C in neutral aqueous media, at which PNIPAM chains undergo a coil to globule transition.67 The incorporation of hydrophilic/hydrophobic components into PNIPAM is expected to yield a different thermoresponsive solution phase transition behavior. In this work, highly hydrophilic ionic liquid was introduced into PNIPAM through random copolymerization. Turbidimetry measurements using UV−vis spectrophotometer are commonly carried out to study the thermoresponsive phase transition of polymer solutions. The phase transition of polymer solution upon heating, based on chain conformation, is a dynamic process.67 Therefore, the change in transmittance as a function of temperature obtained through turbidimetry measurements is heating rate dependent. Before the LCST behavior study, we examined the effect of heating rate on transition temperature, as shown in Figure S5 and Table S2. The transmittance vs temperature curve obtained at a slower heating rate became closer to the equilibrium condition. Therefore, a LCST obtained at nonequilibrium condition could be considered as an apparent LCST (see Supporting Information for details). The effect of TMGA content on LCST behaviors of PNIPAM-based copolymers was investigated through the evolution of transmittance with temperature at a heating rate of 0.5 °C/min, as shown in Figure 3. The LCST summarized in Table 3 was defined as the temperature corresponding to 1% E


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Figure 3. Temperature-based transmittance curves of the aqueous solutions (5.0 g/mL) of (A) PNIPAM-0, (B) PNIPAM-PTMGA-1, (C) PNIPAM-PTMGA-2, and (D) PNIPAM-PTMGA-3.

Table 3. LCSTs and Protonation−Deprotonation Equilibrium Constants of PNIPAM and Poly(NIPAM-co-TMGA) Copolymers in Aqueous Solutions sample PNIPAM-0 PNIPAM-PTMGA-1 PNIPAM-PTMGA-2 PNIPAM-PTMGA-3

LCST (N2) (°C)a LCST (CO2) (°C)a ΔLCST (°C) 30.5 41.1 56.3 73.2

± ± ± ±

0.5 0.3 0.3 0.2

28.7 33.5 36.5 40.1

± ± ± ±

0.4 0.2 0.4 0.4

1.8 7.6 19.8 33.1

apparent pKa

solution pH (N2)

αb (N2)

solution pH (CO2)

αb (CO2)

N/A 5.34 5.25 5.33

N/A 5.98 ± 0.02 6.00 ± 0.01 5.97 ± 0.02

N/A 0.19 0.15 0.19

N/A 5.17 ± 0.02 5.19 ± 0.02 5.17 ± 0.03

N/A 0.60 0.53 0.59

a

The obtained LCST values were the average of three readings on each sample. The uncertainty was estimated by the difference between maximum observed values and the average value as well as the difference between minimum observed values and the average value. bThe degree of protonation (α) of carboxyl groups ([−COOH]/([−COOH] + [−COO−])) was calculated from the solution pH values using the equation α = 1/(1 + Ka/ [H+]), where Ka is acid dissociated constant estimated in this work and [H+] is the concentration of the hydrogen ion determined by the solution pH.

mittance at 90 °C. In other words, the thermoresponsivity of the copolymer weakened with increasing ionic liquid component. The CO2-induced LCST shifts were significant in the copolymers. As shown in Figure 3 (black lines), the phase separation in all copolymer solutions occurred in a relative narrow temperature range. The LCSTs for PNIPAM-PTMGA1, PNIPAM-PTMGA-2, and PNIPAM-PTMGA-3 solutions treated with CO2 for 20 min were 33.5, 36.5, and 40.1 °C, respectively. The tunable ranges of LCST (i.e., ΔLCST) triggered by N2 and CO2 increased from 7.6 to 33.1 °C, as summarized in Table 3. As a control experiment, PNIPAM-0 treated with CO2 gave an LCST of 28.7 °C and ΔLCST of 1.8 °C. The decrease of LCST with CO2 treatment suggested a

decrease in transmittance. The solutions were bubbled with N2 for 30 min before measurement to minimize air contamination. Compared to PNIPAM-0 with LCST of 30.5 °C, the LCST of the copolymer solution increased with increasing TMGA content (red lines). Specifically, the copolymers with 7.62, 11.51, and 14.04 mol % TMGA had the LCST of 41.1, 56.3, and 73.2 °C. It should be pointed that only PNIPAM-PTMGA1 solution reached a complete phase separation with zero percent transmittance (red line) at above 55 °C, as shown in Figure 3B. For the aqueous solution of PNIPAM-PTMGA-2 (Figure 3C), the percent transmittance decreased from 100 to 20% as temperature increased from 20 to 90 °C. There was only a small change of turbidity in PNIPAM-PTMGA-3 aqueous solution (Figure 3D), with 7% decrease in transF


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Figure 4. (A) Visualization of reversible solution phase transition behaviors of the PNIPAM-PTMGA-1 aqueous solution (5.0 g/mL) treated with N2 and CO2 in water bath at different temperatures. (B) CO2/N2-induced temperature-based transmittance curves and (C) CO2/N2-switchable LCST of the PNIPAM-PTMGA-1 aqueous solution (values in parentheses indicated the solution pH after gas treatment).

Scheme 2. Reversible Protonation of Guanidine in the Presence of CO2 and Water

reduced solubility of the copolymer in water. Figure 4A shows that the solution phase transition was reversible. The aqueous solutions of PNIPAM-PTMGA-1 treated with N2 and CO2 exhibited recoverable and different gas-induced thermoresponsive transitions at 20, 40, and 60 °C. In addition to CO2induced LCST shifting, the LCST of the copolymers was gasswitchable. Figures 4B and 4C show their gas-switchable thermoresponsivity. Transmittance curves of three cycles were almost overlapped. The LCST of the solution could readily switch back and forth through alternatively bubbling N2 for 30 min and CO2 for 20 min. Mechanism Consideration of CO2-Induced Switchable LCST Behavior. Guanidine is known to be CO2-responsive.68 It undergoes reversible protonation in aqueous solution through CO2/N2 bubbling, as shown in Scheme 2 (reaction 1). The copolymers synthesized in this work contained guanidine. However, the imine group of guanidine was protonated as cation in TMGA monomer. Therefore, reaction 1 did not happen in our system. Is reaction 2 in Scheme 2 possible? Figure 5 shows no difference in 1H NMR spectra of PNIPAM-PTMGA-1 solution treated with N2 and CO2. There was no change of chemical shift at 2.91 ppm, suggesting no interaction between CO2 and tertiary amine groups in TMGA.

Figure 5. 1H NMR spectra of PNIPAM-PTMGA-1 in D2O treated with N2 and CO2.

On the other hand, it is also known that the tertiary amine groups in poly(N,N-dimethylaminoethyl methacrylate) could be easily protonated.53 Why not the tertiary amine groups in TMGA be protonated then? The strong electron-withdrawing effect of imine group near the tertiary may caused the result. G


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PNIPAM-PTMGA-1 solution. The start and end points were determined by the tangents of a conductivity−volume curve (gray line). Approximately full consumption of excess H+ in the solution was assumed, and all the carboxyl groups were protonated at the start point with pH 4.07. Therefore, a midpoint of the volume from start to end-point was determined. This point with pH 5.34 approximately corresponded to the apparent pKa of PNIPAM-PTMGA-1. Similarly, the apparent pKa of PNIPAM-PTMGA-2 and PNIPAMPTMGA-3 were estimated to be 5.25 and 5.33, respectively (see Figures S6 and S7 for details). The apparent pKa showed no significant difference among these three copolymers. By using the apparent pKa, the protonation degrees of carboxyl groups at different conditions were calculated, as summarized in Table 3. For PNIPAM-PTMGA-1 solution, the protonation degree of −COOH groups increased from 0.19 to 0.60 by bubbling CO2. The mole fractions of −COO− groups in PNIPAM-PTMGA-1/2/3 were calculated to be 3.05%, 5.41%, and 5.76% under CO2 treatment. These results confirmed the relationship between pH and LCST as well as copolymer composition and LCST. A quantitative relationship between protonation degree and LCST was gained as well. However, it could not be ruled out that the dissociated TMG+ could also form hydrogen bonds with amine group in PNIAPM, which contributed to the decrease in LCST as CO2 was used. The mechanism proposed for N2/CO2 switchability is illustrated in Scheme 3. Figure 7 shows the continuous variation of pH of PNIPAMPTMGA-1 solution and protonation degree of carboxylic groups with CO2 and N2 treatments. Because of the chemical equilibrium of CO2 + H2O ↔ H2CO3, the pH of solution could be recovered to its initial state after the removal of CO2 by bubbling the N2. In this demonstration system, the protonation degree of carboxylic groups gradually increased from 0.29 to 0.60, which corresponded to the pH decrease from 5.74 to 5.17 with continuous bubbling of CO2. The successive protonation and deprotonation processes were achieved by alternating the trigger gas. This result demonstrated that gas-induced LCST shifting and its reversibility were ascribed to the recoverable protonation. SO2-Induced Solution Phase Transition Behavior. We have also tried another acidic gas (i.e., SO2), which was

Titration experiments of TMG solution confirmed that TMG is a strong monoprotic base, which has one inflection in the titration curve.69 Thus, the cation of TMGA is proven to be inert in acidic medium. Since there was no protonation effect of imine and tertiary amine groups of guanidine caused by carbonic acid, we hypothesized that the switchable LCST behavior was attributed to the change of solution pH triggered by CO2 and the reversible protonation of the carboxyl group. As the solution was exposed to CO2, hydrogen ions (H+) dissociated from H2CO3 decreased pH, weakening the hydrogen bond between amide groups of NIPAM and water molecules. A fraction of −COO− groups in poly(NIPAM-co-TMGA) were protonated and formed −COOH. The intermolecular hydrogen bonds between carboxylic acid and amide groups consumed some binding sites for water molecules, which caused the decrease in hydrophilicity and thus led to lowered LCST. More carboxylic groups gave higher sensitivity to pH. Similar observations were found in other PNIPAM-based systems.22 To illuminate this proposed mechanism, the apparent pKa of the resulting copolymers were determined through the conductometric titration curves. Figure 6 shows a typical titration curve for

Figure 6. pH/conductometric titration of PNIPAM-PTMGA-1 aqueous solution.

Scheme 3. Proposed Mechanism for CO2/N2 Switchability with and without Intermolecular Hydrogen Bonds between Carboxylic Acid and Amide Groups

H


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transmittance curve (hollow circle) in Figure 8 confirmed the recoverable LCST of 39.5 °C. However, it is noteworthy that an accumulation of salts (“kosmotrope effect”10) could not be avoided through such acid−base titration. pH-Induced Solution Phase Transition Behavior. The solution phase transition behaviors at different pH were investigated to provide further insight into the LCST regulation mechanism. As shown in Figure 9, with increasing pH from 2.14 to 9.11 the LCST shifted from 26.0 to 59.2 °C. Further increase of pH to 10.14 did not lead to a further change of the LCST, which indicated that full deprotonation of PNIPAMPTMGA-1 has a LCST of ∼59 °C. The LCST (26 °C) at pH 2.14 was comparable to that of solution induced by SO2. Additionally, when the pH was adjusted to 5.16 and 5.96, LCST corresponded to 31.3 and 43.2 °C, respectively. The relationship between LCST and pH value fitted well to a sigmoidal curve (Figure 9B). A widest LCST shifting window (∼12 °C) was found between pH 5.16 and 5.96 for PNIPAMPTMGA-1 aqueous solution. This regulation window is exactly the control range of CO2 triggered system as discussed before. These results suggested that LCST of the as-prepared ILcontaining copolymers could be regulated through different approaches, at the fixed pH.

Figure 7. pH of PNIPAM-PTMGA-1 solution (5.0 g/mL) and protonation degree of carboxylic groups treated with CO2 and N2.

introduced into the copolymer solutions for 20 min. The initial pH value of solution with N2 bubbling was 6.04, and it decreased to 2.03 with SO2 bubbling. LCST dropped from 40.0 to 26.6 °C, as shown in Figure 8, which was lower than that of

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CONCLUSION In summary, a series of thermoresponsive copolymers poly(NIPAM-co-TMGA) with different ionic liquid contents (i.e., 7.62, 11.70, and 14.04 mol %) having the number-average molecular weight about 10 000 g/mol were synthesized by RAFT copolymerization. The randomly distributed structure of the copolymers was confirmed by the reactivity ratios of NIPAM (rNIPAM = 2.11) and TMGA (rTMGA = 0.56). Single Tg value of the copolymers followed the Fox equation very well, suggesting homogeneous copolymers. Investigations on the solution phase transition behaviors showed that increasing the ionic liquid content led to the increase in LCST from 30.5 to 73.2 °C. A much slow thermoresponsive behavior was observed as TMGA mole fraction increased up to 14.04%. The apparent pKa values of asprepared IL-containing copolymers, which were closely related to the change of LCST, were determined through the conductometric titration curves. Thus, a quantitative relationship between protonation degree and LCST was gained. In addition, poly(NIPAM-co-TMGA) copolymers possessed CO2switchable thermoresponsive property, as demonstrated by turbidimetry measurements. By excluding chemical interactions between CO2 and specific functional groups (i.e., imine and tertiary amine groups), switchable solution pH was considered as the stimulus source. Since the as-designed copolymer had −COO− groups in aqueous solution, tunable LCST was ascribed to the formation of intermolecular hydrogen bonds between carboxylic acid and amide groups. With the ability to undergo reversible protonation, the system showed good reversibility in LCST, bubbled with CO2 and N2. Further studies on SO2 bubbling showed that LCST could be lowered, and its recovery relied on adding NaOH solution, but not on simple N2 bubbling. Solution phase transition behaviors at different pH provided very good insight into the LCST regulation mechanism, and the relationship between LCST and pH value could be well correlated using a sigmoidal model. Compared to conventional regulation methods, CO2 trigger was proven to be an effective pathway for in situ regulation of LCST behavior of thermoresponsive ionic liquid copolymer.

Figure 8. Temperature-based transmittance curves of the aqueous solution (5.0 g/mL) of PNIPAM-PTMGA-1 with various treatments. (“+” in legend means a continuous treatment was made in one solution).

CO2 bubbling (33.5 °C). With this level of pH, almost all −COO− groups in poly(NIPAM-co-TMGA) were protonated. The calculated protonation degree was 99.95%. Besides, both intermolecular and intramolecular hydrogen bonds between the carboxylic acid and amide groups in PNIAPM as well as the hydrogen bonds between the dissociated TMG+ and amine groups could form and further enhance the hydrophobicity. These results firmly confirmed the previously discussed mechanism. Subsequently, simply bubbling N2 into the same solution for 30 min did not allow to switch LCST back to its initial state. As seen in Figure 8, the temperature-based transmittance curve (hollow square) almost overlapped with solid square curve, suggesting that SO2 could not be removed by N2. The solution remained acidic with pH of 2.05. A concentrated NaOH solution was then added to the above solution, and the pH of solution was adjusted back to 6.10. The temperature-based I


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Figure 9. (A) Temperature-based transmittance curves of the PNIPAM-PTMGA-1 aqueous solution (5.0 g/mL) at different pH. (B) Summarized LCSTs correspond to the transmittance curves.

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(3) Hu, J.; Liu, S. Responsive polymers for detection and sensing applications: current status and future developments. Macromolecules 2010, 43, 8315−8330. (4) Alarcon, C. D. H.; Pennadam, S.; Alexander, C. Stimuli responsive polymers for biomedical applications. Chem. Soc. Rev. 2005, 34, 276−285. (5) Vlatković, M.; Collins, B. S.; Feringa, B. L. Dynamic responsive systems for catalytic function. Chem. - Eur. J. 2016, 22, 17080−17111. (6) Ward, M. A.; Georgiou, T. K. Thermoresponsive polymers for biomedical applications. Polymers 2011, 3, 1215−1242. (7) Roy, D.; Brooks, W. L.; Sumerlin, B. S. New directions in thermoresponsive polymers. Chem. Soc. Rev. 2013, 42, 7214−7243. (8) Halperin, A.; Kröger, M.; Winnik, F. M. Poly (N-isopropylacrylamide) phase diagrams: Fifty years of research. Angew. Chem., Int. Ed. 2015, 54, 15342−15367. (9) Van Durme, K.; Rahier, H.; Van Mele, B. Influence of additives on the thermoresponsive behavior of polymers in aqueous solution. Macromolecules 2005, 38, 10155−10163. (10) Zhang, Y.; Furyk, S.; Bergbreiter, D. E.; Cremer, P. S. Specific ion effects on the water solubility of macromolecules: PNIPAM and the Hofmeister series. J. Am. Chem. Soc. 2005, 127, 14505−14510. (11) Xu, J.; Ye, J.; Liu, S. Synthesis of well-defined cyclic poly (Nisopropylacrylamide) via click chemistry and its unique thermal phase transition behavior. Macromolecules 2007, 40, 9103−9110. (12) Li, J.-J.; Zhou, Y.-N.; Luo, Z.-H. Thermo-responsive brush copolymers with structure-tunable LCST and switchable surface wettability. Polymer 2014, 55, 6552−6560. (13) Zhang, Z. X.; Liu, X.; Xu, F. J.; Loh, X. J.; Kang, E. T.; Neoh, K. G.; Li, J. Pseudo-block copolymer based on star-shaped poly (Nisopropylacrylamide) with a β-cyclodextrin core and guest-bearing PEG: controlling thermoresponsivity through supramolecular selfassembly. Macromolecules 2008, 41, 5967−5970. (14) Rauwald, U.; del Barrio, J.; Loh, X. J.; Scherman, O. A. “Ondemand” control of thermoresponsive properties of poly (Nisopropylacrylamide) with cucurbit[8]uril host-guest complexes. Chem. Commun. 2011, 47, 6000−6002. (15) Ji, X.; Chen, J.; Chi, X.; Huang, F. pH-Responsive supramolecular control of polymer thermoresponsive behavior by pillararene-based host-guest interactions. ACS Macro Lett. 2014, 3, 110−113. (16) Phillips, D. J.; Gibson, M. I. Biodegradable poly(disulfide)s derived from RAFT polymerization: monomer scope, glutathione degradation, and tunable thermal responses. Biomacromolecules 2012, 13, 3200−3208. (17) Jiang, X.; Feng, C.; Lu, G.; Huang, X. Thermoresponsive homopolymer tunable by pH and CO2. ACS Macro Lett. 2014, 3, 1121−1125.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01456. Determination of reactivity ratios, composition curve, DCS curves, temperature-based transmittance curves of aqueous solution of PNIPAM-PTMGA-1 at different heating rates, and pH/conductometric titration curves of PNIPAM-PTMGA-2 and PNIPAM-PTMGA-3 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (Z.-H.L.). *E-mail [email protected] (S.Z.). ORCID

Yin-Ning Zhou: 0000-0003-3509-3983 Zheng-Hong Luo: 0000-0001-9011-6020 Shiping Zhu: 0000-0001-8551-0859 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors sincerely acknowledge the Natural Sciences and Engineering Research Council (NSERC) (RGPIN-201505841) of Canada for supporting this fundamental research. S.Z. thanks the Canada Research Chair (CRC) (950-229035) program for supporting his research. Y.-N.Z. thanks the Shanghai Jiao Tong University for HaiWai ShiZi ChuBei postdoctoral fellowship support and China Postdoctoral Science Foundation (No. 2016M600316, 2017T100299) for his visiting at McMaster.

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N2-Switchable Thermoresponsive Ionic Liquid Copolymer

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