Grafting light-, temperature, and CO2-responsive copolymers from cellulose nanocrystals by atom transfer radical polymerization for adsorption of nitrate ions

Polymer 182 (2019) 121830

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Grafting light-, temperature, and CO2-responsive copolymers from cellulose nanocrystals by atom transfer radical polymerization for adsorption of nitrate ions Zahra Abousalman-Rezvani a, b, Parvaneh Eskandari a, b, Hossein Roghani-Mamaqani a, b, **, Hanieh Mardani a, b, Mehdi Salami-Kalajahi a, b, * a b

Faculty of Polymer Engineering, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran Institute of Polymeric Materials, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran

A R T I C L E I N F O

A B S T R A C T

Keywords: Light-responsivity CO2-sensitivity Temperature-responsivity Nitrate ion removal

Cellulose nanocrystal (CNC)-grafted and free block copolymers of 2-dimethylaminoethyl methacrylate (DMAEMA) and coumarin were successfully synthesized via atom transfer radical polymerization. Copolymers were designed in different chain lengths and block ratios and used in ion adsorption from aqueous solutions. The copolymers are responsive to CO2, pH, temperature, and also light. By inserting CO2 to the system, the pro­ tonated amine groups of poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) caused nitrate ion adsorption by electrostatic forces. The LCST of PDMAEMA blocks has been increased by inserting of CO2, which results in their higher solubility and therefore higher possibility of ion-adsorption. Fourier-transform infrared spectros­ copy, proton nuclear magnetic resonance, and thermogravimetric analysis were used for structural and thermal characterization of the synthesized free and CNC-grafted copolymers. UV–Vis spectroscopy was used for study of nitrate ion-adsorption from the aqueous solution. Responsivity of the free copolymers to temperature, CO2, and light was evaluated by dynamic light scattering. By increasing the PDMAEMA length, ion-adsorption capacity has been increased. CO2 helps for ion-adsorption by protonation of the amine groups and increasing the LCST of PDMAEMA block. However, the temperature and light stimuli can be used in regeneration process by the squeezing and crosslinking processes.

1. Introduction Improving the quality of water is one of the most important issues in the human life. Some industrial, domestic, and agricultural activities are the main reasons for pollution of water resources with toxic pollutants such as inorganic anions, metal ions, and synthetic organic chemicals. Lots of water purification and recycling technologies has been used in the recent years [1–3]. In recent years, wastewater treatment has mostly carried out by using responsive macromolecules. Stimuli-responsive polymers such as temperature-responsive, pH-responsive, light-responsive, and also CO2-responsive polymers have received considerable attentions due to their great potential applications [4–7]. These stimuli-responsive polymers change their physical or chemical properties in response to the stimuli. Among the all stimuli-responsive polymers, CO2-responsive polymers require higher attention since CO2

is abundant, environmental friendly, non-toxic, benign, and inexpensive [7]. In addition, dual and multi-responsive polymers can also be used in ion-removal applications with their dual and multi power of respon­ sivity [8–10]. Poly (2-dimethylaminoethyl methacrylate) (PDMAEMA) is one of the most important temperature-responsive polymers which widely synthesized by control radical polymerization (CRP) methods [11–13]. PDMAEMA shows lower critical solution temperature (LCST) behavior at temperature of about 40 � C. PDMAEMA also shows pH- and CO2-re­ sposnive behavior due to its tertiary amine groups [7,14]. Multi-stimuli-responsive polymers can be prepared by incorporation of light-responsive compounds to temperature- or CO2-responsive poly­ mers. In light-responsive compounds, spiropyran responds to UV light by isomerization between the non-polar ring-closed SP form and ring-opened zwitterionic merocyanine (MC) form, and coumarin

* Corresponding author. Faculty of Polymer Engineering, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran. ** Corresponding author. Faculty of Polymer Engineering, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran. E-mail addresses: [email protected] (H. Roghani-Mamaqani), [email protected] (M. Salami-Kalajahi). https://doi.org/10.1016/j.polymer.2019.121830 Received 21 August 2019; Received in revised form 15 September 2019; Accepted 21 September 2019 Available online 22 September 2019 0032-3861/© 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Schematic representation for the synthesis of (A) CNC-Br and (B) CNC-g-(PDMAEMA-b-PC). Table 1 The sample description and the recipe used for preparation of the samples. Sample PD100 PDC200-10 PDC100-5 C-PD100 C-PDC100-5 C-PDC200-10

Description PDMAEMA(100) PDMAEMA(100)-b-PC(5) PDMAEMA(200)-b-PC(10) CNC-g-PDMAEMA(100) CNC-g-(PDMAEMA(100)-b-PC(5)) CNC-g-(PDMAEMA(200)-b-PC(10))

CNC-Br

DMAEMA

CSA

g

mL

mol

g

mol

g

mmol

mL

mmol

mL

mol

0 0 0 0.120 0.120 0.120

5.5 5.5 11 5.5 5.5 11

0.03 0.03 0.06 0.03 0.03 0.06

0 0.372 0.744 0 0.372 0.744

0 1.5 3 0 1.5 3

0.043 0.043 0.043 0.043 0.043 0.043

0.3 0.3 0.3 0.3 0.3 0.3

0.062 0.062 0.062 0.062 0.062 0.062

0.3 0.3 0.3 0.3 0.3 0.3

0.37 0.37 0.37 0 0 0

0.3 0.3 0.3 0 0 0

responds to UV light by bond formation through 2 þ 2 cycloaddition reactions [15–19]. Coumarin is an aromatic organic compound which shows light-responsivity to irradiation with λ ¼ 365 nm by cross-linking with each other, and it shows a reversible behavior when irradiated with λ ¼ 250 nm. Yuan et al. has synthesized ABCBA pentablock double-hydrophilic copolymer of poly(diethylene glycol monomethyl ether methacrylate)-block-poly(2-(N,N-dimethylamino)ethyl meth­ acrylate)-block-poly(ethylene glycol)-block-poly (DMAEMA)-block-poly (diethylene glycol monomethyl ether methacrylate) (PMEO2­ MA-b-PDMAEMA-b-PEG-b-PDMAEMA-b-PMEO2MA) through atom transfer radical polymerization (ATRP) [20]. They investigated the morphology and temperature-responsivity and shows that copolymer is hydrophilic at low temperature and pH values. Considering multi-responsive polymers with coumarin moieties, Zhao et al. prepared light-responsive shell cross-linked reverse micelles of styrene and DMAEMA by ATRP method [21]. They have also prepared a dual responsive block copolymer by inserting coumarin methacrylate units in PDMAEMA; coronal crosslinking has been achieved by photo-induced dimerization of the coumarin units [22]. Cellulose is a biodegradable and biocompatible natural polymer which can be used in ion adsorption applications [23,24]. By acid hy­ drolyzing of cellulose fibers, cellulose nanocrystal (CNC) as a rod like-crystalline cellulose was obtained which forms highly stable aqueous suspensions [25–28]. In synthesis of biodegradable polymer

CuBr

PMDETA

BiBB

composites based on cellulose, PDMAEMA and other thermo-responsive polymers were grafted at the surface of CNC by SI-ATRP method [29, 30]. Roy et al. reported PDMAEMA-grafted cellulose via reversible addition-fragmentation chain transfer (RAFT) polymerization [31,32]. PDMAEMA, poly(diethylaminoethyl methacrylate), and also poly (dimethylaminopropyl methacrylamide) have been grafted on CNC by Garcia-Valdez and coworkers [33]. By grafting three different CO2- and pH-responsive polymers on CNC and purging CO2 to the system, the stability of the solution increased. In the case of application of PDMAEMA in ion-removal processes, a silica-based adsorbent was pre­ pared by Zhao and coworkers by grafting of PDMAEMA onto silica to remove three heavy metal ions of Cr (VI), As (V), and Hg (II) [34]. The maximum sorption capacities of Cr (VI) and As (V) were determined as 51.9 and 44.3 mg g 1, respectively. PDMAEMA as a CO2-responsive polymer has been used by Bai and coworkers for adsorption of various heavy metal ions [35]. The CO2-responsive octopus-like POS­ S-PDMAEMA adsorbent was used as an eco-friend product due to its responsivity to CO2 for adsorption and desorption of Cu2þ. By bubbling of CO2 into the system, the percentage of free Cu2þ increases to 100% due to the reduction of pH value. The protonated DMAEMA units dissociate the polymer-metal complex and release all the chelated Cu2þ as free ions. Wu and coworkers prepared interpenetrating network hydrogels based on nanofibrillated cellulose and PDMAEMA via cross­ linking free radical polymerization, and used the products for removal of 2

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Fig. 2. SEM images of (A) MCC and (B) CNC, TEM images of (C) CNC and (D) C-PDC20010, microscopy images for (E and F) coumarin and (G and H) aggregated CPDC200-10 in the non-fluorescence and fluorescence modes.

Pb(II) and Cu(II) ions [36]. PDMEMA grafted on the polypropylene film was used to investigate the adsorption and desorption behavior of PO34 and NO3 . By decreasing the pH value, the grafted polymer chains were positively charged and attract the negatively charged ions. They showed that the equilibrium adsorption capacity increases with increasing the adsorption time to 10 h and decreases with increasing the pH of medium [37]. In the case of hydrogels, polycationic PDMAEMA/poly (2-hydroxyethyl methacrylate) hydrogels were synthesized by Tian and coworkers, and nitrate ion has been removed from aqueous solu­ tions in different pH values from 2.4 to 6.5 [38]. Protonation of the PDMAEMA blocks at low pH values resulted in higher adsorptions; however, adsorption decreased at higher pH values. Herein, CNC-grafted and free copolymers of DMAEMA and coumarin with different ratios were synthesized through ATRP. Responsivity of the CNC-grafted and free copolymers to CO2, pH, temperature, and light has been investigated. Removal of nitrate ions and the regeneration process has been investigated by using responsivity of the products to CO2, pH, temperature, and light. Due to the protonation of PDMAEMA amino groups in the presence of CO2, removal of ions was expected to increase. However, effect of the responsivity of the product to the light should be investigated in ion-removal efficiency. Crosslinking of the polycumarin (PC) blocks can be resulted in a simple regeneration pro­ cess especially in the case of CNC-grafted copolymers. The main reason of using CNC is the regeneration process with the aid of stimuli and filtration. The main concept of this work is on the stimuli-driven ionremoval and stimuli-regeneration of the product in addition to its physical regeneration process.

2. Experimental 2.1. Materials Resorcinol, ethyl acetoacetate, 11-bromo-1-undecanol, 1,4-dioxane, potassium carbonate, trimethylamine, acryloyl chloride, dichloro­ methane, and sodium sulfate were purchased from the Merck Company and used in preparation of coumarin and its derivates. Microcrystalline cellulose (MCC, Aldrich), sulfuric acid (H2SO4, Merck), α-Bromoisobu­ tyryl bromide (BiBB, Sigma-Aldrich), 4-dimethylaminopyridine (DMAP, Sigma-Aldrich), and triethylamine (Sigma-Aldrich) has been used in the preparation of CNC and CNC-Br. The 2-dimethylaminoethyl methacry­ late (DMAEMA), N,N,N0 ,N’0 ,N00 -Pentamethyldiethylenetriamine (PMDETA, Sigma-Aldrich), and CuBr (Sigma-Aldrich) were used in the synthesis of the free and CNC-grafted polymers. Sodium hydroxide (NaOH), tetrahydrofuran (THF), dimethyl formamide (DMF), methanol, and ethanol were purchased from the Merck Company and used as received. 2.2. Synthesis of coumarin, 7-((11-hydroxyundecyl) oxy)-4-methyl-2Hchromen-2 (CS), and 11-((4-methyl-2-oxo-2H-chromen-7-yl) oxy) undecyl acrylate (CSA) Synthesis of coumarin, CS, and CSA was carried out using the liter­ ature [39], as shown in Fig. S1. For the synthesis of coumarin, resorcinol (11.0 g, 0.1 mol) and ethyl acetoacetate (13.0 g, 0.1 mol) were completely dissolved in 1,4-dioxane (40 mL). Then, sulfuric acid (3 mL) was slowly added into the mixture, which was subsequently warmed up to 65 � C for 3 h in an oil bath. The mixture was cooled to room tem­ perature and poured in ice water (300 mL) to obtain a yellowish pre­ cipitate. The resulted crude product was dried in a vacuum oven at 50 � C and recrystallized twice in ethanol to yield yellow coumarin crystals. 1H 3

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Fig. 3. 1H NMR spectra for (A) PD100 and (B) PDC100-5 and PDC200-10.

NMR (in DMSO): δ 2.42 (CH3CCHCOO), 6.2 (CH3CCHCOO), 6.9–7 (H-aromatic), and 10.8 (OH-aromatic). For the synthesis of CS, coumarin (4 g, 22.7 mmol) was dissolved in DMF (20 mL) in a 100 mL two-necked round-bottom flask. Then, the diluted 11-bromo-1-undecanol (8.641 g, 34.4 mmol) in DMF (10 mL) and potassium carbonate (6.3 g, 45.6 mmol) were added to the mixture. The system was stirred under nitrogen atmosphere at 88 � C for 18 h, cooled down to room temperature, poured into ice water (70 mL), and filtrated to obtain the crude product. The resulted precipitate was recrystallized twice in ethanol to obtain CS. 1H NMR (in DMSO): δ 1.26, 1.43, and 1.74 (-CH2-), 4.06 (CH2-aromatic), 3.3 (CH2CH2OH), and 4.3

Table 2 Molecular weights of the copolymers calculated from the 1H NMR spectra. Sample PDC200-10 PDC100-5

n* 15 9

m** 9 4

Mn (g.mol 1) PC

PDMAEMA

Total

3600 1600

2355 1413

5955 3013

* Number-average polymerization degree of the PDMAEMA block. ** Number-average polymerization degree of the PC block.

4

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Fig. 4. Effect of the CO2 purging time on the pH of the samples in aqueous solutions (the inlet figure is schematic representation for the protonation and depro­ tonation of the copolymers).

(HO-CH2). For the synthesis of CSA, CS (3.59 g, 20.0 mmol) and triethylamine (5.5 g, 40.0 mmol) were mixed with CH2Cl2 (40 mL). The mixture was cooled in an ice-water bath under stirring for 15 min. Then, the diluted acryloyl chloride (3.62 g, 40.0 mmol) with CH2Cl2 (20 mL) was slowly added into the mixture in 1 h. The reaction was allowed to proceed at room temperature for 24 h. For diluting the solution, 50 mL of dichloromethane was added to the mixture. Then, the solution was washed with distilled water three times (100 mL). The organic phase was dried over anhydrous sodium sulfate overnight. After removing the solvent, the crude product of CSA was achieved. 1H NMR (in DMSO): δ 6.12 and 5.83 (CH2CHCOO) and 1.58 (CH2CHCOO).

24 h, the reaction mixture was centrifuged after dissolving in THF and acetone for 3 times to remove any unreacted BiBB molecules. After washing the precipitate by vacuum filtration and subsequent freezedrying process, CNC-Br was obtained (Fig. 1 (A)). 2.5. Synthesis of the free and CNC-grafted polymers Synthesis of the free and CNC-grafted polymers was carried out in glass vials by using the amounts reported in Table 1. For the preparation of C-PDC100-5, dispersion of CNC in THF (5 mL), CuBr, and DMAEMA were added to the vials and left under purging with N2 for 10 min. PMDETA was injected to the vials, and the reaction was allowed to complete at 70 � C for 24 h. After that, CSA was dissolved in THF (10 mL) and injected to the vials, and the reaction was allowed to complete at 70 � C for 24 h (Fig. 1(B)). For preparation of C-PDC200-10, the same procedure has been used. However, DMAEMA and CSA as the first and second monomers were fed in two-folds of the amounts used for the preparation of C-PDC100-5. To synthesize the free copolymers, the same recipe for the preparation of C-PDC100-5 was used. However, BiBB was used instead of CNC-Br. For the preparation of CPD100 and PD100, the same recipes for the preparation of C-PDC100-5 and PDC100-5 were used, respectively. However, the second monomer (CSA) was not incorporated in the polymerization reactions.

2.3. Preparation of CNC from MCC For preparation of CNC by acid-hydrolysis MCC according to the literature [40–42], sulfuric acid (87.5 mL, 64% w/v) was added to the MCC powder (10.0 g) and left under stirring at 45 � C for 1 h. After addition of water (500 mL), the reaction mixture was centrifuged at 5000 rpm for 15 min. The resulted precipitate was dispersed in water (400 mL) and sonicated. To reach the neutral pH values, NaOH solution (1.0%) was added to the mixture under stirring. The CNC product was obtained after several centrifugation and sonication processes and sub­ sequent freeze-drying process.

2.6. Separation of free chains from the CNC-grafted polymers

2.4. Preparation of BiBB-grafted (CNC-Br)

The composites were dissolved in THF. Free chains were separated from the CNC-grafted polymers by centrifugation at 12,000 rpm and passing the solution through a 0.2 μm PTFE filter. The CNC-grafted polymers were remained on the PTFE filter. The grafted polymers were obtained after drying in a vacuum oven at 50 � C for 24 h.

For preparation of CNC-Br, CNC (1 g) was dispersed in THF (15 mL) and ultrasonically agitated for 1 day. Then, 100 mL two-necked roundbottom flask was charged with CNC dispersion. DMAP (0.26 g, 2.12 mmol) and trimethylamine (5.5 mL, 40 mmol) were added to the system and the reactor was purged with N2 for 15 min. Then system was allowed to stay in ice bath until injection of BiBB (4 mL, 32.2 mmol) and THF (5 mL). After completion of the reaction at room temperature for 5

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Fig. 5. UV transmittance of (A) PD100, (B) PDC100-5, and (C) PDC200-10 initially, after CO2 purging, and after N2 purging against temperature and also (D) comparison of UV transmittance against temperature for all the samples.

2.7. Ion-adsorption study

the UK) with a heating rate of 10 � C/min from room temperature up to 700 � C under a nitrogen atmosphere. A sample weight of about 10 mg was used for all the measurements. Surface morphology of MCC, CNC, and CNC-grafted polymers were studied by scanning electron micro­ scopy (Vega Tescan SEM instrument, Czech Republic, voltage of 10 V). The specimens were prepared by precipitation of a thin sample layer on a mica surface using a spin coater and its subsequent gold-coating. A transmission electron microscope, Philips EM 208, with an accelerating voltage of 120 kV was used to investigate the morphology of the CNC and CNC-grafted polymers. Temperature- and CO2-sensitivity of the polymers and also the ion loading and release experiments were inves­ tigated via a UV/Vis spectrophotometer (Milton Roy Spectronic 60) at a wavelength of 290 nm. For investigating temperature- and CO2-sensi­ tivity analyses, a sample weight of 0.6 mg was dissolved in water (12 mL) to obtain the liquid samples. Dynamic light scattering (DLS) was recorded by Malvern Panalytical system at 25 � C with 170.4 count rate (Kcps). Fluorescence imaging was used for investigation of coumarin crystals, polymer-covering on CNC, and applicability of the coumarincontaining samples as surface active agents in stabilizing oil in water dispersions by using Olympus BX50 Microscope with UV Narrow filter, where excitation and emission were performed in 360–370 and 420 nm, respectively.

Batch adsorption studies for the nitrate ion were carried out via equilibrium experiments by using sodium nitrate. Accordingly, 0.01 g of the samples in dialysis bags was added to the nitrate ion solution with concentration of 500 ppm (0.1 mg/mL). After different intervals, the adsorption of the free and CNC-grafted polymers was measured from the difference in nitrate concentrations in the initial and final solutions by using UV/Vis spectroscopy at wavelength of 290 nm. Adsorption ca­ pacity of the nitrate ion was calculated using Equation (1). q¼

ðCt

Ctþ1 ÞV M

(1)

where, q is adsorption capacity, Ct and Ctþ1 are the initial and equilib­ rium concentrations of the nitrate ions, and V and M are the volume and sample weight [35,43,44]. 2.8. Characterization Chemical compositions of the samples are confirmed by Fouriertransform infrared (FTIR) spectra recorded on a Bomem FTIR spectro­ photometer within the range of 4000–400 cm 1 with a resolution of 4 cm 1. The samples were prepared on a KBr pellet in vacuum desic­ cators by inducing pressure of 0.01 Torr. 1H NMR (300 MHz) spectra were recorded on a Bruker Avance 300 spectrometer using CDCl3 and DMSO as the solvents. A pulse delay of 1 s was used to ensure complete relaxation of spins. Thermogravimetric analysis (TGA) measurement is performed with a thermal analyzer (Polymer Laboratories, TGA 1000,

3. Results and discussion 3.1. Investigation of the synthesis process Successful preparation of CNC through acid hydrolysis of MCC has 6

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Table 3 Hydrodynamic diameter and zeta potential results for the PDC200-10 sample at 25 � C with and without CO2 insertion. Sample

DH (nm)

PDI

Zeta potential (eV)

PDC200-10 (without CO2) PDC200-10 (with CO2)

107.5 448.5

0.225 0.265

27.4 41.4

copolymers (C-PDC20010) are also shown in Fig. 2 (D). Polymer coverage of the nanocrystals is shown as an opaque layer around the CNCs. Microscopy images of the coumarin crystals in the non-fluorescence and fluorescence modes (Fig. 2 (E) and (F), respec­ tively) show successful synthesis of coumarin. In addition, aggregated C-PDCs formed after removal of water from its aqueous dispersions in the sample preparation for microscopy imaging is shown in the non-fluorescence and fluorescence modes for C-PDC200-10 (Fig. 2 (G) and (H), respectively). This can also be a confirmation for successful synthesis of coumarin and also copolymer-grated CNCs. The blue fluo­ rescence of coumarin molecules are shown in Fig. 2 (F) and (H) for coumarin and aggregated C-PDC200-10, respectively. FTIR spectra for CNC, CNC-Br, PD100, C-PD100, PDC200-10, and CPDC200-10 are shown in Fig. S2 (Supporting Information). FTIR spec­ troscopy confirms functionalization of CNC by BiBB and also PDMAEMA and the block copolymers. As it shown in CNC spectrum, the bands at 1370 and 2893 cm 1 are related to the C–H bending and stretching vi­ brations, respectively. In addition, stretching vibration of the carboxyl groups is observed at 1640 cm 1 [46,47]. The broad peak observed in 3342–3419 cm 1 is assigned to the OH groups. The signal at 1725 in the CNC-Br spectrum is attributed to the ester groups of ATRP initiator attached to the nanocrystals [48]. The change in OH signal intensity at 3325 cm 1after BiBB-modification of CNC indicates that some surface hydroxyl groups were substituted [49]. The ester groups of PDMAEMA show a band at 1736 cm 1 in the spectrum of PD100. The peak for the N-CH3 groups of PDMAEMA is observed at 2760 cm 1 [50–52] and the peak at 1150 cm 1 is assign to C–N groups. Moreover, 2819 and 2767 cm 1 belongs to the C–H stretching of the –N(CH3)2 in PDMAEMA spectra [53]. Appearance of these bands in the spectrum of C-PD100 can be a proven for successful attachment of PDMAEMA chains to CNC-Br in the C-PD100 sample. By successive grafting of the PC block to the free and CNC-grafted PDMAEMA homopolymers, the bands related to the C¼O and C-O groups of coumarin appeared at 1618 and 1382 cm 1, respectively [54]. This also shows successful attachment of the second block in the PDC200-10 and C-PDC200-10 samples. 1 H NMR spectra of the synthesized polymers are shown in Fig. 3. The calculated degree of polymerization from 1H NMR spectra for each block and molecular weights of the polymers are shown in Table 2. As shown in Fig. 3 (A), the PD100 sample shows feature signals at δ ¼ 2.36 (CH2N (CH3)2), 2.51 (CH2CH2N(CH3)2), 4.13 (CH2CH2N(CH3)), 0.96 (CH3), and 1.87 ppm (CH2) [55,56]. In addition, the peak observed at the δ ¼ 2.17 ppm is related to the ATRP initiator’s protons. Fig. 3 (B) shows the feature signals of the PDC200-10 and PDC100-5 samples. By increasing the PDMAEMA and PC lengths, the peak area of their char­ acteristic signals is increased. 1H NMR analysis showed that the number-average polymerization degree of PDMAEMA block in PDC200-10 and PDC100-5 equals to 15 and 9, respectively. Also, the number-average polymerization degree of PC block in PDC200-10 and PDC100-5 equals to 9 and 4, respectively. According to these results, the number-average molecular weights of the PDC200-10 and PDC100-5 samples are 5955 and 3013 g mol 1, respectively. TGA and DTG thermograms for MCC, CNC, and CNC-Br are shown in Fig. S3 (A) and (B). CNC shows higher char residue than MCC due to its higher amount of the crystalline parts. In comparison with MCC, the decomposition temperature of CNC is observed at lower temperatures, which originates from its lower particle size, higher specific surface area, and the higher amount of amorphous chains at the surface [25,46]. CNC shows no considerable peak in 150–200 � C, which is an indication

Fig. 6. DLS results for the PDC200-10 sample at (A) 25 � C and (B) 60 � C and also (C) reversibility of CO2-responsivity by insertion of N2 and CO2 for the free copolymers.

been investigated by SEM and TEM. The MCC rods with micrometer size can be observed in SEM image in Fig. 2 (A). After acid hydrolysis pro­ cess, the CNC rods with nanometer size are observed in SEM and TEM images, respectively Fig. 2(B) and (C). Due to the high specific area of CNC and strong hydrogen bonding between the nanocrystals, it shows laterally aggregated rods [45,46]. TEM results of CNC-grafted 7

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Fig. 7. Fluorescence microscopy images for n-hexane in water suspension stabilized by PDC5010 after bubbling with CO2: (A) non-fluorescent and (B and C) fluorescent modes.

of absence of sulfate groups because of the successive sonication and treatment of the product with NaOH [47]. DTG curve of CNC shows two peaks related to the degradation of cellulose chains and the remained solid. By grafting BiBB on the surface of CNC, a considerable mass loss is observed at about 100 � C which is related to the initiator moieties. Re­ sidual char for MCC, CNC, and CNC-Br is about 7.6, 16.6, and 7.4%, respectively. The grafting ratio can be calculated from difference of the sample’s residual weights. The char residue difference between the CNC and CNC-Br can be a rough estimation of the BiBB initiator on CNC (9.2%). Fig. S3 (C) and (D) show TGA and DTG thermograms of the free and CNC-grafted polymers. TGA results related to the PD100 shows three mass loss stages. The mass loss of 8.2% at 321.14 � C is due to the physically adsorbed and interlayer water molecules. The second decomposition stage at 425.7 � C is attributed to the functional amino groups of the PDMAEMA. The mass loss of about 91.8% belongs to the carbon skeleton and amine groups [57]. The PDC200-10 sample shows a similar decomposition trend with PD100, and finally reaches to char residue of about 7.1%. TGA results of the CNC-polymers shows a decomposition profile with three weight loss stages. The first one at about 150 � C is assigned to the PC block decomposition, and the second and third ones at about 270 and 310 � C are related to the PDMAEMA and CNC’s decomposition [33]. Table S1 shows char residue, degradation content, and degradation temperature of the samples as a brief summary of the TGA and DTG details.

sharply observed in the first 2 min. Tang et al. [58] reported variation of LCST in different pH values in their multi-responsive polymer system. They reported increase of LCST by decreasing pH. Different LCST values are also reported for PDMAEMA in the presence of other stimuli [59]. The deprotonation of PDMAEMA can be observed via the simple bubbling of the nitrogen inert gas through the aqueous solution. The schematic illustration for the protonation and deprotonation processes is shown in the inlet of Fig. 4. Effect of CO2 on LCST of the polymers is also investigated by their cloud point investigation (as shown in Fig. 5). A samples aqueous so­ lution with concentration of 0.5 mg/mL was subjected to increasing temperature up to 90 � C. The sharp transition which has been observed for all the free polymers is assigned to the LCST behavior. By adding CO2 to the system and increasing temperature, LCST of the polymers are observed at higher temperatures due to the increase of the samples solubility by protonation. In the case of PD100 (Fig. 5 (A)), the LCST is increased from 43 to 78 � C, after insertion of the CO2 gas. By insertion of CO2 to the system transmittance has also increased because of the pro­ tonation process and solubility increasing. To investigate the revers­ ibility, inert N2 gas has immersed to the system. For all the samples, the transmittance against temperature curve is very similar to the initial sample’s result which has not been bubbled with CO2. The PD100 sample shows LCST of about 46 � C after insertion of N2 gas into the solution. Fig. 5(B) and (C) show LCST behavior of the PDC200-10 and PDC100-5 samples, respectively. According to the results, LCST has changed from 48 to 82 � C, after insertion of the CO2 gas and returned to 48 � C after insertion of N2 gas into the aqueous solution of PDC100-5. In addition, LCST has changed from 44 to 82 � C, after insertion of the CO2 gas and returned to 44 � C after insertion of N2 gas into the aqueous so­ lution of PDC200-10. Fig. 5 (D) is also added to compare the trans­ mittance against temperature curves for all the samples. The transmittance of the PD100 sample is higher than the other samples due to its higher solubility. PDC200-10 and also PDC100-5 have lower transmittance compared to the DP100 sample as a result of PC block which is hardly soluble in water. Therefore, LCST is decreased by the addition of PC blocks which resulted from its very low solubility. CO2-responsive polymers and copolymers can be protonated by bubbling CO2 into its aqueous solutions, and deprotonation has accrued by purging inert gas to the system. Because the deprotonation is an

3.2. Investigation of stimuli-responsivity of the samples Responsivity of PD100, PDC100-5, and PDC200-10 has been studied by UV/Vis spectroscopy. Responsivity of PD100 to CO2 and temperature and also responsivity of the copolymers to CO2, temperature, and light have been investigated at a wavelength of 297 and 350 nm for the PDMAEMA and PC blocks, respectively. PDMAEMA because of its ter­ tiary amine groups is expected to show CO2-responsivity. By bubbling CO2, the solutions changed from opaque to transparent, because hy­ drophobic chains change to hydrophilic chains and cause more solubi­ lity. As shown in Fig. 4, a portion of the amine groups of PDMAEMA are protonated by immersing CO2 to the homo and copolymers aqueous solution (0.5 mg/mL) for 5 min. Then, the solution becomes acidic due to the formation of bicarbonate salt. Effects of the CO2 stimulus are 8

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Fig. 8. UV/Vis spectra for the copolymers in aqueous solutions and their dimerization degree under UV irradiation at λ > 310 nm for (A) PDC100-5 and (B) PDC200-10.

endothermic process, heat is needed to make the deprotonation reaction easier [7]. By passing CO2 through the copolymer solutions, decrease of the pH value implying that a number of protonated species were formed in the copolymer chains [60]. By increasing temperature above the LCST, size of the self-assembled structures has been decreased due to the collapsing of PDMAMA chains. Therefore, all small assemblies tend to be accumulated and stick together because of the tendency for minimizing energy [61]. PDMAEMA chains were protonated by immersing CO2 to the solution, which results in higher solubility of the copolymers and also increase of transmittance. By increasing temperature above the

LCST of PDMAEMA, these blocks were collapsed and resulted in the formation of smaller assemblies. Above the LCST and in the presence of CO2, due to the repulsive electrostatic forces between the protonated PDMAEMA blocks the assemblies were not aggregated. The CO2-re­ sponsive behavior of PDC200-10 is also investigated by DLS at 25 � C and the result is shown in Fig. 6(A) and (B) and also Table 3. DLS results show that size of the copolymer assemblies is about 107.5 nm, which increases to 448.5 nm by adding CO2 to the system. The main reason is protonation of the PDMAEMA blocks that caused repulsion electrostatic forces. Protonation of the PDMAEMA blocks by insertion of CO2 resulted 9

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Fig. 9. Ion-removal of the PDC200-10 and C-PDC200-10 samples in CO2, its release in N2, and also the effects of UV irradiation on the removal and release processes.

crosslinking coumarin units. Increasing temperature has resulted in in­ crease of size because of aggregation of the self-assembled structures, which mainly resulted from squeezing the PDMAEMA chains at tem­ peratures of higher than its LCST. PDI has also largely been increased because of formation of aggregates with different size. As expected, the self-assembled structures were expanded by immersing CO2 even at higher temperatures, as confirmed by expansion of aggregates. As shown in Fig. 6 (C), CO2-responsivity is reversible by insertion of N2 gas into the self-assembled solutions in 3 cycles. The free copolymers were used as stabilizing agent in oil in water dispersions. Such an application was completely proved by fluorescence microscopy results of the n-hexane in water droplets. Fluorescence mi­ croscopy results for the n-hexane in water suspension stabilized by

Table 4 Ion absorption efficiency for the PDC200-10 and C-PDC20010 samples before and after UV irradiation. Sample

Efficiency (%)

C-PDC 200-10 C-PDC 200-10 (UV) PDC 200-10 PDC 200-10(UV)

36 67 51 83

in increase of zeta potential of the PDC200-10 sample from 27.4 to 47.6 eV. Irradiation of the assemblies with UV light resulted in decrease of hydrodynamic radius because of stretching forces applied by

Fig. 10. Regeneration of the CNC-grafted copolymers using their stimuli-responsive characteristics. 10

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PDC200-10 after bubbling with CO2 are shown in Fig. 7. The fluores­ cence mode images confirm the presence of the fluorescent PC at the surface of the droplets, which is a confirmation for its suspending agent role in the prepared suspension. It is an ongoing work in our group to use CNC-grafted copolymers in the presence of CO2 or even at low pH values in Pickering emulsion polymerization systems as hybrid emulsifiers. Light-responsibility of the PDC200-10 and PDC100-5 copolymers was investigated by UV-light irradiation with λ ¼ 365 nm. As shown in Fig. 8, UV–Vis spectra changed when copolymer solutions were irradi­ ated with UV light with 365 nm wavelength. Decreasing the absorption intensity of the copolymers solution at λ~320 nm by increasing the irradiation time, originates from the dimerization and confirms forma­ tion of the cross-linked copolymer assemblies [62–64]. The dimerization degree was increased by increasing the UV light-induction time. Zaho et al. reported dimerization degree of about 75% after 1 h irradiation, where partly reversibility of the coumarin dimerization was confirmed by irradiating at λ > 260 nm. They reported that dimerization degree was decreased from 75 to 38% after 2 h irradiation at λ > 260 nm. Dimerization degree can be calculated as 1-At/A0 according to the Lambert-Beer law, where A0 and At are the initial absorbance and the absorbance after an irradiation time t at 320 nm, respectively [62–64]. According to the results given in the inlet of Fig. 8, the dimerization degree reaches to ~92 and 83% after 20 h for PDC200-10 and PDC100-5, respectively. This shows that dimerization degree of the samples increases by increasing the amount of coumarin content.

characteristics and also solidity of the CNCs. Accordingly, by cross­ linking PC blocks of the CNC-grafted copolymers after ion adsorption by using UV light, CNCs can be attached to each other. Therefore, a simple filtration process can be used for their separation from the wastewater. In addition, the separated samples can be regenerated by the breaking of the networks with UV irradiation with wavelength of about 225 nm and purging with N2 gas. 4. Conclusion ATRP was used to synthesize multi-stimuli-responsive free and CNCgrafted block copolymers with application in nitrate ions removal from aqueous solutions. CO2- and temperature-responsivity of the PDMAEMA block and also light-responsivity of the PC block result in such multistimuli-responsive products. By bubbling CO2 to the aqueous solutions of the copolymer products, LCST shifted to the high temperatures due to the protonation of amine groups, which makes the polymers more sol­ uble. The PD100 sample showed LCST of about 40 � C, which increased to 78 � C by adding CO2 to the system. The PC blocks show lightresponsivity by its crosslinking after induction of UV light for 20 h to the self-assembled structures, where the dimerization degree was increased by increasing UV light induction time. Responsivity of the free copolymers to temperature, CO2, and light was evaluated by dynamic light scattering. Increasing size of the self-assembled structures was shown by insertion of CO2 and increase of temperature because of ag­ gregation of the assemblies. Nitrate ion removal of the free and CNCgrafted copolymers has also been investigated. By adding CO2 to the system, protonation of the amine groups resulted in adsorption of the negatively charged nitrate ions. Due to neutralizing of the system through the N2 bubbling, most of the nitrate ions removed from the selfassembled structure and they were regenerated. Results showed that the CNC-grafted copolymers removed lower contents of nitrate ions compared to the free copolymers, which is because of their lower amount of PDMAEMA contents. However, the regeneration process of the CNC-grafted copolymers would be much easier because of solidity of the CNCs.

3.3. Investigation of ion absorption Nitrate ion-removal from the wastewater has been carried out by using different techniques [65]. Stimuli-responsive polymers can also be useful for removal of ions from the wastewater because of their smart behavior and also simple regeneration [35,43,66,67]. CO2-responsive polymers can be excellent for absorption of heavy ions from the wastewater [35]. For studying the nitrate ions quantity which is removed from the wastewater by absorption to the free and CNC-grafted copolymers at equilibrium (q), nitrate ion solutions with different con­ centrations in range of 300–600 ppm were prepared. The PDC200-10 and C-PDC200-10 samples were selected to remove nitrate ions from the wastewater with and without UV irradiation (λ > 350). Amine groups of PDMAEMA were protonated and positively charged by immersing CO2 to the system; therefore, they absorbed negatively charged nitrate ions from the water solution. As shown in Fig. 9, for studying the amount of ion-absorption by the free and CNC-grafted co­ polymers, CO2 was injected into the system at a constant flow rate up to 5 min. The ion-absorption was considerably increased within the first seconds of bubbling with CO2. However, the absorption rate decreases largely after the two min of bubbling with CO2. To investigate the effect of the inert gas on the ion-removal process, N2 has been immersed to the system for 5 min. Therefore, the protonated amine groups were almost neutralized and desorption of the ions to the water was observed. The light-responsive PC blocks were crosslinked by induction of UV light to the system for 20 h, therefore the polymer assemblies become more stable and prevents from their partial dissolution. Therefore, nitrate ions absorption has been carried out more efficient than the uncrosslinked samples. As shown in Fig. 9, absorption rate of the CNC-grafted samples is lower than the free samples, which is because of lower amount of PDMAEMA in the CNC-grafted samples. Table 4 shows efficiency of the free and CNC-grafted copolymers in the presence and absence of UV light for nitrate ion removal. The PDC200-10 sample after irradiation with UV light has the most efficiency for removal of nitrate ions from the wastewater. It was resulted that PDC 200-10(UV) has the most efficiency n ions removing by removing the 83% of the ions from aqueous solution (Table 4). As shown in Fig. 10, the regeneration can be easily possible for the CNC-grafted copolymers through their stimuli-responsive

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