Synthesis of coumarin-containing multi-responsive CNC-grafted and free copolymers with application in nitrate ion removal from aqueous solutions

Carbohydrate Polymers 225 (2019) 115247

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Synthesis of coumarin-containing multi-responsive CNC-grafted and free copolymers with application in nitrate ion removal from aqueous solutions

T

⁎

Zahra Abousalman-Rezvania,b, Parvaneh Eskandaria,b, Hossein Roghani-Mamaqania,b, , ⁎ Mehdi Salami-Kalajahia,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 LE I N FO

A B S T R A C T

Keywords: Light-responsivity CO2-responsivity Temperature-responsivity Vesicle Nitrate ion adsorption

Cellulose nanocrystals (CNC)-grafted and free copolymers were synthesized in three different ratios of DMAEMA and coumarin monomers (30:5, 40:7, and 50:10) through reversible addition-fragmentation chain transfer polymerization. These multi-responsive polymers to carbon dioxide (CO2), temperature, and light triggers can be used in nitrate ions removal from aqueous solutions. These amphiphilic copolymers were self-assembled to vesicular structures in water. Adsorption of nitrate ions was carried out by protonation of the CO2-responsive block with inserting of CO2. Proton nuclear magnetic resonance and thermogravimetric analysis were used to confirm the synthesis process. Responsivity to temperature, CO2, and light in addition to the adsorption of nitrate ions from aqueous solutions was studied by UV–vis spectroscopy and dynamic light scattering. By increasing the PDMAEMA content, the adsorption capacity has also increased. The CNC-grafted copolymers showed lower adsorption in comparison with the free copolymers. The CNC-grafted copolymers can be regenerated by light and filtration processes.

1. Introduction Water pollution with different ions is among the most important problems which causes serious toxicities to human beings and living organisms (Ju et al., 2009; Nriagu, 1989). By increasing industrial and agricultural activities, the toxic pollutants such as inorganic anions, metal ions, and synthetic organic chemicals have raised critical worries about the quality of ground water. Some of the toxic inorganic anions like nitrate anion may remain in the water even after its treatment which increases the health risks (Velizarov, Crespo, & Reis, 2004). This ion can be reduced to nitrite or other forms by using microbial action (Bhatnagar & Sillanpää, 2011). In recent years, wastewater treatment has mostly carried out by using responsive macromolecules, such as temperature-responsive (Chen, Ahmad, & Ooi, 2013; Oliveira Barud et al., 2015), pH-sensitive (Ali, Rachman, & Saleh, 2017; Jamiu, Saleh, & Ali, 2017), CO2-responsive (Bai, Liang, & Hu, 2017; Tran, Kim, Kim, Kim, & Kim, 2016), pH- and temperature-sensitive (Śliwa, Jarzębski, Andrzejewska, Szafran, & Gapiński, 2017), and also multi-responsive polymers (Cheng, Shan, & Pan, 2017; Liang et al., 2017; Wang, Zhang, Qian, Deng, & Tian, 2016). In addition to the linear polymers, stimuliresponsive polymer gels which are commonly prepared by radical

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polymerization can also be used in wastewater treatment (Jing, Wang, Yu, Amer, & Zhang, 2013; Khan & Lo, 2016; Safavi-Mirmahalleh, Salami-Kalajahi, & Roghani-Mamaqani, 2019). Responsive linear polymers are commonly synthesized by control radical polymerization (CRP), where the chain breaking reactions are minimized and chain propagation is obtained by instantaneous initiation. Nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and addition-fragmentation chain transfer (RAFT) polymerization are three main methods of CRP. The CRP systems are based on the establishment of a dynamic equilibrium between propagating radicals and various dormant species (Braunecker & Matyjaszewski, 2007; RoghaniMamaqani, Haddadi-Asl, & Salami-Kalajahi, 2012). The multi-stimuli-responsive polymers exhibit responsivity toward more than two external stimuli. For example, to prepare a dual-responsive copolymer, another responsive polymer to the same or different external stimuli such as temperature, pH, or light is needed (Mohammadi, Salami-Kalajahi, Roghani-Mamaqani, & Golshan, 2017; Darabi, Jessop, & Cunningham, 2016; Mazloomi-rezvani, Salami-kalajahi, & Roghani-mamaqani, 2018; Mohammadi, Salami-Kalajahi, Roghani-Mamaqani, & Golshan, 2017). Light-responsive compounds can be incorporated to temperature- or pH-responsive polymers to

Corresponding authors at: 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.carbpol.2019.115247 Received 21 June 2019; Received in revised form 23 August 2019; Accepted 23 August 2019 Available online 28 August 2019 0144-8617/ © 2019 Elsevier Ltd. All rights reserved.

Carbohydrate Polymers 225 (2019) 115247

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case of application of PDMAEMA in ion-removal processes, silica-based adsorbent was prepared by Zhao, Sun, Zhao, Xu, and Zhai (2011) through radiation-induced grafting of PDMAEMA onto silanized silica to remove three heavy metal ions of Cr (VI), As (V), and Hg (II). By changing the solution pH, PDMAEMA blocks were protonated and ions were attracted to the positively charged PDMAEMA chains due to the electrostatic force. 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 et al. (2017) for adsorption of various heavy metal ions. The CO2-responsive octopuslike POSS-PDMAEMA adsorbent was used as an eco-friend product due to its responsivity to CO2 for adsorption and desorption of Cu2+. By bubbling of CO2 into the system, the percentage of free Cu2+ increases to 100% due to the reduction of pH value. The protonated DMAEMA units dissociate the polymer-metal complex and release all the chelated Cu2+ as free ions. Nitrate ions have also been removed by temperatureresponsive poly(N-isopropylacrylamid) (NIPAAM) gel and poly(N-isopropylacrylamide-co chlorophyllin) gel in different states. By shrinking the gels, their sorption is dramatically weakened. In this study, CNC-grafted and free copolymers of DMAEMA and coumarin were synthesized through RAFT polymerization. Removal of nitrate ions from aqueous solution has been investigated in different amounts of PDMAEMA and CSA. The light-responsivity, CO2-sensitivity, and temperature- and pH-responsivity of the polymers were studied. The effect of multi-responsivity of the products on the removal of nitrate ions and regeneration process is the main concept of this work. Due to the protonation of PDMAEMA amine groups, removal of ions is expected to increase. However, effect of the responsivity of the product to the light should be investigated in ion-removal efficiency. This is the first time in ion-removal applications that the regeneration process is proposed to be accomplished with CNC filtration in combination with multi-responsivity of the products.

prepare a multi-stimuli-responsive polymer. As the most important light-responsive compounds, spiropyran responds to UV light by isomerization between the ring-closed and ring-opened zwitterionic form (Abdollahi, Sahandi-Zangabad, & Roghani-Mamaqani, 2018; Abdollahi, Sahandi-Zangabad, & Roghani-Mamaqani, 2018), coumarin responds to UV light by bond formation through cycloaddition reactions (Babin, Lepage, & Zhao, 2008; Beattie et al., 2006) and azobenzene responds to UV light by cis to trans isomerization (Tang, Liang, Gao, Fan, & Zhou, 2010). In these photo-responsive compounds, coumarin has attracted a large attention, because of its clean, cheap, and readily available photoreversible reaction (Ling, Rong, & Zhang, 2014). The pH-responsive poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) is known as a CO2-responsive polymer due to its tertiary amine groups. It has also an LCST around 40 °C and known as a temperature-responsive polymer. As some examples of multi-stimuli-responsive PDMAEMA systems, azobenzene-terminated PDMAEMA was synthesized via ATRP by Tang et al. (2010). This pH and temperature-responsive polymer was also responsive to light due to the azobenzene grousps. In such systems, changing pH results in variation of LCST of the system. At high pH values, LCST was observed at 30 °C and LCST was seen at 68 °C at lower pH values. Moreover, LCST has observed at higher temperatures by irradiating with UV light due to the photo-isomerization of azobenzene end-groups. Considering multi-responsive polymers with coumarin moieties, Babin et al. (2008) prepared light-responsive shell crosslinked reverse micelles of styrene and DMAEMA by ATRP method. The polymeric micelle structure is stabilized by cross-linking of the shell by using photo-induced coumarin groups (> 310 nm). For micellar aggregates, de-cross-linking of the shell occurred by photo-induced cleavage of the cyclobutane bridges (< 260 nm). They have also prepared a dual responsive block copolymer (DMAEMA-co-CMA)-b-poly(N-isopropylacrylamide) by inserting coumarin methacrylate (CMA) units in PDMAEMA. By photo-induced dimerization of the CMA groups, coronal crosslinking has been achieved (He, Tong, Tremblay, & Zhao, 2009). Cellulose is a biodegradable and biocompatible natural polymer which shows great potential in preparation of functional materials for removal of metal ions (Zhou, Zhang, Zhou, & Guo, 2004). Cellulose nanocrystals (CNCs) show high modulus of elasticity, high aspect ratio, high specific surface area, and low density. It is commonly prepared by acid hydrolysis of the extremely abundant cellulose resources (Zeinali, Haddadi-Asl, & Roghani-Mamaqani, 2014). Biodegradable polymer composites based on cellulose are mainly synthesized by combination of “grafting from” and CRP methods. Considering the RAFT polymerization method, grafting RAFT agents on CNC to achieve biodegradable composites has been reported (Davis, Barner-kowollik, Barner, Barsbay, & Gu, 2009; Haqani, Roghani-Mamaqani, & Salami-Kalajahi, 2017; Roy, Guthrie, & Perrier, 2005; Stenzel, Davis, & Fane, 2003; Zeinali, Haddadi-Asl, & Roghani-Mamaqani, 2018). Roy et al. (Roy, Guthrie, & Perrier, 2008; Roy, Guthrie, & Perrier, 2006) synthesized PDMAEMA-grafted cellulose via RAFT polymerization by grafting of Smethoxycarbonylphenylmethyl dithiobenzoate as the RAFT agent on the cellulosic substrate. Roy et al. (2006) reported the self-made surface modification of a cellulose substrate by sequent physical sorption of ion compound latexes. PDMAEMA was polymerized through RAFT polymerization and grafted to CNC to achieve a hydrophobic surface with a particular topography. PDMAEMA has also attached to the CNC to achieve hydrophobic morphology in other studies (Utsel et al., 2012; Wang et al., 2011). PDMAEMA has also been attached to the substrate and used as in ion-removal processes (Chen et al., 2007; Singh & Ray, 1997). PDMAEMA, poly(diethylaminoethylmethacrylate) (PDEAEMA), and also poly(dimethylaminopropylmethacrylamide) (PDMAPMAm) have been grafted on CNC through SI-NMP method by Garcia-Valdez et al. (2017). By grafting three different CO2- and pH-responsive polymers on CNC and purging CO2 to the system, protonated blocks made the stability of the solution enhanced. PDMAEMA and other thermo-responsive polymers were also grafted at the surface of CNC by SI-ATRP method (Morits et al., 2017; Zhang, Wu et al., 2017). In the

2. Experimental 2.1. Materials Resorcinol, ethyl acetoacetate, 11-bromo-1-undecanol, 1,4-dioxane, potassium carbonate, trimethyl amine, acryloyl chloride, dichloromethane, 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), N, N-dicyclohexylcarbodiimide (DCC, Aldrich, 99%), 4-dimethylaminopyridine (DMAP, Aldrich, 99%), and S-(thiobenzoylthioglycolic) acid (RAFT agent, Aldrich, 99%) were used in the preparation of functional CNC-CTA. The 2-dimethylaminoethyl methacrylate (DMAEMA, 99%), 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 For preparing 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 suspension was cooled to room temperature and poured in ice water (300 mL) to obtain a yellowish precipitate. The achieved crude product was dried in a vacuum oven at 50 °C and recrystallized twice in ethanol to yield yellow coumarin crystals (Zhang, Hong, & Pan, 2017). 1 H NMR (in DMSO): δ 2.42 (CH3CCHCOO), 6.2 (CH3CCHCOO), 6.9–7 (H-aromatic), and 10.8 (OH-aromatic) (Supporting Information, Fig. S3 (A)).

2

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Table 1 The sample description and the recipe used for preparation of the samples. CTA

AIBN

CSA

DMAEMA

CNC-CTA

mmol

g

mol

g

mol

g

mol

mL

g

0.3 0.3 0.3 0.3 0.157 0.157 0.157 0.157

0.068 0.068 0.068 0.068 0.0335 0.0335 0.0335 0.0335

0.121 0.121 0.121 0.121 0.121 0.121 0.121 0.121

0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

0 0.0015 0.0021 0.0030 0 0.0015 0.0021 0.0030

0 0.6 0.84 1.21 0 0.6 0.84 1.21

0.017 0.017 0.023 0.029 0.017 0.017 0.023 0.029

3 3 4 5 3 3 4 5

0 0 0 0 0.067 0.067 0.067 0.067

Description

Sample

PDMAEMA(30) PDMAEMA(30)-b-PC(5) PDMAEMA(40)-b-PC(7) PDMAEMA(50)-b-PC(10) CNC-g-PDMAEMA(30) CNC-g-(PDMAEMA(30)-b-PC(5)) CNC-g-(PDMAEMA(40)-b-PC(7)) CNC-g-(PDMAEMA(50)-b-PC(10))

PD30 PDC305 PDC407 PDC5010 C-PD30 C-PDC305 C-PDC407 C-PDC5010

Fig. 1. SEM images of (A) MCC, (B) CNC, and (C) C-PDC5010, TEM images of (C) CNC and (D) C-PDC5010, microscopy images for (E and F) coumarine and (G and H) aggregated C-PDC5010 in the non-fluorescence and fluorescence modes.

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 (Supporting Information, Fig. S1) (Zhang, Hong et al., 2017). 1 H NMR (in DMSO): δ 6.12 and 5.83 (CH2CHCOO) and 1.58 (CH2CHCOO) (Supporting Information, Fig. S3 (C)).

2.3. Synthesis of 7-((11-hydroxyundecyl) oxy)-4-methyl-2H-chromen-2 (CS) Coumarin (4 g, 22.7 mmol) was dissolved in DMF (20 mL) in a 100 mL two-neck round-bottom flask. Then, the diluted 11-bromo-1undecanol (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 (Supporting Information, Fig. S1) (Zhang, Hong et al., 2017). 1 H NMR (in DMSO): δ 1.26, 1.43, and 1.74 (-CH2-), 4.06 (CH2aromatic), 3.3 (CH2CH2OH), and 4.3 (HO-CH2) (Supporting Information, Fig. S3 (B)).

2.5. Preparation of CNC from MCC CNC was prepared by acid hydrolysis of MCC, according to the literature (Bai, Holbery, & Li, 2009; Loiseau et al., 2003). 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 a large amount of water (500 mL), the reaction mixture was centrifuged at 5000 rpm for 20 min.

2.4. Synthesis of 11-((4-methyl-2-oxo-2H-chromen-7-yl) oxy) undecyl acrylate (CSA) CS (3.59 g, 20.0 mmol) and triethyl amine (5.5 g, 40.0 mmol) were 3

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Fig. 2. 1H NMR spectra for (A) PD30 and (B) PDC305, PDC407, and PDC5010.

2.6. Synthesis of S-(thiobenzoylthioglycolic) acid-grafted CNC to obtain CNC-CTA

Table 2 Molecular weights of the copolymers calculated from the 1H NMR spectra. Sample

PDC305 PDC407 PDC5010 a b

na

5 7 13

mb

3 5 8

Mn (g mol−1) PC

PDMAEMA

Total

1200 2000 3200

785 1099 2041

1985 3099 5241

CNC (0.5 g) was dispersed in THF (75 mL) and ultrasonically agitated for 1 h. Then, a 100 mL two-necked round-bottom flask was charged with CNC dispersion, S-(thiobenzoylthioglycolic) acid as RAFT agent (0.5 g, 2.3 mmol), DCC (0.8 g, 3.8 mmol), and DMAP (0.26 g, 2.130 mmol). The reaction was carried out in an oil bath at 40 °C for 3 days. The reaction mixture was then cooled to room temperature and precipitated in methanol. The precipitate was dissolved in THF and precipitated again in methanol to remove any unreacted RAFT agents. After washing the precipitate by vacuum filtration and drying the product at 60 °C, CNC-CTA dispersion was obtained (Supporting Information, Fig. S2 (A)) (Loiseau et al., 2003).

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

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 subsequent freeze-drying process.

2.7. Synthesis of the free and CNC-grafted polymers Synthesis of the free and CNC-grafted copolymers was carried out in glass vials which were placed in an oil bath thermostated at 70 °C. For preparation of the CNC-grafted copolymers, CNC was completely 4

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Fig. 3. Effect of the CO2 purging time on the pH of the samples aqueous solutions (the inlet figure is schematic representation for the protonation and deprotonation of the copolymers).

sample weight (Bai et al., 2017; Tran et al., 2016).

Table 3 Estimated morphology of the copolymers self-assembly in water. Sample

n

m

Morphology

fphilic (%)

PDC305 PDC407 PDC5010

5 7 13

3 5 8

Vesicular Vesicular Vesicular

39.57 35.44 38.97

2.10. Characterization Fourier transform infrared (FTIR) spectra were recorded on a Bomem FTIR spectrophotometer within a range of 500-4000 cm−1 with a resolution of 4 cm−1. An average of 32 scans has been reported for each sample and the cell path-length was kept constant during all the experiments. Samples were prepared on a KBr pellet in vacuum desiccators under a 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. Thermal gravimetric analyses were carried out with a PL thermo-gravimetric analyzer (Polymer Laboratories, TGA 1000, the UK). The thermograms were obtained from ambient temperature to 700 °C at a heating rate of 10 °C/min. A sample weight of about 10 mg was used for all the measurements, and nitrogen was used as the purging gas at a flow rate of 50 mL/min. Surface morphology of MCC, CNC, and CNC-g-copolymers were investigated by scanning electron microscopy (Vega Tescan SEM instrument, Czech Republic) using an applied 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 study the morphology of the CNC and CNC-g-copolymers. Temperature- and CO2-sensitivity of polymers and also the ion removal experiments were studied by a UV/ visible spectrophotometer (Milton Roy Spectronic 60) at a wavelength of 290 nm. A sample weight of 0.6 mg was dissolved in water (12 mL) to obtain the liquid samples for temperature- and CO2-sensitivity analyses. Dynamic light scattering (DLS) was recorded by Malvern Panalytical system at 25 °C, with 170.4 count rate (Kcps) and also measurement position reported as 4.65 mm. Fluorescence imaging was used for morphology investigation of the coumarin-containing samples (0.2 mg/ mL) by using Olympus BX50 Microscope with UV Narrow filter, where excitation and emission were performed in 360–370 and 420 nm, respectively.

dispersed in THF (5 mL) and sonicated for 20 min. Then, CTA, AIBN, DMAEMA, and CSA were added to the dispersion. Synthesis of the free copolymers was carried out similarly. However, CTA was used in the absence of CNC-CTA. DMAEMA and CSA were mixed in THF (5 mL). Then, CTA and AIBN were added to the mixture. Finally, all the samples were prepared under stirring at 70 °C for 24 h (Supporting Information, Fig. S2 (B)). Designation of the samples and the recipe used for their synthesis are presented in Table 1. 2.8. Separation of free polymers from the CNC-grafted polymers The composites were dissolved in THF. Free polymer chains were separated from CNC-grafted polymers by centrifugation at 12,000 rpm and passing the solution through a 0.2 μm PTFE filter. The CNC-grafted polymers remained on the PTFE filter. Then, the grafted polymers were dried in vacuum at 50 °C for 24 h. 2.9. Adsorption study Adsorption studies were carried out using a batch equilibrium procedure. Accordingly, 0.01 g of the samples in dialysis bags was added to the NO3− ions solution with concentration of 500 ppm (0.1 g/ L). Adsorption of the NO3− ions was carried out at natural pH under stirring. In different intervals, residual concentration of the NO3− ions in solution was measured by UV/Vis spectroscopy at wavelength of 290 nm. Adsorption capacity of the NO3- ions was calculated using Eq. 1:

q=

(Ct − Ct + 1 ) V M

(1)

where, q is adsorption capacity; Ct and Ct+1 are the initial and equilibrium concentrations of the NO3− ions; and V and M are volume and 5

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Fig. 4. Schematic illustration of the CO2- and temperature-driven self-assembly processes for the free copolymers.

3. Results and discussion

synthesis processes. However, it is required more characterization of the samples by spectroscopic analyses. FTIR was used to confirm the successful surface-modification of CNC by CTA and also preparation of the free and CNC-grafted copolymers (Supporting Information, Fig. S4). The spectrum of CNC shows two bands at 1370 and 2893 cm−1 related to the CeH bending and stretching vibrations, respectively (Golshan, Salami-Kalajahi, RoghaniMamaqani, & Mohammadi, 2017). The symmetric and asymmetric stretching vibrations of the carboxyl groups are also observed at 1410 and 1640 cm-1 (Cha, He, & Ni, 2012). The broad peak observed in 3342–3419 cm−1 is attributed to the OeH groups (Hemmatpour, Haddadi-Asl, & Roghani-Mamaqani, 2015). By grafting the CTA, the carbonyl groups peak is observed at 1730 cm−1 and the benzene ring of the RAFT agent is appeared at 1500 cm−1. In addition, the characteristic peaks at 807 and 1035 cm−1 are related to the CeS and C]S groups of the CTA, respectively (Jafarzadeh, Haddadi-Asl, & RoghaniMamaqani, 2015). These observations are some proves for successful grafting of the CTA at the surface of CNC. The band related to the hydroxyl groups stretching in CNC and CNC-CTA shows insignificant difference, which shows that the RAFT agent is grafted to the surface hydroxyl groups with lower content and the interior hydroxyl groups of the crystallite with higher content is remained unreacted (Jamiu et al., 2017). In the case of PD30, C-PD30, PDC5010, and C-PDC5010, the ester group of PDMAEMA is appeared at 1736 cm−1. Moreover, the peak for the NeCH3 groups of PDMAEMA is observed at 2760 cm-1 (Hu, Yu, Ye, Gu, & Zhou, 2010; Yi, Xu, Zhang, & Zhang, 2009). Appearance of the characteristic peaks of PDMAEMA and CSA in the spectra of the polymer-grafted CNCs originates from the successful grafting of polymers on CNC-CTA. Higher strength of CeH stretching vibrations at 2730–3025 cm−1 is assigned to the higher content of methylene groups

3.1. Investigation of the synthesis process SEM, TEM, and fluorescence microscopy images for the MCC, CNC, coumarin, and PDC5010 are shown in Fig. 1. MCC shows micro-sized crystalline cellulose domains (Fig. 1(A)) (Eichhorn & Young, 2001). After acid hydrolysis of MCC, small crystalline blocks were separated from the amorphous sections. These rod-shape products with average dimensions of about 10–20 nm are known as CNCs (Fig. 1(B and D)). Accordingly, CNCs showed laterally aggregated morphology because of their high lateral surface area and strong hydrogen bonding between the nanocrystals. CNCs have high specific area, strong hydrogen bonding, and also ionic surface charge which caused by acid treatment (Elazzouzi-Hafraoui et al., 2008). After polymer-grafting in the CPDC5010 sample, the CNCs turned to highly opaque nanocrystals with a layer of polymer on its surface (Fig. 1(C and E)). This caused separation of nanocrystals because of the hindrance effect of the polymer layer which prevents from highly strong hydrogen bonding between the layers. Fluorescence microscopy was also used for investigation of the coumarin and the coumarin-containing polymers morphologies. A drop from the solution of the synthesized coumarin in THF was put under microscope after its drying. According to the images (Fig. 1(F and G)), the crystal forms of coumarin with a blue-color fluorescence were observed, which can be a confirmation for successful synthesis of coumarin molecules. After deposition of a dilute dispersion of the CPDC5010 sample in THF on a flat surface and putting it under the fluorescence microscope, the aggregated polymer-covered CNCs and also their blue fluorescence can clearly be observed in the Fig. 1(H and I). These visual characterizations are confirmations for the successful 6

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Fig. 5. UV/Vis spectra of copolymers in aqueous solutions and their dimerization degree under UV irradiation at λ > 310 nm: (A) PDC305, (B) PDC407, and (C) PDC5010.

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Fig. 6. Ion-removal of the PDC5010 and C-PDC5010 samples in CO2, its release in N2, and also the effects of UV irradiation on the removal and release processes.

the product with NaOH. DTG curve of CNC shows two peaks related to the degradation of cellulose chains and the remained solid. By grafting of CTA on the surface of CNC, a mass loss related to the degradation of the CTA moieties is observed in 170–250 °C (Haqani et al., 2017). Residual char for MCC, CNC, and CNC-CTA is about 7.6, 16.8, and 13.3%, respectively. The char difference between the CNC and CNC-CTA can be a rough estimation of the RAFT agent grafted on CNC (3.5%). TGA and DTG thermograms of PD30 and the free copolymers are shown in Fig. S5 (C) and (D) (Supporting Information), respectively. TGA thermogram of the PD30 shows three weight loss stages. The 4% weight loss at 100–200 °C is due to the physically adsorbed water molecules. The 51.1% weight loss at the second decomposition stage at 200–360 °C is attributed to the functional amino groups of the PDMAEMA. The final mass loss of about 35.7% at about 360–450 °C belongs to the decomposition of carbon skeletons especially in the main chain backbone (Yao et al., 2016). As shown in Fig. S5 (C) and (D) (Supporting Information), the block copolymers show different thermal behavior than PD30, which originates from their PC blocks. The decomposition steps of the copolymers in 150–310 °C are associated with pyrolysis of the PC block and also functional amino groups of the PDMAEMA, respectively (Benbettaïeb et al., 2016). Fig. S5 (E) and (F) (Supporting Information) show the TGA and DTG thermograms of the CNC-grafted polymers, respectively. Thermal decomposition profiles of the CNC-grafted polymers exhibit two weight loss stages at about 200–320 and 320–450 °C. The first weight loss is attributed to the nanocrystals and partial copolymer chains decomposition (48%). The second weight loss is related to decomposition of the remaining grafted copolymers (37.5%) (GarciaValdez et al., 2017). In addition, CNC-PDC5010 show higher polymer content with respect to CNC-PDC305 (the char values are 16.7 and 34.3%, respectively). The char residues, weight loss, and DTG peak points (°C) for all the samples are also reported in Table S1 (Supporting Information).

in the polymer-grafted CNCs with respect to the neat and CTA-modified CNCs, which originates from the successful polymer-grafting reactions (Cao, Habibi, & Lucia, 2009). By sequential addition of CSA on the CNCPDMAEMA, the characteristic vibrations of the C]O and CeO units related to the coumarin were appeared at 1618 and 1382 cm−1, respectively (Sinkel, Greiner, & Agarwal, 2010). 1 H NMR spectra for the coumarin, CS, and CSA are shown in Fig. S3 (AeC) (Supporting Information), respectively. The absence of the aromatic peak of HO at δ = 10.8 ppm in CS and CSA is an evidence for proving the successful reactions presented in Fig. S1 (Supporting Information). In addition, the 1H NMR spectra for the PD30 sample and also the block copolymers are shown in Fig. 2(A). According to the results of Fig. 2(A), the characteristic signals of PDMAEMA are located at δ 2.3 (CH2N(CH3)2), 2.58 (CH2CH2N(CH3)2), 4.07 (CH2CH2N(CH3)), 0.95 (CH3), and 1.8 ppm (CH2) (Dong, Mao, Wang, Yang, & Ji, 2015). The aromatic hydrogen atoms of the RAFT agent are observed in the chemical shift range of 5.8 to 6.6 ppm, and the peak at δ = 4.27 ppm is assigned to the HOOCCH2 unit. Fig. 2(B) also shows the feature signals of the PDC305, PDC407, and PDC5010 copolymers. According to the results, by increasing the DMAEMA and CSA content, the signal intensity of their characteristic bands increases (Du & Zhao, 2004; Yuasa & Tsuruta, 2004). Molecular weight of the synthesized copolymers is calculated from the 1H NMR spectra, and the results are presented in Table 2. By increasing the DAMAEMA and CSA content, molecular weight of the copolymers is increased. 1H NMR analysis shows that the number-average molecular weights of PDC305, PDC407, and PDC 5010 are 1985, 3099, and 5241 g.mol−1, respectively. TGA and DTG thermograms of MCC, CNC, and CNC-CTA, are shown in Fig. S5 (A) and (B) (Supporting Information), respectively. According to the results, CNC shows higher char residue than MCC due to its higher amount of the crystalline parts (Zeinali et al., 2018). The decomposition of CNC is observed in a higher temperature range, while its DTG peak point as a measure of degradation temperature is observed at lower temperatures. Such a different thermal decomposition behavior between the CNC and MCC originates from their different particle size. Smaller particle size and higher specific surface area of CNC result in its lower decomposition temperature. Lower thermal stability of CNC can be assigned to the higher amount of amorphous chains at its surface, which is degraded at lower temperatures. DTG thermogram of CNC shows no considerable peak in 150–200 °C, which is an indication of absence of sulfate groups (Cha et al., 2012). This shows that sulfate-free CNC was obtained because of the successive sonication and treatment of

3.2. Investigation of stimuli-responsivity and self-assembly processes The CO2, temperature, and light-responsivity of the copolymers are investigated in aqueous solution by UV/Vis spectrophotometry at a wavelength of 297 and 350 nm for the PDMAEMA and PC, respectively. The pH and CO2-responsivity of the PDMAEMA have been observed by bubbling of CO2 to the polymer aqueous solution (0.5 mg/mL) in different bubbling times. PDMAEMA is a pH- and CO2-responsive polymer due to the presence of tertiary amine groups in its structure. In addition, 8

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25-100 43.2 43.2 25-300 10 10 50-1000 100-200

Ion-exchange Variation of pH Variation of pH Variation of pH Variation of pH Variation of pH Variation of pH Variation of pH

(Alikhani & Moghbeli, 2014) (Cho et al., 2011) (Cho et al., 2011) (Mukherjee & De, 2014) (Rajeswari, Amalraj, & Pius, 2016) (Rajeswari et al., 2016) (Sowmya & Meenakshi, 2013) (Wu et al., 2016)

it has an LCST of around 40 °C. Tang et al. (Tang et al., 2010) reported variation of LCST in different pH values in their multi-responsive polymer system. They reported increase of LCST by decreasing pH and also by irradiation of UV due to the trans-to-cis photoisomerization of the azobenzene moieties. Different LCST values are also reported for PDMAEMA in the presence of other stimuli (Han et al., 2012). As shown in Fig. 3, in the first few seconds, the pH of the solutions reached to its lowest value due to the protonation of the tertiary amine groups. All the solutions were changed from an opaque to transparent liquid after bubbling with CO2. The protonation process of PDMAEMA can be reversed via the simple bubbling of an inert gas such as nitrogen through the aqueous solution. The schematic illustration for the protonation and deprotonation process is shown in the inlet of Fig. 3. LCST behavior of the PDMAEMA and its block copolymers in solution was studied by their cloud point variations. Fig. S6 (Supporting Information) shows the temperature-responsivity of the 0.5 mg/mL aqueous solution of PDMAEMA and its block copolymers by variation of transmittance against temperature. LCST-type soluble-to-insoluble phase transition of the PDMAEMA is investigated, as shown in Fig. S6 (A) (Supporting Information). The transmittances curve against temperature for PD30 shows a sharp transition at around 43 °C, which is considered as the LCST. By CO2-bubbling to the solutions, the LCST of PD30 increases to the temperatures higher than 88 °C due to protonation of the tertiary amines which results in the increase of hydrophilicity. By adding the insoluble PC block, LCST of the copolymers increases to the higher temperatures than the experimental window. As shown in Fig. 6S (B, C, and D) (Supporting Information), due to insolubility of the second block (PC), LCST cannot be observed evaluated for these systems. In the case of the copolymers, the insoluble PC block causes the solution to be cloudy. In addition, higher length of the PC block results in lower transmittance. By purging CO2 to the system transmittance increased due to the protonation of PDMAEMA chains and increasing the block copolymers solubility. However, the protonated PDMAEMA chains deprotonated by adding N2 to the system, and transmittance turns back near to the initial transmittance value. The CO2-responsive behavior of the PDC5010 sample was also investigated via DLS. The measurement was conducted in the protonated and deprotonated states of the PDMAEMA chains at 25 °C (Supporting Information, Fig. S7). The results show that the size of copolymer assemblies in aqueous solution is about 64.7 nm, which increases to 350.6 nm by adding CO2 to the system due to protonation of the PDMAEMA chains and the consequent repulsion electrostatic force. In addition, zeta potential results confirmed protonation of the PDMAEMA chains by adding CO2, where zeta potential of the PDC5010 sample increases from 27.7 to 28.3 eV after purging the solution with CO2 (Supporting Information, Table S2). The free copolymers can act as stabilizing agent in oil in water dispersions. This claim was completely proved by fluorescence microscopy results of the n-hexane in water droplets. Fluorescence microscopy results for the n-hexane in water suspension stabilized by PDC5010 after bubbling with CO2 are shown in Fig. S8 (Supporting Information). According to the results coumarin units are located at the surface and therefore acting as the suspending agent. 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. Amphiphilic copolymers have commonly been used for self-assembly investigations by selection of a certain solvent for dissolution of one block of the copolymers (Smart et al., 2008). In this study, 1H NMR was used to study the morphological transition of the copolymers in aqueous solution by application of CO2 and temperature stimuli. The aggregated geometry of the block copolymers is characterized by using a hydrophilic mass fraction (%fphilic) calculated by Eq. 2 (Feng, Zhan, Yan, Liu, & Yuan, 2014).

26.2 3.20 9.04 15 50.68 35.03 67.5 221.8 Cellular porous vinylbenzyl chloride/divinylbenzene poly(high internal phase emulsions) Granular activated carbon Cationic polymer-modified granular activated carbon Polyacrylonitrile and alumina nanoparticles Poly(ethylene glycol)/Chitosan Poly(vinyl alcohol)/Chitosan Quaternized chitosan beads Modified cross-linked polystyrene

240 90 90 24 10-60 10-60 180 4000

Initial NO3− concentration (ppm) Adsorption time (min) Adsorption capacity (mg/g) Material

Table 4 Comparison of nitrate ions adsorption capacity and time, initial concentration, and mechanism of adsorption for some polymeric ion-removal systems.

Mechanism

Reference

Z. Abousalman-Rezvani, et al.

∑ Nphilic × mphilic ∑ Nphilic × mphilic + ∑ Nphobic × mphobic 9

(2)

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Fig. 7. Regeneration of the CNC-grafted copolymers using their stimuli-responsive characteristics.

microscopy, as shown in Fig. 1 (I). Dimerization of coumarin moieties upon UV irradiation can be further proven by variation of the absorbance peak point at wavelength of about 320 nm in Fig. 5. Dimerization degree of PDC305, PDC407, and PDC5010 are about 39, 82, and 85% respectively. Dimerization degree of the samples increases by increasing the amount of coumarin content.

where, mphilic and mphobic are the monomer mass of the hydrophilic and hydrophobic blocks, respectively. In addition, Nphilic and Nphobic are the repeating unit number for hydrophilic and hydrophobic blocks, respectively. The estimated morphology of the copolymers self-assembly in water is presented in Table 3. Fig. 4 schematically shows the effects of temperature and CO2 stimuli on the self-assembly of the copolymers in the aqueous solution. As concluded in Table 3, samples are assembled as vesicles in aqueous solution. In the presence of CO2 in aqueous system, part of the tertiary amine groups of PDMAEMA are protonated and formed ammonium bicarbonate salt. This responsivity to CO2 is reversible by insertion of N2 gas into the solution (Supporting Information, Fig. S9). Through increase of temperature above the LCST of PDMAEMA, the size of vesicles decreased due to the collapsing of the PDMAEMA chains. In addition, all of the small vesicles tend to accumulate and stick together because of the tendency of PDMAEMA blocks to minimize their energy (Feng et al., 2014). By bubbling CO2 to the system, protonation of the PDMAEMA blocks caused the copolymer to be more soluble and resulted in the increase of transmittance as previously discussed. The protonated PDMAEMA chains at above the LCST temperature could not aggregated due to the repulsive electrostatic forces between the chains. Fig. 5 shows variation of the UV/Vis spectra of copolymer solutions upon irradiation of UV light with wavelength of 365 nm by increasing irradiation time. The results show that the coumarin absorption peak continuously decreases by increasing irradiation time. Zaho et al. (He, Tremblay, Lacelle, & Zhao, 2011) showed coumarin dimerization degree of 75% after 1 h irradiation for their system, which was partly reversible at λ > 260 nm. Dimerization degree was decreased from 75 to 38% after 2 h irradiation at λ > 260 nm. Decreasing the absorption of the coumarin units at wavelength of about 320 nm by increasing the irradiation time shows the dimerization process and formation of the cross-linked copolymers. Dimerization degree is calculated as 1-At/A0, where A0 and At are the initial absorbance at 320 nm and the absorbance after irradiation at time t, respectively (He, Tong, & Zhao, 2009; Ling et al., 2014; Zhao, Tremblay, & Zhao, 2011). The light-responsivity of the CNC-g-copolymers dispersion in water is also observed by

3.3. Investigation of nitrate ions adsorption There are some problems with wastewater treatment such as adsorbent recycling, generation of waste sludge, etc. Such problems can be solved by using eco-friendly CO2-responsive copolymers as the adsorbent for heavy metal ions (Bai et al., 2017). In addition, nitrate ions are among the most widespread groundwater contaminant in the world because of its high water solubility. In different treatment technologies for removing nitrate ions from the wastewater, the adsorption process has largely been considered. Stimuli-responsive polymers are useful in ion-removal processes. The pH-responsive, temperature-sensitive, and CO2-responsive polymers have largely been used as adsorbent in the literature (Bai et al., 2017; Kaşgöz, Durmuş, & Kaşgöz, 2008; Liu et al., 2013; Sánchez et al., 2018; Tran et al., 2016). Therefore, stimuli-responsive adsorption process was used in this work for absorption of nitrate ions by the PDC5010 and C-PDC5010 samples. To obtain the amount of the nitrate ions adsorbed on the samples at equilibrium (q), various aqueous solutions with initial nitrate ion concentrations in the range of 300–600 ppm were prepared. The PDC5010 and C-PDC5010 with and without UV irradiation (λ > 350 nm) were used for nitrate ions removal from the wastewater. Fig. S10 (Supporting Information) shows a schematic illustration for the nitrate ions adsorption and release from the copolymer assemblies by CO2 and N2 purging, respectively. By adding CO2 to the system, the amine groups of the PDMAEMA chains become protonated, and consequently the negative nitrate ions were absorbed by the positively charged chains of the samples. Purging N2 to the system results in deproptonation of the protonated amine groups, and consequently the nitrate ions were released. Such behavior can be used in the removal of nitrate ions and also regeneration of the 10

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(30:5, 40:7, and 50:10) are responsive to CO2, temperature, and light triggers. 1H NMR shows that Mn of these copolymers is 1985, 3099, and 5241 g.mol−1, respectively. In addition, the CNC-grafted copolymers show char residue of 16.7 and 8.3% respectively for the 30:5 and 50:10 monomer ratios. By bubbling CO2, pH of the free polymers solutions reached to near 5.5 due to the protonation of the tertiary amine groups. LCST of PD30 increases to the temperatures higher than 88 °C in the presence of CO2. Also, LCST of the copolymers increases to the higher temperatures than the experimental window. These amphiphilic copolymers were self-assembled in water to vesicular structures, and the light-responsive coumarin-containing blocks cause stabilization of the self-assembled structures. DLS results show that the size of PDC5010 in aqueous solution is about 64.7 nm, which increases to 350.6 nm by adding CO2 to the system. By inserting N2 gas into the protonated system, deprotonation of the protonated amines was observed. Removal of the nitrate ions from the wastewater was carried out by protonation of the CO2-responsive PDMAEMA block with inserting of CO2. Temperature-responsivity of the PDMAEMA block in addition to the N2 insertion can be used in regeneration process. By increasing the PDMAEMA block length, the ions adsorption has also enhanced. The CNC-grafted copolymers showed lower adsorption in comparison with the free copolymers due to their lower PDMAEMA contents. The CNCgrafted copolymers can be easily regenerated by their stimuli-responsive characteristics. They can be crosslinked with each other after ion adsorption by applying UV light. In the future, 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 network with UV irradiation with wavelength of about 225 nm and purging with N2 gas. However, the harmful nature of UV irradiation is a challenge in application of these products in the bio-systems. Therefore, the next step can be emphasized on using safe light sources like near infrared (NIR) lightening on these applications, maybe by using upconverting nanoparticles for changing the NIR lightening to UV.

absorbent. To investigate the quantity of the absorbed ions by free and CNC-grafted copolymers, CO2 was injected into the system at a constant rate in 15–300 seconds. Ion-removal of the PDC5010 and C-PDC5010 samples in CO2, its release in N2, and also the effects of UV irradiation on the removal and release processes are shown in Fig. 6. Accordingly, absorption is strongly increased in the first two-minutes. After the second minute, the absorption rate becomes very low. Here, N2 gas was injected to the system for evaluating the removal process by neutralization of the protonated amine groups and therefore desorption of the nitrate ions to the water. As shown in the Fig. 6, complete release of the adsorbed ions has not been occurred. The light-responsive coumarins were dimerized and result in crosslinking of the PC blocks in both PDC5010 and C-PDC5010 by applying UV light with wavelength of 350 nm to the system. Effect of coumarin dimerization on the nitrate ion absorption has also been investigated. According to the results, the absorption rate was increased by applying light to the system and crosslinking of the second block. As shown in Fig. 6, the final ion absorption content and its rate are higher for the cross-linked samples. The reason is assigned to increasing surface area after cross-linking of coumarin units in the free and CNC-grafted copolymers. By increasing ion adsorption by cross-linked samples, the concentration of ions in the system decreased and cause the C-C0 concentration enhanced (Zeng, Fan, Wu, Wang, & Shi, 2009). The results also show that the absorption rate and content of CNC-grafted sample is lower than the PDC5010 sample, which results from lower amount of PDMAEMA chains in a constant weight of C-PDC5010. According to the ion adsorption efficiency of the samples shown in Table S3 (Supporting Information), the UV-crosslinked PDC5010 sample has the highest efficiency for ion removal. Adsorption of anions by electrostatic forces has frequently been reported. Abou Taleb et al.(Abou Taleb, Mahmoud, Elsigeny, & Hegazy, 2008) reported adsorption and desorption of phosphate and nitrate ions by propylene and DMAEMA graft copolymers through protonation of the amine groups in PDMAEMA backbone by variation of pH. They showed that the adsorption amount was decreased by increasing pH. In our study, pH is decreased by purging CO2 to the system (as shown in Fig. 6) and resulted in higher adsorption of negative nitrate ions by positively charged sites. CO2-responsive polymeric octopus has been synthesized from a silsesquioxane core and PDMAEMA shell for removal of Cu2+ from aqueous solution by varying the pH of solution (Bai et al., 2017). The simulation results showed that increase of pH results in higher adsorption capabilities. By protonation of the system by immersing CO2, the positively charged ions were regenerated from the polymer due to electrostatic forces. Moreover, PDMAEMA-grafted cellulose has been used as a fluoride and arsenic ions absorbent (Meng, Wu, Tian, Kuga, & Huang, 2013). Protonation of the amine groups in low pH values resulted in adsorption of the negatively charged ions. Adsorption of nitrate ions in different studies with their adsorption capacity and time are presented in Table 4 for more evaluation. As it is clear, reduction of pH is the main mechanism for removal of nitrate ions; therefore, CO2 was used in this study for reducing pH and removal of nitrate ions from aqueous solutions. The CNC-grafted copolymers can be easily regenerated by their stimuli-responsive characteristics, as shown in Fig. 7. Accordingly, CNC-grafted copolymers can be crosslinked with each other after ion adsorption by applying UV light. 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 network with UV irradiation with wavelength of about 225 nm and purging with N2 gas.

Declaration of Competing Interest There is no conflict of interest. Acknowledgement Financial support of Sahand University of Technology is highly appreciated (Grant Number: 30/22441). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.carbpol.2019.115247. References Abdollahi, A., Sahandi-Zangabad, K., & Roghani-Mamaqani, H. (2018a). Light-induced aggregation and disaggregation of stimuli-responsive latex particles depending on spiropyran concentration: Kinetics of photochromism and investigation of reversible photopatterning. Langmuir, 34(46), 13910–13923. https://doi.org/10.1021/acs. langmuir.8b02296. Abdollahi, A., Sahandi-Zangabad, K., & Roghani-Mamaqani, H. (2018b). Rewritable anticounterfeiting polymer inks based on functionalized stimuli-responsive latex particles containing spiropyran photoswitches: Reversible photopatterning and security marking. ACS Applied Materials & Interfaces, 10(45), 39279–39292. https://doi.org/ 10.1021/acsami.8b14865. Abou Taleb, M. F., Mahmoud, G. A., Elsigeny, S. M., & Hegazy, E.-S. A. (2008). Adsorption and desorption of phosphate and nitrate ions using quaternary (polypropylene-g-N,Ndimethylamino ethylmethacrylate) graft copolymer. Journal of Hazardous Materials, 159(2–3), 372–379. https://doi.org/10.1016/j.jhazmat.2008.02.028. Ali, S. A., Rachman, I. B., & Saleh, T. A. (2017). Simultaneous trapping of Cr(III) and organic dyes by a pH-responsive resin containing zwitterionic aminomethylphosphonate ligands and hydrophobic pendants. Chemical Engineering Journal, 330, 663–674. https://doi.org/10.1016/j.cej.2017.08.003. Alikhani, M., & Moghbeli, M. R. (2014). Ion-exchange polyHIPE type membrane for removing nitrate ions: Preparation, characterization, kinetics and adsorption studies.

4. Conclusion Free and CNC-grafted block copolymers of DMAEMA and coumarin were synthesized via RAFT polymerization. These multi-responsive polymers in three different ratios of DMAEMA and coumarin monomers 11

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