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How Does a Smart Polymer Respond to Imidazolium-based Ionic Liquids? Reddicherla Umapathi, Awanish Kumar, Payal Narang, and Pannuru Venkatesu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03790 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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ACS Sustainable Chemistry & Engineering

How Does a Smart Polymer Respond to Imidazolium-based Ionic Liquids? Reddicherla Umapathi, Awanish Kumar, Payal Narang and Pannuru Venkatesu* Department of Chemistry, University of Delhi, Delhi – 110007, India Mailing address of authors: Reddicherla Umapathi: Department of Chemistry, University of Delhi, Delhi 110007, India Awanish Kumar: Department of Chemistry, University of Delhi, Delhi 110007, India Payal Narang: Department of Chemistry, University of Delhi, Delhi 110007, India Pannuru Venkatesu: Department of Chemistry, University of Delhi, Delhi 110007, India Corresponding author: e-mail: [email protected]; [email protected] ABSTRACT In this article we have explored the changes in the lower critical solution temperature (LCST) of the poly(N-vinylcaprolactam) PVCL in the presence of imidazolium-based ionic liquids (ILs) with fixed cation 1-butyl-3-methylimidazolium cation [Bmim+], and commonly used anions such as SCN-, I-, Br-, Cl-, CH3COO- and HSO4-. The series of anions also reflect the Hofmeister series of ILs. The results reveal that the LCST of the PVCL increases with increasing IL concentration. Interestingly, the results are the reverse of the PNIPAM with the same ILs where the LCST of PNIPAM decreases linearly with increase in the IL concentration following the Hofmeister series. Herein, we observed that the increase in the LCST of PVCL follows the order of ILs as, HSO4- < CH3COO- < Cl- < Br- < I- < SCN-. This is the reverse order for Hofmeister series anions effect on the polymer. Furthermore, the different kind of interactions occur between ILs and PVCL observed using UV-absorbance, Fluorescence and FTIR spectroscopy. The predicted mechanism suggest that the ILs act as bridge between water and PVCL, due to which the solvation of the polymers is increased leading to increase in the LCST of the polymer. Moreover, DLS and FESEM techniques were employed to examine the morphological changes in the PVCL at different temperature in the presence of various concentrations of the ILs. KEYWORDS: poly(N-vinylcaprolactam) (PVCL), ionic liquids (ILs), phase separation, lower critical solution temperature (LCST), Hofmeister series. 1 ACS Paragon Plus Environment

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INTRODUCTION Stimuli responsive polymers find wide application in the field of research, which include sensors, gels, and several drug delivery devices such as, the nanops.1,2 The physical properties that make stimuli responsive polymers are the property to undergo a reversible coil to globule state (and vice-versa) with changes in temperature, pH, ionic strength, and co-solutes in aqueous medium. In the class of stimuli responsive polymers, temperature responsive (thermoresponsive) polymers can be considered as the polymers of the future.3 The solubility of a thermoresponsive polymer (TRP) decreases with increase in the temperature. The formation of turbidity in the polymer solution with increase in temperature indicates phase transition and the temperature at which the turbidity appears is termed as lower critical solution temperature (LCST).4 The LCST is mainly dependent on the hydrogen bonding between water molecules and the structure of functional monomer units of polymers. At LCST a clear transparent polymer solution turns turbid, as stated, is assumed to be a macroscopic manifestation of a coil–globule transition. Macroscopic phase separation of the solution is observed due to dehydration of the polymer chains at LCST.5-7 Mechanistically, the hydrogen bonding interactions within the polymer chain are broken by thermally enhanced molecular motions. The phase separation behavior of the aqueous solutions of TRPs is generally associated with the temperature dependency of inter and intra molecular hydrogen bonding and hydrophobic interactions in polymer chains.8-10 The most commonly studied TRPs in aqueous solution is poly(N-isopropylacrylamide) (PNIPAM) with an LCST of 32 0C.11,12 Other polymers include, poly(oligoethylene glycol (meth)acrylate)s,13-16 poly(2-oxazoline)s,17-22 poly(vinyl ether)s23 and polypeptides.24,25 Poly(Nvinylcaprolactam) (PVCL), a member of the amphiphilic TRPs is known to exhibit in aqueous solutions a LCST. The phase transition of PVCL, at LCST, is manifested by milk-white turbidity of solutions. At LCST, the PVCL blocks become hydrophobic resulting in collapse of the polymer. PVCL exhibits a LCST at 32 0C in water.26 Below Tc, the polymer is swollen or soluble, while at temperatures higher than Tc, precipitation takes place. Due to this extraordinary characteristics and response to changes in temperature PVCL find varied application in the field of drug delivery systems, biocatalysis, purification of metal ions, and understanding stability in biomolecules. PVCL has its own unique advantage in the biomedical applications because it does not produce toxic low-molecular-weight amines during hydrolysis.27-33 2 ACS Paragon Plus Environment

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Moreover, unlike PNIPAM, the LCST of PVCL is dependent on its molecular weight.34 Therefore, the phase separation in PVCL solutions was studied less often. Moreover, PVCL has a better biocompatibility as compared with PNIPAM. This is due to the hydrolysis of PVCL which will only produce a polymeric carboxylic acid. These properties make the PVCL more suitable for biomedical applications.35 The major application of PVCL includes preparation of PVCL-based copolymers,36-38 gels39-43 and immobilization of enzymes.44 Based on the requirement and application, different fabricated TRPs are required. These fabrications are the morphological behavior of polymers when precipitated using several excipients such as urea or ionic salts.45,46 In the present situation, ionic liquids (ILs) have gained much interest in examining polymers structure and conformational changes in aqueous medium.47-51 The study of polymer properties in ILs provide useful results and information regarding inter and intra-molecular interactions in polymers. In this context, imidazolium-based ILs were observed to decrease the LCST of PNIPAM.52-57 These information are extremely important since ILs have received a great deal of attention as solvents for preservation medium for biopolymers, such as proteins; polymerization and for the solubilization of sparingly soluble polymers such as cellulose.58,59 In the present study, we have examined the changes in the LCST of PVCL in the presence of imidazolium-based ILs. Herein, we have included ILs composed of 1-butyl-3methylimidazolium cation [Bmim+], and commonly used anions such as SCN-, I-, Br-, Cl-, CH3COO- and HSO4-, on the phase transition temperature of PVCL aqueous solution. To our knowledge, this is the first direct measurement to explore the effect of imidazolium-ILs on the LCST of PVCL. Furthermore, we believe that the present work will be useful in tailoring the thermoresponsive behavior of amphiphilic polymers according to the needs of polymer technologies. Here, our main purpose is to explore how the ILs influences the conformations of PVCL, and to explain the mechanism for the shift of the LCST of PVCL following the Hofmeister series. MATERIAL AND METHODS Materials

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N-vinylcaprolactam (VCL), hexane and azobis(isobutyronitrile) (AIBN) were purchased from TCI and used after recrystallization. 1,8-Anilinonapthalene sulfonic acid (ANS), D2O and six ILs [Bmim][HSO4] (≥97.0%), [Bmim][CH3COO] (≥95.0%), [Bmim][Br] (≥97.0%), [Bmim][I] (≥99.0%), [Bmim][SCN] (≥95.0%) and [Bmim][Cl]) (≥99.0%) were purchased from Sigma-Aldrich Chemicals Co. and used without further purification. Sample solutions were prepared by using the high purity of water with a resistivity of 18.3 MΩ cm and which was obtained from a NANO pure water system. Preparation of the PVCL in the absence and in the presence of imidazolium-based ILs samples All the samples in the present study were prepared in distilled deionized water with a resistivity of 18.3 MΩ cm. AND balance with a precision of ±0.00001 g was used for gravimetric measurements. All the sample mixtures were filtered with 0.22 µm disposal filter (Millipore, Millex-GS) through syringe prior to the measurements and were incubated for few hrs at 25 oC in order to obtain complete dissolution before performing experiments. The final polymer concentration for all measurements was 7 mg/mL. The weighed amounts of ILs at various concentrations (5, 10 and 15 mg/mL) were added directly to the aqueous polymer solution. The concentration of external probe used for the UV-vis absorption and fluorescence study was kept at 2×105 M to avoid the probe interference in the measurements. The sample solutions were stored in cool place and kept container tightly to prevent water absorption. Synthesis of PVCL The synthetic reaction of PVCL via solution polymerization is shown in Figure 1. PVCL was prepared by a solution polymerization of NVCL in the presence of AIBN as an initiator. The obtained polymer was a white, powder type polymer. It was soluble in water and common organic solvents. The detailed synthesis of PVCL and its spectral analysis using NMR (1H and 13

C) and IR has been provided in supporting information.

Methods The detailed information and techniques used in the current study have been delineated in our previous papers 26,52,57 and brief description has provided in the supporting information. 4 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION Dynamic Light Scattering Studies of PVCL in the Presence of Imidazolium-based ILs. The effect of temperature on the conformation of PVCL in absence as well as in the presence of ILs was monitored. The hydrodynamic diameter (dH) of the PVCL in aqueous medium is sharply increased at LCST. As represented in Figure 2, the dH of the PVCL in absence and presence of ILs is observed to be 31.0, 31.0, 30.0, 30.5, 31.2, 30.1 and 31.0 nm for PVCL (alone), and in the presence of 15 mg/mL of ILs anions, HSO4-, CH3COO-, Cl-, Br-, I- and SCN-, respectively. The insets in Figure 2 (a,b,c) are the first derivative spectra of each of the sample data and the colored circles indicate the phase transition temperature for polymer samples in absence and presence of the ILs. An another interesting aspect, that is, as provided in the inset of each of the figures, the derivative spectra reveals that the LCST values of PVCL in ILs is higher than that obtained for the polymer in aqueous solution. It seems that at 25 0C the ILs has the minimal effect on the polymer expanded conformation. Further, dH of PVCL increases with the increasing temperature. By definition, the temperature where a large increase in the dH is observed is considered to be LCST of the polymer. As represented in Figure 2, the increase in the dH is observed at 31, 33, 33, 34, 35, 36 and 38 0C for PVCL in aqueous solution, and in the presence of 15 mg/mL of ILs anions, HSO4-, CH3COO-, Cl-, Br-, I- and SCN-, respectively. Further, with the addition of the ILs the increase in the dH is observed to occur at the temperature greater than the LCST of the PVCL alone. It is evident that all investigated ILs increase the LCST curves to different extents, which is reverse of the effects that is, in most cases observed for the Hofmeister series of the anions. The values of dH estimated from a cumulant method for 5,10,and 15 mg/Ml ILs aqueous solutions of were plotted as a function of temperature, increasing from 25 to 40 0C (Figure 2a-c). The dH value for PVCL in the absence of ILs started to increase at 31 0C, and reached a nearly constant dH value at ~1300 nm above 31 0C. Further, the LCST value was increased with increase in the concentration of the ILs. As represented in Figure 2(d), only 5 mg/mL of ILs with Br, I and SCN anions, had an effect on the LCST of PVCL. Moreover, increase in the concentration of ILs to 10 and 15 mg/mL the LCST of the PVCL dramatically increases. These observations suggested that in the presence of imidazolium ILs upon increase in the temperature



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above LCST of pure PVCL, start to form an intramolecular aggregate. The increase in the LCST of PVCL in ILs follows the order as, HSO4- < CH3COO- < Cl- < Br- < I- < SCN-. Spectroscopic Methods: 8-anilino-1-naphthalenesulfonic acid (ANS) is an amphipathic dye used in the detection and analysis of conformational changes in TRPs or proteins.60 In water it is non-fluorescent, but when it interacts with a polymer in aqueous medium, there is an increase in the fluorescence quantum yield. Therefore, we have employed the absorption and fluorescence spectroscopic techniques to examine the phase behavior of PVCL in the presence of ILs. Absorption Spectroscopy Analysis of PVCL in the Presence of Imidazolium-based ILs. PVCL is a polymer having no UV absorbing group, and thus there exist the use of ANS as a probe to examine the conformational changes in the polymer. As represented in Figure 3, the absorption maximum of ANS is obtained at about ~380 nm with a hump at about 350 nm.61 The appearance of the UV bands for ANS at 350 and 380 nm indicates that the probe is well incorporated in stable ground state conformations into the complex chain of PVCL network. With the addition of ILs the intensity of the ANS absorption intensities is decreased (Figure 3 a-c). This decrease in the ANS absorption intensity is dependent on the concentration of the ILs added to the PVCL solution. As represented in Figure 3(d), the decrease in absorption intensities are much larger at higher concentration of the ILs and follow the order HSO4- < CH3COO- < Cl- < Br- < I- < SCN-. However, in spite of the decrease in the absorption intensities, the characteristics bands at 350 and 380 nm still exists for the ANS. It reflects, though the presence of the ILs increases the ionicity of the system, ILs do not affect the PVCL-ANS interactions to a greater extent. Furthermore, due to increased ionic character in the PVCL solutions, as indicated by the ANS absorption, the polymer conformation is not altered to greater extent at 25 0C. Steady State Fluorescence Spectroscopy Analysis of PVCL in the Presence of Imidazoliumbased ILs. The fluorescence spectrum of ANS in solution depends pronouncedly on the nature of the solvent, especially on its polarity.61 Figure 4 shows the fluorescence spectra of ANS in PVCL 6 ACS Paragon Plus Environment

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aqueous solution in the absence and presence of ILs at 25 0C. Since, ANS is having both hydrophobic and a hydrophilic property it is likely that it can bind with the PVCL in aqueous medium.62 Figure 4 reveals that ANS exhibits fluorescence intensity at a wavelength of 510 nm in the PVCL aqueous solution which is consistent with the literature value.62 The fluorescence intensities of ANS increase with increase in the hydrophobicity of the environment. Consequently, it is clear from Figure 4(d), that a complex is formed between ANS and PVCL and, ANS in these complexes are surrounded by nonpolar chain links of the PVCL. Further, with the addition of the ILs the ANS emission intensity is decreased and eventually reaches a maximum decrease with the addition of ILs. This effect may be explained by taking into account that each ANS molecule is solvated in a slightly different way due to ILs. It has been reported in the literature that the fluorescence in ANS is affected by solvent polarity.63 Changes in the fluorescence intensity and wavelength occur due to the changes on the quantum yield of ANS. Fluorescence in ANS occurs due to the intermolecular excitation of the localized electron in the naphthalene ring. In the polar conditions, intramolecular electrontransfer is prominent and results in low intensity and longer wavelengths. The lack of the intermolecular electron transfer in polar solvents in ANS is attributed to the solvent reorientations for larger polar molecules compared to that of water molecules. Due to this reason we obtain a decrease in the fluorescence intensities in ANS in the presence of ILs. High fluorescence intensity of ANS is expected to be due to the hydrophobic interactions of ANS to the PVCL. It is supposed that the presence of ILs changes the water environment around the ANS in PVCL aqueous solution. Therefore, we observe a decrease in the fluorescence intensity with increase in ILs concentrations (Figure 4 a-c). Moreover, due to the presence of the hydrophobic backbone in PVCL, the solvent polarity change due to the presence of ILs is not much affecting the ANS-polymer interactions. This was confirmed by normalization of the fluorescence spectra of the ANS containing PVCL solution in the presence of the ILs. Figure 4 represents the normalized spectra of ANS fluorescence in the presence of ILs. It is observed that there is minimal shift in the ANS wavelength due to ILs and this effect still remains insignificant with the increase in the ILs concentrations (Figure 5). This reflects that



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the ILs do not disturb the integrity of the probe in the vicinity of the PVCL. Also, the PVCL is in its extended conformation in the presence of the ILs at 25 0C. Moreover, as represented in Figure 4(d), the change in the ANS fluorescence intensity decreases with changes in the anion of the ILs. This decrease in the fluorescence intensity for ANS in PVCL solution with ILs follows the order as, pure > HSO4- > CH3COO- > Cl- > Br- > I> SCN-. The series is similar to the Hofmeister series of anions, which is represented as, SO4-2 > HPO4-2 > OH- > F- > HCOO- > CH3COO- > Cl > Br > NO3- > I- > SCN- > ClO4-.64 The anions in the Hofmeister series have the property to change the water structure surrounding them in the solutions. Ions on the left of Cl− in the Hofmeister series are said to be “cosmotropes” or “water structure makers”, whereas ions on the right of Cl− are called “chaotropes” or “water structure breakers”. Moreover, Chowdhuri et al.65 stated that the H-bond is found to be strongest between water and kosmotropes followed by Cl− and, chaotropes such as I− with all of them having a slower H-bond dynamics compared to the water−water H-bond. Keeping this in context, and based on the analysis of Figure 4 and 5, it seems that the anions lying on the left side of the Clanion disturbs the water structure around the chromophore to the lesser extent. Due to this reason we observe a decrease in the fluorescence intensity of ANS with changes in the anions following the Hofmeister series. However, the overall conclusion form the fluorescence intensity graphs is that there is no changes in the PVCL structure at 25 0C due to the addition of ILs, even at their higher concentrations, since, no changes in the wavelength was observed for the probe. Thermal Fluorescence Spectroscopy Analysis of PVCL in the Presence of Imidazoliumbased ILs. The temperature dependence fluorescence intensity of ANS in PVCL aqueous solution in the presence and absence of ILs (5, 10 and 15 mg/mL) is shown in Figure 6. The fluorescence intensity of ANS decreases drastically at the LCST. This is in contrary to ANS fluorescence of the PNIPAM aqueous solutions where the fluorescence intensity of ANS was increased with increase in the temperature of the solution close to the LCST of PNIPAM.57 Due to the structural differences between PVCL and PNIPAM, PVCL is expected to possess more hydrophobic character than PNIPAM. This might be one of the reasons for this anomalous behavior of ANS with PVCL.


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This fact is further supported by the observations of Kirsh et al.62 The authors studied the binding of ANS with various polymers such as, poly-N-methylacetamide (PVMA), poly-Nvinylcaprolactam

(PVCL),

poly-N-vinylpyrrolidone

(PVP)

and

copolymers

of

N-

vinylcaprolactam with N-vinylpyrrolidone (CP VCL VP) and with N-vinyl-Nmethylacetamide (VMA). According to the authors, the stability constant for the complexes with the probe was the maximum for the PVCL, indicating the essential role of hydrophobic interactions in the complex formation. Furthermore, using pyrene fluorescence analysis, Chee et al.66 also supported the fact that PVCL is more hydrophobic than PNIPAM. In their analysis, the author investigated the 3/1 ratio of the pyrene/PVCL dispersion at all temperature ranging in between 10 to 70 0C. The results reflect higher fluorescence intensities for pyrene for PVCL. According to the authors, the spectral data imply that the probe experiences a more hydrophobic environment for PVCL as compared to PNIPAM, and the hydrophobicity in the interior of the PVCL collapse increases with increase in the temperature of the system. PVCL has a repeat unit consisting of a cyclic amide where the amide group nitrogen is directly attached to the hydrophobic polymer backbone.67 Below LCST the polymer is in expanded conformation hence; the hydrophobic interaction of the ANS with the polymer is expected to be high. With increase in the temperature, the polymers collapses and forms large aggregates. It has been reported by Aseyev et al.68 that PVCL has a tendency to form small aggregates termed as mesoglobules close to LCST. According to the author’s investigations, the stability of the mesoglobules upon time and dilution at temperatures above the LCST suggests that the p surfaces possess a hydrophilic character. Since, we are using ANS which is not tagged with the PVCL chemically, it is possible that most of the ANS probe is unable to bind in the interior of the globule of PVCL, upon aggregation remain in the solution. Furthermore, as stated earlier, the outer surface of PVCL upon aggregation becomes hydrophilic, thereby, influencing the fluorescence properties of ANS in the solution. Due to this, there is a sharp decrease in the fluorescence intensity of the ANS upon approaching the LCST of PVCL (Figure 6). Another factor that controls the decrease in the ANS intensities in the PVCL solution during phase transformation may be the decrease in the transparency of the solution. At LCST the PVCL solution is cloudier which hinders the light to pass through the sample under investigation.



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Moreover, with addition of the ILs, the fluorescence intensity of the ANS in PVCL solution is slightly decreased. It is observed that the LCST of the PVCL is increased in the presence of the ILs. As presented in Figure 6(d), the LCST increases with increase in the concentration of the ILs, if considered at a specific concentration, say 5 mg/mL of the ILs, the increase in the LCST follow the order, pure < HSO4- < CH3COO- < Cl- < Br- < I- < SCN-. This order is opposite to that of the Hofmeister effect observed in the case of PNIPAM.57 The increase in the PVCL LCST with ILs addition is not clear yet. However, it is expected that both the structure of PVCL (lactum ring) and ILs play a dominant role in the increase in the LCST of the polymer. According to Li and Wu,69 imidazolium ILs have the tendency to H-bond with both water and PVCL. This can be supported by the fact that incorporation of the hydrophilic imidazole units to PVCL chains would produce globular polymer conformations which remain soluble in the aqueous medium without macroscopic phase separation.

70,71

The correct

mechanism for the increase in the LCST of the PVCL is not known yet, however, the ILs (both cations and anions) are expected to act as a bridge which drags more water molecules towards the PVCL. Hence, addition of ILs to the PVCL strengthens the H-bonding between water and PVCL. With increase in the concentrations of the ILs this tendency is increased, due to which an increase in the LCST is observed. However, due to different solvation tendencies of the anions of the ILs, we observe a different trend in the increase of the LCST with changes in the IL type. Fourier Transform Infrared Spectroscopy Analysis of PVCL in the Presence of Imidazolium-based ILs. FTIR spectroscopic was carried out to study the interactions responsible for the increase in the LCST of PVCL in the presence of ILs at 25 0C. D2O was mainly used instead of H2O for the FTIR measurements in order to avoid the overlap of the OH band of water at about 1640 cm-1 with the (C=O) band of PVCL and facilitates the assessment of the hydration state of the amide C=O groups of PVCL.72 Moreover, it is reported, since, PVCL does not contain any dissociable protons, the position of the IR bands of PVCL measured in D2O were essentially identical with those measured in H2O.73 Additionally, the authors also suggested that hydrogen bonding of lone pair of the lactam ring in PVCL affects the frequency of the amide I band in aqueous medium. Therefore, in the present study two spectral regions are mainly considered here, the C–H stretching band (3000–2815 cm-1) and the amide I (C=O) band (1650–1570 cm-1) to probe the


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molecular mechanism behind the phase transition behavior of PVCL in the presence of imidazolium family ILs. Figure 7 illustrates the variations in absorbance induced by different ILs in the concentration range of 5, 10 and 15 mg/mL in the presence of 7 mg/mL of PVCL aqueous solution. At all concentration of the ILs there is a change observed in the C–H and C=O stretching bands for the polymer. For simplicity, let’s consider the FTIR spectra of PVCL in the presence of 5 mg/mL of the IL. The wavenumber (νC-H) for C–H groups for PVCL without ILs appears at 2930 cm-1. As seen in Figure 7, the νC-H for PVCL shifts to the higher values in the presence of the ILs and this increase in the ν values follow the order as, pure < HSO4- < CH3COO- < Cl- < Br- < I- < SCN-. The increase in the ν values indicates the increased interaction of the PVCL with the water molecules due to the ILs. This is supported by the observations of Maeda et al.73 where in the presence of salts, decrease the ν values of PVCL. Further, Sun and Wu72 also supported the fact that the higher the number of water molecules surrounding C–H groups in PVCL, higher vibrational frequency is observed.72 Furthermore, we did analysis of the C=O stretching bands for the polymer in presence of the ILs. Maeda et al.73 illustrated that the amide I in PVCL contains four vibrational components which are not resolved yet hence, a direct analysis of the particular bond in this region is not possible. However, the authors, predicted that a strong H-bond with C=O group decreases its electron density and thereby, the ν values decreases. In our experiment we did not observed a shift in the ν values, however, the peak intensities are significantly decreased in the presence of the ILs. This indicates that there exists H-bonding between the ILs and the C=O group of the polymer. It seems that the formation a H-bond between the ILs and the C=O group leads to rearrangement of the water molecules around the PVCL. We hypothesize that this rearrangement of water molecules in the presence of the ILs around PVCL helps in increased interaction between PVCL and water molecules, thereby increasing the LCST of the polymers. Our observations are in close agreement with the results of Sun and Wu,72 where the presence of 1-ethyl-3-methylimidazolium

action

with

bis(trifluoromethylsulfonyl)

imide

anion

([EMIM][NTf2]) increased the LCST of the PVCL. The authors argued that this increased change in the LCST was due to the rearrangement of the water molecules induced due to the presence of the ILs around the PVCL structure. Overall, the presence of the ILs significantly 11 ACS Paragon Plus Environment


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modulates the solvation of the polymers, thereby promoting or acting as a bridge to support a strong solvation layer of water around the polymer. The role of the anions may be significant in this case however, it is still not certain to define the extent upto which the anions play role in increasing the LCST of the PVCL aqueous solutions. The concentration of ILs we have used is very low (~0.05 M of ILs). Under such low concentration it is observed that the Hofmeister effect is not followed. It has been reported by Galamba74 at low salt concentrations, the halide anions slightly increase the tetrahedrality of the H-bond network of water in the anionic second hydration shell and I− is found to be the strongest kosmotrope, contrary to its structure breaker reputation. Further, at lower concentrations of sodium salts, for chaotropic anions such as I- and SCN-, the decreased in the LCST of PNIPAM was observed to be nonlinear as compared to the other anions of the Hofmeister series.75 As reported by Zhang et al.75 this change is due to the fact that the most strongly hydrated species are capable of polarizing the water sufficiently to weaken its interactions with the lone pairs on the carbonyl oxygen and amide nitrogen, leading to the partial dehydration of the amide in the polymer. Based on this argument, we expect that at lower concentrations, the interactions of the anions of the Hofmeister series with water molecules are drastically altered. Additionally, based on our experimental observations, we suppose that the water structuring capacity (kosmotropes) is in the order of HSO4- < CH3COO- < Cl- < Br- < I- < SCN-. Hence, we observe an increase in the LCST of PVCL in the presence of I- as compared to the Cl-. Field Emission Scanning Electron Microscopy Analysis of PVCL in the Presence of Imidazolium-based ILs. The surface morphology of PVCL in ILs was further characterized using FESEM technique. We have selected only two ILs, [Bmim][HSO4] and [Bmim][SCN], such that we can gain information for the whole series of the ILs used in our experiment. As represented in Figure 8, the FESEM images show that the microstructures of pure PVCL and PVCL-ILs lyophilized hydrogels are significantly different. The images shown here are representative of the morphology of the whole samples. According to Figure 8, the PVCL is characterized by a porous structure, in which the polymer chains are interconnected with each other. With the addition of ILs, the porosity of the material is increased. The observed increase in the porous structure is probably due to enhanced 12 ACS Paragon Plus Environment

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crosslinking between the hydrated PVCL chains due to the ILs. A comparison between [Bmim][HSO4] and [Bmim][SCN] shows that the pores sizes are more homogeneous in [Bmim][SCN] as compared to [Bmim][HSO4]. Further, the sample becomes more uniform with increase in the concentration of the ILs. Homogeneous gel structures reveal homogeneity in the solvation structure of the polymer in the solution medium. Hence, more water molecules will be distributed surrounding the PVCL in the presence of [Bmim][SCN] than [Bmim][HSO4]. Due to this modification in the morphology of the PVCL in ILs, the LCST of the PVCL is expected to increase as compared to the LCST of the polymer without ILs. Comparison of Present Results with the Results of PNIPAM aqueous solution with same ILs Our results are contradictory to the results of Reddy et al.52,57 where the authors explored that the LCST of the polymer decrease while moving along with the Hofmeister series of anions of imidazolium ILs. Working on the LCST of PNIPAM using a series of ILs, it was observed that basis of the tendency for lowering the LCST, the current studied anions of ILs have been arranged in the following order: SCN- < I- < Br- < Cl- < CH3COO- < HSO4-. The mechanism was predicted in terms of disruption of the hydration layer and the H-bond interactions between polymer chains and water due to ILs. The anions induce self-assembly of PNIPAM into nanoscale range ps in aqueous solution and decrease the LCST of PNIPAM. In our experiments we observed an opposite trend in the LCST of the PVCL due to the same ILs as studied by Reddy et al.52,57 interestingly, the LCST of the PVCL is similar to the LCST of the PNIPAM in the aqueous medium. However, the LCST of PVCL increased in the presence of the ILs at all concentrations in the following order as, HSO4- > CH3COO- > Cl- > Br> I-

>

SCN-. This reverse in the LCST of PVCL following the Hofmeister series anions, is

considered due to the difference in the structure of PVCL as compared to PNIPAM. We observed that, unlike PNIPAM, ILs did not disturb the H-bonding of the PVCL with the water molecules. Rather, the ILs ions act as bridge that holds mole and more water molecules in the solvation sphere of the polymer. The results of increase water solvation layer around the groups of PVCL leads to higher LCST of the polymer. Overall, the Hofmeister series is not obeyed for the changes in the LCST of PVCL aqueous solutions.



Page 14 of 31

Conclusions In this article we have investigated the effect of Hofmeister anions of ILs on the changes in the LCST of PVCL. We have selected the ILs with the fixed cation 1-butyl-3methylimidazolium cation [Bmim+], with combination of anions such as SCN-, I-, Br-, Cl-, CH3COO- and HSO4-. A combination of absorption spectroscopy, fluorescence and thermal fluorescence analysis, DLS, FTIR and FESEM measurements were performed in order to examine the conformational changes in the PVCL in aqueous medium. The DLS results reveal that increase in the LCST of PVCL follows the order of ILs as, HSO4- < CH3COO- < Cl- < Br- < I- < SCN-. Further, results were confirmed using thermal fluorescence analysis of the PVCL with increasing temperature in the presence of ANS. The ANS fluorescence and absorption studies reveal that the solvation structure of the PVCL is enhanced in the presence of the ILs. Decreases in the fluorescence intensities without changes in the ANS fluorescence wavelength reveal that the structural conformation of the PVCL is not disturbed at 25 0C. To confirm further, FTIR analysis of the PVCL in presence of ILs was performed. The analysis reveals that the water structured around the C-H and C=O groups more, in the presence of the ILs. It seems that the ILs act as the bridges between the polymer and the water molecules thereby the IL holds more and more water molecules around the PVCL, due to which the LCST of the PVCL is increased. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected]; Tel: +91-11-27666646-142; Fax: +91-11-2766 6605. ACKNOWLEDGMENTS The authors gratefully acknowledged the SERB, Department of Science and Technology (DST), New Delhi, India through the Grant No. EMR/2016/001149 for financial support.


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Figure 1. Synthetic route of PVCL through solution polymerization from VCL in the presence of AIBN as an initiator.



5000

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5 mg/mL 10 mg/mL 15 mg/mL -

-

HSO4 CH3COO- Cl

-

Br

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I

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SCN

Figure 2: Hydrodynamic diameter (dH), of PVCL aqueous solution in the presence of various Hofmeister anions; {IL free (black colour line), HSO4- (red colour line), CH3COO- (green colour line), Cl- (blue colour line), Br- (cyan colour line), I- (pink colour line), and SCN- (yellow colour line)} as a function of temperature. Concentration of ILs is 5, 10 and 15 mg/mL for Figure a, b and c, respectively. Inset is the d(dH)/dT for each of the respective data in each figures, and the circles in inset figures represents the phase transition temperature of PVCL in absence and presence of various ILs. Figure d contains the LCST of the polymer in various concentrations of ILs.


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Figure 3: UV-absorption of ANS in PVCL aqueous solution and in the presence of various Hofmeister anions; {IL free (black colour line), HSO4- (red colour line), CH3COO- (green colour line), Cl- (blue colour line), Br- (cyan colour line), I- (pink colour line), and SCN- (yellow colour line)} as a function of temperature. Concentration of ILs is 5, 10 and 15 mg/mL for Figure a, b and c, respectively. Figure d contains the Amax at 380 nm for ANS in the polymer solution and in various concentrations of ILs.



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Figure 4: Fluorescence of ANS in PVCL aqueous solution and in the presence of various Hofmeister anions; {IL free (black colour line), HSO4- (red colour line), CH3COO- (green colour line), Cl- (blue colour line), Br- (cyan colour line), I- (pink colour line), and SCN- (yellow colour line)} as a function of temperature. Concentration of ILs is 5, 10 and 15 mg/mL for Figure a, b and c, respectively. Figure d contains the intensity values at 510 nm for ANS in the polymer solution and in various concentrations of ILs.


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Figure 5: Normalized fluorescence of ANS in PVCL aqueous solution and in the presence of various Hofmeister anions; {IL free (black colour line), HSO4- (red colour line), CH3COO(green colour line), Cl- (blue colour line), Br- (cyan colour line), I- (pink colour line), and SCN(yellow colour line)} as a function of temperature. Concentration of ILs is 5, 10 and 15 mg/mL for Figure a, b and c, respectively.



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Figure 6: Thermal fluorescence analysis of ANS in PVCL aqueous solution and in the presence of various Hofmeister anions; {IL free (black colour line), HSO4- (red colour line), CH3COO(green colour line), Cl- (blue colour line), Br- (cyan colour line), I- (pink colour line), and SCN(yellow colour line)} as a function of temperature. Concentration of ILs is 5, 10 and 15 mg/mL for Figure a, b and c, respectively. Figure d contains the LCST of the polymer in various concentrations of ILs.


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Figure 7: FTIR spectra of PVCL and in the presence of various Hofmeister anions; {IL free (black colour line), HSO4- (red colour line), CH3COO- (green colour line), Cl- (blue colour line), Br- (cyan colour line), I- (pink colour line), and SCN- (yellow colour line)} as a function of temperature. Concentration of ILs is 5, 10 and 15 mg/mL for Figure a, b and c, respectively.



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Figure 8: FESEM micrographs of (a) pure PVCL, (b, c and d) in the presence of [Bmim+][HSO4] at a concentration of 5, 10 and 15 mg mL-1 and (e, f and g) in the presence of [Bmim+]SCN-] at a concentration of 5, 10 and 15 mg mL-1.


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Synopsis: A smart polymer PVCL phase transition in the presence of imidazolium-based ionic liquids.

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