Research Article pubs.acs.org/journal/ascecg
Solution Behavior of Triblock Copolymer in the Presence of Ionic Liquids: A Comparative Study of Two Ionic Liquids Possessing Different Cations with Same Anion Reddicherla Umapathi and Pannuru Venkatesu* Department of Chemistry, University of Delhi, Delhi 110007, India S Supporting Information *
ABSTRACT: A comparative study of two ionic liquids (ILs) with same anion and different cations on the phase transition changes of aqueous solution of a commonly used triblock copolymer poly(ethylene glycol)-block-poly(propylene glycol)block-poly(ethylene glycol) (PEG−PPG−PEG) (or Pluronic F-108), is undertaken using fluorescence spectroscopy, dynamic light scattering (DLS), viscosity (η), Fourier transform infrared (FT-IR) spectroscopy and nuclear magnetic resonance (NMR) techniques. Furthermore, to demonstrate it by direct visualization of various self-assembled morphologies, we employed field emission scanning electron microscope (FESEM). The two ILs include 1-allyl-3-methylimidazolium chloride ([Amim][Cl]) and 1-benzyl-3-methylimidazolium chloride ([Bzmim][Cl]). The results demonstrate that the addition of ILs alter the critical micellization temperature (CMT) toward lower temperatures. The extent to which the CMT altered is observed to be significantly more for [Bzmim][Cl] as compared to [Amim][Cl]. The charge and size of cations, presence of ionpair interactions between the ions of ILs and the hydrophobic part PG are the reasons for these observations. It is noted that the changes in the properties of aqueous PEG−PPG−PEG upon addition of IL depend strongly on the nature of the hydrophobic part PG of the copolymer than EG hydrophilic part. The present work demonstrates significant information on amphiphilic block copolymers for influencing temperature-sensitive copolymer CMT and micelle shapes in an IL, with potential applications in many arenas. KEYWORDS: Ionic liquids, Triblock copolymer, Critical micellization temperature, Biophysical studies, Aggregation studies
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INTRODUCTION Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG−PPG−PEG) is a triblock copolymer, which consists of hydrophilic PEG segment and hydrophobic PPG segment and is another important class of temperatureresponsive copolymer.1−5 They can self-assembles into micelles above their critical micelle concentration (CMC) or above their critical micellization temperature (CMT) in aqueous solution that are fascinating arena of extensive investigations due to their unusual physicochemical properties.6,7 Aqueous copolymer solutions have enjoyed more attention from the research community due to their environment friendly nature inherent in the aqueous-based systems. The influence of the temperature, light, pH and ionic strength on the process of selfassembly of polymers has been well documented.8−11 Because of their versatile potential applications, they are being used in diversified fields such as aggregation,12 drug delivery,13 detergency,14 dispersion stabilization,15 foaming,16 emulsification,17 lubrication,18 formulation of cosmetics,19 rechargeable batteries,20 food packaging,21 inks,22 and interaction with nanoparticles.23 Additionally, they have tendencies to accumulate in tumor cells, enhance the efficiency of chemotherapeutics © XXXX American Chemical Society
and decrease the chances of side effects on the immune system.24,25 Most of scientific communities, particularly chemists and biochemists, there is an explosive broadening and importance of applications of ionic liquids (ILs), which are emerged as excellent cosolvent media for many of chemical processes. This broadening is seen in ever-increasing vistas, allowing the advancement of technologically dependent society and providing solutions to society’s important problems in the environment. Some of these broadening is the result of extended understanding applications that are already wellknown;26,27 however, a very little is known about the developmental applications of ILs in polymer science. The exceptional properties of neoteric solvents (ILs) have made it most suitable green solvent media for various physicochemical, biophysical and biochemical processes in comparison to common volatile organic solvents.28−30 Because of routine and often novel versatile applications of ILs as a cosolvent in Received: January 20, 2016 Revised: February 26, 2016
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DOI: 10.1021/acssuschemeng.6b00137 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
[Amim][Cl] and [Bzmim][Cl] that are used for the present study are shown in Figure 1.
polymer chemistry, they have gained much attention recently.2,31−37 The study of the influence of ILs on polymers is being recognized for many possibilities in designing new polymeric smart materials.38,39 The potential practical applications of polymers and ILs have been identified in the areas of supported catalyst,40 polymer electrolyte membrane,41 dye sensitized solar cells,42,43 lithium ion batteries,44 metal ion removal45 and so on. Phase transition properties of polymers alter significantly by the change in concentration of cosolvents and temperature that are mainly influencing on their hydrophilic and hydrophobic character.2,38,39 On the other hand, the studies of the polymers with the novel biocompatible ILs have been less explored.2,46,47 The investigations related to the interactions between polymers and ILs are comparatively more important as compared to those of other cosolvents. The influence of ILs on the CMT of poly(ethylene oxide)block-poly(propylene oxide)-block-poly(ethylene oxide) (PEG− PPG−PEG) has been well explored by various researchers.48−50 However, to the best of our knowledge studies related to the influence of ILs on CMT of PEG−PPG−PEG are limited.2,51 Apparently, PEG−PPG−PEG is a potential drug delivery vehicle among the amphiphilic copolymers due to its excellent biocompatibility and environmental sensitivity.52−54 Therefore, it is essential and necessary to explore the influence of ILs on the CMT of PEG−PPG−PEG copolymer. Studies of imidazolium based ILs are proved to alter the CMT of triblock copolymers.2,46,51 Recently, Madhu et al.2 showed that the ability of imidazolium-based ILs for decreasing the copolymer CMT, which is resulted not only from charge and size of anions of the ILs but also due to the weak ion−ion pair interactions within IL. All of the above studies are carried out with a same cation and different anions; however, studies of the effect of different cations with same anion are still lacking. Thereby, we have made an attempt of comparative study of ILs with different cations with same anion. For the sake of comparison, we have taken two different ILs from the imidazolium family of ILs such as 1-allyl-3-methylimidazolium chloride [Amim][Cl] and 1-benzyl-3-methylimidazolium chloride [Bzmim][Cl]. Both of the ILs are structurally different in respect of the cation. One of the substituent on the nitrogen is unsaturated alkyl in [Amim][Cl], whereas in the case of [Bzmim][Cl] the substituent is an aromatic group. This difference in the cation structure leads to the difference in the physiochemical properties of both ILs such as melting point of [Amim][Cl] is 55 °C whereas that of [Bzmim][Cl] is 78 °C (source from chemical manufacturer). The exceptional characteristics of these particular ILs made us interested to investigate the comparative study on Pluronic F-108. Multiple biophysical studies such as fluorescence spectroscopy, dynamic light scattering (DLS), viscosity (η), Fourier transform infrared (FT-IR) spectroscopy, nuclear magnetic resonance (NMR) and field emission scanning electron microscope (FESEM) have been carried out in order to investigate systematically the temperature-induced micellization and morphology of Pluronic F-108 aqueous solution in the presence of [Amim][Cl] and [Bzmim][Cl]. It is worth mentioning that both [Amim][Cl] and [Bzmim][Cl] decrease the CMT of triblock copolymer aqueous solution. The temperature-induced micellization and morphology of Pluronic F-108−IL mixture aggregates are expected to bring new information and directions for versatile applications. The schematic chemical structures of Pluronic F-108 and the ILs
Figure 1. Chemical structures of PEG−PPG−PEG (Pluronic F-108), [Amim][Cl] and [Bzmim][Cl].
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EXPERIMENTAL SECTION
Materials. The Pluronic F-108 [poly(ethylene glycol)-blockpoly(propylene glycol)-block-poly(ethylene glycol)], pyrene and ILs were purchased from Sigma-Aldrich Chemicals Co. and used without further purification. The polymer molecular weight (Mn) is ∼14.6 kDa with a PEG content of 82.5%, (PEG)133−(PPG)49−(PEG)133. 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. Sample Preparation. All the sample solutions were prepared gravimetrically, by using a Mettler Toledo balance with a precision of ±0.0001 g, the required amount of pyrene and Pluronic F-108 was weighed for stock solutions. Aliquots of these solutions were mixed to prepare the sample solutions at desired concentration of polymer. The final copolymer 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 copolymer solution. All the polymer−IL samples were kept at room temperature for a few hours to equilibrate the samples for the proper incorporation of polymer and IL aqueous solution. The polymer samples were stored in cool place and kept container tightly to prevent water absorption. In fluorescence measurements, the concentration of probe was kept at 6 × 10−7 M to avoid the probe interference in the measurements. Prior to the measurements, each sample was filtered with 0.45 μm disposable filters (Millipore, Millex-GS) through a syringe. Fluorescence Intensity Measurements. Fluorescence intensity measurements of pyrene in aqueous Pluronic F-108 solution in the absence and in the presence of ILs were carried out using a Cary Eclipse fluorescence spectrophotometer (Varian optical spectroscopy instruments, Mulgrave, Victoria, Australia) with an intense xenon flash lamp as light source. All intensity measurements were performed at excitation wavelength (λex) of 335 nm. Emission spectra were recorded with slit width of 5/5 nm and the PMT voltage of 720 V. Scan speed was kept at 1200 nm min−1. Quartz cuvette (QC) containing sample was placed in multi cell holder, which is electro-thermally controlled at precise temperature by a Peltier. The temperature control of the Peltier thermostated cell holders is extremely stable over time, with a typical precision of ±0.05 K. prior to measuring each sample solution was left for 30 min undisturbed at all temperatures to attain thermodynamic equilibrium. Hydrodynamic Radius (Rh) Measurements. The hydrodynamic radius (Rh) of Pluronic F-108 aggregates in the absence and in the presence of ILs was measured by dynamic light scattering (DLS) using a Zetasizer Nano ZS90 (Malvern Instruments Ltd., UK), equipped with He−Ne (4 mW, 632.8 nm). The built thermostatic sample chamber enables us to maintain the desired temperatures within a temperature range of 2−90 °C with great accuracy. This instrument measures the movement of particles under Brownian motion and converts this motion into size (Rh) by using the Stokes−Einstein equation as follows B
DOI: 10.1021/acssuschemeng.6b00137 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 2. Fluorescence spectroscopy of pyrene in PEG−PPG−PEG aqueous solution with and without ILs at 25 °C. Panel a represents [Amim][Cl] and panel b represents [Bzmim][Cl]. IL free (black line), 5 mg/mL of IL (red line), 10 mg/mL of IL (green line), 15 mg/mL of IL (blue line). Rh =
kT 6πηD
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RESULTS AND DISCUSSION Two imidazolium based ILs with same anion and different cations, were chosen to explore the specific cation effects on the phase behavior of triblock copolymer by various biophysical techniques such as fluorescence spectroscopy, DLS, η, FT-IR spectroscopy, NMR and FESEM. Influence of ILs on the CMT of Pluronic F-108 in an Aqueous Solution by Fluorescence Spectroscopic Analysis. To illustrate the effect of ILs on the CMT behavior of Pluronic F-108, we initially employed fluorescence spectroscopy that is a powerful tool to study the critical micellar properties of amphiphilic block copolymers in aqueous media. Previous reports2 lead us to choose pyrene as an extrinsic fluorescence probe for the current study that exhibits five distinct characteristic emission spectra peaks in the wavelength range of 350−500 nm. However, the intensity of the first (374 nm) and third (385 nm) emission peaks in the emission spectrum of pyrene are known to be quite sensitive to the changes in microenvironment and thus these peaks were mainly considered to dictate the polarity variations by inclusion of ILs indicating that pyrene fluorescence is a viable method in the IL systems.55 To demonstrate the impact of different concentrations (5, 10 and 15 mg/mL) of imidazolium-based ILs such as [Amim][Cl] and [Bzmim][Cl], fluorescence steady state emission spectra measurements of pyrene in Pluronic F-108 have been carried out in the absence and presence of ILs at 25 °C, and the results are provided in Figure 2. As evident from Figure 2, copolymer aqueous solution in the absence of IL exhibits relatively less intensity peak in the entire wavelength range indicating an absence of hydrophobic environment for pyrene molecules. Changes in the intensities of the vibrational bands are sensitive to the changes in microenvironment of solvent molecules. The intensities of aqueous copolymer solution were observed to increase substantially with an increase in the concentration of ILs due to the increased hydrophilicity of the segments of the copolymer. This increase in the intensity reflects the incorporation of pyrene into the hydrophobic core region of the micelles. From Figure 2a,b, it is clearly identified that a minimum significant enhancement in intensities was observed in the presence of [Amim][Cl] (∼227, ∼284 and ∼341 au) whereas maximum significant enhancement in intensities was observed in the presence of [Bzmim][Cl] (∼305, ∼372 and ∼430 au) at all the studied three concentrations. The low intensities of [Amim][Cl] as compared to [Bzmim][Cl] higher intensities indicate that the micelle formation was merely
(1)
where k is the Boltzmann’s constant, T is absolute temperature, η is viscosity and D is diffusion coefficient. All data were obtained by the instrumental software. All the reported values are an average of three measurements of the sample. Viscosity (η) Measurements. The viscosities (η) of all the samples are measured using a sine-wave vibro viscometer (model SV10, A&D Company Limited, Japan) with an uncertainty of 1%. The η of sample solutions was collected in the wide temperature range by using a circulating temperature control water bath (LAUDA alpha 6, Japan) with accuracy of temperature of ±0.02 K. The sine-wave vibro viscometer equipped with two gold sensor plates measures the η of sample by detecting the driving electric current necessary to resonate the two sensor plates at a constant frequency of 30 Hz and amplitude of less than 1 mm. All η measurements of the sample were collected at heating rate of 2 K/h after allowing to the thermodynamic equilibrium. All of the viscometer accessories were cleaned and dried before performing each measurement. Fourier Transform Infrared Spectroscopy Measurements. The Fourier transform infrared (FTIR) spectrum was recorded on an iS 50 FT-IR (Thermo-Fisher scientific) spectrometer. The bubble-free samples were placed into an IR cell with two ZnSe windows. A chromel−alumel K-type thermocouple was provided for continuous monitoring of the temperature inside the sample chamber. Each IR spectrum reported here was an average of 200 scans using a spectral resolution of 4 cm−1. The IR spectra were recorded and stored using spectroscopic software (Varian Resolutions, Version 4.10). A background spectrum was obtained directly before the sample spectra. Specifically, for each sample containing copolymer, an otherwise identical IL solution without the copolymer was used as background. The temperature of the cell was maintained for 5 min to allow the sample solution to equilibrate. 1 H NMR Measurements. The 1H NMR spectra of copolymer in the presence and in the absence of ILs are obtained using a JEOL NMR spectrometer. The instrument was operated at a frequency of 400 MHz at 25 ± 0.1 °C. The chemical shifts of samples measured with acquisition time of 1.30387 s and pulse width of 11.91 μs at 90°. 0.3% chloroform in acetone has been used as reference. All the samples were dissolved in D2O. Field Emission Scanning Electron Microscopy Measurements. Field emission scanning electron microscopy measurements (FESEM) studies were carried out using a MIRA3 TESCAN electron microscope operating at 10 kV. For FESEM images, the aqueous IL solution treated sample specimens were subjected to freeze-drying and used for FESEM images. For morphological observation, the samples were loaded onto the surface of copper substrates and sputter coated with a thin layer of gold before observation. All of the FESEM micrographs were taken at a resolution of 50 μm with an accelerating voltage that was optimized to obtain the best images. C
DOI: 10.1021/acssuschemeng.6b00137 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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4. As can be seen in Figure 4, there is no increase in intensity until ∼39.9 °C and on further heating of this solution, there is sudden enhancement in intensity that indicates dehydration of EG and PG groups. The dehydration process has been identified by the onset of intensity in fluorescence spectra that indicates the formation of micelles, and this point is considered as CMT. After the CMT, the intensity values become more or less constant. Surprisingly, the CMT of copolymer was significantly shifted from 39.9 °C (in the absence of IL) to lower temperatures 38.0, 36.7 and 34.9 °C, and 37.0, 35.5 and 34.1 °C in the presence of [Amim][Cl] and [Bzmim][Cl] at different concentrations (5, 10 and 15 mg/ mL), respectively. The results clearly show that the maximum and minimum shift in diminishment in CMT in the presence of [Bzmim][Cl] and [Amim][Cl], respectively. The results in Figure 4 explicitly elucidate that the diminishment was more pronounced in the presence of higher concentration of ILs. From these results, it can be inferred that cation of IL plays a major role in modifying the properties of the aggregates of triblock copolymer in aqueous media. A decrease in CMT indicates a favorable micellization process was obtained copolymer in the presence of ILs. Electrostatic attractive interaction between cation of IL and hydrophilic sites of copolymer results in decreased repulsion among EG and PG groups consequently resulting in more efficient micellization process. The results have clearly shown that the decrease in CMT is more pronounced in the presence of [Bzmim][Cl] as compared to that of [Amim][Cl]. To confirm the changes in micelle formation merely from the copolymer, we have further performed the temperature dependent fluorescence spectroscopy for pure ILs in aqueous medium over a temperature range of 20−50 °C, and the results are presented in Figure 5. As shown in Figure 5, it was indicating that the micelle formation was absent in aqueous solutions of ILs as there was no onset in intensity profile in the experimental temperature range and in turn, it was confirmed that the complete micelle formation was from copolymer only. Generally, the dehydration of hydrophobic portion, PPG is forming micelles with a blob of a partially collapsed PPG attached to a solvating PEG-blocks. Above the CMT, the fluorescence behavior is different for two ILs, the intensity does not change after entering two phase region, i.e., system turns
caused by the ILs, which is dependent on the nature of cation of the ILs. This observed variation in the intensities indicates a strong tendency of the triblock copolymers toward hydrophobic pyrene in the core of micelle. The changes in intensities of these samples (Figure 2) were expected to be resulted from the interactions between the copolymer segments and IL as well as ions of IL and water molecules. To gain further insights into the role of cations in hydrophobic collapse of copolymer, we have compared the emission intensities of I1\I3 in the presence of [Amim][Cl] and [Bzmim][Cl] at room temperature in Figure 3. As shown
Figure 3. Fluorescence emission I1\I3 intensities of pyrene in PEG− PPG−PEG aqueous solution containing [Amim][Cl] (blck line) and [Bzmim][Cl] (red line). The inset represents the dehydration process of PEG−PPG−PEG aqueous solution with ILs and the dehydration of the copolymer is more in [Bzmim][Cl] than that in [Amim][Cl] based on the observed variation in intensity.
in Figure 3, the intensity was maximum in the presence of [Bzmim][Cl] when compared to [Amim][Cl]. This observed variation in intensity was attributed to the dehydration of the copolymer in the presence of ILs is more in [Bzmim][Cl] than that in [Amim][Cl]. Additionally, we have measured the temperature dependent fluorescence intensities of copolymer in the absence and in the presence of ILs and the obtained results are displayed in Figure
Figure 4. Temperature dependent fluorescence emission spectra of pyrene in aqueous solution. Panel a represents [Amim][Cl] and panel b represents [Bzmim][Cl]. IL free (black line), 5 mg/mL of IL (red line), 10 mg/mL of IL (green line) and 15 mg/mL of IL (blue line). D
DOI: 10.1021/acssuschemeng.6b00137 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
In the absence of ILs, the Rh ∼ 25−43 nm (below CMT), ∼61.3 nm (at CMT) and ∼75 nm (after CMT) are observed. These results clearly show that above CMT the copolymer is completely dehydrated and we obtained an aggregated copolymer structure. The addition of ILs to the copolymer aqueous solution causes a slight change in the size of micellar aggregation as can be seen in Figure 6. On the addition of [Amim][Cl], the size of copolymer slightly increases up to ∼55.5 (below CMT), ∼65.6 (at CMT) and ∼90.2 nm (after CMT) in the presence of 5 mg/mL of [Amim][Cl]. This increase in size is due to the incorporation of IL within copolymer micelle, which affects the structure of the micelles, as can be seen in Figure 6a. Furthermore, the aggregated size increases with increasing the concentration of IL. These results indicate that the [Amim][Cl] remains as an ion-pair within the copolymer micelle. The presence of the ion-pairs in copolymer and their electrostatic interaction may be responsible for the aggregation of the triblock copolymer, thereby, we observed a sharp increase in the size of the copolymer in the presence of ILs. On the other hand, the addition of [Bzmim][Cl], the size of copolymer increases up to ∼67.6 nm (below CMT), ∼75.2 nm (at CMT) and ∼98.7 (after CMT) in the presence of 5 mg/mL of [Bzmim][Cl] as shown in Figure 6b. The size of the triblock copolymer increases with increasing the concentration of [Bzmim][Cl] (Figure 6b). The observed changes in Rh values of all the concentrations of ILs in the copolymer solutions are graphically represented in Figure 7. Furthermore, the increase in size of the copolymer micelle with the addition of ILs at various concentrations has been schematically shown in Scheme 1. Obviously, smaller size of micelles in [Amim][Cl] indicates high compact packing of copolymer segments that is caused by the higher solvophobic interactions among the monomers in this IL (Figure 7). Furthermore, the H-bonding between [Amim][Cl] and the monomers of copolymer will incorporate ions of IL into the surroundings of the monomers and lead to relatively less solvated micelle size. However, such effects are not stronger in [Bzmim][Cl] due to the weak interactions between the ions of IL and the monomers of copolymer and also the steric hindrance of the larger structure of [Bzmim][Cl]. Figure 6 shows that Rh values are increasing toward lower temperatures with increase in the concentration of ILs in aqueous copolymer solution. This happens because of slightly expanded IL molecules complexed with the copolymer chains due to the repulsive force between the polymer bound IL
Figure 5. Fluorescence emission spectra of pyrene in aqueous solution with [Amim][Cl] (red line) and [Bzmim][Cl] (green line) in the temperature range of 20−50 °C. Concentration of IL is 10 mg/mL.
more turbid which is influenced by the collapse/aggregation of copolymer. Influence of ILs on the CMT of Pluronic F-108 in an Aqueous Solution by Dynamic Light Scattering. To confirm the micelle formation of Pluronic F-108 in ILs, DLS measurements have been performed to characterize the conformational changes of copolymer and IL complexes during the critical micellar aggregation process. In this study, we employed DLS measurements to determine the hydrodynamic radii (Rh) of copolymer/IL complexes as a function of temperature, and the results are depicted in the Figure 6. Evidently, the Rh value of copolymer aqueous solution in the absence of IL, was about ∼25−43 nm2,54 at lower temperatures (
