From Polymerizable Ionic Liquids to Poly(ionic liquid)s: Structure

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

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From Polymerizable Ionic Liquids to Poly(ionic liquid)s: StructureDependent Thermal, Crystalline, Conductivity, and Solution Thermoresponsive Behaviors Yajnaseni Biswas, Palash Banerjee, and Tarun K. Mandal* Polymer Science Unit, School of Chemical Sciences, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

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S Supporting Information *

ABSTRACT: This contribution describes the synthesis of 3alkyl-1-vinylimidazolium bromide ionic liquid monomers (ILMs), ion exchange with bis(trifluoromethane)sulfonimide anion (NTf 2 −), and their radical polymerization to a homologous series of poly(ionic liquid)s (PILs) containing an imidazolium ion substituted with alkyl chains of different lengths (C2(n=7−11)). Owing to the presence of a long alkyl chain, all the ILMs show crystalline phases whose melting point and melting enthalpy increase sharply with increasing alkyl chain length, and those parameters decrease upon replacing Br− with a NTf2− anion. In addition to the solid crystalline phase, the ILMs with a Br− ion only show liquid crystalline mesophases above the melting of solid crystals. The corresponding PILs are semicrystalline in nature due to crystallizable alkyl side chain showing similar chain length and counteranion-dependent melting behaviors with poor crystallinity compared to those of ILMs. The formation of strong birefringent crystals of various morphologies including Maltese-cross and ring-banded spherulites is observed for ILMs. However, the corresponding PILs with a Br− ion show crystals with fibrillar morphology with weak birefringence, but surprisingly such fibrils are not observed for PILs with a NTf2− ion. The influence of structural modulations of ILMs and their corresponding PILs on their ionic conductivities is also investigated. Moreover, PILs with a Br− ion exhibit an upper critical solution temperature (UCST)-type turbid-totransparent phase transition in CHCl3 with tunable cloud point as observed from turbidity and calorimetric measurements. Such a thermoresponsive solution phase behavior is totally absent for PILs when Br− is replaced with a NTf2− ion.

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INTRODUCTION Recently, poly(ionic liquid)s (PILs) have been an interesting research area because of their multitasking performance in polymer science,1−5 catalysis,6,7 and materials chemistry.5,8 The unique and tunable properties of ionic liquids (ILs) along with properties of polymers make the PILs interesting materials for many applications including batteries,9 supercapacitors,10 gas separation membranes,11 gene therapy,12 mesoporous materials, 13 and thermoresponsive materials.4,5,14,15 In this context, it is worth mentioning that ILs have also received great attention because of their huge potential as battery materials, display materials, and reaction solvents for organic synthesis.16−21 Many of these applications of ILs/PILs require good understanding of their crystalline properties and ionic conductivity those need thorough study. It has been reported that ILs/PILs with reasonably high crystallinity can overcome the problem of leakage during their use as solid electrolytes in battery applications.22 Thus, there have been plenty of studies that were directed toward the development of ILs containing different cations and anions of variable crystallinity.18,23,24 Specifically, the imidazolium and pyridinium ILs containing a long alkyl chain are © XXXX American Chemical Society

noticeable because of their interesting structures and properties and some of them having liquid crystalline mesophases.18,25 In this context, the development of polymerizable IL monomers (ILMs) with good crystallinity is highly important so that the corresponding PILs obtained after polymerization would also exhibit good crystallinity. For example, Luo et al. developed different vinyl-functionalized imidazolium ILs and studied their solid state and liquid crystalline behaviors in monomeric form, but not after their polymerization.26 There are also few recent reports of development of polymerizable ILMs with thermotropic crystal phases and their corresponding PILs with variable crystallinity that could be the potential materials for display or battery applications.1,27,28 Earlier, there have been some reports of ILtype ammonium-based small molecules, which they termed polymerizable surfactants, showing only lyotropic liquid crystallinity.29,30 It is also equally important to have an understanding how a specific property of a ILM changes upon Received: November 2, 2018 Revised: January 4, 2019

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DOI: 10.1021/acs.macromol.8b02351 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of [C2(n=7−11)vim][Br] and [C2(n=7−11)vim][NTf2] ILMs and Their Corresponding PILs

phosphonium styrenesulfonate) (PTPSS),15 and cationic poly(tributyl-4-vinylbenzylphosphonium pentanesulfonate).53 The UCST-type PILs include imidazolium- and DMAEMAbased polycations in the presence of both LiNTf2 and NaCl salts,52 poly(triphenyl-4-vinylbenzylphosphonium chloride) in the presence of halide ion,4 and imidazolium-based poly(zwitterionic ionic liquid)s at different pHs and in the presence of different anions.14 On the other hand, PILs showing thermoresponsiveness in nonaqueous solvent are very less reported. Seno et al. reported the LCST-type phase transition of vinyl ether polymers with imidazolium and pyridinium salt as pendants in organic solvents,57 whereas nonaqueous thermoresponsive ILs are relatively higher in number.58−60 Thus, the studies on the structure and thermoresponsive property relationship of ILs/PILs would be highly interesting for the researchers working in this domain. It is agreed that the presence of long alkyl chain is the origin of crystallinity/liquid crystallinity in alkylimidazolium-based ILs.18,25 Thus, PILs containing these ILMs would certainly be interesting considering their side-chain crystallinity and conductivity. The side-chain crystallinity in nonionic poly(αolefin)s and a recent report of poly(1-octadecene) due to pendent long alkyl chain have also been studied and discussed by different groups.35,61 Thus, in this work, a series of alkylimidazolium ILMs ([C2(n=7−11)vim][Br]s) are synthesized by the nucleophilic substitution reaction of 1-vinylimidazole and n-alkyl bromides. Upon radical polymerization, these ILMs produce a homologous series of imidazolium PILs (P[C2(n=7−11)vim][Br]s) with alkyl pendants of varying lengths. This was followed by the investigation and comparison of crystalline behaviors and morphologies of these ILMs and PILs. Notably, these ILMs, not PILs, show liquid crystalline mesopahse above the melting of crystalline solid phase. The ionic conductivities of these PILs are compared with those of ILMs. Finally, the Br− ion is exchanged with a bulky NTf2− ion to examine the effect of size of counteranion on the crystallinity and spherulitic morphologies of ILMs/PILs. It is observed that P[C2(n=7−11)vim][Br]s PILs are soluble in chloroform, not in water, with the clear existence of a

its polymerization to PIL with a detailed study of their structure−property relationship considering their applications in different areas. In comparison, there are only few reports of ionic semicrystalline polymers or PILs without detail study of their crystalline behaviors.31 Although, there are quite a few examples of liquid crystalline PILs containing mesogenic groups either in the backbone (main-chain liquid crystallinity) or attached to the backbone as pendants (side-chain liquid crystallinity) in the literature.32−34 However, nonionic semicrystalline polymers have been widely studied and discussed for a long time by many researchers because of their high mechanical properties and ease of their processability. Most of these semicrystalline polymers such as aliphatic and aromatic polyester showed main-chain crystallinity, whereas polyolefin as well as syndio- and isotactic-polystyrene exhibited side-chain crystallinity with interesting phase behaviors and structures of different spherulitic morphologies including ring-banded structures.35−38 As mentioned above, the ionic conductivity of IL/PIL is the key parameter that determines its efficiency as an electrolyte in electrochemical devices. Thus, several groups developed ILs/ PILs with high ionic conductivities by varying the counteranions.39−43 Of course, PILs are the most promising candidates for solid electrolytes, since they combine the advantages of ILs and polymers.2,5,44 Therefore, the structural tuning of IL/PIL for obtaining high ionic conductivity is the requirements for obtaining better materials. It would also be interesting to study how the ionic conductivity of polymerizable ILM changes due to its conversion to the corresponding PIL.45 The research on thermoresponsive ionic polymers, especially PILs, is mostly focused on their synthesis and studies of lower critical solution temperature (LCST)-type or upper critical solution temperature (UCST)-type phase behaviors mainly in aqueous solution.4,14,15,46−55 Some notable examples include studies of LCST-type phase behaviors of poly[1-butyl-3vinylimidazolium bis(trifluoromethanesulfonyl)imide] in the presence of cyclodextrin, 56 anionic poly(4-tetrabutylB


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PILs through precipitation in acetone. The resultant PILs were then washed thoroughly with acetone for complete removal of unreacted ILMs. Finally, the PILs were characterized by 1H NMR (Figures S22−S26) and FTIR (Figure S27) spectroscopy and were stored in vacuum desiccator for subsequent use. Synthesis of Poly(3-alkyl-1-vinylimidazolium bis(trifluoromethane)sulfonimide)s (P[C 2(n=7−11) vim][NTf 2]) PILs. These PILs were also synthesized using the CFRP technique. The reaction procedures and the polymerization recipes (monomer:initiator = 2.57:0.05) were exactly similar to those used for P[C2(n=7−11)vim][Br]s, but the polymerizations were continued for 36 h. However, these PILs were isolated and purified by a different procedure as follows: After completion of polymerization, CHCl3 was evaporated in a rotary evaporator, and the entire mass was diluted with acetone; the corresponding PILs were then isolated by the fractional precipitation technique from acetone solutions by adding H2O, in volume ratio of acetone:H2O = 100:20, which actually acted as the bad solvent for these PILs. Finally, all the P[C2(n=7−11)vim][NTf2] PILs were dried in vacuum oven at 70 °C and were characterized by 1H NMR/19F NMR (Figures S28−S37) and FTIR (Figure S27) spectroscopy. Cloud Point Measurements. The cloud points (Tcps) of P[C2(n=7−11)vim][Br]s in CHCl3 solution were determined in a UV−vis spectrophotometer equipped with a fiber-optic probe and a low-temperature thermostated water/alcohol bath. Typically, a transparent CHCl3 solution of any PIL was filtered through a membrane filter (D ∼ 0.45 μm). The solution was then taken in a test tube, and the optical probe was dipped into it. The whole assembly was then placed in a thermostated bath. The Tcp was then measured by recording the absorbance (at λ = 590 nm) in the temperature window of 25 to −42 °C with increasing/decreasing temperature at a scan rate of 3 °C min−1 after equilibration for 5 min at the experimental temperature. The Tcp was considered to be a point at which percent transmittance (%T) of the solution reduced to half of its original value. The phase transition temperatures (Tcps) were also measured from calorimetric measurements performed in a micro-DSC instrument. Characterizations. NMR Spectroscopy. 1H NMR spectra of ILMs and PILs as well as 19F NMR spectra of ILMs and PILs with NTf2− anion were acquired from CDCl3 in a Bruker DPX 500/400 MHz spectrometer using tetramethylsilane (TMS) and hexafluorobenzene (HFB) as internal standard, respectively. Electrospray Ionization Mass (ESI-MS) Spectroscopy. The ESIMS spectra of ILMs were recorded by using a quadrupole time-offlight (Q-TOF) Micro YA263 mass spectrometer. The sample was prepared at a concentration of 1 mg mL−1 in CHCl3. Fourier Transform Infrared (FTIR) Spectroscopy. FTIR spectra of ILMs were recorded from pellets prepared by mixing with KBr in a 1:100 (w/w) ratio and were recorded by using a Spectrum 400 spectrometer (PerkinElmer). Spectra for PILs were acquired from their thin film casted from their CHCl3 on KBr pellets. Size Exclusion Chromatography (SEC). The number-average molecular weights (Mns) and dispersities (Đs) of PILs were determined by the SEC technique using a Waters GPC system containing a Waters 1515 isocratic pump and a Waters 2414 refractive index detector maintained at 35 °C. Four Styragel columns (Waters; HR1, HR3, HR4, and HR5) with size-exclusion limits of 100−5000, 500−30000, 5000−500000, and 50000−5000000, respectively, were connected in a series and placed in a column oven maintained at 35 °C. THF with 20 mM LiBr was used as an eluent to suppress the effect of ionic PIL. The columns were calibrated against six polystyrene standards with peak molecular weights (Mp) of 9960, 30230, 43400, 76300, 139400, 213000, 282000, 791000, and 1640000. Thermogravimetric Analysis (TGA). TGA thermograms of ILs and PILs were recorded using a TA SDT Q600 instrument at a heating rate of 20 °C min−1 under a N2 atmosphere. All samples were dried overnight at 60 °C under vacuum before analysis. Differential Scanning Calorimetry (DSC). The melting points (Tms) and other thermal properties such as glass transition

UCST-type turbid-to-transparent transition. Earlier, there was a report containing structurally similar ILMs with alkyl chains only up to C14 and their dispersion polymerization to PIL latex.44 But, such study did not focus on the crystallinity and conductivity of ILMs and their corresponding PILs as well as their thermoresponsiveness in nonaqueous solution.

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EXPERIMENTAL SECTION

Materials. 1-Bromotetradecane (C14Br; >97%), 1-bromohexadecane (C16Br; >96%), 1-bromooctadecane (C18Br; >97%), 1bromoeicosane (C20Br; >95%), and 1-bromodocosane (C22Br; >98%) were used as received from TCI Chemicals. 1-Vinylimidazole (vim) (Aldrich; ≥95%) was purified by vacuum distillation prior to use. 2,2′-Azobis(isobutyronitrile) (AIBN) (Aldrich; 98%) was recrystallized twice from ethanol prior to use. Bis(trifluoromethane)sulfonimide lithium salt (LiNTf2) (99%) was used without any further purification as received from Aldrich. Chloroform (CHCl3) and dichloromethane (DCM) were pot-to-pot distilled and collected in a round-bottom (RB) flask prior to use. Synthesis of Long Chain Alkylimidazolium Ionic Liquid Monomers (ILMs). Synthesis of 3-Alkyl-1-vinylimidazolium Bromide ([C2(n=7−11)vim][Br]) ILMs. These ILMs were synthesized by a nucleophilic substitution reaction between 1-vinylimidazole and different n-alkyl bromides [C2(n=7−11)-Br] (Scheme 1). For syntheses, 13.24 mmol of an alkyl bromide [such as C14Br (3.6 mL), C16Br (4.0 mL), C18Br (4.41 g), C20Br (4.78 g), and C20Br (5.16 g)] was separately added into a 100 mL RB flask followed by the addition of vim (1 mL; 11.04 mmol), TEMPO (45 mg; 0.28 mmol), and 10 mL of dry CHCl3. The reaction mixtures were then placed in a preheated oil bath at 60 °C for 24 h with continuous magnetic stirring. After cooling, the mixtures were added dropwise into 200 mL of hexane. The process was repeated twice for purification of each ILM. Finally, the obtained white powdery ILMs were kept in a refrigerator under an argon atmosphere for subsequent polymerizations. All the purified ILMs were then dried in a vacuum and were well-characterized by ESI-MS (Figures S1−S5), 1H NMR (Figures S6−S10), and FTIR (Figure S11) spectroscopy, and their analyses are provided in the Supporting Information. Synthesis of 3-Alkyl-1-vinylimidazolium Bis(trifluoromethane)sulfonimide ([C2(n=7−11)vim][NTf2]) ILMs. The exchange of Br− ion of [C2(n=7−11)vim][Br] with NTf2− ion was conducted by slight modification of a previously reported protocol.62 Typically, in a 100 mL conical flask, 0.576 mmol of any [C2(n=7−11)vim][Br] ILMs was dissolved in 10 mL of CHCl3. This solution was then brought in contact with a 10 mL aqueous solution containing 2 equiv of LiNTf2 with respect to ILM. The biphasic solution was then stirred (1500 rpm) at 40 °C for 8 h. After completion of the exchange reaction, the organic phase was separated using a separating funnel and was washed with water several times until all the Br− ion came into aqueous phase (tested by AgNO3). Finally, a yellowish-white waxy solid was isolated from the organic phase under reduced pressure in a rotary evaporator, Yield = ∼98%. All the [C2(n=7−11)vim][NTf2]s ILMs were then characterized by 1H NMR/19F NMR (Figures S12−S21) and FTIR (Figure S11) spectroscopy, and their analyses are provided in the Supporting Information. Preparation of Long Chain Alkylimidazolium Poly(ionic liquid)s. Synthesis of Poly(3-alkyl-1-vinylimidazolium bromide)s (P[C2(n=7−11)vim][Br]) PILs. These PILs were synthesized by conventional free radical polymerization (CFRP) of the as-synthesized ILMs. For syntheses, 5.14 mmol of any [C2(n=7−11)vim][Br] ILM [such as [C14vim][Br] (1.5 g), [C16vim][Br] (1.64 g), [C18vim][Br] (1.78 g), [C20vim][Br] (1.93 g), and [C22vim][Br] (2.0 g)] and AIBN (16.89 mg, 0.1 mmol) were separately taken into 25 mL long-necked RB flasks followed by the addition of 15 mL of dry CHCl3, and the reaction mixtures were then purged with argon gas for 45 min for oxygen removal. After that, the flasks were immediately sealed with a silicone rubber septum and were separately stirred in an oil bath thermostated at 65 °C for 24 h. The polymerization mixtures were then allowed to cool to room temperature followed by the isolation of C


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Electrical Measurements. Electrical properties, such as conductance and capacitance, of ILMs and PILs were measured from their pellets using a LCR meter (Quad Tech, Model 7600) in the frequency range from 10 Hz to 2 MHz in an anhydrous environment. The pellets were kept between two stainless steel blocking electrodes of a conductivity cell for the electrical measurements.

temperatures (Tgs) or cold crystallization temperatures (Tcs) of ILs and PILs were determined on a PerkinElmer Diamond DSC equipped with an intracooler. All samples were dried at 70 °C under vacuum for 2 days before the DSC measurement. Typically, the ILM sample pan was first annealed at 100 °C for 2 min to remove the previous thermal history and was then cooled to −89 °C at a rate of 10 °C min−1 inside the DSC cell. The exothermic peak in this scan was taken as the crystallization temperature (Tc), and the enthalpy of crystallization (ΔHc) was obtained by integration of the exothermic peak. Finally, ILM samples were again heated at a rate of 5 °C min−1 to 100 °C, and the endothermic peak in this second heating thermogram was taken as the melting temperature (Tm). The melting enthalpy of fusion (ΔHf) was obtained by integration of the endothermic peak at Tm. However, for PILs, the measurements were performed as follows. The samples were first annealed at 90 °C for 7 min to remove the previous thermal history and then rapidly quenched down to −89 °C inside the DSC cell, held for 10 min, and rescanned at a rate of 10 °C min−1. Tg and Tm values were then taken from the second heating thermograms. For determination of equilibrium melting point (T0m), the molten PIL samples were subjected to isothermal crystallization for 2 h at the desired Tc and were again rescanned at a scan rate of 10 °C min−1, and the melting points were recorded from the heating thermograms. Polarized Optical Microscopy (POM). The morphologies of ILMs/ PILs crystals were examined under an Olympus polarized optical microscope (Model BX51) equipped with a camera and a heating stage. Typically, a CHCl3 solution of ILMs/PILs (2 wt %) was first drop-casted on a microscopic coverslip and air-dried to prepare a thin film. The dried film was then sandwiched by placing another coverslip on top of it. Subsequently, this film was melted on a microscopic heating stage (Linkam THMS 600 equipped with a T-95 temperature programmer) at 100 °C for 10 min to obtain an isotropic phase. Afterward, it was cooled to a desired Tc and allowed the film to recrystallize for 1 h. The coverslip containing the sample film was then placed under the POM and was imaged at a particular temperature using the ProgRes CapturePro 2.7 software tool. Powder X-ray Diffractometry (PXRD). The crystallinity of ILM/ PIL samples were studied using a Bruker D8 X-ray diffractometer operated at an accelerating voltage of 40 kV using Cu Kα (λ = 1.5405 Å) as the X-ray radiation source with a current intensity of 40 mA. Samples were prepared by drop-casting 4 wt % of ILM/PIL solutions in glass slides and dried in a vacuum oven at 25 °C. Typically, the ILM sample was then examined in the form of powder on a glass slide without any heat treatment. However, PIL samples were melted on a heating stage (Linkam THMS 600 equipped with a T-95 temperature programmer) at ∼100 °C for 10 min, then quenched to the desired Tc, and allowed to recrystallize for 2 h before analysis. Micro-Differential Scanning Calorimetry (Micro-DSC). A microDSC from TA Instruments was used to measure the solution phase transition temperature (Tcps) and ΔHs of the transition of PILs (1 wt %) in CHCl3. All experiments were performed between −30 and 25 °C with a scanning rate of 1 °C/min. Standard vessels were used with an average sample volume of 400 μL. The same mass of sample and reference were weighted to minimize the differences in heat capacities between them. The samples were equilibrated at 25 °C for 2 h before each scan. Turbidity. The turbidity measurements of PILs in CHCl3 were carried on a Cary 60 UV−vis spectrophotometer (Agilent Technologies) equipped with a fiber-optic probe and a lowtemperature thermostated water−alcohol bath. The probe was dipped into the sample solution taken in a test tube. Transmission Electron Microscopy (TEM). Typically, 1 wt % CHCl3 solution of PIL was cooled to below −20 °C to obtain turbidity. 50 μL of the turbid suspension was then injected into 500 μL of cooled DMF to freeze the as-formed nanoaggregates (NAs). Note that PILs were insoluble in DMF. One drop of this dispersion was casted onto a carbon-coated copper grid and allowed to dry in air at room temperature for 24 h. The grid was then observed on a JEOL JEM-2010 electron microscope operated at an accelerating voltage of 200 kV.

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RESULTS AND DISCUSSION Long Chain Alkylvinylimidazolium ILMs. [C2(n=7−11)vim][Br] ILMs were synthesized by a nucleophilic substitution reaction between 1-vinylimidazole and n-alkyl bromides (Scheme 1). The formation of ILMs was initially assessed from ESI-MS spectra (Figures S1−S5) showing only one sharp peak for each sample; those exactly matched with their corresponding molecular weights. 1H NMR spectra of ILMs (Figures S6−S10) showed all the characteristic signals of imidazolium ring protons at δ 10.9, 7.7, and 7.5 ppm and vinyl protons at δ 7.44, 5.9, and 5.4 ppm along with the wellresolved signals at δ 4.39, 1.9, 1.3, and 0.8 ppm for alkyl chains, thus unambiguously confirming their successful syntheses and purifications. FTIR spectra of [C2(n=7−11)vim][Br]s (Figure S11) showed all of their characteristic bands at 1435 cm−1 (N−CH2− deformation), 1650 cm−1 (vinyl CC stretching), and 3110 cm−1 (CN deformation of imidazole ring). In addition, the Br− ion of [C2(n=7−11)vim][Br] was exchanged with a large hydrophobic anion, NTf2−, by a simple anion exchange as described in Experimental Section.62 The formations of [C2(n=7−11)vim][NTf2] ILMs were unambiguously confirmed from 1H and 19F NMR spectra (Figures S12− S21) as they showed the characteristic proton signals of imidazolium (δ 9.0, 7.6, and 7.4 ppm), vinyl (δ 7.14, 5.7, and 5.4 ppm), and alkyl (δ 4.22, 1.8, 1.3, and 0.87 ppm) groups along with a sharp signal at δ −78.94 for the −CF3 group. Moreover, the appearance of two new bands at 1056 cm−1 (C−F stretching of −CF3 group) and 1219 cm−1 (SO deformation of NTf2−) along with bands at 1650 cm−1 (vinyl CC stretching) and 3110 cm−1 (CN deformation of imidazole ring) in the FTIR spectra of [C2(n=7−11)vim][NTf2]s (Figure S11) further confirmed the successful exchange of Br− with a NTf2− ion. Long Chain Alkylimidazolium PILs. Alkylimidazolium PILs, P[C2(n=7−11)vim][Br]s and P[C2(n=7−11)vim][NTf2]s, were synthesized from their corresponding ILMs in CHCl3 using the CFRP technique (Scheme 1). 1H NMR spectra (Figures S22−S26) exhibited the absence of any signals corresponding to vinyl protons (δ 7.44, 5.9, and 5.4 ppm), and the appearance of a broad signal at δ 1.96 ppm corresponding to the backbone methylene and methine protons indicated the successful formation of pure P[C2(n=7−11)vim][Br]s. The FTIR spectrum (Figure S27) of a representative PIL P[C18vim][Br] showed bands at 1625 and 3100 cm−1, corresponding to the C−N stretching and CN deformation of the imidazole ring, and the absence of any band around 1650 cm−1 due to CC stretching further confirmed the successful polymerization of ILMs. The syntheses of P[C2(n=7−11)vim][NTf2]s were also confirmed from 1H and 19F-NMR spectra as shown in Figures S28−S37. The FTIR spectrum of a representative [C18vim][NTf2] sample (Figure S27) also exhibited three bands at 1056 cm−1 (C−F stretching of −CF3 group), 1219 cm−1 (SO deformation of NTf2), and 3110 cm−1 (CN deformation of imidazole ring) as can be seen in [C2(n=7−11)vim][NTf2] samples (Figure S11), but the absence of a band at 1650 cm−1 (vinyl CC stretching) unambiguously confirmed the D


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Figure 1. DSC thermograms of (A)[C2(n=7−11)vim][Br]s and (B)[C2(n=7−11)vim][NTf2]s ILMs in the cooling (dashed curve) at 10 °C min−1 and second heating (solid curve) scans at 5 °C min−1.

Scheme 2. Schematic Representation of the Most Probable Arrangement of Molecules in Their Crystalline Phases for (A) [C2(n=7−11)vim][Br] and [C2(n=7−11)vim][NTf2] ILMs and (B) P[C2(n=7−11)vim][Br] and P[C2(n=7−11)vim][NTf2] PILs

successful synthesis of PILs with the NTf2− ion. These ILMs and PILs were insoluble in water and other highly polar organics such as DMF, DMSO, etc., but were soluble in organic solvents such as CHCl3, THF, etc. SEC analysis of cationic PILs was tricky because of their tendency to aggregate on the column fillers due to the polymer−column interaction.63 In this case, the chromatograms of PILs using THF as the eluent did not show any peak. However, the chromatograms of PILs using THF with 20 mM LiBr as the eluent with a column and detector temperatures of 35 °C exhibited well-resolved peaks with unimodal molecular weight distributions (Figure S38). The analyses of chromatograms revealed production of PILs of very high Mns and dispersities (Đs) of 1.4−1.8 (Table S1). It should be noted that one can polymerize these ILMs by any controlled radical

polymerization (CRP) technique to obtain PILs of varying and controllable Mns. This may allow detailed studies of molecular weight dependency of the PILs’ properties such as crystallinity and conductivity. However, there is usually a limitation in making polymers of high molecular weights by the CRP techniques. Thus, in this case, the CFRP technique was used to obtain high molecular weight PILs considering their various applications where the mechanical strength is an important parameter in addition to their inherent crystallinity and ionic conductivity. Thermal Phase Behaviors of ILMs. The thermal stability of [C2(n=7−11)vim][Br] ILMs was examined by TGA, and the obtained thermograms (Figure S39) clearly exhibited a weight loss around ∼300 °C due to the degradation of imidazolium groups containing long alkyl substituents with a residual weight E


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Macromolecules Table 1. Phase Transition Temperatures and Thermodynamic Data for Different ILMs sample name [C14vim] [Br] [C16vim] [Br] [C18vim] [Br] [C20vim] [Br] [C22vim] [Br]

endothermic phase transition (°C) 61.1, 107.0a

ΔHf (J/g)

exothermic phase transition (°C)

75.8

106.4, 1.07a 121.6, 1.6a 135.3

79.3

141.6

60.9, 39.6b

84.7

143.1

69.5, 64.1b

67.7, 172.5a

ΔHc (J/g)

32.1

99.8

35.4

111.4

49.8, 29.9b

48.6, 30.8b 73.7, 43.2b 74.0, 53.5b

sample name [C14vim] [NTf2] [C16vim] [NTf2] [C18vim] [NTf2] [C20vim] [NTf2] [C22vim] [NTf2]

endothermic phase transition (°C)

ΔHf (J/g)

29.9

85.6

8.6

78.6

42.2

98.6

24.2

87.8

51.3

109.8

28.6

108.1

54.4

112.4

44.5

57.2

63.6

114.6

52.8, 28.9b

62.9, 26.7b

exothermic phase transition (°C)

ΔHc (J/g)

a

Melting temperature and enthalpy of fusion ((ΔHf)LC) of the as-formed liquid crystalline mesophase. bExothermic phase transition (Tcc and ΔHcc) during second heating.

of ∼10% at 600 °C, indicating their quite high thermal stabilities. The corresponding PILs (P[C2(n=7−11)vim][Br]s) samples also have very high thermal stabilities with degradation temperatures of ∼360 °C (Figure S40). The thermal stabilities were found to be almost independent of length of alkyl chain attached with these PILs. TGA thermograms did not show any weight loss near 100 °C, indicating that these ILMs/PILs are not hygroscopic in nature. ILMs ([C2(n=7−11)vim][Br]s) were then examined by DSC to study their crystallinity, melting, and liquid phase crystallinity. The cooling thermogram from 100 to −89 °C showed an exothermic crystallization peak (Tc) at which crystalline solid phase is generated from molten state of ILM (Figure 1A). During crystallization from isotropic molten state, the ILM molecules assembled themselves into bilayer structures as shown in Scheme 2A. It is observed that Tc and enthalpy of crystallization (ΔHc) values were increased monotonically with the increase in the length of the substituted alkyl chain (Table 1). This is because of increase of packing efficiency within the bilayer due to expectedly higher hydrophobic interactions among ILM molecules of higher chain lengths in the formed crystals (Scheme 2A). Figure 1A further showed a prominent melting endotherm for each of these ILMs in their second heating scan; those were the melting temperatures (Tms) of the solid crystal phase of these ILMs formed during slow cooling. As can be seen from Table 1, Tm and ΔHf increased with increasing alkyl chain length associated with the ILM. Again, such an increase can be ascribed to tight packing of alkyl chains in the bilayer assembly of formed crystal due to increase of the length of alkyl chain of ILM (Scheme 2A). A similar arrangement of the alkyl chain in the formed bilayer structure in crystalline ILs has also been proposed by other research groups.25,64 Surprisingly, in addition to the endothermic peak, [C18vim][Br], [C20vim][Br], and [C22vim][Br] ILMs also exhibited an exothermic cold crystallization peak (Tcc) at 29.9, 39.6, and 64.1 °C with enthalpy of crystallization (ΔHcc) of 30.8, 43.2, and 53.5 J/g, respectively (Figure 1A and Table 1), which was thought to be due to transition from a metastable solid phase to a crystalline solid phase.65 Actually, during cooling from the melt, these ILMs would not have enough time to crystallize completely due to fast transition of some percentage of molecules from their melt to glassy amorphous state. Note that the Tcc and ΔHcc also increased with the increase of length of alkyl chain associated with the ILM (Table 1). It was surprisingly observed that the samples [C14vim][Br] and [C16vim][Br] showed very feeble endothermic peaks at higher temperatures (107 and 172.5 °C, respectively) in

addition to their Tms in the second heating scan (Figures 1A and 2). This second peak can be assigned to the melting of

Figure 2. DSC thermograms of all [C2(n=7−11)vim][Br] ILMs up to 200 °C. [C14vim][Br] and [C16vim][Br] show two endothermic peaks and their enlarged view of second endothermic peaks for the melting of liquid crystalline mesophase. POM images show crystals of mesomorphic ILMs.

liquid crystals, generated in the low percentage of mesophase existed in between these two temperatures (Table 1). Note that their corresponding melting enthalpy values ((ΔHf)LC) were also extremely low compared to that of crystalline solid phase (Table 1). From these results, we assume that there is a phase transition from crystalline to smectic and finally to F


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Figure 3. DSC thermograms of (A) P[C2(n=7−11)vim][Br]s and (B) P[C2(n=7−11)vim][NTf2]s in the second heating scan after quenching the samples from 100 to −89 °C and keeping them for 15 min for equilibration. Inset: Tgs of the corresponding PILs.

NTf2− ion was also observed by other research groups.67 In addition, as observed for [C2(n=7−11)vim][Br], DSC cooling thermograms of [C2(n=7−11)vim][NTf2]s also showed a prominent Tc for each sample, which increased along with the increase of ΔHc upon increasing the length of the associated alkyl chain (Figure 1B and Table 1). As expected, the observed Tm and ΔHf for all these samples, due to their melting from as-formed crystalline solid to isotropic liquid phase, increased with the increase of length of alkyl chain (Figure 1B and Table 1). However, in this case, only [C22vim][NTf2] showed a Tcc in addition to its Tm (Table 1). But, [C18vim][NTf2] and [C20vim][NTf2] did not show any Tcc although, as mentioned above, the corresponding ILMs with Br− ion have a clear Tcc (Table 1). Notably, for [C2(n=7−11)vim][NTf2] samples, no crystalline mesophase was traced after the melting of their solid crystal phases via DSC or POM. These results substantiated that the values of both Tms and ΔHfs decreased, which is ascribed to the fact that the replacement of a small Br− ion with a bulkier NTf2− ion disturbs the local ordering among the neighboring alkyl chains of imidazolium cations of ILMs present in the bilayer. Thus, the alkyl chains associated with [C2(n=7−11)vim][NTf2]s are loosely packed in the bilayer of the crystalline solid phase (Scheme 2A),25,64 resulting in a decrease of melting point and enthalpy of fusion. This is also probably the reason why [C2(n=7−11)vim][NTf2]s do not exhibit any mesomorphic phase as there is no possibility of any kind of ordering among alkyl

isotropic for these two ILMs. The other three ILM samples ([C18vim][Br], [C20vim][Br], and [C22vim][Br]) did not show any such peak at higher temperature (∼200 °C up to which these samples can be scanned safely without any degradation), indicating either the absence of any liquid crystalline mesophase or the melting temperature of existing mesophase is beyond the range of measurable temperature (∼200 °C) (Figure 2). However, POM images (Figure 2) revealed birefringent crystals for ILM samples after their melting from solid crystalline phases, which provided further evidence of existence of crystalline mesophase for all samples, which will be discussed later in this section. Such low values of (ΔHf)LC of these two ILMs can be explained from the earlier reports on ILs by the Binnemans group65 and our own report.66 The long alkyl chains of ILMs are flexible and would not exist in their straight chain conformation in the molten state, which reduces the degree of ordering of the molecules substantially in the liquid crystalline mesophase. In addition, the Br− ion of [C2(n=7−11)vim][Br] was exchanged with large NTf2− ion to investigate its effect on their thermal behaviors. TGA thermograms (Figure S41) of representative ILM and its corresponding PIL sample, [C18vim][NTf2] and P[C18vim][NTf2], showed higher thermal stabilities with degradation temperatures (∼450−480 °C) compared to those (∼300−350 °C) of ILMs/PILs with a Br− ion. Such higher thermal stability for imidazolium IL with a G


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Macromolecules Table 2. Phase Transition Temperatures and Thermodynamic Data for Different PILs sample name P[C14vim][Br] P[C16vim][Br] P[C18vim][Br] P[C20vim][Br] P[C22vim][Br]

Tm (2nd run) (°C) 1.2 26.6 38.5 50.6 65.6

ΔHf (J/g)

T0m (°C)

Tg (°C)

14.2 26.1 30.1 47.8 56.7

− 29.4 51.6 54.5 73.6

−35.2 −58.7 −62.5 −55.8 −39.8

a

sample name

Tm (2nd run) (°C)

ΔHf (J/g)

T0m (°C)

Tg (°C)

P[C14vim][NTf2] P[C16vim][NTf2] P[C18vim][NTf2] P[C20vim][NTf2] P[C22vim][NTf2]

− −b 19.0 40.9 56.9

− −b 9.0 30.7 36.2

−b −b −a 44.3 58.2

−50.5 −23.0 −21.0 −33.0 −17.9

b

b

a

No detectable T0m because of very low crystallinity of PIL. bNo detectable melting peak in the DSC thermogram.

Figure 4. POM images of different ILMs crystallized at different temperatures: (A) [C14vim][Br], (B) [C16vim][Br], (C) [C18vim][Br], (D) [C20vim][Br], (E) [C22vim][Br], (F) [C14vim][NTf2], (G) [C16vim][NTf2], (H) [C18vim][NTf2], (I) [C20vim][NTf2], and (J) [C22vim][NTf2].

chains of these ILMs due to the presence of a bulkier NTf2− ion in the liquid state. Thermal Phase Behaviors of PILs in Solid State. In the past, major research has been focused toward the development of crystalline ILs with a special emphasis on their liquid crystalline phase behaviors.18,23,24,66 However, there are not many reports on synthesis of PIL homopolymers as well as the study of their crystallization behaviors. As these ILMs have shown sufficient crystallinity, therefore, it is expected that their corresponding PILs would also show interesting crystalline behaviors. DSC thermograms (second heating) of PILs (P[C2(n=7−11)vim][Br]s) showed a single endothermic melting peak (Tm) (Figure 3A). Table 2 clearly indicates that Tm and ΔHf values increased with the increase of length of substituted alkyl chain. As mentioned above, a similar alkyl chain-length dependency in Tm and ΔHf values was also observed for the cases of their ILM analogues (Table 1). As shown in Scheme 2B, the crystallinity of P[C2(n=7−11)vim][Br] is due to the organization of crystallizable alkyl side chain into bilayer structure through noncovalent hydrophobic interactions. The origin of side-chain crystallinity in these PILs can also be ascribed to the hydrophobic interactions-driven self-segregation of the pendent alkyl segments into bilayer (Scheme 2B). This causes microphase separation between crystalline and amorphous regions. As the extent of crystallization among pendent alkyl side chain is increased with the increase of their length, therefore, it is expected that the values of Tm and ΔHf would also be increased. In the literature, it has been reported that the selfsegregation of the alkyl segments is also the cause of sidechain crystallinity in these types of polymers.68 It has also been explained that in those cases a lamellar morphology is

generated due to the formation of bilayer structure made up of pendent alkyl chains. Therefore, it is expected that these alkylimidazolium PILs would also exhibit a similar structure. These PILs, thus, behave like semicrystalline polymers with low crystallinity compared to those of conventional main-chain semicrystalline polymers such as polycaprolactone (PCL) and aliphatic polyesters.36,69 The highest ΔHf value (56.7 J/g) registered for P[C22vim][Br] was lower than the experimentally observed values ∼84 J/g for neat PCL.70 It is known that the melting behavior of a semicrystalline polymer highly depends on crystallization condition. Therefore, the observed melting point in the second heating scan may vary depending upon the crystallization conditions. Thus, the equilibrium melting point (T0m), which is independent of crystallization conditions of these PILs, was determined by the isothermally crystallization of the samples at different temperatures (Tc) close to Tm for 2 h. T0ms of P[C2(n=7−11)vim][Br]s were then determined from the Hoffman−Weeks Tm versus Tc plots (Figures S42−S45) at the point where Tm = Tc.71 T0ms of P[C2(n=7−11)vim][Br]s are summarized in Table 2. However, it was difficult to determine the T0m value for P[C14vim][Br] because of its very low crystallinity, though a noticeable Tm = 1.2 °C was observed in the second scan of the sample from −89 to 100 °C (Figure 3A). Table 2 clearly revealed that the T0m value increased with increasing length of the alkyl chain of PILs. It should be noted from Table 2 that the melting point and the ΔHf of PILs were much lower than those of ILMs (Table 1) because of dense packing of substituted alkyl chains in the ILM crystals (Scheme 2). As mentioned above, the exchange of Br− with NTf2− ion drastically affected the crystallinity of ILMs. Therefore, such exchange would also affect the crystallization behavior of PILs. H


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Figure 5. POM images of different semicrystalline PILs crystallized at 25 °C: (A) P[C16vim][Br], (B) P[C18vim][Br], (C) P[C20vim][Br], and (D) P[C22vim][Br].

Indeed, Tm, ΔHf, and T0m (as obtained from Figures S46 and S47) values of P[C18vim][NTf2], P[C20vim][NTf2], and P[C22vim][NTf2] were much lower than their corresponding PILs containing a Br− ion (Table 2). However, surprisingly DSC thermograms of P[C14vim][NTf2] and P[C16vim][NTf2] samples did not exhibit any endothermic melting peak (Figure 3B). Thus, it can be concluded that upon exchange of a Br− ion with a bulkier NTf2− ion, the crystallinity of P[C2(n=7−11)vim][NTf2]s decreased sharply because of the less dense packing of alkyl chain in the bilayer of lamellae (Scheme 2B). As the length of the alkyl chain is lower, the introduction of the NTf2− anion completely destroys the ordering of the alkyl chains in the cases of P[C14vim][NTf2] and P[C16vim][NTf2]. This is the reason why these two samples did not show any melting peak. In addition to crystalline melting transitions, as can be seen from the enlarged view of thermograms (insets of Figure 3A,B) that both P[C2(n=7−11)vim][Br] and P[C2(n=7−11)vim][NTf2] PIL samples showed prominent Tgs, whose values are given in Table 2. Although some other research groups previously reported PILs with lower Tgs,5 to the best of our knowledge, there is no report of PIL homopolymer having both Tg and Tm simultaneously. Crystal Morphologies of ILMs and PILs. The morphologies of these crystalline ILMs were examined via POM. To destroy the previous history, thin film samples were melted at high temperature in a heating stage. It was observed that all [C2(n=7−11)vim][Br] samples gave a dark image under polarized light at 250 °C, which is the characteristic of the isotropic phase, as reported by other groups (Figure S48A).72 These samples were then quenched to near room temperature (15−35 °C) for formation of solid crystals. POM images of all samples showed the formation of birefringent crystals of different morphologies (Figure 4). The samples [C14vim][Br] (Figure 4A) and [C16vim]Br] (Figure 4B) gave crystals of

small and large batonnet textures, respectively, while [C18vim][Br] (Figure 4C) and [C20vim][Br] (Figure 4D) showed focal conic type crystals with four lobes. Although, the four lobe spherulitic structures were more prominent in the case of the [C20vim][Br] sample. In addition, [C22vim][Br] crystallized into different streaklike morphologies (Figure 4E). In general, ILMs showed melting of their solid crystal phases upon heating above their Tms; those were very close to that obtained from DSC. In each case, interestingly, we observed that some of the birefringent crystals still remained in the molten liquid phase as can be seen from POM images (Figure 2). An enantiotropic mesophase with small batonnet textures were observed for [C14vim][Br] and [C16vim][Br] at 76 and 98 °C, respectively (Figure 2), while the other ILMs, [C18vim][Br], [C20vim][Br], and [C22vim][Br], also showed formation of liquid crystalline mesophases of either focal conic or streaklike morphologies (Figure 2). We propose that such mesophase is probably a smectic A (SmA) phase as is generally reported for most of other alkyl-substituted imidazolium-based ILs.73,74 Similar to [C2(n=7−11)vim][Br]s, [C2(n=7−11)vim][NTf2] samples, under similar heat treatment (see Experimental Section), also showed the formation of birefringent crystals of various morphologies near room temperature (∼15−30 °C). In this case, crystals of interesting morphologies were observed for ILMs after the exchange of Br− with a NTf2− ion. For example, [C14vim][NTf2] and [C16vim][NTf2] mainly crystallized into four lobe spherulites containing banded rings (Figure 4F,G). Although, the ring bands were not so prominent for [C14vim][NTf2] (Figure 4F) but were very clear for [C16vim][NTf2] sample. The formation of such ringbanded spherulites is very common for many semicrystalline polymers.36,37,69,70 This is probably the first report of formation of such ring-banded spherulitic crystals in small molecular ILs. However, [C2(n=7−11)vim][NTf2]s with alkyl chains of higher lengths (C18, C20, and C22) showed I


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Figure 6. Frequency-dependent conductivity plots of (A) [C2(n=7−11)vim][Br]s and (B) P[C2(n=7−11)vim][Br]s measured at 25 °C.

those of peaks for each sample of [C2(n=7−11)vim][NTf2], which is obvious considering the bulky size of the NTf2− ion. However, the evaluation of the nature of crystal packing from the wide-angle PXRD data was very complex, maybe due to the formation of a bilayer structure of the cationic alkyl chain.76 Therefore, from XRD analyses, it can be assumed that the alkyl chain-substituted imidazolium cations and Br− or NTf2− anions of these ILMs assembled in an interdigitated bilayer structure.64 Within the bilayer unit, the constituent ions are arranged through intermolecular alkyl chain−chain hydrophobic interactions and electrostatic interactions. The WAXS patterns of PILs were recorded at room temperature to elucidate their crystallinity as shown in Figure S51. All P[C2(n=7−11)vim][Br]s showed a moderately sharp diffraction peaks at around 2θ = 21.8, indicating moderate crystalline nature of the samples with reasonably high percentage of amorphous phase (Figure S51A). For example, the peak for P[C14vim][Br] is relatively broad compared to those of other samples, indicating very low crystallinity as also evident from low ΔHf value (Table 2). However, the peak became very sharp with the increase of the length of the alkyl chain of the PILs. This indicated the increase of side-chain crystallinity of the PIL samples as also observed from ΔHf values given in Table 2. Although, the peak is very sharp for P[C22vim][Br], it is not as sharp as that of conventional semicrystalline polymers.70 Also, the semicrystalline polymers usually showed multiple sharp peaks; instead, we just observed a single peak for these PILs. ILMs with the NTf2− ion also showed a single main peak in the ranges of 2θ = 19.3−21.3, except for P[C14vim][NTf2] showing a broad peak at 2θ = 8.6 (Figure S51B). The broad peak for the latter sample can be assigned to its amorphous nature as also evident from its DSC trace with no melting peak (Figure 3B). P[C16vim][NTf2] showed a relatively broad diffraction peak at 2θ = 19.3, indicating very high amorphous content with very low crystalline phase. DSC data of this sample also showed very low crystallinity (Table 2). For other three samples, the sharpness of the peak increased with the increase of length of the alkyl chain, indicating increase of side-chain crystallinity as also observed from the DSC results (Table 2). Ionic Conductivity. Ionic conductivity is one of the most important fundamental physical properties of ILs and PILs, which needs to be investigated thoroughly for their

formation of clear four-lobe Maltese-cross spherulites (Figures 4H−J, respectively). The presence of such Maltese cross in the spherulitic crystals is a very common phenomenon for conventional semicrystalline polymers,36,38,75 but it has been rarely observed in small molecular ILs, especially in ILMs. However, the POM image (Figure S48B) of ILMs ([C2(n=7−11)vim][NTf2]s) after melting of their crystalline solid phases appeared dark without the formation of any mesophase as also not observed from DSC (Figure 1B). On the other hand, the corresponding PILs, P[C2(n=8−11)vim][Br]s, except for P[C14vim][Br], exhibited birefringent crystals with mainly fibrillar texture (Figure 5). The fibrillar morphologies were becoming more prominent in the cases of PILs with higher length of alkyl chains (C16 to C22) probably due to their efficient packing in the bilayer of the lamellae. However, no birefringence pattern was observed for P[C14vim][Br] because of its very low crystallinity. Moreover, it was also found that none of the P[C2(n=7−11)vim][NTf2] samples exhibited birefringent crystals under polarized light. This is also because of their very poor crystallinity due to the disturbance of packing of PIL chains in the crystalline lamellae in the presence of the bulky NTf2− counterion. PXRD. At room temperature, the XRD patterns of ILM samples obtained at low and high angles are presented in Figures S49 and S50. Of them, [C2(n=7−11)vim][Br] ILM samples showed two prominent peaks at the low angle region, and their corresponding d-spacing values are also provided in Figure S49 (marked in red). The first low angle peak in each sample indicated the formation of ordered lamellar in the crystalline phase.26 Interestingly, the corresponding d-spacing values obtained for different ILMs (Figure S49) showed a linear increase with the increase of the length of substituted alkyl chain, indicating structural similarity for all the ILMs in this homologues series. Notably, the d-spacing value corresponding to the second peak also increased linearly with the increase of the length of the alkyl chain of ILMs. Surprisingly, [C2(n=7−11)vim][NTf2]s showed only one peak (Figure S50) for each sample; those were closer to the second peaks of the [C2(n=7−11)vim][Br]s (Figure S49). In the cases of [C2(n=7−11)vim][NTf2]s, there was also a linear increase of dspacing value with increasing length of the alkyl chain (Figure S50). It should be noted that the d-spacing values for second peaks of [C2(n=7−11)vim][Br]s were higher in comparison to J


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from 97 to 0 upon cooling, or vice versa on heating, showing cloud points (Tcps) at −20 °C (heating) and −25.5 °C (cooling) with a little hysteresis (Figure 7). Similarly, 1 wt % CHCl3 solutions of P[C20vim][Br]s, P[C18vim][Br]s, and P[C16vim][Br]s also exhibited a reversible UCST-type transition, and their corresponding Tcp values are summarized in Table S2. This UCST type of behavior was reversible under repeatable heating/cooling cycles for at least four for all the four PIL samples as shown in Figure S53. Above UCST, P[C2(n=7−11)vim][Br] chains remained solvated in CHCl3. Upon decreasing the solution temperature, the mobility of alkyl chains of PIL molecules became restricted, and the intermolecular hydrophobic chain−chain interactions among these pendent alkyl chains predominate over their polymer−solvent interactions, resulting in the formation of nanogel aggregates of PIL. This causes the phase transition from transparent to turbid at low temperature. The formation of nanogel aggregates in turbid suspension of CHCl3 was further confirmed from the TEM study. The images of turbid suspensions of two representative PILs, P[C18vim][Br] and P[C22vim][Br], clearly showed the formation of spherical nanoaggregates of average diameters of 100 and 420 nm, respectively (Figure S54). The formed nanogel aggregates in the turbid suspension below UCST-type Tcp disrupted upon heating, making the solution transparent. Furthermore, by comparing the Tcps from turbidity curves in Figure 7 and Table S2, it can be summarized that Tcp of P[C2(n=7−11)vim][Br] increased with increasing the length of the substituted alkyl chain. These results confirmed that the intermolecular hydrophobic chain−chain interactions of alkyl chains of PILs are the origin for such UCST-type transition behavior. This hypothesis was further supported by the fact that the enthalpy of phase transition, as obtained by micro-DSC (Table S2), was strongly dependent on the length of alkyl chain. The calorimetric measurements of P[C18vim][Br], P[C20vim][Br], and P[C22vim][Br] (Figure 8) showed exothermic peaks at −22, −6.4, and 14.11 °C, respectively. These phase transition temperatures were found to be close to Tcps of PILs obtained from turbidity measurements. It should be noted that the

applications in batteries. This draws our attention to examine the effect of length of substitute alkyl chain on the conductivity of these ILMs/PILs. The ionic conductivity of the imidazolium ILMs/PILs was determined by a LCR meter under anhydrous conditions. The conductivity (σ) values of [C2(n=7−11)vim][Br]s (Figure 6A) obtained from complex impedance plot (Figure S52A) were found to be decreased from σ ∼ 10−6 to 10−9 S cm−1 (at 25 °C) with the increase of alkyl chain length, which is attributable to the low ionic mobility of the larger size imidazolium cation. It has been reported that PILs obtained by polymerization of ILMs generally have lower ionic conductivity compared to that of ILMs due to both considerable elevation of Tg and reduced number of mobile ions after covalent bonding of the polymerizable caions.77 However, PILs are mechanically more superior than ILs, which enables their use in solid state electronic devices.78In this case, it was also found that the σ values of P[C2(n=7−11)vim][Br]s (Figure 6B), obtained from their complex impedance plot (Figure S52B), decreased from 10−8 to ∼10−10 S cm−1 (at 25 °C) with the increase of length of alkyl chain, and these values were much lower than those of the corresponding ILMs. The conductivity of these PILs was found to be in a similar range as that of similar types of PILs.31 However, we were unable to measure the ionic conductivity of P[C2(n=7−11)vim][NTf2] samples in pellet form in the LCR meter since these were sticky and waxy in nature. Thermoresponsiveness of P[C2(n=7−11)vim][Br] in Nonaqueous Solvent. There are quite few reports describing the thermoresponsive phase behaviors of ILs in nonaqueous soltuion.58−60 Thus, it would be very interesting to investigate such properties of these ILMs. These ILMs were soluble in CHCl3. But, the solutions remained transparent over the temperature ranges of −42 to 50 °C, indicating the absence of any LCST- or UCST-type phase transition in CHCl3. However, the transparent CHCl 3 solutions of P[C2(n=7−11)vim][Br] PILs at room temperature became turbid as the temperature was decreased to −40 °C. In addition, these turbid suspensions became transparent upon heating, indicating the presence of reversible UCST-type transition (see photographs in the inset of Figure 7). Typically, the turbidity curves of a representative PIL, P[C22vim][Br](1 wt %), revealed that the percent transmittance gradually decreased

Figure 7. Turbidity curves of different P[C2(n=7−11)vim][Br]s in CHCl3 solution (1 wt %): (a) P[C22vim][Br], (b) P[C20vim][Br], (c) P[C18vim][Br], (d) P[C16vim][Br], and (e) P[C14vim][Br]. Heating (solid curve) and cooling (dashed curve).

Figure 8. Micro-DSC thermograms of P[C18vim][Br], P[C20vim][Br] and P[C22vim][Br]s in CHCl3 solution (1 wt %). K


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interactions are very weak for P[C14vim][Br] with short alkyl chain and did not lead to the formation of PIL nanoaggregates upon cooling. Interestingly, for P[C2(n=7−11)vim][NTf2]s, it was observed that the 1 wt % CHCl3 solutions remained transparent in the above-mentioned temperature ranges without showing any UCST-type transition. This is probably due to the fact that in the presence of bulkier NTf2− anion the substituted alkyl side chains of P[C2(n=7−11)vim][NTf2]s are unable to come closer enough in solution to have any effective hydrophobic chain−chain interactions that could lead to the formation of nanogel aggregates.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.B. thanks IACS for providing a fellowship. Thanks are also due to DST for providing an Inspire fellowship to P.B. The research was supported by the grants from SERB, India. We thank Dr. Sayan Das, IACS, India, for his help to measure the ionic conductivity. The authors also thank DST for the project (DST/SJF/CSA-01/2014-15) and Dr. Pradip Dey of IACS, Kolkata, for his help in the measurement in micro-DSC.

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CONCLUSION In conclusion, long chain alkylvinylimidazolium ionic liquid monomers (ILMs) with Br− and NTf2− counteranions and their corresponding poly(ionic liquid)s (PILs) belonging to a homologous series were synthesized for detail investigation of their chain-length-dependent thermal behaviors and crystallinity. ILMs/PILs were thermally stable, and their melting points and enthalpy of fusions increased sharply with increasing length of associated alkyl chains. However, the side-chain crystallinity of these PILs was very low compared to their corresponding ILMs. The size of the counteranion drastically affected the melting and melting enthalpy for ILMs and PILs. All the [C2(n=7−11)vim][Br] ILMs exhibited a liquid crystalline mesophase in addition to their crystalline solid phase, whereas only solid crystal phase was indentified for [C2(n=7−11)vim][NTf2] ILMs. The ordered molecular packing of these ILMs in different phases was deduced from the XRD study. The assembly of molecules of both types of ILMs into bilayer resulted in the formation of the birefringent crystals of various morphologies including Maltese-cross and ring-banded spherulites. However, a weakly birefringent fibrillar crystal was imaged for P[C2(n=7−11)vim][Br], but not for P[C2(n=7−11)vim][NTf2] PILs. Additionally, the influence of structural tuning on solid state ionic conductivities of these ILMs and PILs was scrutinized. Furthermore, P[C2(n=7−11)vim][Br]s exhibited an UCST-type phase transition in CHCl3, and the Tcp can be tuned by varying the length of alkyl chain of P[C2(n=7−11)vim][Br]. But surprisingly, no such UCST-type phase transition was detected for P[C2(n=7−11)vim][NTf2]s. Thus, these assynthesized ILMs and PILs in combination with their interesting physicochemical properties and their enhanced conductivities make them potential candidates for energystorage materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02351. NMR, ESI-MS, and FTIR spectroscopic data, SEC traces, TGA thermograms, Hoffman−Weeks plots, POM images, small- and wide-angle XRD data, impedance plots, temperature-dependent transmittance data, TEM images (PDF)

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Corresponding Author

*Fax +91-33-24732805; e-mail [email protected] (T.K.M.). ORCID

Tarun K. Mandal: 0000-0003-1626-8637 L


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From Polymerizable Ionic Liquids to Poly(ionic liquid)s: Structure

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