The Effect of Cation n-alkyl Side Chain

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C: Energy Conversion and Storage; Energy and Charge Transport

The Effect of Cation n-alkyl Side Chain Length, Temperature and Pressure on the Glass Transition Dynamics and Crystallization Tendency of [CCPyrr]+[TfN]- Ionic Liquids Family n

1

2

Wenkang Tu, Grzegorz Szklarz, Karolina Adrjanowicz, Katarzyna Grzybowska, Justyna Knapik-Kowalczuk, and Marian Paluch J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02689 • Publication Date (Web): 01 May 2019 Downloaded from http://pubs.acs.org on May 1, 2019

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The Journal of Physical Chemistry

The Effect of Cation n-alkyl Side Chain Length, Temperature and Pressure on the Glass Transition Dynamics and Crystallization Tendency of [C n C 1 Pyrr] + [Tf 2 N] − Ionic Liquids Family Wenkang Tu, 1,2 Grzegorz Szklarz, 1,2 Karolina Adrjanowicz, 1,2 * Katarzyna Grzybowska, 1,2 Justyna Knapik-Kowalczuk, 1,2 Marian Paluch 1,2 1

Institute of Physics, University of Silesia, 75 Pulku Piechoty 1, 41-500 Chorzow, Poland

2 Silesian

Center for Education and Interdisciplinary Research (SMCEBI), 75 Pulku Piechoty 1a, 41-500

Chorzow, Poland * Corresponding authors: Wenkang Tu ([email protected]), Karolina Adrjanowicz ([email protected])

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Abstract This work focuses on the influence of non-polar side-chain length on the glass transition dynamics and crystallization behavior of a family of non-aromatic pyrrolidinium-based ionic liquids bearing bis(trifluoromethanesulfonyl)imide counterion, [CnMPyrr]+[Tf2N]−, where n varies from 4-8. A combination of different experimental techniques, including differential scanning calorimetry, shear-mechanical and dielectric spectroscopy, enabled us to investigate the fundamental relationship between charge transport and structural relaxation as well as its impact on the dynamic properties of the investigated samples as approaching the glass transition. In contrast to recent experimental findings reported for imidazolium-based analogs, we demonstrate no evidence of the slow supramolecular mode in the rheological and dielectric response of the studied pyrrolidinium cation-based ionic liquids (ILs). Finally, we studied the crystallization tendencies for each sample under both ambient and elevated pressures, and further compared behaviors of the samples of good glass forming ability under isochronal conditions, characterizd by the same conductivity relaxation time, 𝜏𝑀. Pressure seems to not exert pronounced impact on the overall crystallization rate for the tested samples. Obtained results so as proposed methodology may shed new light on the properties affecting the physical stability of ionic liquids at various thermodynamic conditions which is a key prerequisite for their rational and efficient applications in different sectors of technology.


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Introduction Ionic liquids (ILs) are called salts which below some arbitrary temperature, usually defined as 100 °C, are in the liquid state. Historically, term ‘ionic liquid’ was introduced in 1943 by New Zealand’s chemist R. M. Barrer,1 but the first reported compound with this property was ethanolammonium nitrate dated back to 1888 by S. Gabriel and J. Weiner.2 From a chemical point of view, ILs are characterized by having weakly coordinated ions with delocalized charge. Moreover, usually in ILs at least one of the ions is an organic component.3 Ionic liquids have attracted increasing attention as due to their unique physicochemical properties which make them very promising materials for numerous applications, e.g. in the catalysis, electrochemistry, or energy storage.4–11 However, so far, only a small number of ILs are usable for large-scale applications in the industry,12 as this requires excellent thermal and physicochemical stability, power efficiency that comes together with environmentally friendly features (including relatively low toxicity).13,14 The most promising candidates that might fulfilled these criteria are low-melting points ILs (often termed as room temperature ionic liquids, RTILs) composed of massive, asymmetric cations which are mostly imidazolium-, phosphonium-, pyridinium-, or pyrrolidinium-based, and a variety of inorganic/organic anions. In general, RTILs are characterized by high conductivity, low vapor pressure, good chemical and thermal stability, broad electrochemical window and non-flammability. However, these may vary strongly depending on the molecular composition of the comprising ionic species, as well as thermodynamic conditions to which they are exposed to. Therefore, a detailed understanding of their physicochemical properties is essential for successful usage of ionic liquids-based strategies in various fields of modern technology. RTILs, as well as many other ILs, show glass-forming ability on cooling which makes this group of substances very interesting for studying the fundamental aspects related to charge transport and glass-transition dynamics.15–17 However, glass-formation, just like coin’s head and tail sides, comes together with crystallization ability emerging either on cooling or 3 ACS Paragon Plus Environment


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glass reheating. This causes a number of challenges as for some of the applications one would require great physical stability of ionic liquids exposed to varying thermodynamic conditions, including increased pressure. On the other hand, glass-forming tendency of ionic liquids is not essentially the key point when one wishes to use crystallization as an efficient and nondestructive method of purification and retrieval of ILs.18,19 From the above, it becomes evident that understanding glass-formation/crystallization phenomena in ionic liquids is extremely important. However, studies devoted to crystallization or phase transition behavior of ionic liquids are still relatively rare, especially when it comes to the crystallization mechanism, its kinetics, or factors controlling crystallization/glass-formation tendency.20–23 For example, it remains unclear if there is any relationship between the structure of the constituent ions and the phase behavior of ILs or, in general, recognizing the stability limits at varying T-p conditions to design appropriate anti-crystallization and purification strategies whenever needed. Among the most promising family of aprotic ILs, i.e. characterized by the mechanism of charge transport without of a proton transfer and low vapor pressure,16,24 we choose for this study

methyl-pyrrolidinium

cation

based

ionic

liquids

with

hydrophobic

bis(trifluoromethansulfonyl)imide (bistriflimide, [Tf2N]−, TFSI) anion. In recent years, pyrrolidinium-based ILs with TFSI anion has received an increasing attention, in particular, because of similar physicochemical behavior to more common and well-studied imidazoliumbased analogs, e.g. in terms of the melting behavior or low viscosity.25–28 Pyrrolidiniumbased ILs are even termed as a lower cost alternative to imidazolium-based ionic liquids. Based on the literature data, bis(trifluoromethylsulfonyl)imides with either pyrrolidinium or imidazolium cation are the subject of intensive research as environmentally friendly engineering liquids in a variety of industrial applications, including heat-transfer fluids, or solvent-free electrolyte materials.28–30 However, as a matter of fact, some biological and physiochemical properties of pyrrolidinium-based ILs seem to be often superior to imidazolium-based analogs (e.g. lower toxicity, wider electrochemical window, higher 4 ACS Paragon Plus Environment

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electrochemical and thermal stability). 28,31–33 Some of these features can be directly related to a particular structure of five-membered heterocyclic pyrrolidinium cation, which in contrast to imidazolium ring is non-aromatic and non-planar (see Figure 1). In turn, bis(trifluoromethansulfonyl)imide is hydrophobic, weakly coordinating anion, which allows synthesis RTIL’s with a wide range of cations.15 Ionic liquids which contain [Tf2N]− anion are characterized by lower toxicity and higher electrochemical stability in comparison to other IL with different anions.34–36 Furthermore, due to their structure which implies the possibility of existence of two conformers (cis/trans)23,37–40 [Tf2N]− anion-based ILs are an interesting group of materials to study crystallization and phase transformations.

Figure 1. Structures of (a) methyl-pyrrolidinium and (b) methyl-imidazolium cations, (c) bis(trifluoromethylsulfonyl)imide anion, n is the number of carbon atoms (n=4, 5, 6, 8) in the side chain of the cation. The material under present investigation was composed of [CnC1Pyrr]+ cation and [Tf2N]− anion. The aim of this work is to investigate the glass-transition dynamics and crystallization kinetics as a function of temperature and pressure in the family of n-alkyl-1methylpyrrolidinium bis(trifluoromethansulfonyl)imide ([CnMPyrr]+[Tf2N]−) with moderate length of the non-polar side tail ([C4MPyrr]+, [C5MPyrr]+, [C6MPyrr]+ and [C8MPyrr]+). The motivation to vary the length of alkyl chain while keeping fixed ionic backbone is related to the fact that the tail groups of the cationic part of many RTILs tend to self-aggregate resulting in the structural heterogeneity, i.e. the occurrence of separated non-polar mesoscopic domains composed of alkyl chains and polar networks formed by anions and cationic headgroups



connected by the hydrogen bonds. Such self-organization of hydrophobic alkyl tails has been observed in molecular simulations as well as in the experimental studies for a variety of ILs, including both aromatic (imidazolium) and non-aromatic (pyrrolidinium) cations.41–45. Mesoscopic aggregates in ILs are responsible for the presence of additional slow relaxation mode detected in fast terahertz or neutron dynamics of such materials.46,47 More recently, evidence of additional relaxation mode, slower than the -relaxation, was also observed in the dielectric and viscoelastic response of supercooled imidazolium-based ILs, meaning that the size and lifetime of mesoscale organization/clustering might be quite extensive as lowering the temperature.48 Nevertheless, its role in determining the physicochemical properties of ionic liquids is relatively weakly recognized. In this context, we have investigated charge transport, glass transition dynamics, and viscoelastic properties of pyrrolidinium-based IL with different length of the alkyl side chain. Differences in the length of the non-polar chain are expected to produce changes in the longrange organization and hence affect the overall behavior of the considered family of ILs which was probed using complementary experimental techniques. To study the crystallization kinetics, we performed non-isothermal measurements and isothermal cold crystallization in a wide temperature range from the glass transition temperature to the melting temperature. This allows us to determine the temperature dependence of the crystallization rate in the entire supercooled liquid regime and verify if there is any connection between the length of the cationic side-chain and glass-forming/crystallization tendency of the studied ILs. In order to verify to what extent temperature and pressure affect ionic mobility and crystallization of pyrrolidinium-based ILs, we have also performed dielectric studies at varying thermodynamic conditions. Thus, providing versatile information regarding their dynamic features and phase transition behavior have not only fundamental meaning but numerous potential applications.


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Experimental Materials 1-Butyl-1-Methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C4MPyrr]+ [Tf2N]−, Mw=422.41 g/mol, C11H20F6O4S2N2) of purity 98% and declared water content ≤0.2% was purchased

from

Sigma-Aldrich,

while

N-Pentyl-N-Methylpyrrolidinium

bis

(trifluoromethanesulfonyl)imide ([C5MPyrr]+[Tf2N]−, Mw=436.44 g/mol, C12H22F6O4S2N2), 1-Heksyl-1-Methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([C6MPyrr]+[Tf2N]−, Mw=450.47

g/mol,

C13H24F6O4S2N2)

and

1-Oktyl-1-Methylpyrrolidinium

bis

(trifluoromethanesulfonyl)imide ([C8MPyrr]+[Tf2N]−, Mw=478.52 g/mol, C15H28F6O4S2N2) of 99.9% purity were ordered from SOLVIONIC SA (France). Since investigated ILs are hydroscopic, before measurements the samples were dried under laboratory vacuum at 373 K for 2h. Figure 1 presents structures of cation and anion investigated in this work. Based on the literature data, as well as material safety data sheets provided by the supplier’s thermal stability of pyrrolidinium-based ion liquids exceeds 250 oC. Standard DSC Measurements Thermodynamic properties of the investigated samples were examined by using Mettler-Toledo DSC System equipped with a liquid nitrogen cooling accessory and a HSS8 ceramic sensor (a heat flux sensor with 120 thermocouples). The temperature and the enthalpy calibrations were performed by using indium and zinc standards. Measurements were carried out in aluminum crucibles (40 µL) under nitrogen purge (flow rate 60 mL/min). Prior standard DSC scans were performed, investigated samples were heated up to 373 K and kept at this temperature under LN flow to remove residual water absorbed upon sample preparation for calorimetric studies. After that, each sample was rapidly cooled (with the cooling rate of 50 K/min) to the temperature range well below the glass transition temperature and then heated up with 10 K/min to the room temperature. At least three DSC runs following the same experimental protocol were performed for studied ILs to ensure data reproducibility. 7 ACS Paragon Plus Environment


Temperature-modulated differential scanning calorimetry Stochastic temperature-modulated differential scanning calorimetry (TMDSC) technique implemented by Mettler-Toledo TOPEM was employed to obtain the temperature dependences of calorimetric structural relaxation times for investigated materials. The quenched samples were analyzed in one single measurement near their corresponding Tgs at a heating rate of 0.5 K min−1. In the experiment, a temperature amplitude of 0.5 K for the pulses was selected at a switching time range with minimum and maximum values of 15 and 30 s, respectively. Within the frequency range from 4 mHz to 30 mHz, different results of the real part of the complex heat capacity, 𝑐′𝑝(𝑇), were obtained at several frequencies, f. From each 𝑐′𝑝(𝑇) curve, a corresponding temperature is determined as the temperature of the half step height of 𝑐′𝑝(𝑇). The relaxation times, 𝜏𝛼_𝑇𝑀𝐷𝑆𝐶, were thereafter determined according to the equation, 𝜏𝛼 = 1 2𝜋𝑓. For details on the frequency dependent TMDSC measurements the reader is referred to the earlier literature.49

Shear mechanical spectroscopy Dynamic mechanical measurements were performed by using an ARESG2 rheometer (TA Instruments), in which the sample was placed between two 8 mm diameter parallel circular plates with a controlled gap distance (7-10 mm). During the experiments, we measured a viscoelastic response of the material by applying an oscillatory strain and measuring the resultant stress. The strain was produced by rotating the bottom plate, whereas the stress was determined from the torque, required to hold the top plate in its position. The frequency dependences of 𝐺′ and 𝐺′′ were measured for various temperatures in the frequency range 0.01–100 Hz with 0.01-1 strain %. The temperature inside the test chamber was controlled using the liquid nitrogen cooling device with a precision better than 0.1 K. Shear deformation was applied under conditions of controlled strain and linear viscoelastic response. As due to crystallization tendency of the investigated ILs experiments were


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performed upon heating from the glassy state. However, prior dynamic-mechanical spectra were recorded, samples were kept inside rheometer at 373 K with a constant nitrogen from for roughly one hour to ensure that the residual water absorbed upon setting up the experiment was removed. Broadband Dielectric Spectroscopy Dielectric measurements at ambient pressure were carried out in the frequency range of 10-2 –106 Hz using a Novocontrol Alpha Analyzer. The temperature was controlled by the Quattro system, with the use of a nitrogen gas cryostat. The temperature stability was within ± 0.1 K. The sample cell for dielectric study consists of two stainless steel electrodes (20mm diameter) separated by the Teflon spacer (distance ~0.1 mm). The dielectric response of investigated pyrrolidinium-based ILs were collected over a wide temperature range. For isothermal crystallization studies, experimental protocol involves cooling of the investigated samples from the room temperature down to the desired crystallization temperature. After completing of the liquid-crystal transformation, the temperature was increased to melt the crystal. In the next step, the entire protocol was repeated again but for different crystallization temperature. The applied measuring ac-voltage upon dielectric studies of ILs did not exceed 1.5 V (i.e. all dielectric measurements were done within the linear-response regime, 𝜏𝛼_𝐺 > 𝜏𝑀. Similar 𝜏–T phenomenon has also been reported for other ionic liquids, such as (EMIm)2[Co(NCS)4] and (BMIm)2[Co(NCS)4],64 and it might be attributed to the diverse mechanisms involved in the investigated relaxation process with various measuring methods used. To be specific, the dielectric measurements provide information to ionic charge fluctuations while rheological technique detects the structural relaxation process. Nevertheless, the longest relaxation time, 𝜏𝛼_𝑇𝑀𝐷𝑆𝐶 determined in the calorimetric measurements, can signify the whole dynamics of structural relaxation together with other secondary relaxation processes.


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Table 1. Comparison of the glass transition temperature and the steepness indexes obtained for various methods DSC

BDS

RH

Tg Materials Tg

(𝜏𝑀=

𝐺′ 𝐺′′

Tg mp

(η=1010

mp

Pa*s)

10 s)

TMDSC

(𝜏𝛼_𝐺=

Tg mp

𝜏𝛼_𝑇𝑀𝐷𝑆𝐶 mp =10 s

188.0 K 108

10 s)

[C4MPyrr]+[Tf2N]-

188 K

182.2 K

79

184.8 K

100

184.8 K

82

[C5MPyrr]+[Tf2N]-

189 K

-

-

-

-

-

-

-

-

[C6MPyrr]+[Tf2N]-

189 K

185.4 K

80

187.7 K

98

189.0 K

81

190 K

115

[C8MPyrr]+[Tf2N]-

191 K

187.2 K

78

189.2 K

110

190.6 K

86

191.9 K

92

Figure 4. Relaxation map for IL’s obtained by various methods. Left vertical axis: squares represent the conductivity relaxation times obtained for dielectric spectroscopy, triangles represent times of molecular relaxation obtained by TMDSC and pentagons by RH, respectively. Right vertical axis: stars represent viscosity obtained by RH. Olive points represent results obtained for [C4MPyrr]+[Tf2N]-, red for [C6MPyrr]+[Tf2N]- and blue for 15 ACS Paragon Plus Environment


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[C8MPyrr]+[Tf2N]-, respectively. Inner panel represent results of relaxation times for βprocess obtained by dielectric spectroscopy, lines are Arrhenius fits. Further, we analyzed all the 𝜏 – T correlations by using the VFT equation, while 𝜂 – T correlations were fitted by an analogue of the VFT equation,65 𝐷 𝑇𝑇 0

𝜂 = 𝜂∞exp (𝑇 ― 𝑇0)

(3)

where 𝜂∞, 𝐷𝑇 and 𝑇0 are fitting parameters. Typically, Tg is counted as the temperature at which structural α-relaxation time equals to 100 s or the temperature at which the viscosity equals to 1012 Pa*s. However, in this work, due to the fact that no structural α-relaxation (see Figure 5) but only conductivity relaxation is observable in the dielectric spectra, we used the latter type of movement to define Tg as the temperature 𝑇(𝜏𝑀 = 10 𝑠), according to the earlier literature reported for other ionic liquids.57 And for consistency, Tg data determined by other techniques are assumed by this relaxation time of 10 s or the viscosity value of 1010 Pa*s. As revealed in Table I, the obtained Tg results show good correspondence to each other. Moreover, regardless of different test methods, a trend is noticeable that larger Tg values correspond to the longer cationic alkyl groups. The relaxation maps established in Figure 4 also allow us to determine another parameter, fragility (steepness index) mP, which is defined in terms of the deviation of the relaxation time (or viscosity) - temperature dependence from the Arrhenius behavior at Tg,66 𝑚𝑝 =

) |

∂𝑙𝑜𝑔10𝜏 (𝑜𝑟 𝜂)

(

∂

𝑇𝑔 𝑇

(4) 𝑝 = 𝑐𝑜𝑛𝑠𝑡, 𝑇 = 𝑇𝑔

Table I collects the obtained 𝑚𝑝 values for the tested samples. We can find that the 𝑚𝑝 values determined for the same sample by various methods differ greatly. For some imidazolium-based ionic liquds, such as [C6C1im]+[NTf2]- and [C4C1im]+[BF4]-,67 analogous situations have also been observed. Besides, among these ILs there is no explicit link between the alkyl side-chain length and the 𝑚𝑝 values ascertained by the same method. Usually, on the basis of the 𝑚𝑝 value, glass-forming liquids are classified as ‘strong (𝑚𝑝 ≤ 30)’, 16 ACS Paragon Plus Environment

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‘moderately fragile (30 < 𝑚𝑝 < 100)’ and ‘fragile (𝑚𝑝 ≥ 100)’.68 Here, these three pyrrolidinium-based ionic liquids belong to moderately fragile or even fragile glass formers. As shown in Figure 3a, the electrical modulus 𝑀′′ spectra of [C8MPyrr]+[Tf2N]- reveal a pronounced secondary (β-) relaxation process, of which the relaxation times 𝜏𝛽 can be estimated based on the peak positions. In the inset of Figure 4, we established the temperature dependences of relaxation times 𝜏𝛽 for these [CnMPyrr]+[Tf2N]- (n= 4, 6 and 8) samples with the linear 𝜏𝛽–T correlations being analyzed in terms of Arrhenius equation, 𝜏 = 𝜏∞exp (𝐸𝛽 𝑅 𝑇).69 The corresponding glassy state activation energy 𝐸𝛽 are determined for each IL, which are 31 kJ/mol for [C4MPyrr]+[Tf2N]-, 38 kJ/mol for [C6MPyrr]+[Tf2N]- and 39 kJ/mol for [C8MPyrr]+[Tf2N]-, respectively. As reported in early literature, flexible molecules show secondary (β-) relaxation process correlated to the motion of only a part of the molecule.70 It is noted that Rivera et al.71 studied the dynamics of a series of glass-forming ILs based on the same 1-butyl-3-methyl imidazolium cation, [C4MIM]+ but different anions, such as [Cl]-, [PF6]- and [BMSF]-, by using BDS and observed a fast secondary process with the same activation energy (21 kJ/mol) and same time constants as well. This relaxation process was proposed to correlate with the butyl group in the cation. In addition, in the work of Kwon et al.72 on some glass-forming ILs based on the 1-alkyl-3-methyl-imidazolium family [CnMIM]+ cations and the same anion [Tf2N]-, they also noticed that local domain formation composed mainly of the long enough alkyl chains got involved in the secondary relaxation dynamics. In this work, higher 𝐸𝛽 is determined for the [CnMPyrr]+[Tf2N]- sample with larger n, which may indicate that increased alkyl side chains provide stronger steric hindrance for the local motions of molecules in the glassy state.



Figure 5. Derivative spectra, ε′′der = ( ―π 2)[∂ε′ ∂ln (f)], of [C8MPyrr]+[Tf2N]- at various temperatures above the glass transition. Panels (b) and (c) represent the corresponding real, ε′ and imaginary, ε′′ parts of the complex permittivity, respectively. In this section, it is of necessity to review the studies of Cosby et al. on a series of 1alkyl-3-methylimidazolium-based ionic liquids,48 among of which the ILs with hexyl or octyl side chains displayed the existence of an additional sub-α relaxation at low frequencies in both dielectric and dynamic-mechanical spectra. Such unique slow dynamics indicated the existence of some long-lived mesoscopic aggregates in the studied ILs. In the present work, it is of interest to investigate whether such slow relaxation mode will also occur in the pyrrolidinium-cation-based ionic liquids. Therefore, in Figures 5 and 6, the representative dielctric, shear modulus and dynamic shear viscosity spectra for [C8MPyrr]+[Tf2N]- are studied. As shown in the dielectric ε′′ spectra in Figures 5c, no well-resolved structural αrelaxation is observable due to the effect of high ionic conductivity. However, the derivative 𝜀′′𝑑𝑒𝑟( = ( ―𝜋 2)[∂𝜀′ ∂𝑙𝑛 (𝑓)]) spectra in Figure 5a, which is ascertained from the ε′ spectra in Figure 5b, reveal notable but single α-relaxation peaks at temperatures of 193 K, 198 K and 203 K. Though the spectra of T= 218 K shows some additional slow processes, they should be a consequence of the sample crystallization since it has been demonstrated in 18 ACS Paragon Plus Environment

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Figures 3a and 3c that crystallization starts at this temperature. In addition, the drastically reduced static permittivity 𝜀𝑠 at T= 218 K in Figure 5b verifies the occurrence of crystallization.

Figure 6. Frequency dependent shear mechanical data for [C8MPyrr]+[Tf2N]- at a temperature of T= 195 K:the real 𝐺′ (blue triangles) and imaginary 𝐺′′ (green circles) parts of complex shear modulus. Inset: dynamic shear viscosity normalized by the viscosity estimated from the Maxwell relation for [C4MPyrr]+[Tf2N]- (T= 190 K), [C6MPyrr]+[Tf2N]- (T= 194 K) and [C8MPyrr]+[Tf2N]- (T= 195 K). At selected temperatures, the positions of the shear peak maximum (𝑓𝑚𝑎𝑥) for all materials are almost the same. Frequencies f are normalized by the corresponding 𝑓𝑚𝑎𝑥. In Figure 6, the shear modulus 𝐺′ and 𝐺′′ spectra of [C8MPyrr]+[Tf2N]- recorded at T= 195 K is presented. Apparently, the low-freqency regions show no distinguishable signs of extra mechanical relaxations, and the slopes are determined to be 1.94 and 0.98 for the 𝐺′ and 𝐺′′ spectra, respectively. It is worth noting that these values are consistent with the results, 2 and 1, widely reported for simple molecular glass-forming liquids without supramolecular orgnization, such as polyphenyl ether (5PPE) and di-ethyl-phthalate (DEP).73,74,75



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Additionally, in the inset of Figure 6, we present the shear viscosity data (calculated as 𝜂′ = 𝐺′′(𝑓) 𝑓) normalized by the steady-state viscosity values determined from the Maxwell equation

(𝜂0 = 𝐺∞𝜏𝐺 ≈ 𝐺∞ (2𝜋 ∗ 𝑓𝑚𝑎𝑥))

for

[C8MPyrr]+[Tf2N]-

together

with

[C4MPyrr]+[Tf2N]- and [C6MPyrr]+[Tf2N]-, and none of these ILs show substantial departure from the simple viscosity mechanism in the low-frequency regions. Therefore, the absence of the slow supramolecular mode dynamics is demonstrated from the rheological and dielectric response of the studied pyrrolidinium cation-based ionic liquids (ILs). As reported in numerous simulation and experimental studies, the cation tails of pyrrolidinium-based ionic liquids with long alkyl chains can also form mesoscopic aggregation.41–45 Nevertheless, according to the investigation of Russina et al.,76 the characteristic sizes of mesoscopic structural heterogeneity in non-aromatic ILs, such as pyrrolidinium-based ones, are distinctly smaller than those in aromatic ILs, such as imidazolium-based ones. Hence, it can be spectulated that the slow supramolecular mode dynamics may become detectable for [CnMPyrr]+[Tf2N]- samples with much longer alkyl chain length (n>8) through the dielectric and shear-mechanical techniques, which deserves further investigations. Conductivity relaxation behaviors of ILs at ambient and elevated pressure


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Figure 7. Panel (a) represents isochronal superposition of dielectric spectra of IL’s collected at similar relaxation times above Tg. The fitting results by using the Kohlrausch stretched exponential function are shown as dashed lines. Panels (b) – (d) present isochronal superposition of spectra obtained under ambient and elevated pressure at similar relaxation times for (b) [C4MPyrr]+[Tf2N]-, (c) [C6MPyrr]+[Tf2N]- and (d) [C8MPyrr]+[Tf2N]-. Panels (e) – (g) show the pressure dependence of the glass transition temperature Tg= T(τ= 10s) obtained from isobaric and isothermal dielectric measurements for (e) [C4MPyrr]+[Tf2N]- (f) [C6MPyrr]+[Tf2N]- (g) [C8MPyrr]+[Tf2N]-, respectively. The solid lines in panels (e) – (g) are Anderson-Anderson function fits. As we know, pressure is also a significant factor governing the molecular dynamics. In order to check how the ionic liquids behave under elevated pressures, we conducted the pressure-dependent dielectric measurements by using the BDS technique, and studied the conductivity relaxation process for these three glass-forming IL’s. Samples were put inside a capacitor of the same geometry as that used for ambient pressure measurements, and then placed into the pressure chamber immediately after being wrapped by Teflon tapes. We performed experiments by varying the temperature along various isobars from 0.1 to 215 MPa and also by varying the pressure along various isotherms from 188.15 to 210.15 K. It should be mentioned that these experimental protocols, which resulted in that the positions of the electrical loss modulus 𝑀′′ peaks would not exceed 100 Hz during the measurements, were designed to avoid sample crystallization. Figure 7a presents the electrical loss modulus 𝑀′′ spectra characterized by the same peak frequency (~ 2 Hz) for the IL’s. The spectra obtained at ambient pressure but different temperatures are normalized by corresponding maximum peak values. To analyze the frequency dispersion of the conductivity relaxation, we applied the Kohlrausch stretched exponential function,77 𝛽𝐾𝑊𝑊

ϕ(t) = exp [ ― (𝑡 𝜏)

]


(5)


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where 𝛽𝐾𝑊𝑊 is the fractional exponent. The narrower the width of the loss peak, the larger is the 𝛽𝐾𝑊𝑊 value. Here, the fits yield a 𝛽𝐾𝑊𝑊 value of 0.56 for [C4MPyrr]+[Tf2N]while same 𝛽𝐾𝑊𝑊 values of 0.62 for [C6MPyrr]+[Tf2N]- and [C8MPyrr]+[Tf2N]-. It shows that the 𝑀′′ spectra become narrower with increasing cationic side alkyl length. In order to visually express how pressure affects the 𝑀′′ spectra for each IL’s, we compared the spectra with similar conductivity relaxation times obtained from various temperature and pressure combinations. As shown in Figures 7b-d, unlike the valid isochronal superposition of conductivity relaxation or 𝛼-relaxation commonly observed for other glass formers,78,79 the 𝑀′′ spectra of each studied IL become narrower with increasing pressures, which indicates that compression drives the ion mobility of these systems to be more uniform. In addition, it is worth noting that such unconventional breakdown in the temperature-pressure superposition rule has ever been observed for a protic ionic liquid, verapamil hydrochloride. To explain this phenomenon, we need to take into account the fact that the density of ions varied under different thermodynamic conditions.80 Thereafter, through analysis to the modulus spectra collected in both isobaric and isothermal experiments, two important factors, glass transition temperature 𝑇𝑔 and glass transition pressure 𝑃𝑔, can be estimated based on the definition as temperature or pressure at which conductivity relaxation time 𝜏𝑀 reaches 100s, as described above. As presented in Figures 7e-g, the Tg values are plotted as functions of Pg data for [C4MPyrr]+[Tf2N]-, [C6MPyrr]+[Tf2N]- and [C8MPyrr]+[Tf2N]-, respectively. For each ionic liquid, Tg is observed to increase with pressure in a nonlinear fashion. To describe the dependence of Tg on pressure, we applied the empirical Andersson and Andersson equation, which is expressed as,81 𝑘2

1

𝑇𝑔 = 𝑘1(1 + 𝑘3𝑝)

𝑘2

(6)

where k1, k2 and k3 are fitting parameters. According to the fitting results, we calculated the quantity, dTg/dp|p→0, which reflects the pressure sensitivity of glass transition temperature in the limit of zero pressure, for each studied IL. The dTg/dp|p→0 values for 22 ACS Paragon Plus Environment

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[C4MPyrr]+[Tf2N]-, [C6MPyrr]+[Tf2N]- and [C8MPyrr]+[Tf2N]- are determined to be 89.3 K/GPa, 97.7 K/GPa and 84.2 K/GPa, respectively. The result obtained for [C4MPyrr]+[Tf2N]is consistent with 80 K/GPa as reported by Lima et al.82 Besides, the dTg/dp|p→0 values for these studied ILs are comparable to the results of some other aprotic ILs, especially polymerized ILs involving [PBuVIm][Tf2N] (90 K/GPa),83 Poly-EGIm TFSI (90 K/GPa), and Poly-EtVIm TFSI (75 K/GPa).60 On the other hand, much higher dTg/dp|p→0 values have been reported for some protic ionic liquids, including carvedilol di-(dihydrogen phosphate) (170 K/GPa)63, verampamil hydrochloride (208 K/GPa),84, procainamide hydrochloride (150 K/GPa).78 Nevertheless, there exists no evident correlation between the dTg/dp|p→0 values and the cationic alkyl chain lengthes. Crystallization studies of ILs by BDS at ambient pressure

Figure 8. Results of the non-isothermal BDS measurements collected at frequency of 10 kHz in a function of dielectric permittivity for (a) [C4MPyrr]+[Tf2N]-, (b) [C5MPyrr]+[Tf2N]- c) [C6MPyrr]+[Tf2N]-, (d) [C8MPyrr]+[Tf2N]- with fixed cooling/heating rates of 1 K/min and 5 K/min. Blue circles are experimental point recorded for cooling rate of 1K/min, red circles 23 ACS Paragon Plus Environment


for heating rate of 1K/min, purple diamonds for cooling rate of 5K/min and olive diamonds for heating rate of 5K/min. For a comprehensive understanding of the crystallization process of IL’s, we performed measurements under both ambient and elevated pressures by using the BDS technique. Firstly, we have investigated the non-isothermal cold crystallization behaviors for each studied IL at ambient pressure. In these experiments, samples were cooled down to temperatures below Tg (~163 K) at a cooling rate of 1 K/min, and then heated up to room temperature at the same rate. The results obtained in this cooling-reheating cycle were shown in Figure 8 in representation of the real part of dielectric permittivity, 𝜀′. As a next step, we repeated the experiments at another fixed cooling/heating rate of 5 K/min for each ionic liquid. Noteworthily, prior to the second cooling/heating cycle, samples were heated up above the melting points and kept for some time to erase the previous thermodynamic history. From the crystallization behaviors of the studied IL’s shown in Figure 8, we can see that during the cooling process at a rate of 5 K/min [C5MPyrr]+[Tf2N]- crystallizes (with onset at 241 K), as indicated by the significant decrease in dielectric permittivity ε′. This observation has already been mentioned in the above section. Nevertheless, no crystallization is detected for other three tested ILs in the same cooling process. When the cooling rate is 1 K/min, the cooling scan of [C5MPyrr]+[Tf2N]- shows a faster crystallization (with onset of 251 K) than that at rate of 5 K/min. Apparently, the easy crystallization of [C5MPyrr]+[Tf2N]- makes it difficult to reach low temperatures at large degree of supercooling within the BDS experiments. Similar to [C5MPyrr]+[Tf2N]-, [C4MPyrr]+[Tf2N]- also crystallizes (with onset of 230 K) during the cooling process at a rate of 1 K/min. Nevertheless, crystallization still does not occur upon the slower cooling process of other two IL’s. Also, these IL’s differ greatly from each other in the behaviors during the heating process. In the case of [C4MPyrr]+[Tf2N]-, the crystallization process starts at 213 K and then 24 ACS Paragon Plus Environment

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completes rapidly upon heating at a rate of 5 K/min. The further heating shows that dielectric permittivity ε′ drops slightly again at 244.6 K, which indicates the occurrence of a polymorphic transition.23 Afterwards, the melting process of the [C4MPyrr]+[Tf2N]- sample, which is identified as the sudden increase in ε′, is observed to occur at 262 K. In contrast, the scan upon the heating of [C4MPyrr]+[Tf2N]- at a rate of 1 K/min shows only a melting signal emerged at 262 K, so it was crystallized before. By comparing these two heating spectra of [C4MPyrr]+[Tf2N]- in Figure 8a, we can find that when the temperature is lower than the polymorphic transition temperature T= 244.6 K the spectra collected at a heating rate of 5 K/min is always higher than that recorded at a rate of 1 K/min. However, the two spectra overlap in the temperature regions after the polymorphic transition. These behaviors can be understood when considering the fact that only glassy state is achieved for [C4MPyrr]+[Tf2N]after the cooling process at 5 K/min while the sample is totally crystalline after the cooling process at 1 K/min. As a consequence, the heating spectra for the amorphous sample is above that for the crystalline sample. Furthermore, it can be speculated that the crystal form obtained in the cooling process of 1 K/min might be originally a mixture of two crystal polymorphs, while the amorphous sample obtained from the cooling process of 5 K/min will completely transform into the same crystal mixture only after the polymorphic transition. In Figure 8b for [C5MPyrr]+[Tf2N]-, it is clear that only one melting process can be detected at the same temperature of 281 K due to the fact that the samples are totally crystallized during the prior cooling processes. In the case of [C6MPyrr]+[Tf2N]- in Figure 8c, when the heating rate is 1 K/min, a noticeable crystallization signal (with onset of 241 K) together with a follow-up polymorphic transition at T= 249 K is observed. The subsequent melting process starts at T= 275 K. When the heating rate is 5 K/min, though the melting process at 275 K is easily recognizable, the prior crystallization signal is somewhat unidentified. Notwithstanding, it is distinguishable that the cooling and heating scans of [C6MPyrr]+[Tf2N]- at rate of 5 K/min deviate from each other within the region of 253 - 275 K. To explain this behavior, we can make assumptions from two aspects: (i) the sample is just 25 ACS Paragon Plus Environment


partially crystallized before melting in the heating process with a rate of 5 K/min; (ii) the original crystallization region at low temperatures shifts to higher temperature, which is possibly the region of 253 – 275 K, due to faster heating rate. In Figure 8d, we can also notice the shift of crystallization region due to higher heating rate for [C8MPyrr]+[Tf2N]-. The sample starts to crystallize at T= 222 K upon heating at a rate of 1 K/min, while a crystallization signal at higher temperature of 230 K is observed when the heating rate is 5 K/min. The melting behavior of [C8MPyrr]+[Tf2N]- is later observed at T= 258 K. For these ionic liquids, it is worth mentioning that the melting points determined from these nonisothermal dielectric measurements agree well with the DSC results as shown in Figure 2.

Figure 9. Frequency dependence of the (a) dielectric permittivity, (b) dielectric loss, (c) real part of conductivity, (d) real part and (e) imaginary part of the complex electrical modulus collected upon crystallization process of [C8MPyrr]+[Tf2N]- at T= 213.15 K. Spectra were collected at every 600 s. Violet line and dark red line indicate the begining and end of crystallization process, respectively. Plot (f) represents time evolution of normalized real part of various representations at frequency 104 Hz for [C8MPyrr]+[Tf2N]- at T= 213.15 K. Black squares, red circles and blue triangles represent the normalized real part of dielectric permittivity, conductivity and electrical modulus, respectively. 26 ACS Paragon Plus Environment

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Besides the non-isothermal measurements, we performed isothermal measurements to characterize the crystallization dynamics for these ionic liquids at ambient pressure. The samples held inside the capacitor were heated up to 373 K to eliminate the absorbed water, and subsequently cooled at a rate of 10 K/min to the desired crystallization temperature at which the complete crystallization process were monitored. In Figure 9, the time-dependent crystallization data for [C8MPyrr]+[Tf2N]- recorded at T= 213.15 K are shown in various representations, viz., real (Figure 9a) and imaginary (Figure 9b) parts of complex dielectric permittivity 𝜀 ∗ (𝑓), real part (Figure 9c) of conductivity σ*(f), as well as real (Figure 9d) and imaginary (Figure 9e) parts of complex electrical modulus 𝑀 ∗ (𝑓). Apparently, various behaviors are observed in these representations. For example, the evolution of crystallization process can be identified as the gradual decline of 𝜀′ spectra (Figure 9a) in the whole frequency region over time. Similarly, Figure 9b shows that the overall 𝜀′′ spectrum also drops with the proceeding crystallization though it lacks of a well-resolved relaxation peak. Also, Figure 9c shows that the plateau occurred in the 𝜎′ spectra drops continuously as the crystallization process proceeds. Nevertheless, as seen in Figure 9d and 9e, the spectra in the representations of 𝑀′ and 𝑀′′ reveal more complicated behaviors. We can divide the sigmoid 𝑀′ spectrum into the high-frequency region (above 103 Hz) and low-frequency region (below 103 Hz), aiming for a better understanding of the evolution of the crystallization process. At the beginning of the crystallization, the high-frequency part increases dramatically while the change of the low-frequency part with time is much slower. As the crystallization process goes on, more parts in the low frequency region get involved and start to increase over time. Afterwards, the rise of the high-frequency part slows down, while the low-frequency part still keeps rising. At the end of the crystallization, all the obtained sigmoid 𝑀′ spectra overlap at the upper right side. For the 𝑀′′ spectra depicted in Figure 9e, we can see an evident conductivity relaxation peak located at a frequency of f~ 104 Hz in the initial stage of the crystallization process. As the crystallization process proceeds, the peak is observed to shift towards lower frequency with its peak amplitude first enhanced and then decreased. To 27 ACS Paragon Plus Environment


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understand how crystallization develops as time progresses, normalized parameters, namely 𝜀′𝑁(𝑡), 𝜎′𝑁(𝑡) and 𝑀′𝑁(𝑡), are, respectively, employed to evaluate the crystallization degree at a certain time according to the following equations, 𝜀′(𝑡 = 0) ― 𝜀′(𝑡)

𝜀′𝑁(𝑡) = 𝜀′(𝑡 = 0) ― 𝜀′(𝑡 = ∞) 𝜎′(𝑡 = 0) ― 𝜎′(𝑡)

𝜎′𝑁(𝑡) = 𝜎′(𝑡 = 0) ― 𝜎′(𝑡 = ∞) 𝑀′(𝑡) ― 𝑀′(𝑡 = 0)

𝑀′𝑁(𝑡) = 𝑀′(𝑡 = ∞) ― 𝑀′(𝑡 = 0)

(7) (8) (9)

where 𝜀′(𝑡 = 0), 𝜎′(𝑡 = 0) and 𝑀′(𝑡 = 0) are the values of the static dielectric permittivity, conductivity and electrical modulus at the initial stage of crystallization while 𝜀′(𝑡 = ∞), 𝜎′ (𝑡 = ∞) and 𝑀′(𝑡 = ∞) are the corresponding values at the ultimate stage of crystallization, and 𝜀′(𝑡), 𝜎′(𝑡) and 𝑀′(𝑡) are the values at time. As a next step, we established the time dependence of the normalized parameters, 𝜀′𝑁(𝑡), 𝜎′𝑁(𝑡) and 𝑀′𝑁(𝑡), which are ascertained at a fixed frequency, f= 104 Hz, as presented in Figure 8f. These sigmoid curves are further analyzed in terms of a first-order crystallization kinetics equation:85–88 𝑉 = 1 ― exp ( ― 𝐾(𝑡 ― 𝑡0)𝑛)

(10)

where V, n and 𝑡0 denote the crystallization degree, Avrami parameter and the incubation time, respectively. 𝐾 is a rate constant, and equals to kn, where k is crystallization rate. Subsequently, for each curve in Figure 8f, the k, n and 𝑡0 parameters are determined, viz., k= 1.52×10-4 s-1, n= 2.25, 𝑡0= 520 s for the case of 𝜀′𝑁(𝑡), k= 1.51×10-4 s-1, n= 2.30, 𝑡0= 370 s for 𝜎′𝑁(𝑡) and k= 1.23×10-4 s-1, n= 2.98, 𝑡0= 390 s for 𝑀′𝑁(𝑡). Obviously, despite the different representations (𝜀′, 𝑀′ and 𝜎′) of dielectric spectra used for analysis, the crystallization parameters seem comparable to each other.


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Figure 10. Panel (a): Evolutions of crystallization rate k obtained from various representations for [C6MPyrr]+[Tf2N]- as functions of temperature. Panel (b) and (c): Evolutions of Avrami parameters and crystallization rate k obtained from electrical modulus M′ representation for [CnMPyrr]+[Tf2N]- (n= 4, 5, 6, 8) samples as functions of temperature. Error bars in panel (b) cover the differences between various Avrami parameters when diverse representations are used. Next, we conducted isothermal measurements for these four ionic liquids in a series of temperatures, and analyzed the experimental data by the same method as used in Figure 9. The fitting results of k and n for the studied IL’s are collected in Figure 10 as functions of the degree of supercooling, ∆𝑇 = 𝑇𝑚 ―𝑇. As illustrated in Figure 10a, we compared the k values (𝑘𝑀′, 𝑘𝐸′ and 𝑘𝜎′), which are respectively determined from the normalized spectra in representations of 𝜀′, 𝑀′ and 𝜎′, for [C6MPyrr]+[Tf2N]-. It is apparent that the k values are close to each other, especially when samples are at large degrees of supercooling. Accordingly, the 𝑘𝑀′ results determined for these four ionic liquids are representatively selected for further comparison in this work. Figure 10c shows changes of 𝑘𝑀′ along with degree of undercooling for all these four IL’s, of which three other samples except [C8MPyrr]+[Tf2N]- show arch-shaped 𝑘𝑀′ - ∆𝑇 correlations. More interestingly, the maximum crystallization rates of these three ILs occur approximately at the same degree of



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supercooling, namely ∆𝑇~ 25 K. As for [C8MPyrr]+[Tf2N]-, crystallization is only detectable in a narrow temperature region of 25 K ≤ ∆𝑇 ≤ 55 K. Besides, in Figure 10c, it is clearly seen that at the same ∆𝑇 the 𝑘𝑀′ values rank in the order of [C5MPyrr]+[Tf2N]- > [C4MPyrr]+[Tf2N]-

>

[C6MPyrr]+[Tf2N]-

>

[C8MPyrr]+[Tf2N]-.

Especially,

𝑘𝑀′

of

[C5MPyrr]+[Tf2N]- is at least 1.5 orders of magnitude higher than those of other three samples. This observation strengthens the conclusion that [C5MPyrr]+[Tf2N]- has the highest crystallization tendency. During the course of experiment, we also noticed that the isothermal crystallization studies of [C5MPyrr]+[Tf2N]- are only available in a narrow temperature region since the sample would become partially crystallized before reaching the desired crystallization temperature where ∆𝑇 is larger than 40 K. As illustrated in Figure 10b, we studied the variation of Avrami parameter n determined in the crystallization experiments of these IL’s along with temperature. Here, the n values are determined from the modulus 𝑀′ representation of dielectric spectra, and the error bars cover the differences in the n values obtained from diverse representations. It appears that there exists a gradually decreasing trend for all these n - ∆𝑇 correlations. Analagous observations have also been reported in earlier literature.89,90 Crystallization studies of ILs by BDS at elevated pressure


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Figure 11. Panels (a) and (b) represent time evolutions of conductivity during isothermal cold crystallization process for [C6MPyrr]+[Tf2N]- at 190 MP and at ambient pressure, respectively. Panel (c) shows comparison of the crystallization parameters at ambient pressure (red circles) and elevated pressure (black squares). Panel (d) presents the dependence of melting temperatures as functions of the pressure. Panels (e) and (f) present the heating behaviors of the crystallized samples in the conductivity representation collected at the frequency of 100 Hz under ambient and elevated pressures, respectively. Olive squares represent results obtained for [C4MPyrr]+[Tf2N]-, red circles for [C6MPyrr]+[Tf2N]- and blue triangles for [C8MPyrr]+[Tf2N]-. To gain a better insight into the crystallization mechanism of these ionic liquids, we conducted the isothermal crystallization measurements under isochronal conditions (𝜏𝑀 = 𝑐𝑜𝑛𝑠𝑡.) with various combinations of temperature and pressure. For example, Figures 11a and 11b present real-time evolution of the crystallization process for [C6MPyrr]+[Tf2N]under two conditions, involving T= 223.15 K, p= 190 MPa and T= 198.15 K, p= 0.1 MPa. It should be noted that the modulus 𝑀′′ peaks under these two conditions locate at the same frequency in the range of 102 - 103 Hz (see Figure S1). Here, the dielectric spectra are recorded in the representation of real part of complex conductivity 𝜎′. In addition, the crystallization experiments under both ambient and elevated pressures were conducted in the same pressure chamber to reduce the experimental uncertainties with a temperature control system (PrestoW85, Julabo) used. However, due to the limitation of this system, the highest cooling rate of ~5 K/min is insufficient to avoid the crystallization of [C5MPyrr]+[Tf2N]-. We studied the crystallization behaviors of [C4MPyrr]+[Tf2N]- at isochronal conditions of T= 193.15 K, p= 0.1 MPa and T= 208.15 K, p= 200 MPa while those of [C8MPyrr]+[Tf2N]- at isochronal conditions of T= 203.15 K, p= 0.1 MPa and T= 238.15 K, p= 190 MPa with all the corresponding modulus 𝑀′′ peaks located close to 102 Hz (see Figure S1). The crystallization results are analyzed in terms of the aforementioned empirical equations with parameters, viz., crystallization rate k and Avrami parameter n, determined. 31 ACS Paragon Plus Environment


And subsequently, in Figure 11c, we compared the dependences of k and n on the alkyl numbers in the cationic groups under both ambient and elevated pressures. We can see that as the increasing alkyl numbers these two parameters, k and n, varied irregularly in each pressure condition. Notwithstanding, the k values determined for each sample under ambient and elevated pressures are close to each other, suggesting that pressure does not exert pronounced impact on the overall crystallization rate of these ionic liquids. In addition, the n values of [C4MPyrr]+[Tf2N]- and [C8MPyrr]+[Tf2N]- determined at ambient pressure are lower than those obtained at elevated pressures. On the other hand, the n values for [C6MPyrr]+[Tf2N]- are almost the same. As reported in literatures, the Avrami parameter n is linked to the crystallization mechanism.89 Thus, it seems that [C6MPyrr]+[Tf2N]- has a crystallization mechanism less sensitive to pressure than [C4MPyrr]+[Tf2N]- and [C8MPyrr]+[Tf2N]-. After the complete crystallization of each ionic liquid under different conditions, the obtained crystalline sample was immediately heated up in the BDS to detect the melting process. Figures 11e and 11f illustrate the conductivity 𝜎′ spectra collected upon slowly increasing the temperature at a rate of 0.1 K/min under ambient and elevated pressures, respectively. It is interesting that the two spectra for each sample show marked differences, as reflected by the occurrence or disappearance of polymorphic transitions. In the case of [C4MPyrr]+[Tf2N]-, we can observe a polymorphic transition at T= 236.2 K within the measurement at ambient pressure, while such kind of transition is not visible in the elevatedpressure spectra. Different from [C4MPyrr]+[Tf2N]-, the heating scans of [C8MPyrr]+[Tf2N]show a polymorphic transition at T= 246.9 K under elevated pressure while no polymorphic transition under ambient pressure. In the case of [C6MPyrr]+[Tf2N]-, the crystalline sample is observed to experience a polymorphic transition under ambient pressure at T= 244.6 K, and the transition temperature shifts to T= 283.7 K under elevated pressure. From the heating scans depicted in Figures 11e and 11f we can also determine the corresponding melting points, which are established in Figure 11d as functions of pressure for 32 ACS Paragon Plus Environment

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these three ionic samples with the ambient pressure data used. The melting point determined for [C4MPyrr]+[Tf2N]- under a pressure of p= 175 MPa is also added in this plot. To describe the experimental data, we applied the Clausius – Clapyeron equation, ∆𝑉𝑚

𝑇𝑚(𝑝) = ∆𝑆𝑚 (𝑝 ― 𝑝0) + 𝑇𝑚(𝑝0)

(11)

where 𝑝0 denotes atmospheric pressure (0.1 MPa), ∆𝑉𝑚 and ∆𝑆𝑚 are the volume and entropy differences between the liquid and crystalline states at Tm, respectively. Within a small pressure change, i.e. several hundreds of megapascals, we can assume that ∆𝑉𝑚/ ∆𝑆𝑚 ratio is constant, so the temperature evolution of Tm(p) can be approximated by a linear dependence. The fits also yield the results of a pressure coefficient, 𝑑𝑇𝑚 𝑑𝑝|𝑝→0, which are 100 K/GPa for [C4MPyrr]+[Tf2N]-, 190 K/GPa for [C6MPyrr]+[Tf2N]-, 160 K/GPa for [C8MPyrr]+[Tf2N]-. Similar to the observervation for [C4MPyrr]+[Tf2N]- in the work of Lima et al.,82 we also find that the 𝑇𝑔(p) and 𝑇𝑚(p) curves follow the same pressure dependence and the result of 𝑑𝑇𝑔 𝑑𝑝|𝑝→0 ≈ 𝑑𝑇𝑚 𝑑𝑝|𝑝→0 as well. However, in the cases of both [C6MPyrr]+[Tf2N]- and [C8MPyrr]+[Tf2N]-, the 𝑑𝑇𝑚 𝑑𝑝|𝑝→0 results are approximately twice as the values of 𝑑𝑇𝑔 𝑑𝑝|𝑝→0, which might be associated with the longer catonic alkyl tails. Conclusion In this work, we studied a series of aprotic ionic liquids composed of a hydrophobic bis(trifluoromethansulfonyl)imide

anion,

[Tf2N]−

and

methyl-pyrrolidinium

cations,

[CnMPyrr]+ with varied length of alkyl groups (n=4, 5, 6 and 8). To approach a comprehensive understanding of the glass transition dynamics and crystallization kinetics for these ILs, we conducted measurements with complementary experimental techniques used. Firstly, the glass forming ability/crystallization tendency of these ILs were investigated in the non-isothermal calorimetric and dielectric measurements. Except for [C5MPyrr]+[Tf2N]−, other three ILs are good glass formers, for which the glass transition dynamics were then probed by dielectric, calorimetric and shear-mechanical techniques. Despite difference in the



obtained results, we observed the same positive correlation between the length of the cationic side-chain and the corresponding Tg values. Considering the fact that the tail groups of the cationic part of many RTILs tend to selfaggregate, which can even result in an additional slow relaxation mode, we examined these glass-forming [CnMPyrr]+[Tf2N]− (n= 4, 6 and 8) samples by focusing on their dielectric and viscoelastic response in the supercooled liquid state close to Tg. Unfortunately, we did not find the existence of additional slow sub-a relaxation. Additionally, we conducted isothermal cold crystallization measurements by applying the BDS techniques in a wide temperature range from the melting temperature down to glass transition temperature at ambient pressure. The data were analyzed in various representations (𝜀 ∗ , 𝑀 ∗ , 𝜎 ∗ ) of the dielectric spectra with comparable crystallization parameters obtained. Also, the crystallization tendency of these ILs, as reflected by the overall crystallization rate k, is observed to decrease with the increasing cationic alkyl side chains. As a next step, we studied the crystallization tendencies for the samples under isochronal conditions with various temperature and pressure combinations used, and found that pressure did not exert pronounced impact on the overall crystallization rate for each sample. Supporting Information Imaginary part of the complex electrical modulus with the same conductive relaxation time collected for the studied samples at various combinations of temperature and pressure. Acknowledgements W. T and K. A are grateful for the financial support from the National Science Centre within the framework of the SONATA BIS project (Grant No. 2017/26/E/ST3/00077).

Author Information: Corresponding Author E-mail: [email protected], [email protected] ORCID 34 ACS Paragon Plus Environment

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Karolina Adrjanowicz 0000-0003-0212-5010 Wenkang Tu 0000-0001-8895-4666 Grzegorz Szklarz 0000-0002-9657-8055 Notes: The authors declare no competing financial interest.

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149x159mm (300 x 300 DPI)


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289x202mm (300 x 300 DPI)



254x177mm (300 x 300 DPI)


Page 48 of 55

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253x194mm (300 x 300 DPI)



272x208mm (300 x 300 DPI)


Page 50 of 55

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279x139mm (300 x 300 DPI)



279x195mm (300 x 300 DPI)


Page 52 of 55

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279x195mm (300 x 300 DPI)



272x208mm (300 x 300 DPI)


Page 54 of 55

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279x141mm (300 x 300 DPI)


The Effect of Cation n-alkyl Side Chain

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