Influence of Chain Rigidity and Dielectric Constant on the Glass Transition Temperature in Polymerized Ionic Liquids

Article abstract of DOI:10.1021/acs.jpcb.7b09423

Polymerized ionic liquids (PolyILs) are promising candidates for a wide range of technological applications due to their single ion conductivity and good mechanical properties. Tuning the glass transition temperature (T_g) in these materials con

Full text of DOI:10.1021/acs.jpcb.7b09423

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Article
The Influence of Chain Rigidity and Dielectric Constant on the
Glass Transition Temperature in Polymerized Ionic Liquids
Vera Bocharova, Zaneta Wojnarowska, Peng-Fei Cao, Yao Fu, Rajeev Kumar, Bingrui Li, Vladimir N Novikov,
Sheng Zhao, Alexander M Kisliuk, Tomonori Saito, Jimmy W. Mays, Bobby G. Sumpter, and Alexei P. Sokolov
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b09423 • Publication Date (Web): 28 Nov 2017
Downloaded from http://pubs.acs.org on November 30, 2017
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The Influence of Chain Rigidity and Dielectric Constant on the Glass
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Transition Temperature in Polymerized Ionic Liquids
V. Bocharova1*, Z. Wojnarowska1,2,3, PengꢀFei Cao1*, Y. Fu4 , R. Kumar5,6, Bingrui Li,1 V. N.
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Novikov7, S. Zhao , A. Kisliuk1, T. Saito1, Jimmy W. Mays1,7, B.G. Sumpter5,6, A. P.
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Sokolov1,7*
1Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831,
USA
2 Institute of Physics, University of Silesia, Uniwersytecka 4, 40ꢀ007 Katowice, Poland
3Silesian Center for Education and Interdisciplinary Research, 75 Pulku Piechoty 1A, 41ꢀ500
Chorzow, Poland
4Department of Aerospace Engineering & Engineering Mechanics, University of Cincinnati,
Cincinnati, OH 45220
5Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge,
Tennessee 37831, US
6Computational Sciences & Engineering Division, Oak Ridge National Laboratory, Oak Ridge,
Tennessee 37831, USA
7Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USA
Corresponding Authors
*Eꢀmail: bocharovav@ornl.gov; caop@ornl.gov; sokolov@utk.edu
Abstract
Polymerized ionic liquids (PolyILs) are promising candidates for a wide range of technological
applications due to their single ion conductivity and good mechanical properties. Tuning the
glass transition temperature (Tg) in these materials constitutes a major strategy to improve room
temperature conductivity while controlling their mechanical properties. In this work, we show
experimental and simulation results demonstrating that in these materials Tg does not follow a
universal scaling behavior with the volume of the structural units Vm (including monomer and
counterion). Instead, Tg is significantly influenced by the chain flexibility and polymer dielectric
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constant. We propose a simplified empirical model that includes the electrostatic interactions and
chain flexibility to describe Tg in PolyILs. Our model enables design of new functional PolyILs
with the desired Tg.
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Introduction
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Polymerized ionic liquids (PolyILs) are a relatively new class of materials that
combines a high charge density of ionic liquids (ILs) with superior mechanical properties of
polymers 1,2. Due to a decreased mobility of one of the ions achieved by its chemical bonding to
the polymer chain, PolyILs are essentially single ion conductors which facilitate their application
in electrochemical devices 3,4,5. One of the critical parameters for PolyILs, as for any other
polymers, is the glass transition temperature, Tg. In polymers, Tg defines a transition from soft
rubbery state to a “brittle” solid glassy state. Moreover, in case of PolyILs it also controls the
room temperature ion conductivity. In general, a lower Tg provides for higher ion conductivity, σ,
at ambient conditions 6,7,8. Such correlation is achieved because conductivity is controlled by the
rate of polymer segmental mobility at ambient temperature 9,10 which increases with decrease in
Tg. Designing polymers with lower Tg is one of the traditional ways to achieve higher ion
conductivity in polymer electrolytes11,12. Thus, understanding the dependence of Tg on chemical
structure of PolyIL is critical for their design.
In earlier studies, it has been demonstrated that Tg in the imidazoliumꢀbased PolyILs
scales well with the molecular volume of structural unit, Vm, (monomer plus counterion)7,8: Tg
decreases with the increase in Vm. However, the physical origin of this behavior and whether
such dependence holds for PolyILs of diverse chemical structures remain unknown. Similar
behavior of Tg vs Vm has been reported for molecular ionic materials such as monomers of ionic
liquids, ionic liquids, and inorganic salts7, 13 . However, in spite of their ionic nature, the
dependence of Tg on Vm in PolyILs and other ionic liquids doesn’t collapse on a single master
curve suggesting that there might be other structural factors controlling the glass transition. For
instance, it is known that in polymers with prevailing van der Waals (vdW) interactions, the
chain rigidity and side group bulkiness play an important role in affecting Tg14,15. However, the
results from nonꢀionic polymers cannot be directly applied to polymers with prevailing
electrostatic interactions, and therefore detailed studies are required to elucidate the role of
different structural parameters on Tg behavior in these materials.
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In this Letter, we analyze several PolyILs with different chemical structures, counterion
sizes and charges, and chain rigidity (Figure 1). To unravel microscopic details, we also use
coarseꢀgrained molecular dynamics (MD) simulations. Our analysis clearly demonstrates that Vm
is not the only parameter controlling Tg in PolyILs. The latter can change by a factor of two at
the same Vm, depending on the polymer chain rigidity. To understand this dependence, we
present a simple physical model that accounts for chain flexibility, electrostatic and vdW
interactions. This model accurately describes the behavior of Tg vs Vm for PolyILs, their
monomers, and ionic liquids, and unveils the importance of Vm, dielectric constant ε, and chain
rigidity in controlling Tg of PolyILs.
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Figure 1. Chemical structures of PolyILs investigated in this paper.
Materials and Methods
1ꢀvinyl imidazole was purchased from Alfa Aesar and distilled under vacuum prior to use. N,Nꢀ
dimethylformamide (DMF, Alfa Aesar), 1ꢀbromoꢀ4ꢀmethoxy butane (C4OC, Matrix Scientific),
1ꢀbromoꢀ3ꢀmethoxy propane (C3OC, Matrix Scientific), 2ꢀbromoethyl ethyl ether (C2OC2,
Oakwood Chemical), bromoethane (Sigma Aldrich), diethylene glycolꢀ2ꢀbromoethyl methyl
ether (TCI), lithium bis(trifluoromethanesulfonimide) (LiTFSI, Acros Organics), lithium
hexafluorophosphate (LiPF6, Alfa Aesar), dimethylformamide (DMF, anhydrous, Alfa Aesar),
ethyl acetate (BDH), methanol (BDH), hexanes (BDH), tetrahydrofuran (THF, BDH) were used
as received. Reversible Addition Fragmentation Chain Transfer Agent (RAFTꢀCTA) was
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synthesized according to the previous reference16. Azobisisobutyronitrile (AIBN, SigmaꢀAldrich)
was recrystallized from ethanol prior to use. 2ꢀ(Dimethylamino) ethylacrylate (> 97%, TCI) was
deꢀinhibited via passing through activated alumina column and inhibitor remover.
1. Synthesis of STF-based polyanions; PolyILs 1-3
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Scheme 1. Synthesis of (4ꢀstyrenesulfonyl)(trifluromethaneꢀsulfonyl)imide triethylamine (STFꢀ
TEA)
Synthesis of 4ꢀstyrene sulfonyl chloride 17: 8.8 g (43 mmol) of 4ꢀstyrenesulfonic acid sodium salt
was slowly added to the thiolyl chloride (21.7 mL, 300 mmol) in a round bottom flask that was
immersed in an ice bath. With magnetic stirring, the 12 mL dry DMF was added dropwise to the
solution. The mixture was stirring at 0°C for 2 hours and ambient temperature for another 6
hours. The mixture was stay in the freezer overnight before it was poured into the ice water. The
aqueous solution was extracted by diethyl ether for three times, and the combined organic layer
was washed by water for additional two times. The organic solution was dried by Na2SO4, and
after filtering, the ether solution was concentrated by rotary evaporation.
Synthesis of (4ꢀstyrenesulfonyl)(trifluromethaneꢀsulfonyl)imide triethylamine (STFꢀTEA)18: The
4ꢀstyrene sulfonyl chloride (12.9g, 64 mmol) was dissolved in 20 mL of dried acetonitrile
(CH3CN) and placed in a threeꢀneck round bottom flask that was immersed in an ice bath. 10 g
trifluoromethylsulfonamide (68 mmol), 27 mL triethylamine (194 mmol) and 0.80 g (6.5 mmol)
4ꢀdimethylaminopyridine was dissolved in 45 mL of dried CH3CN, and the mixture was slowly
added to the acetonitrile solution of the 4ꢀstyrene sulfonyl chloride via a dropping funnel. After
two hours stirring in the ice bath, the mixture was stirred at room temperature for another 18
hours before the solvent was removed by rotary evaporation. The product was reꢀdissolved in
150 mL of dimethyl chloride (DCM), and the solution was washed by the aqueous solution of
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NaHCO3 (2%, 3×35mL), HCl (1mol/L, 4×30 mL). The H NMR spectrum was shown in Figure
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S1 in Supplementary Information (SI).
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Scheme 2. Synthesis of poly(4ꢀstyrenesulfonyl)(trifluromethaneꢀsulfonyl)imide with different
counter ions
Synthesis of poly(4ꢀstyrenesulfonyl)(trifluromethaneꢀsulfonyl)imide with different counter ions
(PolyILs 1ꢀ3): 119 mg (0.5 mmol) RAFTꢀCTA, 10.1g (25 mmol) STFꢀTEA and 8.2 mg (0.05
mmol) AIBN were dissolved in 50 mL dry dimethylformamide (DMF). After the solution in
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schedule tube was degassed for 30 min, the reaction was performed at 65 C for 24 hours. The
crude product was obtained by evaporating the solvent. The poly(STFꢀLi+), poly(STFꢀK+), and
poly(STFꢀCs+) was obtained by adding excess LiOH, K2CO3 and Cs2CO3 to the water/THF
°
solution of the poly(STFꢀTEA) and stirring at 40 C for 24 hours. The products were purified by
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dialysis against DI water/CH3OH before further characterization. The typical H NMR spectrum
analysis was shown in Figure S2 of SI, and end group analysis suggested the DPn of the
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poly(STFꢀLi+) is 116.08/4=29.02. The complete disappearance of the TEA signals after the
couterion exchange (Figure S2 vs Figure S1) suggested the complete counter ion exchange.
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Mn of poly(STFꢀLi+) =322.22×29.02+238.38= 9,589 Da.
Mn of poly(STFꢀK+) =354.38×29.02+238.38= 10,522 Da
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Mn of poly(STFꢀCs2+)=448.19×29.02+238.38= 13,244 Da
2. Synthesis of poly(N-vinyl imidazole) based poly(ionic liquid)s; PolyILs 4-9.
Scheme 3. Synthesis of poly(Nꢀvinyl imidazole), poly(Nꢀvinyl ethyl imidazolium), poly(Nꢀ
vinyl diethylene glycol ethyl methyl ether imidazolium bromide)
Synthesis of Poly(Nꢀvinyl imidazole): 1ꢀvinyl imidazole (50 g, 0.531mol) and AIBN (0.1 mol%,
0.0872 g, 5.31×10ꢀ4 mol) dissolved in ~150 mL of DMF were charged into a roundꢀbottomed
flask equipped with a magnetic stirrer. The solution was purged with argon for 20 min and
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placed in an oil bath. The reaction proceeded at 65 C for 17 h. The resulting reaction solution
was precipitated into ethylacetate, reꢀdissolved in methanol, and reꢀprecipitated into ethylacetate.
The precipitation process was repeated 3 times. A resulting poly(Nꢀvinyl imidazole) was white
powder and dried under reduced pressure at 40°C overnight. The isolated yield was ~54%.
Unfortunately, direct measurements of the molecular weight of PolyILs synthesized from this
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precursor is very challenging and wasn’t perform here, because it requires a combination of
certain solvents to neutralize the charge of the polymer. According to previous reference19, the
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reaction condition would afford the polymer with Mn of 42000 g/mol and PDI of 1.84. H NMR
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(400 MHz, CD3OD, δ): 1.8ꢀ2.3 (br,ꢀCH2CHꢀ), 2.8ꢀ3.7 ppm (br, ꢀCH2CHꢀ), 6.6ꢀ7.0 ppm (br, ꢀ
NCHCHNꢀ), 7.0ꢀ7.3 ppm (br, ꢀNCHNꢀ); here and below br stays for a broad peak in the NMR
spectra.
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Synthesis of poly(Nꢀvinyl ethyl imidazolium, polyILs 4-6): Poly(Nꢀvinyl imidazole) (5 g) was
dissolved in DMF (75 mL) in a roundꢀbottomed flask equipped with a magnetic stirrer.
Bromoethane (1.5 equivalent, 8.68 g, 5.95 mL) was charged into the reaction flask and the
reaction proceeded at 38 °C for 90 h. Over the course of reaction, the solution became milky due
to its low solubility of the product. The resulting reaction solution was precipitated into
ethylacetate, redissolved in methanol, and reprecipitated into ethylacetate. The precipitation
process was repeated 3 times. The resulting poly(Nꢀvinyl ethyl imidazolium bromide) was dried
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under reduced pressure at 40°C overnight. Based on the chemical shift and integration in H
NMR spectrum, the conversion to imidazolium bromide was nearly quantitative, more than 96ꢀ
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97 %. H NMR (400 MHz, CD3OD, δ): 1.3ꢀ1.8 (br,ꢀCH2CH3), 2.3ꢀ3.2 (br,ꢀCH2CHꢀ), 4.1ꢀ4.4
(br,ꢀCH2CH3), 4.5ꢀ4.7 ppm (br, ꢀCH2CHꢀ), 7.3ꢀ8.2 ppm (br, ꢀNCHCHNꢀ), 9.1ꢀ9.8 ppm (br, ꢀ
NCHNꢀ)
TFSI exchange and PF6 exchange were conducted in methanol with 1.5 equivalents of LiTFSI or
LiPF6, respectively. The product was precipitated into DI water, dissolved in acetone and
precipitated into DI water. Precipitation and water rinsing were repeated several times.
Synthesis of poly(Nꢀvinyl diethylene glycol ethyl methyl ether imidazolium, polyILs 7-9): Poly(Nꢀ
vinyl imidazole) (1.77g) was dissolved in DMF (40 mL) in a roundꢀbottomed flask equipped
with a magnetic stirrer. Diethylene glycolꢀ2ꢀbromoethyl methyl ether (1.17 equivalent, 5 g) was
charged into the reaction flask and the reaction proceeded at 80°C for 48 h. The resulting reaction
solution was precipitated into ethylacetate, redissolved in methanol, and reprecipitated into
ethylacetate. The precipitation process was repeated 3 times. The resulting poly(Nꢀvinyl
diethylene glycol ethyl methyl ether imidazolium bromide) was dried under reduced pressure at
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40°C overnight. Based on the chemical shift and integration in the H NMR, the conversion to
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imidazolium bromide was quantitative. H NMR (400 MHz, CD3OD, δ): 2.3ꢀ3.2 (br,ꢀCH2CHꢀ),
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3.4 (br,ꢀOCH3), 3.5ꢀ3.9 (br,ꢀOCH2CH2Oꢀ), 3.9ꢀ4.1 (br,ꢀNCH2CH2Oꢀ), 4.2ꢀ4.6 (br,ꢀNCH2CH2Oꢀ),
4.6ꢀ4.8 ppm (br, ꢀCH2CHꢀ), 7.3ꢀ8.2 ppm (br, ꢀNCHCHNꢀ), 9.0ꢀ9.8 ppm (br, ꢀNCHNꢀ)
TFSI and PF6 exchange were conducted in methanol with 1.5 equivalents of LiTFSI and LiPF6,
respectively. The product was precipitated into DI water, redissolved in acetone and precipitated
into DI water again. Precipitation and water rinsing were repeated several times. Similar PolyILs
where oligomer of ethylene glycol was attached to the styrene type imidazoliumꢀbased monomer
were reported earlier showing promising RT conductivity 20.
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3. Synthesis of poly(ammonium acrylate) based poly(ionic liquid)s: PolyILs 10-12.
Scheme 4. Synthesis of poly(ammonium acrylate) based poly(ionic liquid)s: PolyILs 10ꢀ12
Synthesis of poly(2ꢀ(Dimethylamino) ethylacrylate): 2ꢀ(Dimethylamino) ethylacrylate (113 g,
120 mL) and AIBN (0.2 wt%, 226.3 mg) were dissolved in ethyl acetate (190 mL). Argon was
°
bubbled through the solution for 20 min. The reaction proceeded for 5 h at 65 C (Scheme 4).
The resulting poly(dimethylamino ethylacrylate) was precipitated into hexanes for 5 times. The
precipitated polymer was dried at 40 °C in vacuum for 24 h and the isolated yield was 52% (58
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g). H NMR (400 MHz, D2O, δ) Poly(dimethylamino ethylacrylate): 1.3ꢀ2.0 ppm (br,ꢀCH2CHꢀ),
2.1ꢀ2.2 ppm (br,ꢀN(CH3)2), 2.2ꢀ2.4 ppm (br,ꢀCH2CHꢀ), 2.4ꢀ2.8 ppm (br,ꢀCH2CH2N(CH3)2), 3.8ꢀ
4.3 ppm (br,ꢀCH2CH2N(CH3)2).
Quaternization of poly(dimethylamino ethylacrylate): The reaction was performed via reaction
with alkyl ether bromide (C3OC, C4OC, C2OC2). All the quaternization reactions were performed
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with 1.1 equimolar of alkyl ether bromide in anhydrous DMF at 80 C for 24 h. The resulting
quaternized poly(dimethylamino ethylacrylate) bromide was washed with THF several times.
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The isolated polymer was dried at 40 C in vacuum for 24 h. The synthesized quaternary
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ammonium bromide ionic liquid polymers include poly[(2ꢀ(acryloyloxy)ethyl)ꢀN,N’ꢀdimethylꢀ
Nꢀmethoxy propylꢀammonium bromide] (C3OC), poly[(2ꢀ(acryloyloxy)ethyl)ꢀN,N’ꢀdimethylꢀNꢀ
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methoxy butylꢀammonium bromide] (C4OC), poly[(2ꢀ(acryloyloxy)ethyl)ꢀN,N’ꢀdimethylꢀNꢀ
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ethoxy ethylꢀammonium bromide] (C2OC2). H NMR (400 MHz, D2O, δ):
poly[(2ꢀ
(acryloyloxy)ethyl)ꢀN,N’ꢀdimethylꢀNꢀmethoxy propylꢀammonium bromide], 1.5ꢀ2.0 ppm (2H,
br, ꢀCH2CHꢀ), 2.0ꢀ2.1 ppm (2H, br, ꢀN(CH3)2CH2CH2CH2OCH3), 2.2ꢀ2.7 ppm (1H, br, ꢀ
CH2CHꢀ), 3.0ꢀ3.2 ppm (6H, br, ꢀN(CH3)2ꢀ), 3.2ꢀ3.3 ppm (3H, br, ꢀN(CH3)2CH2CH2CH2OCH3),
3.3ꢀ3.6 ppm (4H, br, ꢀN(CH3)2CH2CH2CH2OCH3), 3.6ꢀ3.8 ppm (2H, br, ꢀOCH2CH2N(CH3)2ꢀ),
4.2ꢀ4.6 ppm (2H, br, ꢀOCH2CH2N(CH3)2ꢀ); poly[(2ꢀ(acryloyloxy)ethyl)ꢀN,N’ꢀdimethylꢀNꢀ
ethoxy ethylꢀammonium bromide], 0.9ꢀ1.4 ppm (3H, br, ꢀN(CH3)2CH2CH2OCH2CH3), 1.5ꢀ2.0
ppm (2H, br, ꢀCH2CHꢀ), 2.2ꢀ2.7 ppm (1H, br, ꢀCH2CHꢀ), 3.0ꢀ3.3 ppm (6H, br, ꢀN(CH3)2ꢀ), 3.4ꢀ
3.6 ppm (2H, br, ꢀN(CH3)2CH2CH2OCH2CH3),
3.6ꢀ3.7 ppm (2H, br,
ꢀ
N(CH3)2CH2CH2OCH2CH3), 3.7ꢀ3.8 ppm (2H, br, ꢀOCH2CH2N(CH3)2ꢀ), 3.8ꢀ4.0 ppm (2H, br, ꢀ
N(CH3)2CH2CH2OCH2CH3), 4.3ꢀ4.6 ppm (2H, br, ꢀOCH2CH2N(CH3)2ꢀ)
Counter ion exchange: Ion exchange from bromide ion to bis(trifluoromethane)sulfonimide
(TFSI) ion was performed using 1.2 equivalent of LiTFSI in water for 5 days (Scheme 4). The
TFSIꢀexchanged ionic liquid polymers include poly[(2ꢀ(acryloyloxy)ethyl)ꢀN,N’ꢀdimethylꢀNꢀ
methoxy propylꢀammonium TFSI] (C3OC), poly[(2ꢀ(acryloyloxy)ethyl)ꢀN,N’ꢀdimethylꢀNꢀ
methoxy butylꢀammonium TFSI] (C4OC), poly[(2ꢀ(acryloyloxy)ethyl)ꢀN,N’ꢀdimethylꢀNꢀethoxy
ethylꢀammonium TFSI] (C2OC2). Based on elemental analysis results detailed elsewhere 21 the
content of Br in TFSI containing PolyILs doesn not exceed 1.6 %
4. Synthesis of poly(methylhydrosiloxane-graft-imidazolium); PolyILs 13, 14
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Scheme 5. Synthesis of poly(ammonium acrylate) based poly(ionic liquid)s: PolyILs 13,14.
Synthesis of polymethylhydrosiloxaneꢀgraftꢀ5ꢀbromoꢀ1ꢀpentene (polyꢀMHS B5B): To a preꢀdried
two neck round bottom flask equipped with a condenser, a rubber septum and a magnetic stirrer
was added PMHS (2.0 g, 0.84 mmol) and B5B (5.0 g, 33.6 mmol, 40 eq) The reactor was purged
with Ar for 20 min followed by the addition of a platinumꢀdivinyltetramethyldisiloxane complex
°
(Pt[dvs]) (30 ꢁL). The reaction mixture was then stirred at 60 C. The reaction progress was
1
monitored using HꢀNMR spectroscopy. Upon the completion of reaction, the solvent was
1
removed and precipitated into excess methanol to obtain brown oil. Yield: 5.9 g, 98%. HꢀNMR
δ (CDCl3, 500MHz): 3.41 (ꢀCH2ꢀBr); 1.86 (ꢀCH2CH2ꢀBr); 1.28ꢀ1.53 (ꢀCH2CH2ꢀ); 0.53 (SiꢀCH2);
0.07 (SiꢀCH3).
Synthesis of polymethylhydrosiloxaneꢀgraftꢀ5ꢀimidazoliumꢀ1ꢀpentene bromide (polyꢀMHSImi Br,
PolyIL 13). To a two neck round bottom flask equipped with a condenser and a magnetic stirrer
was added the solution of PMHSꢀB5B (2.4 g, 0.36 mmol) in DCM (30 mL) and 1ꢀ
methylimidazole (1.03 g, 12.5 mmol, 35 eq). The reaction was stirred at 70 °C for 72 h. Upon the
completion of the reaction, the solvent was removed under reduced pressure and the product was
recovered by washing with DCM three times and drying in vacuum oven at 40 °C. Yield: 3.2 g,
98%. 1HꢀNMR δ (D2O, 500MHz): 8.95 (NCHN); 7.56 (CH3NCHCHN); 4.26 (NCH2ꢀ); 3.95 (Nꢀ
CH3); 2.14ꢀ1.27 (4H, ꢀCH2ꢀCH2ꢀ); 0.58(SiꢀCH2); 0.10 (SiꢀCH3). Mass (ESIꢀ) m/z: 78.92 (Br).
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Synthesis of polymethylhydrosiloxaneꢀgraftꢀ5ꢀimidazoliumꢀ1ꢀpentene bis (trifluoromethane)
sulfonamide (polyꢀMHSIm TFSI, PolyIL 14). To a one neck round bottom flask equipped with a
magnetic stirrer was added the solution of PMHSꢀImiꢀBr (1.0 g, 0.11mmol) in deionized water
DIꢀH2O (20 mL) followed by the addition of LiTFSI (1.3 g, 6.9 mmol, 64 eq). The mixture was
stirred at room temperature for 7 days. The precipitation was further washed with DIꢀH2O 3
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times and dried in vacuum oven at 40 °C. Yield: 1.5 g, 99%. HꢀNMR δ (acetoneꢀd6, 500MHz):
8.87 (NCHN); 7.66 (CH3NCHCHN); 4.30 (NCH2ꢀ); 4.02 (NꢀCH3); 2.03ꢀ1.30 (4H, ꢀCH2ꢀCH2ꢀ);
0.61 (SiꢀCH2); 0.13 (SiꢀCH3). Mass (ESIꢀ) m/z: 180.89 (TFSI). The details of ion exchange are
in 22.
Synthesis of monomers is described in SI.
Polymer Characterization:
Size exclusion chromatography (SEC) measurements were performed in THF/5% triethylamine
°
mobile phase at 40 C at flow rate of 1 mL/min using a Polymer Labs GPCꢀ120 size exclusion
chromatograph. The GPCꢀ120 is equipped with Polymer Laboratories PL gel; 7.5mm x 300 mm
(column size); 10 µm (particle size); 5,000 Å (pore size), a Precision Detector PD2040 (two
angle static light scattering), Precision Detector PD2000DLS (dynamic light scattering),
Viscotek 220 differential viscometer, and a Polymer Labs refractometer calibrated with narrow
polydispersity polystyrene standards. The RI increment (dn/dc) was calculated online.
Elemental analysis for carbon, nitrogen, hydrogen, and sulfur were performed by Galbraith
Laboratories, Inc. (Knoxville TN).
Differential Scanning Calorimetry (DSC) measurements were performed on TA Q2000. The
samples were hermetically sealed in aluminum pans while using an empty pan for reference.
These samples were initially equilibrated at 383 K for thirty minutes, then cooled down to 183 K
and heated up to 453 K. The heating and cooling cycle was performed with a rate of 10K/min
and repeated three times to make certain the reproducibility of the results. The glass transition
temperature was estimated as the midꢀpoint of the step in specific heat on cooling.
Measurements of density: The mass of the samples were measured using a weight balance
(Mettler Toledo NewClassic MF Model MS105DU) with an accuracy of 0.01 mg. The gas
pycnometer (Micromeretics Accupyc II 1340) measured the volume of penetrable helium gas
within the sample and sample cup (total volume 0.1cm3) until the pressure did not vary more
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than 0.005psig/min during the equilibration period. The accuracy of the volume measurement is
0.0001 cm3. This cycle was repeated 5 times for reproducibility. The density was then
determined from the mass to volume ratio and the error of this measurement is less than 0.3%.
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°
All the density data were obtained at atmospheric pressure and at T = 20 C. The molecular
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volume Vm was calculated based on the following equation ꢀꢁ = ꢄꢂꢅꢃ , where Mwꢀmolecular
ꢆ
weight of the monomer and anion, ρ is the density, and NA is the Avogadro constant.
Broadband Dielectric Spectroscopy (BDS) measurements of static dielectric constant: Dielectric
measurements at ambient pressure from 10−1 to 106 Hz were carried out using a Novocontrol
GMBH Alpha dielectric spectrometer. The sample was placed between two stainless steel
electrodes of the capacitor with a gap of 0.1 mm. The dielectric spectra of PolyILs were collected
over a wide temperature range. The temperature was controlled by the Novocontrol Quattro
system, with the use of a nitrogen gas cryostat with a temperature stability of the samples of 0.1
K. The applied voltage was 0.1 V. The static dielectric constant was determined from fitting of
the real part permittivity spectra at temperature above Tg using HavriliakꢀNegami23 function. An
example of fit is presented in the Fig. S3 in SI. The obtained values of the static dielectric
constants of studied here PolyILs and monomers are presented in the Fig. S4 in SI.
Results & Discussion
Fourteen PolyILs with different backbones, side groups, and counterions of different
charges and compositions [Fig.1 and Table 1] were synthesized in our lab and details of the
synthesis are presented in21,22, in the Materials and Methods section, and in the Supplementary
Information (SI). Differential scanning calorimetry (DSC) was employed to estimate their glass
transition temperature. Each sample was first cycled in the calorimeter to remove any prehistory,
and then Tg was measured on cooling as the midpoint of the jump in the heat flow (Fig. 2). To
calculate the molecular volume Vm of the PolyILs, the density was measured by pycnometry.
The obtained values, including Tg, density and calculated molecular volume, are listed in the
Table 1. In addition, we used Tg and Vm data for PolyILs, monomers, and ionic liquids reported
in the literature7, 8,13,24,25 (Figure 3). It is important to note that the major difference between ionic
liquids and monomers lies in the presence of double bond in monomers which makes them
polymerizable. To provide additional microscopic insight, we performed coarseꢀgrained MD
simulations using a beadꢀandꢀspring model with every forth bead having a charge. The radius of
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the counterions was changed from 0.05 σ to 0.5 σ (σ is the bead diameter), so that the ratio of the
counterion size to the chain bead size varies from 0.1 to 1. In addition, the dielectric constant and
the chain rigidity were tuned in MD simulations. The Tg in MD simulations was estimated by the
change in the slope of the specific volume with respect to temperature. Details of the simulations
are presented in 26.
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Figure 2. Representative thermograms of PolyILs studied in this work. Tg was obtained as a midpoint of
the jump in heat flow.
Table 1. The physicochemical parameters of PolyILs studied herein, molecular weight, density,
molecular volume, glass transition temperature.
PolyIL
Cation
M
g/mol
Vm
Tg
K
ρ,
No
nm3
g/cm3
Anion
Rigid PolyILs
353.1
1.78
1.60
2.33
0.33
0.33
0.31
500
507
470
1
2
3
K
Li
Cs
polySTF
320.94
446.9
Semi-Flexible PolyILs
202.9
267.97
403.15
320.9
1.48
1.58
1.61
1.52
1.48
1.53
0.22
0.28
0.41
0.35
0.43
0.56
451
432
344
348
296
261
4
5
6
7
8
9
Br
PF6
TFSI
Br
polyEtVIm
polyEGVIm
385.97
521.15
PF6
TFSI
polyAcrEtAm+
C2OC2
polyAcrEtAm+
496
496
1.53
0.54
270
270
10
11
TFSI
1.597 0.51
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C3OC
polyAcrEtAm+
C4OC
510
1.513 0.56
256
12
Flexible PolyILs
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9
491.15
290.15
526
1.48
1.38
1.6
0.55
233
276
240
249
13
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16
TFSI
Br
polyHMSIm
0.35
0.54
0.61
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I-EOT*
I-AT*
508
1.37
Poly
Triazolium
879
1.5
0.97
211
17
TFSI**
Tg was determined by standard DSC measurements (10K/min);
*Data were taken from ref. [24];
** Data were taken from ref. [25] assuming density of 1.5 g/cm3.
The dependence of Tg on structural unit volume Vm shows a general behavior for
polymerized ionic liquids and their monomers (Figure 3): Tg decreases with increase of Vm and
starts to level off at Vm ~0.8ꢀ1nm3. In the case of ionic liquids, Tg dependence goes through a
clear minimum that is not that obvious for other systems (Fig. 3). Analysis of the data shows that
Vm is not the only parameter controlling Tg in PolyILs because Tg can change by almost a factor
of two at the same Vm. Flexible siloxaneꢀbased and phosphazeneꢀbased24 PolyILs (open circles
Fig.3) exhibit Tg comparable to that of monomers, while rigid polystyreneꢀbased chains (open
diamonds Fig.3) show significantly higher Tg at the same Vm (Fig. 3). This behavior is consistent
with the known tendency in nonꢀionic polymers, where rigid chains show much higher Tg than
the flexible ones 14,15,27,28,29
.
The increase in Tg with increase in chain rigidity was also observed in our MD
simulations (Fig. 4). Moreover, simulations also reveal the initial decrease in Tg with the increase
in Vm, reaching a minimum and then leveling off at higher Vm. The Tg(Vm) dependence remains
qualitatively the same for different dielectric constant εs of the simulated polymer. However, an
increase in εs results in suppression of Tg in both rigid and flexible chains. These MD results are
very important because in experiment it is difficult to tune only dielectric constant without
affecting other structural parameters of the polymer. The strong dependence of Tg on the
dielectric constant emphasizes the importance of electrostatic interactions in PolyILs.
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1110
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polymers
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monomers
ionic liquids
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Vm ( )
nm3
Figure 3 Tg as a function of molecular volume (monomer + counterion) for a number of PolyILs: open
symbols present the data for PolyILs studied herein: open diamonds present rigid PolyILs, triangles for
semiꢀflexible, and circles are for flexible PolyILs. The numbers in the open symbols correspond to the
PolyILs in Table 1. Solid blue, green, orange symbols and open symbols 15ꢀ17 are literature data from
[7,8,13] and [24,25], respectively. Solid lines are fits of the ionic liquids and monomers data to the Eq.
(2). The fitting parameters for ionic liquids are A=ꢀ97±49 K, B=137±20 K⋅nm, and C=177±38 K/nm2;
and for monomers are A=ꢀ297±118 K, B=354±72 K⋅nm, and C=153±47 K/ nm2. The dashed lines are
curves calculated using the Eq. 4 with A, B, and C parameters of monomers while K were fixed to K=4
⋅10ꢀ4 , 2⋅10ꢀ4, and 1⋅10ꢀ4 nm3/K for rigid (orange), semiꢀflexible (blue), and flexible (black) chains,
respectively.
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Figure 4. The results of MD simulations with two different values of the static dielectric constant εs. The
asterisk on Tg and Vm means these quantities obtained from the MD simulations are in reduced Lennardꢀ
Jones units [26]. Figure was reproduced from [26].
To understand the observed behavior of Tg in PolyILs we note that the glass transition
temperature usually correlates with the cohesive energy of the material30,31. There are two major
contributions to the cohesive energy in ionic systems. The first one comes from electrostatic
interactions of ions 32 . In a rough approximation, the electrostatic energy scales with the
molecular volume Vm as
ꢊ
ꢈ
ꢇꢈ ꢉ
∝
(1)
ꢎ/ꢏ
ꢍ
ε ꢌ
ꢋ
Here e is the ion charge and εs is the static dielectric constant. The second contribution comes
from the vdW interaction. For molecular van der Waals liquids, it was demonstrated
experimentally that vdW interaction is roughly proportional to the surface area of the unit, EvdW
∝ Vm2/3 33. As a result, the dependence of Tg on the molecular volume of structural units in ionic
liquids can be approximated by the following expression:
ꢕ
ꢐꢑꢒ = ꢓ ꢔ ꢎ/ꢏ ꢔ ꢖꢌꢊ/ꢏ
(2)
ꢍ
ꢌ
ꢍ
where A, B and C are constants. In that case (Eq. 2), the electrostatic term dominates at small Vm
leading to a sharp decrease in Tg(Vm) with increasing Vm, while the vdW term should dominate
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at larger Vm. This competition should lead to a broad minimum in Tg(Vm). This minimum is
indeed observed in the case of simple ionic liquids13 (Fig. 3) where it was also explained by the
competition between the electrostatic and Van der Waals forces. It should be noted that the term
for vdW interaction used here is a result of empirical analysis of Tg behavior of simple vdW
liquids and substantial theoretical work is required to understand the origin of this term.
However, in spite of empirical nature of Eq.2 it describes Tg(Vm) in ionic liquids and monomers
(Fig. 3) quite well. The fitting revealed that the electrostatic term (B) in the case of monomers is
much stronger than in ionic liquids. This term is inversely proportional to the static dielectric
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ꢎ
constant (ꢗ. ꢈ. , ꢕ ∝ ꢘ ), and the obtained difference in B suggests that ionic liquids has εs higher
ꢋ
than the typical values obtained for monomers. Our analysis of the dielectric constant (Fig. S4 in
SI) indeed confirms that on average εs in ionic liquids is higher than in monomers. It is important
to note that monomers were specifically synthesized for these studies. Detailed description of
monomer synthesis and structures are presented in the SI. In the present model, we neglected
variation of εs for the same class of material. The fact that we were able to fit data for polymers,
monomers, and ionic liquids with constant B suggests that variations of εs within each class are
not very significant. Generally, it is possible to improve the model by adding εs as variable.
However, it requires additional guidance from theory and further experimental validation.
It has been shown in 34 that Tg of a PolyIL increases with molecular weight (length of the
chain) in a manner similar to usual vdW polymers 27, 29. Thus to go from a monomer to a PolyIL
we can use approximations developed for usual polymers. Among several ways to describe
dependence of Tg on molecular weight (MW), we chose the KanigꢀUeberreiter equation 35:
ꢎ
ꢎ
ꢜ
=
ꢔ ꢙꢚ
(3)
ꢐ (ꢙꢚ)
ꢐ
ꢑꢛ
ꢑ
where Tg is the glass transition temperature at infinitely high MW, D is the coefficient that
∞
depends on the chain rigidity. This equation describes the dependence Tg(MW) better than the
35
FoxꢀFlory equation, especially at low MW . We can invert eq. 3 and express the polymer Tg
∞
via monomer Tg :
0
ꢢꢎ
ꢊ
ꢎ
ꢎ
ꢜ
ꢎ
ꢝ
ꢕꢎ
ꢝ
=
−
=
−
= ꢞꢓ ꢔ
ꢔ ꢖꢌꢏꢍꢡ
−
ꢌ
ꢍ
(4)
ꢎ
ꢐ
ꢐ
ꢑꢒ
ꢙꢒ
ꢐ
ꢑꢒ
ꢌ
ꢑꢛ
ꢍ
ꢏ
ꢟ ꢌ
ꢠ
ꢍ
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where M0 is the monomer mass, K = D/ρ, and ρ=M0/Vm is the monomer density. Here we
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introduced B1=Bꢁεs to explicitly emphasize the dependence of Tg on the dielectric constant of the
PolyILs. Assuming no strong dependence of density and dielectric constant on molecular weight,
we can now predict the dependence of Tg on Vm in PolyILs (Fig. 3).
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Figure 3 presents three different dashed curves calculated using the Eq.4 and Tg0(Vm) for
monomers (keeping parameters A, B, and C of the eq.2 the same) with only one adjustable
parameter K. They describe the experimental data for PolyILs well, with the parameter K being
the smallest for flexible (e.g. siloxaneꢀ and phosphazeneꢀbased) backbone and relatively large for
rigid (e.g. polystyreneꢀbased) chains. It is important to note that the flexibility parameters (K)
obtained from this analysis are close to that of usual vdW polymers. For instance, the Tg(MW)
data for polystyrene 27 gives for the parameter K ~0.001 [nm3/K] (using Vm of polystyrene ~0.17
nm3 and molecular weight of monomer 104 g/mol). This is about two times larger than K found
for PolyILs with rigid chains (Fig. 3), but taking into account that K is found for different
polymers in a broad range of Vm this is a reasonably good agreement. Thus the proposed model
(eqs. 2, 4) provides a reasonable description of the Vm dependence of Tg in PolyILs. Given the
fact that we could fit the data for the monomers and PolyILs using the same value of B, it is
logical to suggest that there are no significant differences in their static dielectric constants.
Indeed, the measured dielectric constants of monomers and PolyILs are very close (Fig S4).
Obviously, explicit account for the difference in the static dielectric constant of various PolyILs
could significantly improve the model description of their Tgs. However, the latter requires
rigorous theory development. In contrast, we attempted to present a simplified qualitative picture
of the role of chemical structure of PolyILs on their glass transition temperature that provides an
excellent basis for further theoretical exploration.
Our analysis of the experimental and MDꢀsimulations results revealed that in addition to
Vm chain rigidity plays a significant role in defining Tg of PolyILs. However, from Fig 3 it also
became apparent that by tuning Vm and chain flexibility it will be difficult to synthesize PolyILs
with Tg below 200K since most of the monomers have Tg above this value (Fig. 3). This strongly
limits the value of ionic conductivity that can be reached at ambient conditions. At the same
time, our analysis (Fig. 3 and Fig. S4) shows that ionic liquids display a higher static dielectric
constant than that of monomers, reflecting their overall lower Tg. As a consequence, strong
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increase in the dielectric constant might be a promising route to drop Tg below 200K in PolyILs,
especially at smaller Vm. In other words, only tuning the chain flexibility is not enough to
significantly reduce Tg of PolyILs. Combination of polymer flexibility with high dielectric
constant implemented into design of PolyILs may offer further lowering of Tg, providing high
segmental and ion mobility at ambient temperature. An increase in the dielectric constant will
also provide better ion dissociations, additionally improving ionic conductivity. However, further
studies are required to ascertain similar behavior in other PolyILs. Another important alternative
can be a decoupling of ion transport from segmental mobility10,36,37,38,39,40,41,42,43,44. It might
provide significant enhancement of ion conductivity even at rather slow segmental dynamics of
the polymer. However, this topic is out of scope of the current paper.
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Conclusions
In conclusion, the presented detailed analysis of experimental and MDꢀsimulations data
revealed three important parameters controlling the glass transition temperature in PolyILs: chain
rigidity, dielectric constant, and the size of the structural unit, Vm. To address the role of these
parameters in determining Tg in PolyILs, we proposed a simplified empirical model that was
shown to adequately describe the Tg vs Vm dependence of ionic liquids, monomers, and PolyILs.
This model is important predictive tool that can help in design of PolyILs with a desired Tg. In
particular, our results suggest that a strong increase in the dielectric constant of PolyILs is a
promising direction for dropping Tg which can ultimately result in improved room temperature
ion conductivity.
ASSOCIATED CONTENT
Supporting Information: The Supporting Information is available free of charge on the ACS
1
Publications website at DOI: Monomer synthesis and structure (Scheme S1), NMR spectra
(Figure S1 and S2), Dielectric spectra (Figure S3), and Dielectric constants (Figure S4)
Acknowledgements
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This work was supported by Laboratory Directed Research and Development program of Oak
Ridge National Laboratory, managed by UTꢀBattelle, LLC, for the U.S. Department of Energy
(DOE). APS, BGS, AK acknowledge partial financial support by the U.S. Department of
Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.
MD simulations were performed at the Center for Nanophase Materials Sciences, which is a US
DOE Office of Science User Facility. VNN thanks the NSF Polymer Program (DMRꢀ1408811)
for funding theoretical part. Z.W. is deeply grateful for the financial support by the National
Science Centre within the framework of the Opus8 project (Grant DECꢀ2014/15/B/ST3/04246).
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TOC Graphic
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