Revealing the Charge Transport Mechanism in Polymerized Ionic Liquids: Insight from High Pressure Conductivity Studies

Article abstract of DOI:10.1021/acs.chemmater.7b01658

Polymerized ionic liquids (polyILs), composed mostly of organic ions covalently bonded to the polymer backbone and free counterions, are considered as ideal electrolytes for various electrochemical devices, including fuel cells, supercapacitors, and batteries. Despite large structural diversity of these systems, all of them reveal a universal but poorly understood feature: a charge transport faster than the segmental dynamics. To address this issue, we studied three novel polymer electrolyte membranes for fuel cells as well as four single-ion conductors, including highly conductive siloxane-based polyIL. Our ambient and high pressure studies revealed fundamental differences in the conducting properties of the examined systems. We demonstrate that the proposed methodology is a powerful tool to identify the charge transport mechanism in polyILs in general and thereby contribute to unraveling the microscopic nature of the decoupling phenomenon in these materials.

Full text of DOI:10.1021/acs.chemmater.7b01658

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Article
Revealing the Charge Transport Mechanism in Polymerized
Ionic Liquids: Insight from High Pressure Conductivity Studies
Zaneta Wojnarowska, Hongbo Feng, Mariana Diaz, Alfredo Ortiz, Inmaculada Ortiz, Justyna Knapik-
Kowalczuk, Miguel Vilas, Pedro Verdía, Emilia Tojo, Tomonori Saito, Eric W. Stacy, Nam-Goo. Kang,
Jimmy W. Mays, Danuta Kruk, Patryk Wlodarczyk, Alexei P. Sokolov, Vera Bocharova, and Marian Paluch
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b01658 • Publication Date (Web): 05 Sep 2017
Downloaded from http://pubs.acs.org on September 6, 2017
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Chemistry of Materials
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Revealing the Charge Transport Mechanism in Polymerized Ionic
Liquids: Insight from High Pressure Conductivity Studies
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Zaneta Wojnarowska1,2,3*, Hongbo Feng4, Mariana Diaz5, Alfredo Ortiz5, Inmaculada Ortiz5,
Justyna Knapik-Kowalczuk2,3, Miguel Vilas6, Pedro Verdía6, Emilia Tojo6, Tomonori Saito1, Eric
W. Stacy7, Nam-Goo. Kang4, Jimmy W. Mays1,4, Danuta Kruk7, Patryk Wlodarczyk8, Alexei P.
Sokolov4,9, Vera Bocharova1, Marian Paluch2,3
1Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
2Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland
3Silesian Center for Education and Interdisciplinary Research, 75 Pulku Piechoty 1A, 41-500 Chorzow, Poland
4Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USA
5Chemical and Biomolecular Engineering Department. University of Cantabria. Avenida de los Castros s/n 39005
Santander, Spain
6Organic Chemistry Department, University of Vigo, Marcosende 36210 Vigo, Spain
7Faculty of Mathematics and Computer Science, University of Warmia and Mazury in Olsztyn, Sloneczna 54, Olszt-
yn PL-10710, Poland
8Institute of Non-Ferrous Metals, Sowinskiego 5, 44-100, Gliwice, Poland
9Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA
ABSTRACT: Polymerized ionic liquids (polyILs), composed mostly of organic ions covalently bonded to the polymer
backbone and free counterions, are considered as an ideal electrolytes for various electrochemical devices, including fuel
cells, supercapacitors and batteries. Despite large structural
diversity of these systems, all of them reveal a universal but
poorly understood feature - a charge transport faster than the
segmental dynamics. To address this issue, we have studied
three novel polymer electrolyte membrane for fuel cells as well
as four single-ion conductors including highly conductive
siloxane-based polyIL. Our ambient and high pressure studies
revealed fundamental differences in the conducting properties
of the examined systems. We demonstrate that the proposed
methodology is a powerful tool to identify the charge
transport mechanism in polyILs in general and thereby con-
tribute to unraveling the microscopic nature of the decoupling
phenomenon in these materials.
INTRODUCTION
unlimited combination of cations, anions and polymer
chains provides the possibility to design numerous
combinations of chemically distinct polyILs materials of
technological importance.4 The rational design of poly-
ILs requires fundamental understanding of the micro-
scopic parameters controlling their macroscopic proper-
ties, including conductivity. For this reason, in recent
years much effort has been dedicated to fundamental
understanding of the charge transport mechanism in
PolyILs.
Efficient generation and utilization of energy has be-
come one of the greatest challenges of our time. This in
turn creates a growing demand for various types of en-
ergy storage devices in household and in industry.
Therefore, a significant research effort during the past
decade has been focused on development of new func-
tional materials for diverse energy storage applica-
tions.1,2,3 The family of polymerized ionic liquids (poly-
ILs), combining unique futures of ionic liquids with the
outstanding mechanical properties of polymers, might
offer an ideal solution for these purposes. An almost
It has been found that in contrast to low-molecular
weight aprotic ionic liquids (composed solely of ions),
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where the charge transport is fully controlled by viscosi-
conducting properties of polymeric ILs, both being cru-
ty5, the ion mobility in polyILs can be strongly decou-
cial in their potential electrochemical applications. We
demonstrate that the proton transfer mechanism can be
easily distinguished from vehicle conduction in any low-
molecular weight or macromolecular ionic system.
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7
8
pled from segmental (structural) dynamics.6,7,8,9,10, As a
11
consequence, the classical Walden theory predicting
similar temperature dependence for the dc conductivity
and mass diffusion (fluidity), fails for macromolecular
conductors. In particular, when the segmental mobility
slows down due to the glassy freezing, the rate of ion
motions in polyILs can be many orders of magnitude
faster12, resulting in dc conductivity significantly higher
than the conductivity in an ideally coupled system (at Tg
EXPERIMENTAL
In this paper we have investigated three novel protic
polymerized ionic liquids obtained by UV polymeriza-
tion of corresponding monomers:
i) polyanion 1-butylsulfonate-3-methylimidazolium
poly(2-sulfoethylmetacrylate) (SBMIm poly-SEM);
ii) polymer blend poly(1-butylsulfonate-3-
vinylimidazolium) poly(2-sulfoethylmetacrylate)
(poly-SBVIm poly-SEM)
iii) polyanion 1-butylsulfonate-3-vinylimidazolium
poly(vinylphosphonic acid) (SBMIm poly-HPO3).
Additionally, we have studied the charge transport
mechanism in the group of aprotic polyILs:
i) poly(1-ethyl-3-vinylimidazolium
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~10-14-10-15 S/cm)13,14,15
.
From a fundamental point of view, two types of con-
ductivity mechanisms have been recognized to decouple
charge transport from segmental dynamics in polyILs. It
was hypothesized that the decoupling can be caused by
a frustration in chain packing (free volume) that enables
ion diffusion even when the segmental dynamics are
frozen. In such case, the positive or negative species can
freely move through the holes in the matrix formed by
counterions covalently bonded to the polymer back-
bone. The decoupling phenomenon is also known for
polymer electrolyte membranes for fuel cells obtained
by polymerization of protic ionic liquids (PILs) and in
low-molecular PILs themselves.16 In these cases the
difference in time scale of conductivity relaxation and
segmental dynamics is usually attributed to the proton
transfer mechanism through the H-bonded network and
thereby it does not require diffusion of entire molecular
units (vehicles).17,18,19 According to the literature reports
two mechanisms of proton transport can be distin-
guished. Namely, so-called Grotthuss picture, originally
proposed to explain high conductivity of water, where
the protonic defect is moved from one electrode to an-
other while a given proton is transferred only between
two neighboring molecules; and the other case where
the same proton is sequentially transferred within hy-
drogen bonds from one “vehicle” to the other. Im-
portantly, among the computational studies of phos-
phoric acid and imidazolium-based compounds20 no
clear experimental verification of the Grotthuss mecha-
nism has been provided so far. Additionally, in many
protic ionic systems an efficient proton transport does
not occur although it was expected on the basis of
chemical composition21.
Since in disordered ionic polymers there are many
potential charge carriers, it becomes a challenge to
clearly define the dominating mechanism.22 We empha-
size that the usual conductivity measurements do not
provide any microscopic details of the conductivity
mechanism, being critical for understanding their be-
havior in fuel cell and battery applications.
In this paper we investigate the conducting proper-
ties of seven polymerized ionic liquids that can be classi-
fied in three categories: aprotic polycations, protic poly-
anions and polymer blends (see Table 1 for chemical
structures). To reveal the charge transport mechanism
in these materials we employ high pressure conductivity
measurements. We show that isothermal compression
offers a unique possibility to recognize the dominating
charge transport mechanisms as well as to control the
bis(trifluoromethylsulfonyl)imide) (poly-EtVIm
TFSI)
ii) poly(N-vinyl diethylene glycol ethyl methyl ether
imidazolium) bis(trifluoromethylsulfonyl)imide
(poly-EGIm TFSI),
iii) poly(1-butyl-3-vinylimidazolium
bis(trifluoromethylsulfonyl)imide) (poly-BuVIm
TFSI),
iv) polymethylhydrosiloxane-graft-5-imidazolium-1-
pentene bis (trifluoromethane) sulfonamide (PDMS
TFSI).
The synthesis procedures of poly-EtVIm TFSI, poly-
BuVIm TFSI and poly-EGIm TFSI are provided in refs.
23,24
[
]
Table. 1 Chemical structures of investigated polyILs.
Chemicals: 1-methylimidazole (Acros Organics, 99%),
1-vinylimidazole
(Sigma-Aldrich,
≥99%),
1,4-
butanesultone (Acros Organics, 99+%), acetonitrile
(Sigma-Aldrich, ACS reagent, ≥99%) and 2-sulfoethyl
methacrylate (Polysciences, > 90%), were obtained from
the commercial suppliers and used without any pre-
treatment.
General procedure for the synthesis of the 1-alkyl-3-
butylsulfonate imidazolium zwitterions
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Chemistry of Materials
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acrylate as a viscous ionic liquid (3.91 g, 99% yield). H
NMR (400 MHz, D2O) δ 8.59 (s, 1H, H-2), 7.35 (d, J = 1.5
Hz, 1H, H-4*), 7.29 (d, J = 1.2 Hz, 1H, H-5*), 6.01 (d, J =
0.9 Hz, 1H, CH3C(CHH)CO2CH2CH2SO3), 5.57 (d, J = 1.4
Hz, 1H, CH3C(CHH)CO2CH2CH2SO3), 4.37 (t, J = 6.1 Hz,
2H, CH3C(CH2)CO2CH2CH2SO3), 4.10 (t, J = 7.0 Hz, 2H,
NCH2), 3.74 (s, 3H, NCH3), 3.16 (t, J = 6.1 Hz, 2H,
1
2
3
4
5
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7
8
9
Synthesis of 1-butylsulfonate-3-methylimidazolium zwit-
terion: 1-methylimidazole (1.00 mL, 12.5 mmol, 1.00 eq.)
was dissolved in CH3CN (8 mL) and cooled down to 0 ºC
with an ice bath. 1,4-butanesultone (1.28 mL, 12.5 mmol,
1.00 eq.) was added dropwise and the mixture was al-
lowed to stir at 0 ºC for 10 minutes. Then, it was stirred
at 85 ºC under reflux for 3 days. It was then cooled down
to -20 ºC and the resulting precipitate was filtered and
dried under high vacuum. 1-butylsulfonate-3-
methylimidazolium was obtained as a white solid (2.29
g, 84% yield). 1H NMR (400 MHz, D2O) δ 8.77 (s, 1-H, H-
2), 7.53 (s, 1H, H-5*), 7.47 (s, 1H, H-4*), 4.28 (t, J = 6.9
Hz, 2H, NCH2), 3.92 (s, 3H, NCH3), 2.97 (t, J = 7.6 Hz,
2H, N(CH2)3CH2SO3), 2.16-1.97 (m, 2H, N(CH2)2CH2), 1.83
– 1.72 (m, 2H, NCH2CH2); 13C NMR (101 MHz, D2O) δ
136.0 (C-2), 123.7 (C-5*), 122.2 (C-4*), 50.0
(N(CH2)3CH2SO3), 48.9 (NCH2), 35.7 (NCH3), 28.1
(NCH2CH2), 20.9 (N(CH2)2CH2).25
CH3C(CH2)CO2CH2CH2SO3), 2.85
N(CH2)3CH2SO3H), 1.94
–
1.82
=
2.75 (m, 2H,
(m, 2H,
0.9 Hz, 3H,
1.54 (m, 2H,
–
N(CH2)2CH2CH2SO3H), 1.77 (d,
CH3C(CH2)CO2CH2CH2SO3), 1.65
J
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–
NCH2CH2CH2CH2SO3H); 13C NMR (101 MHz, D2O) δ
169.3 (CH3C(CH2)CO2CH2CH2SO3), 135.9 (C-2), 135.6
(CH3C(CH2)CO2CH2CH2SO3),
(CH3C(CH2)CO2CH2CH2SO3), 123.6 (C-5*), 122.1 (C-4*),
60.0 (CH3C(CH2)CO2CH2CH2SO3), 50.0
127.0
(N(CH2)3CH2SO3), 49.5 (CH3C(CH2)CO2CH2CH2SO3),
48.8 (NCH2), 35.6 (NCH3), 28.0 (NCH2CH2), 20.8
(N(CH2)2CH2), 17.2 (CH3C(CH2)CO2CH2CH2SO3); ESI-MS
m/z (%) 219.07986 ([C8H15N2SO3]+, calcd. for C8H15N2SO3
= 219.08001, 17), 437.15242 ([(C8H15N2SO3)(C8H14N2SO3)]+,
cald. for C16H29N4O6S2 = 437.15230, 100), 438.15587
([(C8H15N2SO3)(C8H14N2SO3)
+
1]+, 17) 655.22503
33), 656.22876
([(C8H15N2SO3)(C8H14N2SO3)2]+,
Synthesis of 1-butylsulfonate-3-vinylimidazolium zwitter-
ion:1-butylsulfonate-3-vinylimidazolium was prepared
employing the same general synthetic method described
([(C8H15N2SO3)(C8H14N2SO3)2 + 1]+, 8).
Synthesis of 1-butylsulfonate-3-vinylimidazolium 2-
sulfoethylmetacrylate: 2-sulfoethyl methacrylate (2.45
gr, 12.6 mmol, 1.00 eq.) was dissolved in CH3CN and
added over 1-butylsulfonate-3-vinylimidazolium (3.22 gr,
14 mmol, 1.10 eq.). The mixture was stirred at room tem-
perature overnight. Then, it was filtered. The organic
phase was collected and the solvent was evaporated
under reduced pressure at room temperature, in order
to avoid product decomposition. The resulting IL was
dried under high vacuum to obtain 1-butylsulfonic-3-
methylimidazolium 2-sulfoethyl methacrylate as a vis-
cous ionic liquid (4.85 g, 91% yield). 1H NMR (400 MHz,
D2O) δ 8.96 (s, 1H, H-2), 7.67 (s, 1H, H-5*), 7.49 (s, 1H,
H-4*), 7.03 (dd, J = 15.6, 8.7 Hz, 1H, , NCH=CH2), 6.04 (s,
1H, CH3C(CHH)CO2CH2CH2SO3), 5.69 (d, J = 15.6 Hz, 1H,
NCH=CHH), 5.61 (s, 1H, CH3C(CHH)CO2CH2CH2SO3),
5.31 (d, J = 8.7 Hz, 1H, NCH=CHH), 4.40 (t, J = 5.9 Hz,
2H, CH3C(CH2)CO2CH2CH2SO3), 4.18 (t, J = 7.0 Hz, 2H,
for
the
synthesis
of
1-butylsulfonate-3-
methylimidazolium, employing 1-vinylimidazole instead
of 1-methylimidazole, and obtained as a white solid (1.61
g, 63% yield). 1H NMR (400 MHz, D2O) δ 9.05 (s, 1H, H-
2), 7.74 (d, J = 1.7 Hz, 1H, H-5*), 7.57 (d, J = 1.6 Hz, 1H, H-
4*), 7.10 (dd, J = 15.6, 8.7 Hz, 1H, NCH=CH2), 5.76 (dd, J =
15.6, 2.6 Hz, 1H, NCH=CHH), 5.38 (dd, J = 8.7, 2.6 Hz,
1H, NCH=CHH), 4.26 (t, J = 7.0 Hz, 2H, NCH2), 2.92 (t, J
= 7.6 Hz, 2H, N(CH2)3CH2SO3), 2.16 – 1.87 (m, 2H,
N(CH2)2CH2), 1.90 – 1.55 (m, 2H, NCH2CH2); 13C NMR
(101 MHz, D2O) δ 134.5 (C-2), 128.2 (C-5*), 122.7 (C-4*),
119.5
(NCH=CH2),
109.2
(NCH=CH2),
49.9
(N(CH2)3CH2SO3), 49.2 (NCH2), 27.9 (NCH2CH2), 20.8
(N(CH2)2CH2).26
General procedure for the synthesis of the sul-
foethylmethacrylate ionic liquids
NCH2),
3.19
(t,
J
=
5.9
Hz,
2H,
CH3C(CH2)CO2CH2CH2SO3), 2.84 (t, J = 7.5 Hz, 2H,
N(CH2)3CH2SO3H),
N(CH2)2CH2CH2SO3H),
2.01
–
1.81
1.89
–
(m,
2H,
3H,
(s,
CH3C(CH2)CO2CH2CH2SO3), 1.72
1.59 (m, 2H,
NCH2CH2CH2CH2SO3H); 13C NMR (101 MHz, D2O) δ
169.3 (CH3C(CH2)CO2CH2CH2SO3), 135.6 (C-2), 134.4
Synthesis of 1-butylsulfonate-3-methylimidazolium 2-
sulfoethylmetacrylate: 2-sulfoethyl methacrylate (1.84 g,
9.5 mmol, 1.00 eq.) was dissolved in CH3CN and added
over 1-butylsulfonate-3-methylimidazolium (2.29 g, 10.5
mmol, 1.10 eq.). The mixture was stirred at r.t. overnight
and then filtered to remove the unreacted excess of the
zwitterion. The organic phase was collected and the
solvent was evaporated under reduced pressure at room
temperature, in order to avoid product decomposition.
The resulting IL was dried under high vacuum to obtain
1-butylsulfonic-3-methylimidazolium 2-sulfoethyl meth-
(CH3C(CH2)CO2CH2CH2SO3),
(CH3C(CH2)CO2CH2CH2SO3),
128.2
122.8
(NCH=CH2),
(C-5*),
(C-4*),
127.0
119.5
60.0
(NCH=CH2),
109.3
(CH3C(CH2)CO2CH2CH2SO3), 50.0 (N(CH2)3CH2SO3),
49.5 (CH3C(CH2)CO2CH2CH2SO3), 49.2 (NCH2), 27.9
(NCH2CH2),
20.8
(N(CH2)2CH2),
17.2
(CH3C(CH2)CO2CH2CH2SO3); ESI-MS m/z (%) 231.08
([C9H15N2SO3]+, 8), 461.15 ([(C9H15N2SO3)(C9H14N2SO3)]+,
100), 462.15 ([(C9H15N2SO3)(C9H14N2SO3) + 1]+, 17), 463.15
([(C9H15N2SO3)(C9H14N2SO3)
+
2]+,
8),
691.22
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([(C9H15N2SO3)(C9H14N2SO3)2]+,
19),
692.22
ed on an apex-Qe spectrometer. All spectral data were
supplied by the Centre of Research Support (CACTI) of
([(C9H15N2SO3)(C9H14N2SO3)2 + 1]+, 5).
1
2
the University of Vigo.
3
4
Procedure for the synthesis of 1-butylsulfonate-3-
methylimidazolium vinylphosphonate
Dielectric measurements. Isobaric dielectric meas-
urements at ambient pressure from 10−1 to 106 Hz were
carried out using a Novocontrol GMBH Alpha dielectric
spectrometer. For the isobaric measurements, the sam-
ple was placed between two stainless steel electrodes of
the capacitor with a gap of 0.1 mm. The dielectric spec-
tra of polyILs were collected over a wide temperature
range. The temperature was controlled by the Novocon-
trol Quattro system, with the use of a nitrogen gas cryo-
stat. Temperature stability of the samples was 0.1 K. For
the pressure dependent dielectric measurements, the
capacitor filled with the studied samples was placed in
the high-pressure chamber and compressed using the
silicone oil. Note that during the measurement the sam-
ples were in contact with stainless steel and Teflon.
Pressure was measured by the Nova Swiss tensometric
pressure meter with a resolution of 0.1 MPa. The tem-
perature was controlled within 0.1 K by means of a liq-
uid flow provided by a Weiss fridge and Julabo thermal
bath. The general scheme of the apparatus used to in-
vestigate the properties of polyILs under high pressure
conditions is presented in ref. [29]. The dielectric exper-
imental data for poly-BuVIm TFSI were taken from ref.
[23].
5
6
7
8
9
Vinylphosphonic acid (10 mL, 122.6 mmol, 1.00 eq.) was
added drop-wise, under inert atmosphere, over a solu-
tion of 1-butylsulfonate-3-methylimidazolium (26.7 g,
122.6 mmol, 1.00 eq.) in deionized water (50 mL). The
mixture was stirred at 90 ºC overnight. Then, solvent
was evaporated under reduced pressure and he resulting
IL was washed with diethyl ether. Residual solvent was
eliminated under reduced pressure and the IL was dried
by heating at 50 ºC under high vacuum for 12 h. 1-
butylsulfonate-3-methylimidazolium vinylphosphonate
was obtained as a colorless viscous liquid (48.0 g, 99%).
1H NMR (400 MHz, D2O) δ 8.66 (s, 1H, H-2), 7.41 (m,
1H, H-5*), 7.35 (m, 1H, H-4*), 6.26 - 5.88 (m, 3H,
CH2=CHPO3H), 4.16 (t, 2H, J = 7.1 Hz, NCH2), 3.80 (s,
3H, NCH3), 2.86 (m, 2H, N(CH2)3CH2), 1.94 (m, 2H,
NCH2CH2), 1.66 (m, 2H, N(CH2)2CH2); 13C NMR (101
MHz, D2O) δ 135.8 (C-2), 133.5 (CH2=CHPO3H), 127.4 (d,
J(C,P) = 176.8 Hz, CH2=CHPO3H), 123.5 (C-5), 122.0 (C-4),
50.0 (N(CH2)3CH2), 48.8 (NCH2), 35.6 (NCH3), 28.0
(NCH2CH2), 20.8 (N(CH2)2CH2); 31P NMR (162 MHz,
D2O) δ 13.87; ESI-MS m/z (%) 219.07925 ([C8H15N2SO3]+,
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Calorimetric measurements. Calorimetric measure-
ments of the studied materials were carried out by using
a Mettler-Toledo DSC apparatus equipped with a liquid
nitrogen cooling accessory and a HSS8 ceramic sensor (a
heat flux sensor with 120 thermocouples). Temperature
and enthalpy calibrations were performed using indium
and zinc standards. Using a stochastic temperature-
modulated differential scanning calorimetry (TMDSC)
technique the dynamic behavior of the glass−liquid
transition of the studied materials was analyzed in the
frequency range from 4 mHz to 40 mHz in a single
measurement at a heating rate of 0.5 K/min. In these
experiments, the temperature amplitude of the pulses of
0.5 K was selected.
calcd. for C8H15N2SO3
=
219.07979, 100), 220
([(C8H15N2O3S) + 1]+, 8), 545 ([(C8H15N2SO3)2(C2H4O3P)]+,
21).
General procedure for the polymerization of protic
ionic liquids. Protic ionic liquids were polymerized
with UV light. For this purpose, ionic liquid monomers
were mixed with 5 wt% of photoinitiator 2-hydroxy-2-
methyl propiophenone and 5 wt% of crosslinker glycerol
dimethacrylate to improve the mechanical stability. The
mixture was extended over a flat and flexible surface
and exposed to 354 nm UV light (KAIS) for 30 min. After
polymerization, the polymer was removed from the
surface using a razor blade.
The procedure of water content evaluation in ILs:
The protic polyIL samples were prepared in an open
aluminum crucible (40 μL) outside the DSC apparatus.
Next, several calorimetric measurements in the temper-
ature range of 223 – 523 K at the heating rate of 10 K/min
under dry nitrogen purge (60 mL/min) were performed.
Each measurement was finished with a 5-minute long
isothermal scan of the sample at a temperature of 523 K
to remove water residue. The examined material was
weighed before each measurement, and the content of
water remaining in the sample was determined. The
above procedure was repeated until the polymer mass
and Tg value had become stable. The determined water
content in the examined polymer membranes is pre-
sented in the Table 1. The obtained results are in good
agreement with the water content estimated from TGA
analysis.
Determination of pKa of protic ionic monomers. For
pKa determinations in triplicate, a stock solution (0.01
mol/L) of the complex was prepared in degassed water.
The solution was then titrated with aqueous KOH solu-
tion (0.100 mol/L). The pH of the solution was measured
using a calibrated glass electrode on a Metrohm 780 pH
meter at 295.0 K. The pKa for each compound was calcu-
lated by the procedure described by Albert and Ser-
jeant.27
1
13
NMR characterization. The H, C NMR spectra of the
purified products were recorded in D2O on a Bruker
ARX at 400.1621 (1H) and 100.6314 (13C) MHz. Chemical
shifts are given in ppm and coupling constants (J) are
1
given in hertz (Hz). Data for H NMR are reported rela-
tive to D2O (δ 4.79 ppm). ESI mass spectra were record-
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Chemistry of Materials
1
NMR relaxometry studies. The H spin-lattice relaxa-
tion data were collected by a STELAR Fast Field Cycling
(FFC) relaxometer in the frequency range of 10kHz –
25MHz, at 283K, 288k, 294K and 303K. The data are
shown in Supplementary Information (Fig.S1).
the parameters affecting conductivity. In the poly-
SBVIm poly-SEM system, where both cations and anions
are covalently bonded to two different vinyl chains, the
structural reorganization is strongly suppressed. This
together with the fact that there are no others freely
mobile ions besides OH- and H3O+ has resulted in the
development of novel material with pure proton con-
ductivity. This is in contrast to polyanions mentioned
above where the SBMIm cation also contribute to dc-
conductivity and thus it is difficult to unambiguously
recognize the dominating charge transport mechanism.
To analyze the ability of the studied materials to at-
tach/detach proton we have performed DFT calculations
of the SBMIm cation, SEM, and vinyl-HPO3 anions. The
SEM demonstrates the strongest acidity with pKa=-2.16.
Slightly weaker acid properties is present in SBMIm with
pKa=-1.42, while vinylHPO3 is a very weak acid with first
pKa1 equal to 9.02. It should be noted that the strong
interactions e.g. hydrogen bonds between polar groups
in polyanions can change the pKa value. Therefore, in-
fluence of hydrogen bonds on pKa of dimers has been
also studied. In case of SBMIm, an additional confor-
mation with the SO3H group in the close vicinity of the
imidazole ring has been also evaluated (see Table 1 in
SI). We have found that acidity of vinyl-HPO3 dimers is
suppressed i.e. the pKa value is increasing from 9.02 to
13.94. The increase in the acidity has been found in SEM
dimer (pKa decreases from -2.16 to -4.76) as well as in
SBMIm compound. For the conformation C2 of SBMIm
pKa has been changed from -1.42 to -4.56. Very low val-
ues of pKa found for both SEM and SBMIm ions suggest
that the proton can be easily dissociated. Furthermore,
terminal HSO3 groups in cations exposed to water can
also generate protons. Consequently, SEM-based poly-
mer membranes are mixture of imidazolium-based zwit-
terions (being mobile in SBMIm poly-SEM and linked to
1
2
3
4
5
6
7
8
DFT calculations. DFT calculations were used to de-
termine the pKa of acids and based used to prepare pro-
tic ionic samples. The calculations were performed in
water environment, therefore the pKa values are related
to the situation when the proton is solvated by water.
The G(A-) and G(AH) energies were calculated using the
Orca 4 program.28 The geometry optimizations of all
acids and related bases were done on the B3LYP/6-
311G(2d,2p) level of theory. For final energy evaluation,
PBE0 gradient functional and 6-311G(2d,2p) basis set was
used. The solvent model CPCM was used during optimi-
zation and energy evaluation run. Vibrational, thermal
and entropic corrections were evaluated on the
B3LYP/6-311G(2d,2p) level of theory. The energy of a
proton was calculated analytically based on literature
values. Free energy of proton used in the calculations
was equal to -270.29 kcal/mol.
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RESULTS
Samples characterization. The materials examined
herein were designed as model systems in order to un-
derstand the molecular mechanism of charge transport
in polymerized ionic liquids We can classify them into
three categories: protic polyanions, aprotic polycations
and polymer blends, composed of cations and anions
covalently attached to the polymer chains.
The first group is represented by imidazolium-based
polycations with mobile TFSI anion and the covalently
attached to the polymer chain anions with limited mo-
bility. Such chemical design favors a single ion conduc-
tivity in these polyILs. The second group of polyILs
studied herein includes two novel polyanions with the
chemical moieties promoting proton transport. Specifi-
cally, the butyl-imidazolium cation terminated by the
sulfonate group (SBIm) was chosen to create two
Brønsted pairs with sulfoethylmetacrylate (SEM) and
vinylphosphonic poly-acids, respectively. There are at
least two reasons to justify this selection. Firstly, the
chemical structure of the sulfobuthylimidazolium cation
has a potential to favor proton hopping in ionic systems.
It is well-known that the terminal acidic groups SO3H
located in the side chains of polymer electrolyte mem-
branes such as Nafion exposed to water dissociate into
immobile SO3− anions and free protons that move
through the hydrogen-bonded network and thereby are
responsible for the high conductivity of the system.
Likewise, of great importance is vinylphosphonic acid,
due to the highly efficient Grotthuss mechanism con-
firmed in phosphoric acid. Furthermore, sulfoethyl-
methacrylate anion was chosen to duplicate the amount
of proton acceptor groups in SBMIm SEM system which
could result in enhancement in conductivity. Another
great advantage of SEM is the possibility of formation of
a structure of polymer blend with complex architecture
that advantageous for fundamental understanding of
-
the alkyl chain in polymer blend), poly-SO3 anions and
H3O+. On the other hand, high pKa value of HPO3 sug-
gests that the protons are likely transferred from SBMIm
to polyanion and are not moving along the polymer
chains. As a result, the water saturated SBMIm poly-
HPO3 membrane is composed mostly of zwitterions,
neutral poly-H2PO3 structure and water molecules that
strongly participate in proton transport.
To characterize materials we employed TGA and
calorimetric measurements. The DSC scans obtained
during heating of all polymerized IL are presented in
Figure 1. As can be seen, the DSC curves of polyILs re-
veals heat flow jump which is a characteristic of the
liquid-glass transition. The exact values of Tg are listed
in Table 2. Since the examined protic polyILs are water
saturated, in the thermograms one can also observe
broad endotherms related to the water evaporation.
However, it can be easily discerned that in SBMIm pol-
yHPO3 this effect is less pronounced, comparing to two
other protic polyILs with lower water content. The wa-
ter fraction in the examined protic polyILs was deter-
mined by the weight difference between not-dried and
dried sample (see the Experimental Section for details).
We have found that the sulfoethylmetacrylate-based
polyILs contain 13.8 wt % of water, while the material
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Chemistry of Materials
Page 6 of 12
with the poly-HPO3 group is characterized by a water field with frequency interval of milli- to megahertz and
1
2
3
4
5
6
7
8
content equal to 11 wt %. From Figure 1, the glass transi-
tion for anhydrous membranes is 60-70K higher com-
paring to hydrated samples.
a wide temperature range, covering the supercooled
liquid regime as well as glassy state, enables a thorough
analysis of relaxation dynamics of any macromolecular
system. For most conventional polymers, valuable in-
sight into segmental relaxation and local intramolecular
motions can be obtained from dielectric loss ε”(f) spec-
tra. However, if the molecular dynamics are strongly
dominated by translational displacement of charge car-
rying species or proton transport, as observed for poly-
ILs, dc conductivity gives rise to a sharp increase in the
dielectric loss function and completely masks the relaxa-
tion of polymer segments. Therefore, information ob-
tained from dielectric studies of polyILs is limited to
conductivity behavior.
The most convenient approach to define conductivi-
ty contribution to dielectric response, and thereby to
characterize the translational motions of charge carriers
in the studied material is related to analysis of complex
electric modulus, M*(f)=M′(f)+iM″(f)=1/ε*(f) and electric
conductivity σ*(f)=σ’(f)+σ”(f) formalisms.29,30 The dielec-
tric data of SBMIm polySEM are presented in Fig.2.
In contrast to protic polyILs, special care was taken
to remove any water from TFSI conductors. Before the
measurements the samples were extensively dried in a
vacuum oven at temperature above 393K for 3-4 days.
Additionally, in the DSC system polyILs were further
pre-condition at ~393K for 30-60 min in nitrogen at-
mosphere to remove surface water. Several heating-
cooling scans were performed to ensure that both Tg of a
sample and its mass do not change. As presented in the
Figure 1, the obtained DSC scans of aprotic ionic liquids
studied herein reveal only liquid-glass transition with-
out any sign of the presence of water.
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34
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45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Hydrated
↑endo
Protic Ionic Polymers
Tg
water
evaporation
poly-SBVIm poly-SEM
SBMIm poly-HPO3
SBMIm poly-SEM
-4
SBMIm poly-SEM
heating (ꢀT=9K)
A.
B. 10
dc-conductivity
T>Tg
T>Tg
298K
10-7
10-10
10-13
10-1
250K
Anhydrous
Protic Ionic Polymers
250K
223K
conductivity
relaxation
PDMS TFSI
Anhydrous
Aprotic Ionic Polymers
poly-EGIm TFSI
poly-EtVIm TFSI
poly-BuVIm TFSI
223K
178K
10-2
β
-process
T<Tg
178K
γ
-process
T<Tg
100
SBMIm poly-SEM
101 103
102
104
106
10-1
105
frequency /Hz
frequency /Hz
200
250
300
350
400
Fig.2 Dielectric loss modulus (A) and conductivity (B)
spectra of SBMIm polySEM recorded over a wide tem-
perature range. Spectrum recorded at Tg is denoted by
circles.
temperature /K
Figure 1 DSC curves of polyILs. The green arrows indicate
the liquid-glass transition of given system.
Table. 2 Characteristics of polyILs used in this study.
σdc at
300K
S/cm
σdc
at Tg
S/cm
As can be seen, the imaginary part of modulus function
M″(f) has the form of a well-resolved asymmetric peak,
the so-called conductivity relaxation peak ascribed to
the translational ionic motions. On the other hand, two
clearly visible regions characterize the real part of the
complex conductivity spectra: the power law behavior
observed at higher frequencies and frequency-
independent plateau that corresponds to the dc-
conductivity σdc. Note that σdc, directly related to the
number of free ions and their mobility, is inversely pro-
portional to the conductivity relaxation times τσ esti-
mated from the frequency of M″ peak maximum
τσ=ε0εs/σdc= 1/2πfmax, where εs is static permittivity.29,30
Typically, the position of the modulus peak maxi-
mum (as well as dc-conductivity) is strongly dependent
on thermodynamic conditions i.e. σ-mode moves to-
ward lower frequencies with cooling/squeezing and
approaches f≈0.15mHz (τσ≈1000s) at the liquid-glass
transition, if only the ion transport is fully controlled by
viscosity - like in the case of classical low-molecular ILs.
However, as can be clearly seen in Fig.2 M″(f) of SBMIm
poly-SEM collected at Tg appears at frequencies marked-
ly higher than fmax of ideally coupled systems.
Tdec
/K
Tg
/K
polyILs
R(Tg)
Poly-
SBVIm
poly-SEM
529
229*
224*
224*
344
9.5·10-6
2.5·10-5
5.5·10-6
2.9·10-11
9·10-11
4.7·10-12
9·10-13
5
SBMIm
poly-SEM
3.8
3
533
-
SBMIm
poly-HPO3
Poly-EtVIm
TFSI
520
1.7·10-9
6.5
Poly-
BuVIm
TFSI
-
335
259
233
9.1·10-11
3.9·10-7
4.6·10-5
1.7·10-9
1.5·10-11
7.4·10-14
6.5
4
Poly-EGIm
TFSI
551
417
2
PDMS TFSI
Tg, glass transition temperature (from DSC, heating rate 10K/min); *Tg of
hydrated sample; Tdec decomposition temperature obtained from TGA
measurements; σdc dc-conductivity; R(Tg) decoupling index calculated at Tg
Dielectric data at ambient pressure. In general, the
dielectric experiments performed in the external electric
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Chemistry of Materials
Additionally, from Figs. 3A it appears that the M”(f)
between τσ and τα. The values of σdc measured at Tg of all
polyILs investigated in this paper are listed in Table 2.
1
2
3
4
5
6
7
8
spectra of all studied herein polyILs recorded at their
calorimetric Tg cover the frequency range of about five
decades, and importantly none of them is close to the
frequency expected for a coupled systems (solid line). At
the same time, in each examined case the σdc(Tg) is
higher than 10-15 Scm-1 – the value typical for ionic con-
ductors with the charge transport strongly coupled to
structural relaxation (see Fig. 3B). These are the first
signs that generally the rate of charge diffusion is differ-
ent than the motions of polymer segment, and specifi-
cally the efficiency of this process is changing among
examined samples. Nevertheless, to characterize the ion
dynamics in more details the dielectric data of all poly-
mers tested herein both in the supercooled liquid re-
gime as well as in the glassy state need to be analyzed.
B. 4
A. 4
segmental dynamics at Tg
τα=1000s
τσ=1000s
τα(DSC)
τα(DSC)
2
2
Arrhenius-type
behavior
Rτ=2
Rτ=3
Rτ=4
0
-2
-4
-6
-8
0
-2
-4
-6
-8
Rτ=3.8
Arrhenius-type
Rτ=5
behavior
Aprotic
polyIL
Rτ=6.5
Arrhenius behavior
(glassy state)
ion dynamics
decoupling
index
9
Tg
Tg
Isobaric dielectric
measurements
at 0.1 MPa
VFT-type
ProticPolyIL
10
11
12
13
14
15
16
17
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55
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57
58
59
60
Tg
supercooled
liquid state
VFT-type
2.8
0.1 MPa
4.8 5.2
0.8
1.0
1.2
3.2
3.6
4.0
4.4
1000/T K-1
Tg/T
Figure 4 The conductivity relaxation times determined
from M″(f) spectra, presented vs. 1000/T for polyBuVIm
TFSI and SBMIm poly-SEM(A), and vs Tg/T for all studied
polyILs (B). Open stars are τα data determined from TM
DSC technique. Symbols presents the same samples as in
the Fig. 3A.
Aprotic Ionic Polymers
PDMS TFSI
Modulus data at Tg
conductivity relaxations
poly-EGIm TFSI
poly-BuVIm TFSI
Conductivity data at Tg
A.
τσ=1000s
B.
poly-EtVIm FTSI
Protic Ionic Polymers
SBMIm poly-HPO3
SBMIm poly-SEM
10-7
dc-conductivity (σdc
)
100
poly-SBVIm poly-SEM
10-10
secondary
relaxations
conductivity
of fully
coupled system
10-13
229K
fmax=1/2πτσ
10-1
σ
dc=10-15 Scm-1
secondary relaxations
10-16
10-5 10-3 10-1 101
10-3 10-1
101
103
105
107
103 105 107
frequency /Hz
frequency /Hz
Figure 3 Dielectric loss modulus (A) and conductivity (B)
spectra of seven polyILs measured at their calorimetric Tg.
The dotted lines in panel B are fit of the conductivity data
to the Dyre model [31]. The solid lines show spectra ex-
pected for systems with ion dynamics strongly coupled to
segmental relaxation.
Figure 5 The temperature dependence of dc-conductivity
for protic and aprotic polyILs determined by means of BDS
technique.
The temperature dependences of conductivity relaxa-
tion time of all seven polyILs exhibit qualitatively simi-
lar behavior (Figs. 4A, 4B), i.e. a well-defined crossover
from the Vogel-Fulcher-Tamman (VFT) to Arrhenius
like behavior at the glass transition temperature Tg,
being a typical behavior observed in ionic systems.6,32,33
More importantly, τσ(Tg) differs in studied polyILs, but
in all cases it is faster than 102-103 s usually identified
with the freezing point of the structural/segmental re-
laxation at Tg. The direct comparison between the re-
sults of temperature modulated DSC experiments (see
stars in Fig. 4A) - representing the time scale of segmen-
tal dynamics, and conductivity relaxation data clearly
indicate that the ion diffusion is significantly faster than
the rate of segmental motions at Tg in studied polyILs.
According to many studies, a simple way to quantify the
decoupling of charge diffusion from structural relaxa-
tion in the vicinity of the glass transition temperature is
to calculate the decoupling index defined as Rτ(Tg)=3-
logτσ(Tg).14,15,34
Among the studied polyILs, PDMS TFSI shows the
highest value of dc-conductivity at room temperature
conditions (Fig. 5). The σdc around 5 × 10−5 S cm−1 ob-
tained at 303K most likely results from the combination
of the low Tg of the siloxane backbone and highly delo-
calized, non-coordinating and asymmetric TFSI anion.
Interestingly, the room temperature conductivity of
studied protic polyILs is also high (see Table 2), even in
the case of polymer blend where only fast proton trans-
fer is expected to give a contribution to charge
transport.
Dielectric data at elevated pressure. According to the
standard experimental protocol, all polyILs were com-
pressed isothermally at several different temperatures
above the calorimetric Tg. Although both modulus M″(f)
and conductivity σ′(f) formalisms provide the same
information about the ion dynamics (Fig. 3 and Fig. 4),
the former enables a more accurate analysis close to the
liquid-glass transition and therefore it was chosen for
use in presenting the high pressure data for polyILs. The
representative high pressure results are depicted in Fig.
6. It is apparent that isothermal compression has basi-
cally the same effect on ion dynamics as isobaric cool-
ing. Namely, the conductivity relaxation times τσ of
supercooled polyILs are getting longer with squeezing
Importantly, the time scale separation between τσ
and τα is also reflected in the conductivity behavior. In
Fig. 5 σdc data taken directly from the frequency-
independent part of σ’(f), are plotted as a function of
inverse temperature. As can be seen σdc(Tg) is signifi-
cantly higher than expected for coupled systems ~10-14-
10-15 S/cm, and the difference is similar to that reported
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and markedly slow down when a material solidifies.
Low-molecular
protic ionic systems
3
2
A.
B.
1
2
3
4
5
6
7
8
Consequently, the experimental points exhibit two line-
ar regimes that intersect at the glass transition pressure,
Pg. Interestingly, the markedly lower pressure sensitivity
is evident for τσ of hydrated membranes. This agrees
with the general rule indicating that systems with well-
expanded H-bonded network are characterized by
weaker sensitivity to a compression.35 The further in-
spection of high pressure conductivity relaxation data
reveals some universal features of ion dynamics in test-
ed polyILs. Namely, it is evident that the value of con-
ductivity relaxation time at the inflection point, being a
manifestation of the glass transition, is continuously
changing with pressure. This fact, together with the
recent report demonstrating that regardless of T-P
thermodynamic conditions the characteristic crossover
of logτσ occurs at isochronal structural relaxation time
(τα ≈103s)36, indicates that compression strongly affects
the decoupling between τσ and τα in the studied polyILs.
PR-HCl
LD-HCl
C-HCl
ACB-HCl
LD-succinate
CLR-HCl
1
STR-succinate
-
C-H2PO4
0
Aprotic Ionic Polymers
PDMS TFSI
poly-EtVIm TFSI
poly-BuVIm TFSI
poly-EGIm TFSI
-1
-2
-3
-4
-5
Low-molecular Ionic
Protic Ionic Polymers
SBMIm poly-SEM
Liquids (vehicle transport)
lidocaine docusate
CKN
SBMIm poly-HPO3
poly-SBVIm poly-SEM
BuMIm TFSI
9
0
100 200 300 400 500
0
200 400 600 800 1000
pressure /MPa
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
pressure /MPa
Figure 7. Frames (A) and (B) present the pressure varia-
tions of τσ measured at Pg of protic and aprotic ionic poly-
mers, respectively (closed symbols). In addition, the results
of another 11 low-molecular ionic systems were taken from
literature (open symbols). Abbreviations are as follows:
prilocaine HCl (PR-HCl)37, lidocaine HCl (LD-HCl)38, car-
vedilol HCl (C-HCl)45, acebutolol HCl (ACB-HCl)36, lido-
caine succinate (LD-SUCC), chlorpromazine HCl (CLR-
HCl), sumatriptan succinate (STR SUCC)39, carvedilol di-
HIGH PRESSURE
B.
hydrogen phosphate (C-H2PO4 )45, lidocaine docusate,
-
3
A.
Protic Ionic Polymer
SBMIm poly-SEM
decoupling decreases under pressure
Ca(NO3)2-KNO3 (CKN)40,41, (butylmethyl imidazole TFSI)
BuMIm TFSI)42.
2
1
ISOTHERMS
255 K
Pg
244 K
233 K
Pg
0
0.1 MPa
-1
-2
-3
-4
-5
-6
ISOTHERMS
270 K
281 K
DISCUSSION
To provide the physical explanation for experimental
results one needs to consider the charge transport
mechanism in all of the studied polymerized ionic liq-
uids. From the chemical point of view, polyILs (except
for the polymer blend), can be treated as single-ion
conductors due to the only one type of freely moving
ions (TFSI anion or SBIm cation). As a consequence, the
origin of the decoupling phenomenon can be ascribed to
the different contribution of cations and anions to the
ionic conductivity and segmental relaxation. Namely,
the motions of charged polymeric segments are ex-
pected to govern segmental dynamics but at the same
time these are too slow to provide significant contribu-
tion to σdc. The latter parameter is dominated by the
diffusion of counterions, presumably through free vol-
ume associated with frustration in polymer chains pack-
ing. As a consequence of such an assumption any reduc-
tion of free volume, including compression-induced,
should result in slowing down of charge transport at the
same segmental relaxation time, and hence increase of
τσ at Pg. However, the experimental data collected in
Figs. 7 clearly demonstrate that only three of seven poly-
ILs follow this assumption ; these are BuVIm, EtVIm and
EGIm polycations with TFSI counterion dominating
charge transport. Interestingly, the PDMS-based polyIL
with the same TFSI counterion exhibits the smallest
decoupling, consistent with the expectation that the
most flexible chain (siloxane-based) should reveal better
packing and lower free volume. Moreover, in this par-
ticular case the decoupling appears to be independent of
T-P conditions (Fig.7B), again consistent with the idea
of free volume controlling the decoupling.
303 K
decoupling
increases
under pressure
charge transport
Aprotic Ionic Polymer
poly-EGIm TFSI
charge transport
0
100 200 300 400 500
0
150 300 450 600 750 900
pressure/MPa
pressure/MPa
Figure 6 Pressure dependence of ion dynamics. Frames (A)
and (B) present the pressure behavior of conductivity re-
laxation times measured at isothermal conditions for protic
and aprotic ionic polymer, respectively. The single experi-
mental point at logτσ ~-1 indicates the value of τσ(Tg) rec-
orded at 0.1 MPa.
As presented in Figs. 6 and summarized in Fig. 7, the
application of pressure as an external thermodynamic
variable makes the τσ(Pg) faster in protic ionic polymers,
and thus enhances the time scale separation between
charge transport and segmental dynamics in these sys-
tems. However, in the cases of TFSI-based materials
isothermal compression brings the crossover of logτσ(P)
to longer conductivity relaxation times, and thereby the
decoupling between τσ and τα becomes markedly re-
duced under pressure. Interestingly, at high enough
pressure the time scales of charge transport and seg-
mental dynamics became practically coupled, as it is
shown for poly-EGIm TFSI at ~1GPa. These results stand
in sharp contrast with the behavior of PDMS TFSI,
where the time scale of conductivity relaxation is almost
the same as for segmental dynamics. In this special case,
compression of the sample up to 200 MPa does not
affect τσ(Pg), and ion dynamics remains coupled to seg-
mental dynamics.
Most important, in contrast to aprotic polyILs, an in-
crease of pressure strongly accelerates τσ(Pg) in all water
saturated membranes. This result clearly demonstrates a
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lently attached to rigid vinyl chains, the structural reor-
significant difference in conductivity mechanisms be-
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tween aprotic polyILs and hydrated protic polyILs: It
reveals that translational diffusion of free ions is no
longer crucial and suggests that an effective proton
transfer is involved in charge transport of examined
water saturated polymer membranes. This is because
only the proton transfer speeds up when hydrogen bond
length decreases43 and the H-bonded network is
stressed,44 both taking place under high pressure condi-
tions. An additional argument emerges from the recent
studies of low molecular weight protic ionic liquids and
solids, showing that τσ(Pg) becomes faster under high
pressure only if the conductivity is governed by a fast
proton transfer mechanism.45 The comparison of high
pressure data for low-molecular and macromolecular
ionic systems is presented in Fig. 7.
Surprisingly, there is no correlation between the val-
ue of the decoupling index at ambient conditions and its
pressure dependence. For example, the HPO3-based
protic polyIL with relatively small time scale separation
between τσ and τα at ambient conditions exhibits a very
strong change in decoupling under elevated pressure
(Fig.7A). Furthermore, despite the fact that Rτ(Tg) is
smaller for poly(EGIm)-TFSI than in poly(EtVIm)-TFSI
and poly(BuVIm)-TFSI, it reduces in the similar way
with pressure (Fig.7B). Interestingly, the lack of correla-
tion between Rτ(Tg) and its pressure behavior can be
also found for low-molecular ionic systems.
To further advance our understanding of high pres-
sure behavior of decoupling phenomena we focus on
two variables that quantify the pressure sensitivity of
both charge transport and glass transition temperature.
These are pressure coefficient of decoupling index
dlogRτ/dP=dlogτ(Pg)/dP and pressure coefficient of glass
transition temperature dTg/dP, both calculated at 0.1
MPa. As illustrated in Fig. 8, the studied herein polyILs
(except PDMS TFSI) are characterized by relatively low
values of dTg/dP parameter (70-90 KGPa-1); markedly
lower than those found for low-molecular ionic conduc-
tors (see Fig. 7) and non-ionic polymers46. These results
reflect weak pressure sensitivity of segmental dynamics
that can be explained in terms of poor packing of vinyl
backbones under pressure as well as the expanded H-
bonded network in PEMFCs. Despite the fact that gen-
erally Tg of polyILs does not increase much with squeez-
ing, the substantial changes in charge transport are
observed for these systems under pressure (the absolute
value of dRτ/dP is between 2 and 4 per GPa). This cre-
ates a possibility to obtain polymerized ionic liquids
with super protonic properties directly by isothermal
compression. This is in contrast to low-molecular
weight protic ILs where an efficient proton mobility
under pressure can be achieved only for systems with
large value of dTg/dP coefficient.
ganization is strongly suppressed in this system. As a
consequence, the H-bonds are not prone to breaking
and can be treated as highways for fast proton transport.
The idea of efficient proton migration in a polymer
blend is also supported by experimentally determined
ΔpKa values. Among the tested protic polyILs, SBVIm
SEM structure reveals the smallest ΔpKa=1.17 and there-
by the lowest free energy of proton dissociation. On the
other hand, the largest value of ΔG (corresponding to
the highest ΔpKa=2.11) characterizes proton transport of
HPO3-based compound. Poor proton dissociation in this
system is consistent with the dielectric data revealing
the smallest decoupling index for SBMIm poly-HPO3 at
ambient pressure.
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4
VEHICLE
CONDUCTION
2
0
-2
-4
Protic Ionic
Polymers
-6
PROTON TRANSPORT
-8
60 80 100 120 140 160 180
dTg/dPP=0.1MPa /KGPa-1
Figure 8. The pressure coefficient of decoupling index as a
function of dTg/dP (for procedure of dTg/dP calculation see
ref.[33]). The dashed line presents fully coupled systems
and divides the examined materials into two categories of
dominating conductivity mechanism: proton transfer
(dRτ/dP<0), and free volume controlled (dRτ/dP≥0).
This clearly demonstrate that dielectric studies, es-
pecially under conditions of high compression, are ideal
experimental tool for empirical verification of dominat-
ing conductivity mechanism in ionic systems in general.
However, at the same time it should be noted that from
dielectric measurements it is difficult to separate contri-
bution of proton motions and simple vehicle transport
to overall dc-conductivity. Nevertheless, knowing the
temperature behavior of conductivity relaxation times τσ
and the “jump” length λ of ions one can employ the
Random Barrier Model (RBM) D=λ2/6τσ to calculate
diffusivity (D) that reflects the translational motions of
all conducting species in studied material. Note that
RBM provides a good agreement between D parameter
estimated by BDS and measured by NMR for various
aprotic IL and polymerized ionic liquids47. Therefore, to
provide more insight into charge transport in studied
protic systems we have used SBMIm HPO3 monomer as
a model compound and compared the translational
diffusion coming from NMR relaxometry studies and
DRBM calculated from BDS data. The obtained NMR
results are presented in SI and summarized in Table 2.
Interestingly, due to the large value of the decou-
pling index at ambient pressure conditions (Rτ(Tg)=5)
and significant change of τσ accompanying squeezing,
the polymer blend (poly-SBVIm poly-SEM) can be re-
garded as the most efficient proton conductor among
the studied here protic systems over a wide T-P thermo-
dynamic space. Since both cations and anions are cova-
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Table 2: Parameters obtained from the analysis of H
spin-lattice relaxation data for SBMIm HPO3 monomer.
SBMIm poly-SEM
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4
5
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8
2
0
anhydrous
13.8 wt.% of H2O
Tg increases
Rτ(Tg) does not
T /K
283
288
294
303
λ /Å
2.95
2.89
2.86
2.82
Dtrans*/m2s-1
6.64 · 10-13
1.0 · 10-12
DRBM / m2s-1
8.93· 10-12
1.36· 10-11
ꢀTg=68K
depend on
water content
-2
-4
-6
-8
drying
1.03 · 10-12
2.66 · 10-12
1.83· 10-11
3.19· 10-11
9
Tg
Tg
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*Because of no significant differences between Dtrans of
cations and anions the presented values denotes 2Dtrans of
cations.
2.5
3.0
3.5
4.0
4.5
1000/T K-1
Herein, it should be noted that the NMR results for λ,
being in good agreement with values reported by Sango-
Figure 9. The conductivity relaxation times presented vs.
1000/T for SBMIm poly-SEM. Open symbols are τα data for
anhydrous sample.
ro et al.48 for other ILs, were used to calculate DRBM
.
Interestingly, we have found at least one order of magni-
tude difference between NMR and BDS diffusivity coef-
ficients. Namely, Dtrans reflecting only the translational
diffusion of SBMIm cations and vinylHPO3 anions is
slower than the DRBM. Since both vehicle conduction and
proton motions were found to contribute to the dielec-
tric relaxation data one can expect that the observed
difference between Dtrans and DRBM comes from proton
hopping between the conducting species. Taking into
account that the value of the decoupling index was
found to be practically the same for SBMIm HPO3 mon-
omer and polymer (see Fig. 3 in SI), with a similar acti-
vation energy for charge transport in the glassy state,
one can assume that proton motions give similar contri-
bution to dc-conductivity in both these compounds.
Consequently, DRBM is expected to be faster than Dtrans
also in SBMIm poly-HPO3.
Another interesting aspect of ion dynamics in protic
polyILs is the effect of water on the decoupling between
segmental and ionic relaxation. To investigate this issue
we have performed additional dielectric measurements
of dry SBMIm poly-SEM membrane. As can be clearly
seen in Fig. 9 evaporation of water results in dramatic
increase of Tg (68K) accompanied by decrease of dc-
conductivity; however at the same time the crossover of
logτσ(1/T) dependence occurs at the same conductivity
relaxation time. This result clearly indicate that at the
liquid-glass transition the charge transport in the an-
nealed protic polyIL is still faster than the segmental
dynamics and consequently, the conducting properties
of the examined dried-polymer are still governed by the
fast proton hopping through the H-bond network. The-
se results are consistent with the data obtained for other
protic polyILs published in ref [17].
CONCLUSIONS
The studies of seven protic and aprotic polymerized
ionic liquids combined with literature data for 11 low-
molecular ionic systems have revealed the great poten-
tial of high pressure dielectric techniques for developing
fundamental understanding of the conductivity mecha-
nism in ionic glass-formers, in general. It has been
demonstrated that the characteristic crossover of τσ(Pg)
appears at shorter relaxation times with elevating pres-
sure for proton conducting systems. This observation
suggests a dominating proton transfer mechanism in
newly synthesized hydrated protic polyILs that becomes
even more efficient under compression due to shorten-
ing and stressing of H-bonds. On the other hand, the
opposite behavior, i.e. a decrease in decoupling, was
found in aprotic ionic polymers. These results confirm
the dominating role of the chain packing and free vol-
ume in charge transport of aprotic systems. As a conse-
quence, isothermal compression offers a unique possi-
bility to recognize the dominating charge transport
mechanisms as well as to control the conducting prop-
erties of polyILs, both being crucial in their potential
electrochemical applications. Additionally, the high
decoupling between segmental dynamics and ion
transport found in poly-SBVIm poly-SEM demonstrates
that polymer blends in general provide a possibility to
design polyILs with exceptionally high proton conduc-
tivity and satisfactory mechanical stability.
ACCOCIATED CONTENT
Supporting Information The Supporting Information is
available free of charge on the ACS Publications website.
From closer look at Fig. 9 one can also see that the
apparent activation energies determined for conductivi-
ty relaxation in the glassy state of anhydrous and hy-
drated material differs from each other. Namely, it is a
little higher for the dried sample. This indicates that
below Tg when the segmental dynamics is completely
frozen it is more difficult for proton to hop through the
dry material. That can be very easily explained in terms
of the amount of proton active sites which is continu-
ously decreased with decrease of water content.
The details of NMR relaxonetry measurements, DFT
calculations and dielectric data of monomer SBMIm
vinylHPO3 (PDF).
AUTHOR INFORMATION
Corresponding Author
* zaneta.wojnarowska@smcebi.edu.pl
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Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final ver-
sion of the manuscript.
1
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the Dynamics of Polymerized Ionic Liquids and their Mon-
5
6
omers Electrochimica Acta, 2015, 175,55-61.
Notes
10
Frenzel, F., Folikumah, M.Y., Schulz, M., Anton, A.M.,
Any additional relevant notes should be placed here.
7
8
9
Binder, W.H., Kremer, F. Molecular Dynamics and Charge
Transport in Polymeric Polyisobutylene-Based Ionic Liq-
uids. Macromolecules 2016, 49 (7), pp 2868–2875.
11Fan, F., Wang, W., Holt, A.P., Feng, H., Uhrig, D., Lu, X.,
Hong, T., Wang, Y., Kang, N-G., Mays, J., Sokolov, A.P.
Effect of Molecular Weight on the Ion Transport Mecha-
nism in Polymerized Ionic Liquids, Macromolecules 2016,
49, 4557 - 4570.
ACKNOWLEDGMENT
This work was supported by Laboratory Directed Research
and Development program of Oak Ridge National Labora-
tory, managed by UT-Battelle, LLC, for the U.S. Depart-
ment of Energy. APS, acknowledge partial financial support
by the U.S. Department of Energy, Office of Science, Basic
Energy Sciences, Materials Sciences and Engineering Divi-
sion. Z.W. and M.P. are deeply grateful for the financial
support by the National Science Centre within the frame-
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12 Fan, F., Wang, Y., Hong, T., Heres, M. F., Saito, T.,
Sokolov A. P. Ion Conduction in Polymerized Ionic Liquids
with Different Pendant Groups. Macromolecules 2015, 48
(13), 4461–4470.
work
of
the
Opus8
project
(Grant
DEC-
13 Mizuno, F., Belieres, J. P., Kuwata, N., Pradel, A., Ribes,
M., Angell, C. A. Highly decoupled ionic and protonic solid
electrolyte systems, in relation to other relaxing systems
and their energy landscapes. Journal of Non-Crystalline
Solids, 2006, 352, 5147-5155.
2014/15/B/ST3/04246). E. Tojo is grateful to the Xunta de
Galicia (REGALIs Network R2014/015) for their financial
support, as well as the research support services of the
University of Vigo (CACTI) for the NMR and MS divisions
work. M.D., A.O. and I.O acknowledge Spanish Ministry of
Economy and Competitiveness for the project CTQ2015-
66078-R (MINECO/FEDER, UE). M. D. is grateful to the
Spanish Ministry of Education, Culture and Sport for the
FPU2012-3721.
14 Angell, C.A. Dynamic Processes in Ionic Glasses. Chem.
Rev. 1990, 90, 523-542.
15 Malcolm, D. Ingram Ionic conductivity and glass struc-
ture. Philosophical Magazine Part B, 1989, 60, 6, 729-740.
16 Díaz, M., Ortiz, A., Ortiz, I. Progress in the use of ionic
liquids as electrolyte membranes in fuel cells. Journal of
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