Imidazole polymers derived from ionic liquid 4-vinylimidazolium monomers: Their synthesis and thermal and dielectric properties

Article abstract of DOI:10.1021/ma300862t

The synthesis of 1-ethyl-3-methyl-4-vinylimidazolium triflate, its polymerization, and ion exchange to yield a family of 4-imidazolium polymers with a variety of anions are described. For comparative purposes, the synthesis, polymerization, and ion exchange of an analogous set of 1-vinylimidazolium polymers are also presented. The comparative thermal and dielectric characteristics of the 4-vinyl- and 1-vinylimidazolium salts were evaluated. The trends in the glass transition (T_g) characteristics of the various 4-vinylimidazolium and 1-vinylimidazolium polymers were similar; however, the glass transition temperatures of poly(4-vinylimidazolium) BF₄ ^-, PF₆^-, AsF₆^-, and CF₃SO₃^- salts were significantly higher than those of the corresponding poly(1-vinylimidazolium) salts. This difference and the increase in T_g in going from BF₄^- to AsF₆^- in the 4-vinylimidazolium series were attributed to enhanced intramolecular bridging between imidazolium moieties positioned 1,3 or 1,5 along the polymer chain. In the dielectric spectra of 1-vinylimidazolium salts at temperatures in excess of 30 C, one relaxation mode distinct from that for electrode polarization is observed. The single mode appears to correspond to the α-relaxation peak in poly(3-ethyl-1-vinylimidazolium salts) recently identified and attributed to ion-pair motion by Nakamura et al. In the 4-vinylimidazolium polymer spectra set, at temperatures in excess of 30 C, two relaxation modes, distinct from that for electrode polarization, are apparent: the α peak also observed in the 1-vinylimidazolium polymer set and a new relaxation peak observed at lower frequency. The lower frequency relaxation peak is identified in this work as the α′-relaxation and is also associated with ion-pair motion. Assuming the relaxation processes to be Arrhenius in nature, the activation energy of the α-relaxation in poly(4-vinylimidazolium) BF₄^-, PF₆^-, CF₃SO₃^-, TFSI^-, and C ₂N₃^- salts ranged from 83 to 28 kJ/mol and appears to scale with the glass transition temperature.

Full text of DOI:10.1021/ma300862t

Article
pubs.acs.org/Macromolecules
Imidazole Polymers Derived from Ionic Liquid 4‑Vinylimidazolium
Monomers: Their Synthesis and Thermal and Dielectric Properties
,†
‡
†
†
§
Thomas W. Smith,* Meng Zhao, Fan Yang, Darren Smith, and Peggy Cebe
†
‡
School of Chemistry & Materials Sciences, College of Science, and Microsystems Engineering Department, Kate Gleason College of
Engineering, Rochester Institute of Technology, Rochester, New York 14623, United States
§
Physics and Astronomy Department, Center for Nanoscopic Physics, Science and Technology Center, Tufts University, Medford,
Massachusetts 02155, United States
*
S Supporting Information
ABSTRACT: The synthesis of 1-ethyl-3-methyl-4-vinylimidazolium
triflate, its polymerization, and ion exchange to yield a family of 4-
imidazolium polymers with a variety of anions are described. For
comparative purposes, the synthesis, polymerization, and ion exchange
of an analogous set of 1-vinylimidazolium polymers are also presented.
The comparative thermal and dielectric characteristics of the 4-vinyl-
and 1-vinylimidazolium salts were evaluated. The trends in the glass
transition (T ) characteristics of the various 4-vinylimidazolium and 1-
g
−
−
vinylimidazolium polymers were similar; however, the glass transition temperatures of poly(4-vinylimidazolium) BF , PF ,
4
6
−
−
AsF , and CF SO salts were significantly higher than those of the corresponding poly(1-vinylimidazolium) salts. This
6
3
3
difference and the increase in T in going from BF4 to AsF6− in the 4-vinylimidazolium series were attributed to enhanced
−
g
intramolecular bridging between imidazolium moieties positioned 1,3 or 1,5 along the polymer chain. In the dielectric spectra of
1
-vinylimidazolium salts at temperatures in excess of 30 °C, one relaxation mode distinct from that for electrode polarization is
observed. The single mode appears to correspond to the α-relaxation peak in poly(3-ethyl-1-vinylimidazolium salts) recently
identified and attributed to ion-pair motion by Nakamura et al. In the 4-vinylimidazolium polymer spectra set, at temperatures in
excess of 30 °C, two relaxation modes, distinct from that for electrode polarization, are apparent: the α peak also observed in the
1
-vinylimidazolium polymer set and a new relaxation peak observed at lower frequency. The lower frequency relaxation peak is
identified in this work as the α′-relaxation and is also associated with ion-pair motion. Assuming the relaxation processes to be
−
−
−
−
Arrhenius in nature, the activation energy of the α-relaxation in poly(4-vinylimidazolium) BF , PF , CF SO , TFSI , and
4
6
3
3
−
C N salts ranged from 83 to 28 kJ/mol and appears to scale with the glass transition temperature.
2
3
INTRODUCTION AND BACKGROUND
The plasticization strategy has been applied in poly-
vinylidene fluoride) and vinylidene fluoride/hexafluoropropy-
■
(
Ionic liquids are salts with low melting points (often below
room temperature) and are typically composed of quaternary
sulfonium, phosphonium, or ammonium (imidazolium, pyr-
idinium, pyrrolidinium) cations paired with anions of low Lewis
1
0−12
lene copolymer membranes
for lithium secondary
13
14
batteries and fuel cells, transducers, sensors, and electro-
1
5,16
chromic devices.
There are a number of recent reviews
the history and advances in polyionic liquids since the first
intentional synthesis by Watanabe et al. of a viscoelastic
rubbery) polymer electrolyte comprising a poly(N-butylpyr-
idinium tetrachloroaluminate) eutectic. Regarding the present
research, the recent publication by Elabd et al., which detailed
the results of the synthesis and polymerization of (1-[(2-
methacryloyloxy)ethyl]-3-butylimidazolium bromide) and its
subsequent ion exchange with fluoride-containing anions, TFSI,
17−21
−
−
−
−
which document
basicity (BF , PF , CF SO (CF SO ) N , etc.). Today the
4
6
3
3
3
2 2
1
utility of ionic liquids in electrochemical devices ranging from
22
2
3
4
5
lithium ion batteries, to fuel cells, capacitors, solar cells, and
actuators is being explored. Because of the mobility of both the
anionic and cationic components of ionic liquids, the function
of some devices might be improved if conventional ionic liquids
were replaced by film-forming ionic liquid/polymer gel
electrolytes or ionic liquid polymers in which the mobility of
one or both the ions is constrained. There are effectively two
options for the realization of ionic liquid-based polymer
electrolytes: (1) plasticization of a preformed nonionic polymer
(
23
−
−
−
BF , CF SO , and PF , is particularly relevant. These
4
3
3
6
workers investigated the effect of anion type on the chemical,
thermal, and conductive properties of poly(1-[(2-
methacryloyloxy)ethyl]-3-butylimidazolium)-based polyionic
6
with an ionic liquid, or polymerization of a nonionic monomer
7
,8
in an ionic liquid to form “ion gels”, and (2) synthesis of
ionic liquid polyelectrolytes by polymerization of ammonium,
9
sulfonium, or phosphonium monomers, or quaternization of
Received: April 27, 2012
neutral precursor polymers to yield the analogous ammonium,
sulfonium, or phosphonium polymers.
Revised: December 24, 2012
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ma300862t | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
liquids. The glass transition, T , was found to decrease in the
diligence and care. In the present research, rough correspondence in
heating cycle and cooling cycle heat capacity changes were generally
required to validate a heat capacity change as a glass transition.
Additional validation was garnered by way of a final heating cycle from
g
−
−
−
−
following order: Br > PF6 > BF4 > CF SO > TFSI.
3
3
−
Overall, there was a 95° depression in T from the Br salt (T
g
g
=
102 °C) to the TFSI salt (T = 7 °C). While the glass
g
−
50 to 200 °C, heating at 40 °C/min. Under these conditions heating
cycle heat capacity changes associated with the glass transition will be
amplified and shifted to a somewhat higher temperature.
transition temperature was found to play a dominant role in
determining ion conductivity, other factors, including the size
and symmetry of the anion and dissociation energy of the ion
pair, were cited as important factors.
Size Exclusion Chromatography. Molecular weight and poly-
dispersity were determined using an Agilent 1100 series gel
permeation chromatograph with two Agilent Zorbax PSM 60-S
columns (in series). The samples were eluted at 35 °C, using N,N′-
dimethylformamide as the solvent. Molecular weight values reported
are styrene equivalent molecular weights based on hydrodynamic
radius.
Dielectric Spectroscopy. Dielectric properties were measured with
an ARES rheometer system from TA Instruments coupled with an
Agilent LCR Meter 4284A. Polymer films whose thickness ranged
from 250 to 1600 μm were prepared by solution casting from ethanol
or DMF on Al foil substrates at ambient temperature. The films were
dried in a fume hood at room temperature and cut to 22 mm by 24
mm before a final drying process in which they were further dried at
The present report also builds on three recent publications
2
4−26
by Nakamura et al.
in which the dielectric relaxation and
viscoelastic characteristics of poly(1-vinyl-3-ethylimidazolium
+
−
trifluoromethylsulfonylimide) (PVIM TFSI ) and a family of
poly(1-vinyl-3-butylimidazolium salts) were studied.
The dissymmetric, aprotic ionic liquids in this report differ
from those previously described in the literature in that they are
1,3-dialkyl-4-vinylimidazolium salts. Figure 1 displays a pentad
1
20 °C on the surface of a digital hot plate for at least 24 h. The dried
films were then stored in a desiccator until use. The samples were
placed between two stainless steel electrodes in the ARES and
6
isothermally measured as a function of frequency, between 20 and 10
Hz. The temperature was controlled by the ARES with liquid nitrogen
and compressed air as the gas at low and high temperatures,
respectively
2
7
The Havriliak−Negami (HN) model is used to analyze the
dielectric relaxation measurements. The complex dielectric function,
ε*(ω) = ε′ − iε″, is written as
Δε
1 + (iωτ) )
σ
ω ε
0
ε*(ω) = ε∞ + (
− i
s
Figure 1. Pentad segment of the 1-ethyl-3-methyl-4-vinylimidazolium
polymer.
β
γ
0
(1)
where ω = 2πf is the radian electric field oscillation frequency, ε∞ is
the high-frequency limit of the dielectric constant, Δε is the relaxation
strength, defined as Δε = ε − ε , and τ is the relaxation time which is
of a poly(1-ethyl-3-methyl-4-vinylimidazolium salt) in which
the counterion is sandwiched between pendant functional
moieties positioned 1,3 on the polymer backbone. The
asymmetric imidazolium moiety is tethered to the polymer
backbone at the 4-position of the imidazolium ring.
This tends to result in additional degrees of freedom,
increased free volume, and enhanced lateral overlap between
proximate imidazole residues that may be situated 1,3 or 1,5
with respect to each other on the carbon chain.
s
∞
the reciprocal of the radian frequency of maximal loss, ω . β and γ (0
max
< β ≤ 1, 0 < βγ ≤ 1) are the shape parameters of the relaxation spectra.
For a purely Debye relaxation process, both β and γ would be equal to
s
unity. The added term, −i(σ /ε ω ), refers to effects of conduction,
0
0
where σ is related to dc conductivity and ε is the permittivity of
0
0
vacuum. The exponential parameter, s, is equal to 1 for ohmic
28
conductivity and less that 1 for nonohmic effects in the conductivity.
The relationship between frequency at maximum loss, fmax, which is
known as the relaxation rate, and the corresponding temperature can
be described by an Arrhenius function. The frequency of maximum
loss, fmax, was most often evaluated directly from plots of tan δ (= ε″/
ε′) vs frequency. In three instances [poly(1-ethyl-3-methyl-4-vinyl-
EXPERIMENTAL SECTION
■
Instruments. NMR. Proton NMR spectra were obtained using a
Bruker DRX-300 spectrometer. Unless noted, all samples were
dissolved in chloroform-d (Aldrich, 99.8 atom % D, 0.05% v/v TMS).
Differential Scanning Calorimetry (DSC). Glass transition thermo-
grams were obtained under nitrogen using a TA Instruments DSC
Q100 equipped with a liquid-nitrogen cooling system. All samples
were prepared in an Ar-filled, Vacuum Atmospheres glovebox.
Polymer samples were placed in an open, hermetically sealable,
aluminum pan and heated to 100 °C for 15 min on the surface of a
digital hot plate in the glovebox. The aluminum pan was then capped
and sealed. In the DSC, samples were ramped to 200 °C and then
cooled to −50 °C at a rate of 20 °C/min. Each sample was held at −50
and 200 °C, respectively, for 1 min in between each heating and
−
−
imidazolium PF ), poly(3-ethyl-1-vinylimidazolium PF ), and poly-
6
6
−
(1-ethyl-3-methyl-4-vinylimidazolium BF )] additional data were
4
obtained by fitting plots of ε″ vs frequency, according to eq 1, with
Δε, τ, β, γ, σ , and s as fitting parameters.
0
Materials. Reagent Chemicals. Unless otherwise noted, all reagent
chemicals were used as received without further purification.
1,1,1,3,3,3-Hexamethyldisilazane (99.9%), 1-vinylimidazole (99+%),
2,2′-azobisisobutyronitrile (AIBN, 98%, recrystallized from methanol),
4-imidazoleacrylic acid (99%), calcium hydride (coarse granules, 0−20
mm, 95%), ethyl trifluoromethanesulfonate (99%), lithium hexafluor-
ophosphate (LiPF , 98%), lithium trifluoromethanesulfonimide
6
cooling cycle. T values are reported as midpoint glass transition
(LiTFSI, 99.95%), picric acid (99+%), and tetrafluoroboric acid (48
wt % solution in water) were purchased from Sigma-Aldrich. 4-tert-
Butyl catechol (4-TBC, 99%), ammonium sulfate (reagent ACS),
iodomethane (stabilized, 99%), isopropanol (IPA, analysis), methyl
alcohol (reagent ACS, 99.8%), potassium carbonate (reagent grade
ACS, anhydrous), sodium bicarbonate (p.a.), and sodium dicyanamide
(97% pure) were purchased from Acros Organic. Ethyl acetate (EA,
AR ACS), hydrochloric acid (AR ACS), and dichloromethane (AR
ACS) were obtained from Mallinckrodt Chemicals. Magnesium sulfate
g
temperatures, T -mid. The analysis was a seven-step process: (1)
g
heating from 22 to 200 °C at 20 °C/min; (2) holding for 1 min at 200
°C; (3) cooling from 200 to −50 °C at 20 °C/min; (4) holding for 1
min at −50 °C; (5) heating from −50 to 200 °C at 20 °C/min; (6)
holding for 1 min at 200 °C; and (7) cooling from 200 °C to −50 °C
at 20 °C/min. It cannot be emphasized too strongly that obtaining
accurate, repeatable, measures of the glass transition in poly(ionic
liquid) 1-vinyl- and 4-vinylimidazolium salts requires substantial
B
dx.doi.org/10.1021/ma300862t | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
(
anhydrous powder) was obtained from J.T. Baker, N,N-Dimethylfor-
hot plate at 90 °C for 1 h in an Ar-filled, Vacuum Atmospheres
glovebox. Yield = 3.07 g, 88%, M = 8730 g/mol, M = 14 722 g/mol,
PD = 1.7.
mamide (Spectrograde), benzene (GR ACS), acetonitrile (GR ACS),
and hexanes (GR ACS) were obtained from EMD.
n
w
4
(5)-Vinylimidazole. 4(5)-Vinylimidazole was synthesized by a
3-Ethyl-1-vinylimidazolium Triflate. 3-Ethyl-1-vinylimidazolium
triflate was synthesized in a reaction analogous to that for the
synthesis of 1-ethyl-3-methyl-4-vinylimidazolium triflate. Thus, freshly
distilled 1-vinylimidazole (1.73 g, 18.4 mmol) was charged to a 250
mL three-neck round-bottom flask, equipped with gas inlet valve,
Teflon adapter with thermometer, addition funnel, and magnetic stir
bar. The solution was cooled to 0 °C by immersion in an ice−water
bath and stirred under an argon blanket. Ethyl trifluoromethanesul-
fonate (3.93 g, 22.1 mmol, 1.2 equiv) and dichloromethane (50 mL)
were added to the addition funnel, and this solution was added
dropwise over a 30 min period. The reaction mixture was held at 0 °C
and stirred for 1.5 h. The addition funnel was replaced by a short-path
distillation head, and the solvent was removed in vacuo, yielding 5 g,
29
procedure analogous to that of Overberger et al. by means of the
decarboxylation of urocanic acid. In a typical procedure, urocanic acid
5.00 g, 36.2 mmol) was decarboxylated in a 100 mL single-neck
round-bottom flask with attached elbow. This flask was heated in an oil
bath at 230 °C under vacuum (10 μmHg), and crude 4(5)-
vinylimidazole was collected, as a pale yellow viscous liquid, over a
period of about 3 h. The 4(5)-vinylimidazole was then cooled
overnight at 10 °C to obtain a yellow crystalline material. Yield = 2.03
g, 60%.
(
1-Trimethylsilyl-4-vinylimidazole. The synthesis of 1-trimethylsilyl-
4
-vinylimidazole was carried out in a procedure analogous to that of
Kawakami and Overberger. Thus, crude 4(5)-vinylimidazole (5.00 g,
3.1 mmol), 1,1,1,3,3,3-hexamethyldisilazane (8.57 g, 53.1 mmol),
30
5
99%, of 3-ethyl-1-vinylimidazolium triflate.
+
−).
benzene (20 mL, 225 mmol), ammonium sulfate (catalytic amount),
and 4-tert-butyl catechol (catalytic amount) were charged to a 50 mL
three-neck round-bottom flask. The three-neck flask was equipped
with a Teflon adapter with thermometer, 14/20 ground-glass stopper,
reflux condenser with gas inlet valve, and a magnetic stir bar. The
reaction mixture was blanketed with argon, heated, and stirred in an oil
bath at 95 °C for 20 h. The reaction mixture was then allowed to come
to room temperature, and the solvent was removed by rotary
evaporation. The remaining clear oil was purified by vacuum
distillation. The resulting oil crystallized when stored at 10 °C. Yield
Poly(3-ethyl-1-vinylimidazolium triflate) (P1VIm CF
3
SO
3
+
−
P1VIm CF
to that employed in the synthesis of P4VIm CF
99%, M = 15 706 g/mol, M = 18 968 g/mol, PD = 1.21.
SO was polymerized and isolated by a process analogous
3
3
+
−
SO . Yield = 4.90 g,
3
3
n
w
Ion Exchange. The ion-exchange process employed with 4-vinyl-
and 1-vinylimidazolium triflate polymers was the same. In a typical
procedure, 5 mL of a methanol solution containing 1.84 mmol of
imidazolium triflate polymer was added to a centrifuge tube, to which a
4 M excess of a methanol or methanol/water solution of a TFSI,
dicyanamide, hexafluorophosphate, or hexafluoroarsenate salt was
added. The solution was shaken vigorously, and a precipitate formed.
The precipitate was isolated by centrifugation, decanted, and subjected
to at least three methanol wash/centrifugation cycles prior to drying
=
7.47 g, 85%; bp = 50 °C, 0.13 mmHg.
-Methyl-5-vinylimidazole. The synthesis of 1-methyl-5-vinyl-
imidazole was carried out by a procedure analogous to that of
1
30
under Ar in a Vacuum Atmospheres glovebox, < 1 ppm H O. The
Kawakami and Overberger. Thus, pure 1-trimethylsilyl-4-vinyl-
imidazole (7.47 g, 44.9 mmol) was reacted with iodomethane (6.06
g, 42.7 mmol) to give 1-trimethylsilyl-3-methyl-4-vinylimidazolium
iodide, which was hydrolyzed to give 1-methyl-5-vinylimidazole; crude
yield = 2.52 g, 64%. Picric acid was added, and the crude product was
stored at 10 °C until purified by distillation.
2
tetrafluoroborate salt was prepared by a similar procedure mixing 5 mL
of a methanol solution containing 1.84 mmol of imidazolium triflate
polymer with 5 mL of water containing 79.5 mmol of tetrafluoroboric
acid.
1
-Ethyl-3-methyl-4-vinylimidazolium Triflate. Freshly distilled 1-
RESULTS AND DISCUSSION
■
methyl-5-vinylimidazole (4.22 g, 39.1 mmol) and dichloromethane (50
mL) were charged to a 250 mL three-neck round-bottom flask,
equipped with gas inlet valve, Teflon adapter with thermometer,
addition funnel, and magnetic stir bar. The solution was cooled to 0
The present effort to prepare 4(5)-vinylimidazolium polymers
was initially motivated by the idea that, as compared to 1-
vinylimidazolium polymers, attachment at the 4(5)-position of
the imidazole ring would afford increased rotational and
translational freedom for the imidazolium moiety. Moreover,
increased degrees of freedom and greater asymmetry may allow
for greater free volume and greater ability for the imidazolium
moiety to interact cooperatively in the transport of target ions.
The cylindrical volume required by a rotating imidazolium
group tethered to the polymer backbone at the 4 and 1
positions of the imidazolium ring and the bonds around which
rotation is possible are depicted in Figure 2.
°
C by immersion in an ice−water bath and stirred under an argon
blanket. Ethyl trifluoromethanesulfonate, [6.1 mL (8.37 g), 47 mmol]
and dichloromethane (50 mL) were added to the addition funnel, and
this solution was added dropwise over a 30 min period. The reaction
mixture was held at 0 °C and stirred for 2 h. The addition funnel was
replaced by a short-path distillation head, and the solvent was removed
in vacuo, yielding 10.64 g, 95%, of 3-ethyl-1-methyl-5-vinylimidazolium
triflate as a white crystalline solid; mp = 41 °C. The reaction vessel was
1
immersed in a 0 °C ice/water bath throughout this process. H NMR
(
(
(
in THF-d ) 1.47 (3H, t, N−CH CH ), 3.81 (3H, s, N−CH ), 4.18
8
2
3
3
3
The focus of this paper is the 4-vinylimidazolium
homopolymer and understanding how its glass transition and
dielectric properties, as compared to 1-vinylimidazolium
homopolymers, change with variation of the counterion.
Accordingly, the synthesis of 1-ethyl-3-methyl-4-vinylimidazo-
2H, q, N−CH CH ), 5.54 (1H, d, J 11.31 Hz, cis-vinyl H), 5.88
2
3
3
3
3
1H, d, J 17.46 Hz, trans-vinyl H), 6.54 (1H, J 11.31, J
17.46,
cis
trans
vinyl H−C), 7.75 (1H, s, C-4H), 8.92 (1H, s, C-2H). The alkylation
reaction can be carried out at 0 °C in ethyl acetate. The reaction in
ethyl acetate is slower, and one should allow for 6 h to drive the
reaction to completion. Reaction in ethyl acetate enables one to
proceed directly to polymerization without the need for solvent
removal.
Poly(1-ethyl-3-methyl-4-vinylimidazolium triflate)
+
−
(
P4VIm CF SO ). 1-Ethyl-3-methyl-4-vinylimidazolium triflate (5.3
3 3
g, 39 mmol) was dissolved in ethyl acetate (30 mL) and transferred to
a polymerization tube at 0 °C, under an argon blanket. A solution was
made of AIBN (0.03 g, 0.183 mmol) by dissolution in ethyl acetate
(
10 mL). The AIBN solution (1 mL) was added to the polymerization
tube and mixed. The solution was then degassed by three freeze−thaw
cycles by freezing the contents in liquid nitrogen (−192 °C). The tube
was then flame-sealed and immersed in a water bath at 65 °C for 20 h.
The resulting polymer gel was isolated by dissolution in methanol and
precipitation in methyl tert-butyl ether. The precipitate was dried on a
Figure 2. Asymmetry and rotational degrees of freedom in P4VIm+
and P1VIm .
+
C
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Macromolecules
Article
Figure 3. Synthesis of 1-ethyl-3-methyl-4-vinylimidazolium triflate.
lium triflate, its polymerization, and ion exchange to yield a
family of 4-imidazolium polymers with a variety of anions are
described. The glass transition, thermal stability, and dielectric
relaxation characteristics of these related polymers are also
reported and discussed. For comparative purposes, the
synthesis, polymerization, ion exchange, and thermal and
dielectric characteristics of an analogous set of 1-vinyl-
imidazolium polymers are also presented. The goal is to better
understand how the relative degree of freedom of the
imidazolium moiety tethered to the polymer backbone and
the size and character of the counterion influence the glass
transition and molecular relaxations in these “polyionic liquids”.
Synthesis, Polymerization, and Ion Exchange of 4-
Vinyl- and 1-Vinylimidazolium Monomers. The synthesis
of a quaternary 4-vinylimidazolium monomer was first reported
prepared by ion exchange of aqueous solutions of their halide
salts. Nevertheless, in order to obtain the 1-vinylimidazolium
monomer in a pure and dry state, the process of direct
alkylation with ethyl triflate, which was used to prepare our 4-
vinylimidazolium triflate, was also used to synthesize 3-ethyl-1-
vinylimidazolium triflate.
1-ethyl-3-methyl-4-vinylimidazolium triflate and 3-ethyl-1-
vinylimidazolium triflate were homopolymerized free-radically
in ethyl acetate, initiating with AIBN. The triflate polymers that
precipitate from ethyl acetate were dissolved in methanol and
purified by precipitation in ether. Molecular weights were
evaluated by GPC. Details are presented in the Experimental
Section.
The various poly(1-ethyl-3-methyl-4-vinylimidazolium) and
poly(3-ethyl-1-vinylimidazolium) salts were obtained by simply
ion-exchanging methanolic solutions of the 4-vinyl- or 1-
vinylimidazolium triflate polymers with methanol or water
solutions of protic or alkali metal salts of the desired anion. The
31
in 2004 by Wang and Smith. In that preliminary work, 1-
butyl-3-methyl-4-vinylimidazolium iodide was simply prepared
by alkylation of 1-methyl-5-vinylimidazole with butyl iodide.
However, the iodide anion interfered with its free-radical
polymerization. The aqueous solution of the iodide monomer
−
−
−
−
water or methanol insoluble (CF SO ) N , C N , BF , PF ,
3
3 2
2
3
4
6
and AsF6− ion-exchanged polymers precipitated and were
isolated and purified, as described in the Experimental Section,
by centrifugation and washing. The anions that were employed
differ in geometry and polarizability. The triflate salt is a classic
semispherical “hard anion” in which the electrons surrounding
the sulfonate group are not highly polarizable. Trifluorome-
thylsulfonylimide is a bent, tetrahedral, plasticizing anion in
which the anionic charge on the nitrogen atom is soft and
polarizable. Dicyanamide is a less solvating, bent, tetrahedral,
anion in which the anionic charge is soft polarizable and
delocalized over an ambident anion. Tetrafluoroborate,
hexafluorophosphate, and hexafluoroarsenate are a family
non-nucleophilic complex anions derived from Lewis acids in
which the volume of the anion increases from 0.073 to 0.109 to
was ion-exchanged with LiPF6 to yield an ionic liquid
−
monomer, Tg = −12 °C. The PF monomer tends to
6
polymerize spontaneously as it oils out of the aqueous solution.
However, the exploratory work served both to confirm that an
ionic liquid 1-alkyl-3-methyl-4-vinylimidazolium salt can be
synthesized and to elucidate the fact that, as was the case with
32
4
-vinylpyridinium salts, the key problem that must be
overcome was to suppress its spontaneous polymerization.
33
The work of Fife et al. in synthesizing 4-vinylpyridinium
salts teaches that isolable, storage stable, 4- and 5-vinyl-1-
methyl-3-alkylimidazolium salts might be synthesized if the
alkylation is carried out under anhydrous conditions with
triflate esters. This has proven to be the case. Thus, 1-ethyl-3-
methyl-4-vinylimidazolium triflate was quantitatively synthe-
sized, as depicted in Figure 3, from 4(5)-vinylimidazole.
3
38
0
.121 nm , respectively.
The Glass Transition. It is often said that the glass
1-Methyl-5-vinylimidazole was purified by vacuum distillation
transition in polymeric liquids is a poorly understood
phenomenon. This is particularly true for ionic polymers
whose glass transition characteristics have received only limited
prior to direct alkylation with ethyl triflate, at 0 °C, in
methylene chloride or ethyl acetate. The crystalline, ionic liquid
monomer, mp = 41 °C, was obtained, in a pure dry (water-free)
state, by removal of solvent in vacuo at 0 °C.
39
attention since the early analysis by Eisenberg et al. in which
the glass transitions of ionic polymers were found to be
proportional to the ratio of the counterion charge, q, to the
distance between the centers of cations and anions, a.
The preparations of 1-vinylimidazolium salts, which have
34−37
been described in the literature,
synthetic challenges inherent in the synthesis of the 1,3-dialkyl-
-vinylimidazolium analogues in that they do not sponta-
do not present the
40
4
Following on the work of Eisenberg, Tsutsui and Tanaka
neously polymerize at ambient temperatures. Indeed, the
various ionic liquid 1-vinylimidazolium monomer salts can be
reported that the glass transition temperatures of ionic
polymers could be correlated with cohesive energy density
D
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ρ q
⎛
⎜
⎞
⎟
2
c
CEDionic = NAe
⎝
⎠
(2)
(3)
M a
in accordance with eq 2
n
⎛
⎛ ρ q ⎞⎞
2
c
T = K ⎜N e
⎜
⎟
⎟
g
1
A
⎝
⎝ M a ⎠⎠
where N is Avogadro’s number, e is the electronic charge, ρ is
A
the polymer density, M is the molecular weight per skeletal ion,
and a is the equilibrium distance between the center of the
anion and the cation. Equation 3 shows that the glass transition
temperature is directly proportional to the cohesive energy
density multiplied by a coefficient, K . The coefficient varies
1
from polymer to polymer, and in keeping with the analysis by
Eisenberg, the glass transition temperature is predicted to be
inversely proportional to the distance between the anion and
the cation.
Agapov has studied the effect of polar interactions on the
temperature dependence of structural (segmental) and chain
41
dynamics in polymeric liquids. The glass transition temper-
ature, T , and fragility index, m, were found to depend on the
monomer’s polarity and the relative position of the polar group.
g
+
−
Figure 4. DSC scans of P4VIm TFSI during heating and subsequent
cooling.
The effect of polar interactions on T and m is discussed in
g
terms of a balance between changes in the cohesive energy and
conformational rigidity.
respectively. This result cannot be rationalized on the basis of
the change in the ratio of the counterion charge, q, to the
distance between the centers of cations and anions, a.
Enhanced intersegmental and intramolecular interactions
(bridging between imidazolium ions by the anion) may be a
In the present research, the glass transition in 4-vinyl- and 1-
vinylimidazolium polymers was evaluated by differential
scanning calorimetry (DSC). The results of a comparative
DSC analysis of the glass transition temperature of the full set
of ion-exchanged 4-vinylimidazolium polymers and 1-vinyl-
imidazolium polymers are presented in Table 1. Heating cycle
dominant factor in driving the T increase in this series. The 4-
g
−
−
vinylimidazolium polymers with TFSI and C N anions
2
3
exhibit the lowest glass transition temperatures. The lower glass
transition temperatures of these two polymers may be a result
of plasticization by large solvating anions, which, because of
their soft nucleophilic character, allows for association between
anion and cation over a larger distance.
As a complement to our studies of poly(1-ethyl-3-methyl-4-
vinylimidazolium salts), the glass transition characteristics of
corresponding poly(1-vinylimidazolium salts) were analyzed.
Table 1. Glass Transition Temperatures of
Poly(imidazolium salts)
+
+
P4VIm salts
P1VIm salts
heating cycle cooling cycle heating cycle cooling cycle
anion
Tg-mid (°C) Tg-mid (°C) Tg-mid (°C) Tg-mid (°C)
−
BF4
186
200
213
153
88
178
180
208
140
63
106
141
122
140
97
97
−
Table 1 also displays the heating cycle and cooling cycle T -mid
PF6
113
g
−
values for the family of 3-ethyl-1-vinylimidazolium salts. Heat
capacity changes in the cooling cycle, consistent with the
heating cycle glass transition temperatures, were observed in
AsF6
−
CF SO3
3
124
74
−
(
CF SO ) N
3 2 2
−
−
−
−
−
−
the BF , PF , CF SO , TFSI, and C N . In the AsF salt
C N
89
103
105
95
4 6 3 3 2 3 6
2
3
of the 3-ethyl-1-vinylimidazolium polymer, a heat capacity
change that can be attributed to the glass transition was not
observed in the cooling cycle.
and cooling cycle glass transition values are tabulated. Cooling
cycle T -mid values are somewhat lower than the heating cycle
The first difference of note between the glass transition
characteristics of the 1-vinyl- and 4-vinylimidazolium polymers
is that, in spite of the greater free volume required for the 1-
ethyl-3-methyl-4-vinylimidazolium moiety, the glass transition
g
Tg-mid values. This may be a result of a longer time constant
for the reformation of the glass, which results in a heat capacity
42
change over broader temperature range in the cooling cycle.
Figure 4 displays the second heating cycle and subsequent
−
−
−
temperatures of the 4-vinylimidazolium BF , PF , AsF , and
4
6
6
+
−
−
cooling cycle in the DSC thermogram for P4VIm TFSI . The
thermogram is typical of those observed across the series of
CF SO3 salts are higher than those of the corresponding 1-
3
vinylimidazolium salts. Another difference of note is that, in the
43
poly(4-vinylimidazolium salts).
1-vinylimidazolium polymer set, the glass transition of the
−
−
−
The consistency of heating cycle and cooling cycle data
BF , PF , and AsF series does not increase monotonically
4 6 6
provides significant support for the validity of the T values
with increase of anion size. Instead, while the glass transition in
this series of non-nucleophilic complex fluoride anions
increases significantly in going from the BF4 salt to the PF6−
g
−
−
presented. Tetrafluoroborate, BF , hexafluorophosphate, PF ,
4
6
−
−
and hexafluoroarsenate, AsF , comprise a set of complex
6
−
fluoride anions of increasing size. In the 1-ethyl-3-methyl-4-
salt, the T of the AsF salt is dramatically lower than that of
g
6
vinylimidazolum polymer set, the heating cycle T -mid
the PF6− salt.
g
increased from 186 °C to 200 °C and 213 °C when the triflate
The glass transition temperatures of the TFSI− and
dicyanamide derivatives of the 1-vinylimidazolium polymer
−
−
−
polymer was ion-exchanged to BF , PF , and AsF ,
4
6
6
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+
−
are slightly higher than those for the corresponding 4-
dielectric relaxation mode (the α-relaxation in P1VIm TFSI )
−
−
vinylimidazolium salts. As with the TFSI and C N salts of
was associated with ion-pair motion and not with segmental
motion of the imidazolium polymer chain.
In the present research, the dielectric relaxation behavior of a
series of ionic liquid polymers [P4VIm CF SO , TFSI ,
C N , BF , PF , and AsF ] derived from 1-ethyl-3-methyl-
2 3 4 6 6
2
3
25
the 4-vinylimidazolium polymers, the 1-vinylimidazolium salts
−
−
with plasticizing TFSI and C N anions exhibit the lowest
2
3
+
−
−
glass transition temperatures.
3 3
−
−
−
−
Thermal Gravimetric Analysis of Imidazolium Poly-
mers. The comparative thermal gravimetric analysis (TGA) of
the triflate polymers, presented in Figure 5, shows that the 4-
+
−
−
4-vinylimidazolium triflate and [P1VIm TFSI , C N , and
2
3
−
PF ] derived from 1-ethyl-3-vinylimidazolium triflate was
6
evaluated in the frequency range of 20 Hz to 1 MHz and the
temperature range of 30−210 °C. Dipolar relaxations were
analyzed by fitting the dielectric loss ε″ using the Havriliak−
Negami model, in eq 1. Figure 6 shows experimental ε″ vs
frequency data, the deconvoluted α-relaxation peak, and the fit
+
to the Havriliak−Negami equation for P4VIm PF at 130 °C.
6
+
−
Figure 5. TGA of P1VIm CF SO
[dashed blue line], and
3
3
+
−
P1VIm CF SO [solid violet line].
3
3
Figure 6. ε″ vs frequency for poly(1-ethyl-3-methyl-4-vinylimidazo-
vinyl imidazolium polymer has greater thermal stability than the
-vinylimidazolium polymer. The TGA results that were
obtained in the present research for poly(3-ethyl-1-vinyl-
−
lium PF ) at 130 °C. Points refer to measured data. Solid blue curve
6
1
is the summation of individual fitted curves for the α′- and α-
relaxations and dc conductivity reflecting the best fit to the data.
Dashed red curve: deconvoluted α′-relaxation peak (Δε = 6.4, τ = 1.9
+
−
imidazolium triflate), P1VIm CF SO , correspond well with
3
3
3
5
−3
those published by Marcilla et al.
Under nitrogen,
× 10 s, β = 0.83, γ = 0.6); dashed blue curve: deconvoluted α-
+
−
−6
P1VIm CF SO suffers extensive mass loss at 400 to 448
relaxation peak (Δε = 0.18, τ = 3.0 × 10 s, β = 0.74, γ = 0.86);
3
3
−9
dashed green line: dc conductivity (σ = 2.1 × 10 S/cm; s = 0.76).
°
C. Significant mass loss in poly(1-ethyl-3-methyl-4-vinyl-
+
imidazolium triflate), P4VIm CF SO , occurs in the 467−527
3
3
°C temperature range. The increased thermal stability is
Figure 7a−f displays dielectric spectra (ε′, ε″, and tan δ
+
−
apparently due to the difference in the tethering point of the
imidazolium ring to the polymer backbone. The energy
required to break the carbon−carbon bond is some 40 kJ/
versus frequency at 30−210 °C) for P4VIm PF and
6
+
−
P1VIm PF . Below 150 °C, the dielectric constant, ε′, in
6
these two polymers is of the order of 70 and is substantially
4
4
mol greater than that required to break a carbon−nitrogen
single bond. The modest reduction in mass below 400 °C is
due to loss of absorbed moisture.
invariant with frequency. Above 110 °C, the dielectric constant
5
increases dramatically at frequencies below 10 Hz and is
indicative of one or more relaxation processes. These relaxation
processes are apparent in both the ε″ and tan δ spectra;
however, relaxation peaks are most clearly resolved in the tan δ
Dielectric Studies. Dielectric spectroscopy provides
simultaneous measurements of dc conductivity and relaxation
24
+
−
processes as a function of temperature. Nakamura et al. have
reported on the dielectric relaxation characteristics
spectra. In the ε″ and tan δ spectra of P1VIm PF (Figure
6
7c,e) one relaxation mode distinct from that for electrode
polarization is observed in the temperature range 30−130 °C.
This relaxation appears well below the DSC glass transition;
accordingly, it is clearly not associated with segmental motion.
+
−
P1VIm TFSI in the frequency range of 10 mHz to 2 MHz
and the temperature range of −90 to 90 °C. Three relaxation
modes were observed and identified as β, α, and EP (electrode
polarization). The β relaxation mode, which is observed at low
temperature (−90 to 0 °C), was attributed to the rotational
relaxation mode of the imidazolium moiety. Expected dielectric
relaxation peaks related to an ion-pair relaxation mode and
+
−
The single peak in the dielectric spectra of the P1VIm PF set
6
may correspond to the relaxation that Nakamura assigned as
2
4
+
−
the α-relaxation in P1VIm TFSI and attributed to ion-pair
25
motion.
−
+
−
rotational relaxation motion of TFSI in the bulk polymer were
In the P4VIm PF6 spectra set (Figure 7d,f) two relaxation
modes, distinct from that for electrode polarization, are
apparent, the α-relaxation (observed in the 30−150 °C
temperature range) and a new relaxation peak (observed at
lower frequency in the 130−210 °C temperature range) that is
2
4
not observed. More recently, using dynamic viscoelastic
25
measurements, Nakamua investigated the same material.
Upon comparison of the viscoelastic master curves and the
dielectric relaxation spectra, they concluded that the slow
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+
−
+
−
Figure 7. (a−f) Frequency dependence of dielectric characteristics, ε′, ε″, and tan δ of P1VIm PF (a, c, e) and P4VIm PF (b, d, f). Data points
6
6
of different shape and color connected by solid or dashed lines of the same color are measured data at different temperatures: ● blue solid, 30 °C; ■
red solid, 50 °C; ▲ green solid, 70 °C; ◆ black solid, 90 °C; ● blue dashed, 110 °C; ■ red dashed, 130 °C; ▲ green dashed, 150 °C; ■ purple
dashed, 170 °C; ● blue dotted, 190 °C; ◆ orange dotted, 210 °C.
2
4−26
+
−
labeled as α′. Nakamura
has clearly shown that there are
transition temperatures are lower than that of P4VIm PF ,
6
additional β-relaxation modes in the lower temperature regime.
In our work, the temperature range examined was 30 −230 °C,
and the lower temperature β-relaxation modes were not
accessible.
The α′-relaxation is revealed in P4VIm PF (but not in
P1VIm PF ) because of its greater asymmetry and higher glass
transition temperature. At very high temperatures of 190 and
10 °C, the dielectric spectra of poly(4VIm PF6 ) show an
additional peak at lower frequency (10 −10 Hz) that, at this
time, has neither been labeled nor attributed it to any specific
relaxation phenomenon. P4VIm+ polymers, whose glass
do not show this lower frequency peak.
+
−
Plots of tan δ versus frequency for P4VIm C N and
2
3
+
−
P4VIm BF4 are displayed in Figures 8 and 9. The tan δ versus
frequency plots for P4VIm CF SO , BF , TFSI , and C N
+
−
−
−
−
3
3
4
2
3
−
+
−
are similar to that for the PF
6
salt in that α (lower
6
temperature/higher frequency) and α′ (higher temperature/
lower frequency) relaxation peaks were generally observed.
In order to garner some insight into the nature of the
relaxation peaks, an Arrhenius analysis was used as an
approximation to allow an estimate of the activation energy
to be made. The log of the maxima in each relaxation peak in
the PF6− spectra were thus plotted against the reciprocal of the
+
−
6
+
−
2
3
4
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+
−
Figure 8. Plot of tan δ versus frequency: P4VIm C N . Data points of different shape and color connected by solid or dashed lines of the same
2
3
color are measured data at different temperatures: ◆ black solid, 90 °C; ● blue dashed, 110 °C; ■ red dashed, 130 °C; ▲ green dashed, 150 °C.
+
−
Figure 9. Plot of tan δ versus frequency: P4VIm BF . Data points of different shape and color connected by solid or dashed lines of the same color
4
are measured data at different temperatures: ■ black solid, 90 °C; ●blue dashed, 110 °C; ■red dashed, 130 °C; ▲ green dashed, 150 °C; ■ purple
dashed, 170 °C.
+
−
temperature, T, in kelvin. Arrhenius plots for the α-relaxation
dependence than α′, especially in P4VIm PF . The increase in
6
(
(
observed in Figures 7c,e) and the α- and α′-relaxations
relaxation strength with temperature for the α- and α′-
relaxations is not expected for a segmental mode relaxation.
The conductivity increases with temperature by several orders
observed in Figures 7d,f) are presented in Figures 10a,b. The
plots displayed in Figure 10a,b include a least-squares fit line
that, over the temperature range (303−483 K) at which
measurements were taken, is a good fit to the data (correlation
coefficients =0.94 and 0.95, respectively, for the α and α’
relaxation data, in Figure 10 b).
+
−
of magnitude in P4VIm PF but increases only slightly in
6
+
−
P4VIm BF .
4
46
Fragiadakis et al. studied a system of more conventional
copolymer ionomers and report that large values of the
dielectric increment may be associated with ion motion.
Indeed, for the α′-relaxation, Δε values range from 30 to 890
Table 2 presents fitting parameters, Δε (dielectric relaxation
strength) and σ (dc conductivity), for the α- and α′-relaxations
0
+
−
+
−
+
−
+
−
in P4VIm PF and P4VIm BF . The high-temperature, low-
for P4VIm PF and from 28 to 92 for P4VIm BF .
6
4
6
4
frequency data may have large errors in the fit parameters
because of the strong conduction effect and weak relaxation
peak seen there. The α′-relaxation is “stronger” than α in
Accordingly, the α- and α′-relaxations are most likely
attributable to ion-pair motion, with the onset of segmental
motion perhaps being correlated with ion-pair motion. Plots of
Δε vs temperature are included in the Supporting Information.
In any discussion of the origin of relaxation peaks in the
dielectric spectra of ionic polymers one is obligated to consider
+
−
+
−
P4VIm PF and in P4VIm BF . The Δε values for α′
6
4
increase with increasing temperature in both materials;
however, the α process has a much weaker temperature
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+
−
Figure 10. Frequency-max versus 1/T (K): (a) P1VIm PF (green ▲, α-relaxation, data from tan δ plots; red ▲, α-relaxation, data from
6
+
−
deconvoluted ε″ vs frequency plots); (b) P4VIm PF (green ◆, α-relaxation, from tan δ; blue ◆, α-relaxation, from deconvoluted ε″);
6
+
−
P4VIm PF (red ■, α′-relaxation, from tan δ). Points refer to measured data. Lines are least-squares fit to f data from deconvoluted ε″ plots.
6
max
+
Table 2. Temperature Dependence of Conductivity, σ , and
Table 3. E of α- and α′-Relaxations in P4VIm Salts
0
a
Relaxation Strength, Δε, for the α- and α′-Processes in
+
−
+
−
E
a
(kJ/mol), α-
relaxation
E
a
(kJ/mol), α′-
heating cycle
P4VIm PF and P4VIm BF
P4VIm+
6
4
relaxation
Tg-mid, °C
−
polymer
T (K)
Δε(α)
Δε(α′)
σ0 (S/cm)
BF
4
72
28
109
18
186
200
213
88
−
a
+
−
−11
PF6
83
P4VIm PF6
343
5.1
5.7
8.0
7.3
8.1
7.1
96
2.8 × 10
−
9.9 × 10−10
CF SO
3
40
3
3
4
4
4
4
63
83
03
23
43
63
3
−
b
2.3 × 10−8
C N
31
102
141
2
3
−
−
8
7
7
6
TFSI
28
81
6.1 × 10
a
+
−
b
+
−
30
74
2.0 × 10−
E
a
of P1VIm PF
6
= 107 kJ/mol. E
a
of P1VIm C
2
N
3
= 12 kJ/mol.
−
4.0 × 10
1.4 × 10−
−
−
−
890
in the TFSI and C N salts and lowest with the CF SO and
2
3
3
3
+
−
−9
−
P4VIm BF4
363
2.4
3.2
2.8
3.2
4.8
8.1 × 10
2.9 × 10
BF4 salts. The differences between the activation energies of
−
8
8
8
8
+
−
3
4
4
4
83
03
23
43
28
46
51
92
the α- and α′-processes were least pronounced in P4VIm PF
6
4.7 × 10−
+
−
and P4VIm CF SO . The activation energies of the α-
3
3
−
+
−
+
−
5.2 × 10
relaxation process in P4VIm PF and P4VIm C N are
similar to those in P1VIm PF6 and P1VIm C N . These
6 2 3
7.1 × 10−
+
−
+
−
2
3
differences in the apparent activation energies of the α- and α′-
relaxation processes may be related to how tightly the anion is
coupled to the imidazolium cation.
the possibility of ion aggregation. Ion aggregates would be most
expected in the triflate system in which the anion is “hard”, i.e.,
not highly polarizable. “Soft” anions like TFSI and dicyanamide
would be least likely to suffer from ion aggregation. The
similarity of dielectric spectral characteristics across the series of
In light of the substantial difference in the activation energies
+
−
of the α- and α′-relaxations of the P4VIm C N and
2
3
+
−
P4VIm TFSI , one might speculate these large soft (highly
polarizable) anions are not tightly coupled and that the nature
of the ion-pair motion associated with α- and α′-relaxations are
inverted with the α-relaxation being more reflective of ion-pair
motion preceding segmental motion.
+
ion-exchanged P4VIm salts leads us to conclude that the
impact of ion aggregation and any consequent interfacial
+
polarization peaks is minimal in the P4VIm salt system. While
X-ray diffraction or viscoelastic studies which could rule out ion+
47
aggregation have not yet been carried out in our P4VIm2
Agapov and Sokolov have proposed that, in some glass-
6
system, the recent viscoelastic study by Nakamura et al.,
forming systems, ion diffusion can be decoupled from structural
relaxation, with the extent of decoupling being characterized by
the deviation of the temperature dependence of the character-
istic structural relaxation time, τ, from a simple Arrhenius
behavior. This deviation can be uniquely captured in the
fragility index, m (a measure of the steepness of the
temperature dependence of τ at the glass transition temper-
indicating the absence of ion aggregates in poly(1-vinyl-3-
butylimidazolium salts), supports the probability that ion
aggregation is not a factor in any features of the dielectric
+
spectra of the P4VIm salts.
Arrhenius plots of frequency versus 1/T (K) for α- and α′-
processes in the triflate, tetrafluoroborate, trifluoromethylsulfo-
nylimide, and dicyanamide salts are indicative of greater
differences in activation energy than for the α- and α′-processes
in the PF6− derivatives. Activation energies were calculated
from the slope of the plots of frequency vs 1/T (K) for five 4-
48
ature, Tg). At this juncture, the molecular origins of the
relaxation processes in 4- vinylimidazolium polymer salts and
the degree of correlation between anion mobility and the glass
transition can only be unequivocally elucidated by more
extensive dielectric relaxation studies over a broader temper-
ature range. While such measurements are planned, the present
work represents a significant body of new data on the glass
transition and dielectric relaxation characteristics of families of
poly(1-ethyl-3-methyl-4-vinylimidazolium) and poly(3-ethyl-1-
vinylimidazolium) salts.
−
−
−
−
vinylimidazolium polymer salts (CF SO , BF , PF , C N ,
3
3
4
6
2
3
−
and TFSI ). These data are presented in Table 3. An important
finding is that the activation energy of the α-relaxation appears
to scale with the glass transition temperature, with E -α being
a
−
−
lowest for the TFSI and C N salts. The activation energy of
2
3
the α′-relaxation process was variable; however, it was highest
I
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SUMMARY AND CONCLUSIONS
AUTHOR INFORMATION
Corresponding Author
E-mail twssch@rit.edu.
■
■
A robust process for the synthesis of 1-ethyl-3-methyl-4-
+
−
*
vinylimidazolium triflate (EM-4-VIm CF SO ) was devel-
3
3
oped. This new monomer was synthesized in a pure, dry
water-free) state by direct alkylation of 1-methyl-5-vinyl-
imidazole in CH Cl or ethyl acetate solution with ethyl triflate
Notes
(
The authors declare no competing financial interest.
2
2
ACKNOWLEDGMENTS
at 0 °C. While the monomer tends to polymerize
spontaneously at ambient temperature or when exposed to
water, it can be stored and handled without polymerization as
long as it is kept dry in its crystalline state and held at
■
This research was enabled and supported by the National
Science Foundation. Work at RIT and Tufts was funded by
DMR-0938957 and DMR-0906455, respectively. This support
is gratefully acknowledged.
+
−
temperatures below 0 °C. EM-4-VIm CF SO is readily
3
3
polymerized with a free-radical initiator, and the resulting
polymer has been ion-exchanged to create a family of 4-
vinylimidazolium polymers in which the counterion ranged
REFERENCES
■
(1) Fernicola, A.; Scrosati, B.; Ohno, H. Potentialities of ionic liquids
as new electrolyte media in advanced electrochemical devices. Ionics
2006, 12, 95−102.
−
−
−
−
from BF , PF , and AsF to dicyanamide (C N ),
4
6
6
2
3
−
−
bistrifluoromethylsulfonamide (TFSI ), and triflate (CF SO ).
3
3
(
2) Hayashi, K.; Yasue Nemoto, Y.; Akuto, K.; Sakurai, Y. Ionic
This same process was employed in the synthesis of 3-ethyl-
liquids for lithium secondary batteries. NTT Tech. Rev. 2004, 2 (9),
48−54.
3) Martinelli, A.; Matic, A.; Jacobsson, P.; Borjesson, L.; Fernicola,
−
1
-vinylimidazolium triflate (1-VIm+ CF SO ) and its sub-
3
3
sequent polymerization and ion exchange.
(
The trends in the glass transition characteristics of the
various 4-vinylimidazolium and 1-vinylimidazolium polymers
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P4VIm BF , PF , AsF , and CF SO
salts were
significantly higher than those of the corresponding poly(1-
4
6
6
3
3
vinylimidazolium) salts. This difference and the increase in T
g
in going from BF4− to AsF6 in the 4-vinylimidazolium series
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−
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−
25
P1VIm TFSI by Nakamura et al. The second relaxation
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(
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P4VIm BF , PF , CF SO , TFSI, and C N salts ranged
4
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3
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(
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+
−
−
energy of the α′-relaxation in P4VIm TFSI and C N (141
2
3
and 102 kJ/mol), respectively, was dramatically higher than that
2
(
4−33.
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−
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24
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