Discotic side group liquid crystal polymer electrolytes

Article abstract of DOI:10.1080/15421400490425801

The synthesis and characterization is reported of a novel discotic side group polymer based on a predominantly poly(ethylene oxide) backbone to which are attached triphenylene-based mesogenic groups via flexible spacers. The thermal and mechanical behavio

Full text of DOI:10.1080/15421400490425801

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DISCOTIC SIDE GROUP
LIQUID CRYSTAL POLYMER
ELECTROLYTES
Corrie T. Imrie a , Rachel T. Inkster a , Zhibao Lu a &
Malcolm D. Ingram a
a Department of Chemistry, University of Aberdeen,
Meston Walk, Old Aberdeen AB24 3UE, Scotland, UK
Published online: 15 Jul 2010.
To cite this article: Corrie T. Imrie , Rachel T. Inkster , Zhibao Lu &
Malcolm D. Ingram (2004) DISCOTIC SIDE GROUP LIQUID CRYSTAL POLYMER
ELECTROLYTES, Molecular Crystals and Liquid Crystals, 408:1, 33-43, DOI:
10.1080/15421400490425801
To link to this article: http://dx.doi.org/10.1080/15421400490425801
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Mol. Cryst. Liq. Cryst., Vol. 408, pp. 33–43, 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/1542140049025801
DISCOTIC SIDE GROUP LIQUID CRYSTAL POLYMER
ELECTROLYTES
Corrie T. Imrie*, Rachel T. Inkster, Zhibao Lu,
and Malcolm D. Ingram
Department of Chemistry, University of Aberdeen, Meston
Walk, Old Aberdeen AB24 3UE, Scotland, UK
The synthesis and characterization is reported of a novel discotic side group
polymer based on a predominantly poly(ethylene oxide) backbone to which are
attached triphenylene-based mesogenic groups via flexible spacers. The thermal
and mechanical behavior of this polymer and its complex with lithium per-
chlorate are compared. The addition of salt increased both the glass transition
temperature and clearing point of the host polymer. The temperature depen-
dence of the ionic conductivity of the polymer–salt complex is reported and
reveals that ionic conduction can occur in essentially solid-like liquid crystal
polymer electrolytes. Ion transport in this material presumably involves
assistance from local segmental motions of the backbone but does not rely upon
long range cooperative motions.
Keywords: side group liquid crystal polymers; polymer electrolytes; discotic; ionic conductivity;
ion transport mechanism
INTRODUCTION
A polymer electrolyte is an ionically conducting solid or elastomeric phase
comprised of a salt dissolved in a polymeric solvent [1]. The demand for all
solid-state electrochemical devices and, perhaps most notably, lithium
batteries continues to stimulate interest in this class of materials, although
it has not proved possible to design solvent-free solid polymers with ionic
conductivities reaching the target level of ca 10ꢀ3 S cmꢀ1 at ambient
temperature. To understand the root of this problem we must consider the
mechanism of ion transport in polymer electrolytes, which typically consist
of lithium salts in polyether systems. In such a system it is generally
Presented at the International Conference on Discotic Liquid Crystals, ICTP, Trieste, 25–29
November 2002.
*Corresponding author. E-mail: c.t.imrie@abdn.ac.uk
33

34
C. T. Imrie et al.
accepted that ion transport involves segmental motions of the host polymer
and thus significant levels of ionic conductivity will exist only above the
glass transition temperature, Tg [2]. We have described recently, however, a
new class of liquid crystal polymer electrolytes which exhibit significant
levels of ionic conductivity in the liquid crystalline and glassy states [3,4]. In
these polymers, mesogenic (4-methoxybiphenyl) groups are attached via
alkyl spacers to a predominantly poly(ethylene oxide)-based backbone
containing isophthalate linking groups [5–7].
In order to investigate how the nature of the liquid crystalline phase
effects ionic conductivity in this class of polymers, here we report the
synthesis and characterization of a novel polymer electrolyte containing
triphenylene-based discotic moieties attached via alkyl spacers to a pre-
dominantly poly(ethylene oxide) backbone Scheme 1.
The acronym used to refer to this polymer is TPC10G6. The thermal,
mechanical, and conducting properties of the 14:1 TPC10G6: LiClO4
complex are also reported. The concentration of lithium ions in the com-
plex is described as the ratio of active oxygens to lithium ions. An active
oxygen is defined in terms of its ability to coordinate the lithium ion, which
is influenced both by steric factors and donicities of individual oxygens.
Rather arbitrarily, we have assumed that all the oxygen atoms in the
polymer backbone may be counted except for the carbonyl oxygens. Thus,
TPC10G6 contains 7 active oxygens per repeat unit.
EXPERIMENTAL
TPC10G6 was prepared using the synthetic route shown in Scheme 1. The
syntheses of 1,2-dihexyloxybenzene, 1, [8] 1-hexyloxy-2-methoxybenzene,
4, [9] 2-methoxy-3,6,7,10,11-pentahexyloxytriphenylene, 5, [9] 2-hydroxy-
3,6,7,10,11-pentahexyloxy-triphenylene, 6, [9] and 2-(10-bromodecyloxy)-
3,6,7,10,11-pentahexyloxy-triphenylene, 7, [10] have been described in
detail elsewhere.
4-Iodo-1,2-dihexyloxybenzene, 2
1,2-dihexyloxybenzene (55.7 g, 0.20 mol) was added to a solution of glacial
acectic acid 150 mL), water (25 mL), dichloromethane (30 mL) and con-
centrated sulphuric acid (1.0 mL). Iodine (20 g) and then iodic acid (8 g)
were added and the mixture heated with stirring at 40ꢁC overnight. The
reaction progress was monitored by 1H NMR spectroscopy. After the
reaction was complete, sodium hydrogensulphite solution (ꢂ5 g sodium
metabisulphite in ꢂ100 mL water) was added and the red color
disappeared after stirring for 10 min. The product was extracted using
dichloromethane and the organic layer rinsed with potassium carbonate

Discotic Polymer Electrolytes
35
SCHEME 1 Synthetic route for the preparation of TPC10G6.

36
C. T. Imrie et al.
solution to neutralize the remaining acids. The organic layer was dried over
magnesium sulfate, concentrated and passed through a short column (silica
gel, dichloromethane) to give the product 4-iodo-1,2-dihexyloxybenzene,
a yellow oil (76 g, 94%).
dH: 7.14 (dd, J3 ¼ 8.3 Hz, J4 ¼ 2.0 Hz, 1H), 7.12 (d, J4 ¼ 2.0 Hz, 1H), 6.58 (d,
J3 ¼ 8.3 Hz, 1H), 3.99 (4H, t, ArOCH2, J¼ 6.7), 1.81 (4H, m, ArOCH2CH2),
1.2–1.6 (12H, m, ArOCH2CH2(CH2)3), 0.90 (6H, t, CH3, J¼ 6.9).
3,30,4,40-Tetrahexyloxybiphenyl, 3
4-iodo-1,2,-dihexyloxybenzene (19.9 g, 49.2 mmol) was added to copper
powder (20 g) and mixed thoroughly. The mixture was heated carefully to
ꢂ270ꢁC, and the temperature increased to ꢂ310ꢁC due to the exothermic
reaction. After cooling, the product was extracted with dichloromethane
and concentrated under vacuum. Ethanol was added to initiate crystal-
lization to give 3,30,4,40-tetrahexyloxbiphenyl (5.9 g, 43%, m.p. 68–70ꢁC).
dH: 7.04 (4H, m, ArH), 6.90 (2H, d, ArH, J ¼ 6.9), 4.03 (8H, t, ArOCH2,
J ¼ 7.3), 1.82 (8H, m, ArOCH2CH2), 1.2–1.6 (24H, m, ArOCH2CH2(CH2)3),
0.89 (12H, t, CH3, J ¼ 6.7).
Discotic Monomer, 8
A mixture of 2-(10-bromodecyloxy)-3,6,7,10,11-pentahexyloxytriphenylene
(8.0 g, 8.3 mmol), dimethyl-5-hydroxyisophthalate (2.0 g, 9.5 mmol) and
potassium carbonate (4.7 g, 34 mmol) in acetone (50 mL) was heated at
reflux with stirring for 48 h. The cooled reaction mixture was poured into
water and filtered. The residue was recrystallized from hot ethanol. Due to
the low melting point of the product, an oil tended to form, and form this
mixture, crystals were decanted off. This procedure was repeated several
times giving white crystals of 8.
dH: 8.24 ppm (1H, s, ArH), 7.82 (6H, s, ArH), 7.72 (2H, s, ArH), 4.22
(12H, t, ArOCH2, J ¼ 6.6), 4.02 (2H, t, ArOCH2, J ¼ 6.7), 3.92 (6H, s,
ArCOOCH3), 1.93 (14H, m, ArOCH2CH2), 1.3–1.6 (42H, m, CH2), 0.92
(15H, t, CH3, J ¼ 6.9). There was some ethanol exchange with the methyl
ester groups, which gave rise to a quartet at 4.4 ppm. IR(KBr) n 1723 cmꢀ1
(CO stretch). Melting point (DSC) 37ꢁC, a glass transition observed at
ꢀ17ꢁC.
TPC10G6 Polymer, 9
8 (5.1 g, 4.7 mmol) and freshly distilled PEG 300 (1.3 g) were flushed with
argon. This mixture was heated with stirring until fully molten (ꢂ100ꢁC).

Discotic Polymer Electrolytes
37
Two 0.5 mL portions of catalyst (10%v /v titanium(IV) isopropoxide in
2-methoxyethyl ether) were added approximately an hour apart and the
temperature increased to 120ꢀC. The pressure was then reduced over 5 h to
l mmHg; finally the temperature was increased to 150 ꢀC. The reaction was
held at this temperature and pressure for 24 h until the production of a
liquid was no longer evident. The polymer was purified by dissolving the
reaction mixture in a small volume of chloroform and precipitating into a
large volume of hexane.
dH: 8.23 ppm (1H, ArH), 7.81 (6H, ArH), 7.71 (2H, ArH), 4.46 (4H,
COOCH2), 4.21 (12H, ArOCH2), 4.0 (2H, ArOCH2), 3.8 (4H, COOCH2-
CH2O), 3.6 (16H, PEG OCH2), 1.7–2.0 (14H, ArOCH2CH2), 1.3–1.7 (42H,
(CH2)21), 0.91 (15H, CH3). IR(KBr) n 1723 cmꢁ1 (CO stretch). GPC
(polystyrene standards) Mw ¼ 10,800 gmolꢁ1, Mn ¼ 5,200 gmolꢁ1
.
Preparation of TPC10G6:LiClO4 Complexes
The polymer TPC10G6 and salt LiCIO4 were codissolved in freshly distilled
THF. The mixture was stirred and flushed with argon until most of the
solvent was removed. The complex was dried by heating under vacuum for
at least a week. The conductivity cells were prepared by melting the
complex onto the electrode by heating under vacuum for a week, to ensure
the sample was dry.
Characterization
The proposed structures of all the intermediates and the final polymer have
1
been verified using H-NMR and IR spectroscopy. The thermal character-
ization of TPC10G6 and its salt complex was performed by differential
scanning calorimetry (DSC) and dynamic mechanical thermal analysis
(DMTA). DSC measurements were made using a Mettler Toledo 821 DSC
equipped with a liquid nitrogen cooling system and calibrated using indium
as a standard. All heating and cooling rates were 10ꢀC minꢁ1. The DMTA
measurements were made using a PC-controlled Rheometric Scientific
DMTA equipped with a Mk III analyzer operating in the shear mode.
Measurements were recorded at a heating rate of 2ꢀC minꢁ1. Phases were
studied using polarized light microscopy using an Olympus BH2 polarized
light microscope equipped with a Linkam THMS heating stage, LNP2
cooling pump and TMS control unit.
The ionic conductivity of the polymer salt-complex was measured by
standard ac impedance methods using a Schlumberger 1260 impedance
analyzer over a frequency range 1–106 Hz. Complex impedance plots were
obtained using Zview software.

38
C. T. Imrie et al.
RESULTS AND DISCUSSION
The DSC trace for the initial heat of TPC10G6 is shown in Figure 1. A weak
glass transition occurs at ꢀ17ꢁC, and an endotherm is observed at 47ꢁC
with an associated enthalpy change of 33.4 J gꢀ1. On Cooling from the
isotropic phase a poorly defined birefringent texture is observed when
viewed under the polarizing microscope (see Figure 2(a)). This texture
remained unchanged after annealing for prolonged periods and was not
suitable for the assignment of the phase. The phase is highly viscous,
strongly suggesting either a highly ordered columnar phase or a disordered
crystal phase.
The 14:1 TPC10G6:LiClO4 complex shows a glass transition at ꢀ11ꢁC
and a clearing transition at 52ꢁC with an associated enthalpy change of
24.0 J gꢀ1, (see Figure 1). A broad week peak is also evident with a max-
imum at 21ꢁC. A characteristic optical texture could not be obtained for the
mesophase (see Figure 2(b)), and again it is probable that the phase is a
FIGURE 1 DSC reheat traces for TPC10G6 (initial heat, lower trace) and 14:1
TPC10G6:LiC1O4 complex (reheat, upper trace).

Discotic Polymer Electrolytes
39
highly ordered columnar arrangement. X-ray diffraction experiments are in
progress to determine the phase structure.
The addition of salt increased both the glass transition temperature
and clearing point of the host polymer. Similar effects were observed for
calamitic liquid crystal polymer electrolytes [5–7]. The increase in the glass
transition temperature reflects the loss of segmental mobility arising from
the coordination of lithium ions by the ether oxygens of the backbone. The
concurrent increase in the clearing point may be accounted for in terms of
the preferential dissolution of the ions in the backbone domains, which
serves to reinforce the microphase separation between the backbones and
discotic side groups that drives mesophase formation.
FIGURE 2 Optical textures for (a) TPC10G6 and (b) 14:1 TPC10G6:LiClO4
complex. (See COLOR PLATE I)

40
C. T. Imrie et al.
The temperature dependence of the real component of the shear mod-
ulus for TPC10G6 is shown in Figure 3. A softening of the polymer is seen
at ꢀ10ꢁC presumably associated with the glass transition. The major
decrease in the modulus, however, occurs at the clearing temperature. A
shoulder is evident on the trace, suggesting that there is a relaxation just
below the clearing point. The addition of salt shifts the onset of softening to
ca 0ꢁC (see Figure 3), consistent with the observed increase in the glass
transition temperature observed using DSC. Again the sudden decrease in
modulus is associated with the clearing transition, only now the change in
modulus associated with the second event is more pronounced than that
seen for the host polymer (Figure 3). This change in modulus is pre-
sumably associated with the transition observed at 21ꢁC by DSC.
The ionic conductivity of the 14:1 TPC10G6:LiClO4 complex is shown as
an Arrhenius plot in Figure 4. Two regions of Arrhenius behavior can be
seen with a kink separating these at a temperature essentially coincident
with the clearing point. The Arrhenius plot is steeper below the clearing
temperature, i.e., the activation energy for ion transport is larger, which
suggests that the onset of self-assembly is bad for ionic conductivity. It is
FIGURE 3 Temperature dependence of the real component of the shear modulus
of TPC10G6 (lower trace) and the 14:1 TPC10G6:LiClO4 complex.

Discotic Polymer Electrolytes
41
FIGURE 4 Arrhenius plot for conductivity of the 14:1 TPC10G6:LiClO4 complex.
remarkable, however, that given the first-order nature of this transition in
terms of both the thermodynamic and mechanical properties a step change
in ionic conductivity is not clearly evident in these data.
The atypical behavior of the TPC10G6:LiClO4 complex is further
emphasized when the thermal, mechanical, and conductivity data are
overlaid (Figure 5). It seems again that there is no clear discontinuity in the
conductivity data at the clearing point. Furthermore, appreciable con-
ductivities are observed at temperatures for which the shear modulus is
approaching that of a solid-like material.
We have suggested in the past that the decoupling of ion transport
from the segmental motions of the polymer matrix in calamitic side group
crystal polymer electrolytes stems from the suppression of helix formation
involving the poly(ethylene oxide)-based backbones, the suppression
arising from the jacketing effect of the mesogenic side chains [4]. The
resulting open structure and multiplicity of ion sites may be giving rise to a
conductivity mechanism somewhat analogous to that found in inorganic
glasses. It may seem reasonable to invoke a similar mechanism to account
for the ionic conductivity of the discotic material seen here at low
temperatures.

42
C. T. Imrie et al.
FIGURE 5 Dependence of the log (ionic conductivity) (open circles), log (real
component of the shear modulus) (dashed line), and heat flow in the DSC
experiment (solid line) on reciprocal temperature of the 14:1 TPC10G6:LiClO4
complex.
CONCLUSIONS
We have shown that ionic conduction can occur in essentially solid-like
liquid crystal polymer electrolytes. The challenge is now to establish the
mechanism of ion transport in these systems, which presumably involves
assistance from local segmental motions of the backbone but does not
involve long-range cooperative motions. To clarify this further we are now
undertaking measurements of the ionic conductivity as a function of
pressure, where we will gain further insights from variations in activation
volume [11].
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[1] Armand, M. (1994). Solid State Ionics, 69, 309–319.
[2] Angell, C. A., Imrie, C. T., & Ingram, M. D. (1998). Polym. Int., 47, 9–15.
[3] Imrie, C. T., & Ingram, M. D. (2000). Mol. Cryst. Liq. Cryst., 347, 443–454.

Discotic Polymer Electrolytes
43
[4] Imrie, C. T., & Ingram, M. D. (2001). Electrochim. Acta, 16, 1413–1417.
[5] McHattie, G. S., Imrie, C. T., & Ingram, M. D. (1998). Electrochim. Acta, 43, 1151–1154.
[6] Imrie, C. T., Ingram, M. D., & McHattie, G. S. (1999). J. Phys. Chem. B, 103, 4132–4138.
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