A discotic liquid crystal triblock copolymer consisting of a central main chain triphenylene-based liquid crystal block capped at both ends by blocks of poly(ethylene oxide) (PEO) (MW=2000gmol−1) has ...been doped with lithium perchlorate in an EO:Li 6:1 ratio. The polymer electrolyte exhibits a phase separated morphology consisting of a columnar hexagonal liquid crystal phase and PEO-rich regions. The polymer electrolyte forms self-supporting, solid-like films. The ionic conductivity on initial heating of the sample is very low below ca. 60°C but increases rapidly above this temperature. This is attributed to the melting of crystalline PEO-rich regions. Crystallisation is suppressed on cooling, and subsequent heating cycles exhibit higher conductivities but still less than those measured for the corresponding lithium perchlorate complex in poly(ethylene glycol) (MW=2000gmol−1). Instead the triblock copolymer mimics the behaviour of high molecular weight poly(ethylene oxide) (MW=300,000gmol−1). This is attributed, in part, to the anchoring of the short PEG chains to the liquid crystal block which prevents their diffusion through the sample. Temperature and pressure variations in ion mobility indicate that the ion transport mechanism in the new material is closely related to that in the conventional PEO-based electrolyte, opening up the possibility of engineering enhanced conductivities in future.
A fresh analysis of literature data shows how the influences of temperature and pressure on ion transport and structural relaxation in glass-forming systems may be combined within the framework of ...‘master plots’ based on the equation
E
A
=
M
·
V
A
, to reveal new insights into coupling and decoupling effects in a wide range of systems.
E
A,σ
and
V
A,σ
are, respectively, instantaneous activation energies and volumes for ionic conductivity and the parameter,
M
σ
, is a corresponding ‘process modulus’. For structural relaxations occurring at the glass transition, the appropriate modulus is given by
M
s
=
T
g
·
d
P/d
T
g
. We can now identify typical behaviour patterns for fragile liquids on the one hand, and typical inorganic glasses on the other. Thus, the parameters,
M
σ
and
M
s
, for fragile systems such as molten Ca(NO
3)
2:KNO
3 (CKN) or a typical polymer electrolyte such as a complex of LiCF
3SO
3 in PPG, are found to remain constant over a wide range of temperatures down to
T
g
, despite changes in the temperature (and pressure) dependences of the ionic conductivities, as indicated for example by a return to Arrhenius behaviour in the case of CKN, or by so-called Stickel plots and changes in the VTF parameters for the polymer electrolytes. If
E* and
V* are activation energies and volumes assigned to elementary steps, when again
E*
=
M
·
V*, we can go further and identify the microscopic processes driving forward structural relaxation. In the case of inorganic glasses, where usually we find the decoupling index
R
τ
≈
10
12, we identify two distinct decoupling paradigms represented by strong and fragile systems respectively, where in both cases the activation volumes for ion transport are very similar to the corresponding ionic volumes. In the former case (typified by the strongly cross-linked silicate and aluminosilicate systems), the negative activation volumes for structural relaxation (negative values of d
T
g
/d
P) are clearly indicative of a ‘water-like’ behaviour attributable to the collapse of the network under pressure. On the other hand, for the more fragile fast-ion conducting silver iodomolybdate glass, the experimental results show that
M
s
(at
T
g
)
≈
M
σ
(in glass), implying some recoupling of structural relaxation to ion transport. Arguments based on the dynamic structure model lead us to predict that a similar close link should exist between
M
s
(at
T
g
) and
M
σ
in the relatively fragile lithium and sodium borate glasses, thus highlighting the need for more information concerning the effects of pressure on the glass transition temperatures of common inorganic glasses.
►
E
A
=
MV
A
where
M is a process modulus for ion transport in glasses and polymers. ► Two decoupling paradigms describe strong and fragile glassy behaviours. ► For fragile melts and polymers
M
s
is independent of temperature and equal to
M
s
=
T
g
d
P/d
T
g
. ►
M
s
is a constant in polymer electrolytes exhibiting liquid-liquid transitions. ► We predict
M
s
=
M
s
=
T
gdP
/
dT
g
for the fragile alkali-rich borate glasses.
Several strategies are investigated for ‘activating’ polypyrrole electrodes for use in electrochemical supercapacitors. These include: the development of columnar morphologies by micellar deposition, ...self-doping by attachment of anions, and the use of aryl sulfonates to promote cross-linking and hydrophilicity. The key to improved performance, especially in this last example, is the apparent coupling of doping processes to structural relaxations that encourage solvent uptake by the polymer and ready access for dopant ions to all available sites. Thick (15–20 μm) films of polypyrrole activated in this way can be charged and discharged reversibly at scan rates up to 300 mV s
−1, indicating a possible use in high-power supercapacitors.
Polypyrrole (pPy) electrodes containing polysulfonated aromatic anions are investigated by cyclic voltammetry as electrodes for use in electrochemical supercapacitors. These ‘ladder-doped’ materials ...are deposited as thick films (ca. 10
μm) having open structures that permit the rapid insertion/ejection (up to 300
mV
s
−1) of cations and anions from aqueous solution, and give an effective or ‘geometric’ capacitance of up to 0.40
F
cm
−2. The ‘dual mode’ doping behaviour seems essential for good capacitive response. These electrodes show a remarkable tendency to perform better at high cycling rates, an effect attributed to the way the structure ‘self-organises’ during the self-doping process. Good electrode response depends on protecting the open structure containing hydrophilic ion-conducting channels.
A range of analytical techniques (DSC, conductivity measurement, Raman spectroscopy, small- and wide-angle X-ray diffraction (S-WAXS), quasi-elastic neutron scattering (QENS), and single-crystal ...X-ray diffraction) are applied to the characterization of the phase behavior of the low-melting-point liquid crystalline salts 1-hexadecyl-3-methylimidazolium hexafluorophosphate (C16mimPF6) and 1-methyl-3-tetradecylimidazolium hexafluorophosphate C14mimPF6. This is the first time that QENS has been applied to the structural analysis of this type of ionic liquid crystal. For the first time in this class of salts, a low-temperature phase transition is identified, which is assigned to a crystal−crystal transition. Conductivity and QENS data for C16mimPF6 suggest that the higher-temperature crystalline phase (CII) has greatly increased freedom in its long alkyl chain and anion than the lower-temperature crystalline phase (CI). This conclusion is supported by single-crystal X-ray diffraction results for C14mimPF6. In both crystalline phases, as well as in the higher-temperature mesophase, the structure maintains a monodispersed layer structure with interdigitated alkyl chains. The structure of the mesophase is confirmed as smectic A by the S-WAXS and Raman spectroscopy results. Detailed analysis suggests that in this phase the alkyl chains undergo complete conformational melting.
Kinetic data for structural relaxation in silver iodomolybdates at the glass transition temperature (T g) are obtained by high-pressure differential scanning calorimetry (HP-DSC) and are compared ...with activation energies (E A) and volumes (V A) obtained earlier from conductivities below T g. The results are fitted to an empirical equation, E A = MV A, and displayed in the form of a master plot of E A versus V A, an approach previously applied to strongly coupled systems, including polymer electrolytes and molten salts above their glass transition temperatures. The parameter M emerges as a localized modulus, expressive of interatomic forces within the medium, linking together E A, σ, V A, σ and E A,s, V A,s, the “apparent” activation parameters for ionic conductivity and structural relaxation, respectively. The V A and E A values for ion transport are much smaller than the corresponding values for structural relaxation. However, remarkably close agreement emerges between the “process parameters”, M s and M σ, both close to 8 GPa, thus establishing a quantitative link between ion transport and structural relaxation in this highly decoupled system. A new E A versus V A master plot is constructed, which points the way to a unified approach to ion transport in polymers and glasses.
Positron annihilation lifetime spectroscopy (PALS) and impedance spectroscopy (IS) have been employed to study the effect of temperature and pressure on the DC conductivity (
σ
DC) and the mean hole ...volume (
V
h) of a NaPF
6 ethylene oxide based polyurethane electrolyte. The DC conductivity of the polymer electrolyte displayed a characteristic non-Arrhenius temperature dependence yielding acceptable values for both the “pseudo-activation energy” (
B) and the “zero mobility temperature” (
T
0) from a VTF fit.
V
h(
T) showed a linear increase of 0.53
cm
3
(mol
K)
−1. When extrapolating
V
h(
T) to 0
K a temperature very close to
T
0 from the VTF fit was obtained, which suggests a free volume mediated conductivity mechanism. This suggestion is further supported by the linear dependence of ln(
σ
DC(
T)) on
V
h
−
1
(
T
)
. Conductivity was measured as a function of pressure (
σ
DC(
P)) with ln(
σ
DC(
P)) showing a characteristic decrease with increasing pressure. The activation volumes (
V
A) calculated from these measurements ranged from 45 to 20
cm
3
mol
−1 over a temperature from 304 to 365
K. Critical volumes calculated from two current free-volume models were found to be unrealistic. Combining the extra volume required for ionic motion (
V
A) with the available free volume (
V
h) at the same temperature poses a realistic and ‘model-free’ figure of 117
cm
3
mol
−1 for the critical volume at 304
K. This equates roughly to the volume of 3–4 EO units. The pressure dependence of free volume (
V
h(
P)) for a polymer electrolyte has been measured for the first time, and yielded a linear decrease in
V
h with increasing pressure. A linear dependence of
σ
DC(
P) on
V
h
−
1
(
P
)
was also found. A comparison of the isothermal and isobaric dependence of
σ
DC on
V
h
−
1
illustrates the contribution of factors other than free volume have on charge carrier number and mobility. This comparison shows that the variation of
V
h with temperature and the variation of
V
h with pressure affect the conductivity in very different ways. These results clearly show that free volume cannot be considered the sole factor responsible for conductivity in polymer electrolytes.
Evidence is presented for site relaxations occurring in mixed alkali (cation) glasses based on activation volumes, VA(σ)=−RTdlnσ/dpT, which are determined for sodium aluminoborate glasses of varying ...Na2O content, and for mixed alkali glasses where Na+ is partially replaced by Li+, K+ or Cs+ ions. In accordance with the ‘updated’ dynamic structure model, activation volumes are identified here with local expansions that accompany the opening up of C′ sites to admit incoming ions. ‘Anomalous’ increases in activation volume in mixed cation glasses correlate with the size of minority (guest) cations. This anomaly is interpreted in terms of a ‘leader follower’ mechanism that involves dynamic coupling between the faster (majority) and slower (minority) cations. Because of mismatch effects in mixed cation glasses this coupling requires the opening up of additional cation sites by the slower follower cations. The resulting disturbances in the glass network are responsible for many characteristic features of the mixed alkali effect, including the appearance of high temperature internal friction peaks and observed minima in glass transition temperatures and melt viscosities.
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