We report the results of a combined work based on density functional theory (DFT) calculations and experiments of the factors that influence the glass temperature, T g , and the associated ion conductivity in polymerized ionic liquids bearing imidazolium salts in the side group. This study consists of four different N-alkyl side-chain lengths [with n = 4 (butyl), 6 (hexyl), 8 (octyl), and 10 (decyl)] and seven different counteranionsDFT calculations of the anion−cation complexation energies were combined with thermodynamics (differential scanning calorimetry), structural (X-ray scattering), as well as temperature-and pressure-dependent dielectric spectroscopy measurements of ion conduction. Our results show that ion conduction is facilitated by local anion jumps with a length scale on the order of the charge alteration distance. Ion complexation strongly influences the backbone dynamics and the associated T g . A simple "stick and jump" model can account for the increased backbone mobility (reduced T g ) and the concomitant enhanced ion conductivity for anions with intermediate size. Among the different anions, [TFSI] − with its comparably large size and broad charge delocalization is only weakly coordinated with the cation. This best facilitates anion motion within the "ion paths" of the hexagonally packed cylinders and smectic morphologies.
Single ion conductors, based on polymerized ionic liquids (PILs) with a polythiophene backbone bearing imidazolium salts with butyl, hexyl, octyl, and decyl side groups and six different counteranions ([Br]−, [BF4]−, [ClO4]−, [PF6]−, picrate, and [B(Ph)4]−), are synthesized and studied with respect to the thermal, structural, and ion-conductivity properties. PILs bearing the polythiophene backbone are unique as they can simultaneously conduct electronic charge and ions at nanometer length scales. In addition, the π–π stacking of the polythiophene backbones results in exceptional smectic-like order. Increasing side group length from butyl to decyl increases the room temperature conductivity by 4 orders of magnitude (internal plasticization). Anion size (anionic radii from 0.19 to 0.44 nm) affects both the structure (from smectic-like to amorphous with increasing anion radius) and the ionic conductivity. Conductivity values differ by 6 orders of magnitude by varying anion size at ambient temperature. As a result, conductivities as high as 2 × 10–3 S/cm could be obtained at high temperatures. Differences in conductivity are discussed in terms of changes in glass temperature (T g), anion size, and value of dielectric permittivity. Overall ion transport in PILs based on polythiophene backbones is controlled by the low T g, value of dielectric permittivity, smectic layering, and ion association lengths not exceeding a single smectic layer.
Densely grafted poly(ethylene oxide) (PEO) brushes on a poly(hydroxylstyrene) (PHOS) backbone (PHOS-g-PEO) as well as block copolymers with polystyrene (PS) (PS-b-(PHOS-g-PEO)) are designed as model systems for Li ion transport. This macromolecular design suppresses the propensity of PEO chains for complex crystal formation with LiTf as well as for crystallization. Li ion conductivities similar or even exceeding those in the archetypal electrolyte poly(ethylene oxide)/lithium triflate (PEO/LiCF3SO3 (LiTf)) are obtained for a range of temperatures and LiTf compositions. At the same time, PHOS-g-PEO and PS-b-(PHOS-g-PEO) show improved mechanical stability. Typically, at 333 K, the ionic conductivity is ∼6 × 10–5 S/cm and the modulus at ∼2 × 106 Pa for a [EO]:[Li+] = 8:1 composition. In the endeavor for suitable solid polymer electrolytes macromolecular architecture seems to play a decisive role.
The segmental dynamics and the corresponding glass temperature, T g, were investigated in a monocyclic and in the corresponding linear polystyrene as well as in a series of multicyclic polystyrenes, all with the same total molecular weight, with dielectric spectroscopy and DSC. There is a strong reduction of T g with decreasing molecular weight for linear chains but only a moderate reduction for cyclic chains and this below a certain critical molecular weight (M n ∼ 18 000 g/mol). These data contradict the Gibbs–Di Marzio lattice model predicting an increasing glass temperature with decreasing molecular weight of cyclic polymers. In multicyclic polystyrenes the results emphasize the role of constrained segments at the coupling sites (linkers) on determining practically all features of segmental dynamics: the exact temperature dependence of relaxation times and associated T g, the dielectric strength, the distribution of relaxation times, and fragility. A nearly linear increase of T g was found with increasing number of intramolecular constraints. Furthermore, the total molecular weight is an irrelevant parameter in discussing the dynamics of multicyclic polymers. An alternative approach that is based on the concept of free volume emphasizes intermolecular contributions and predicts the same amount of fractional free volume for multicyclic polystyrenes at their respective glass temperature (3.3%) but differences in the respective thermal expansion coefficient of free volume.
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