Pressure Effects on the Dynamic Heterogeneity of Miscible Poly(vinyl acetate)/Poly(ethylene oxide) Blends

The poly­(vinyl acetate) (PVAc) segmental dynamics is studied as a function of composition, temperature, and pressure in thermodynamically miscible blends with poly­(ethylene oxide) (PEO) by dielectric spectroscopy. In the PVAc-rich blends all short-range correlations are dominated by the PVAc component. An invariant frequency dispersion is found when the spectra at each blend composition are compared under isochronal conditions. The self-concentration model with a fixed PVAc self-concentration of ∼0.22 qualitatively describes the temperature dependence of the PVAc segmental dynamics both at atmospheric and at elevated pressures

Relating Structure, Viscoelasticity, and Local Mobility to Conductivity in PEO/LiTf Electrolytes

The phase state, local structure, local mobility, and viscoelastic response have been studied in the archetypal polymer electrolyte (PEO)_<i>x</i>LiCF₃SO₃ with ether oxygen to lithium ion ratio of 2 ≤ [EO]/[Li] ≤12 over a broad temperature range in an effort to explore the factors controlling ionic conduction. We confirm that the crystal structure of the complex is identical to the (PEO)₃LiCF₃SO₃ polymer electrolyte independent of the [EO]:[Li] content. Heating the nonstoichiometric compositions result in progressive melting of the complex, whereas the complex formed at or near the stoichiometric composition remains stable up to the liquidus temperature. The temperature dependence of dc conductivity is neither Arrhenius nor VFT. Its temperature dependence is more complex reflecting the underlying structural changes. Surprisingly, ionic conduction takes place both within the crystalline complex and in the amorphous phase with the latter having the major contribution. The (PEO)₁₂LiCF₃SO₃ polymer electrolyte is the one with the highest conductivity at all temperatures investigated. The linear viscoelastic properties were studied as a function of temperature at two compositions. The different phases have distinct viscoelastic signatures. The complex formed at or near stoichiometric composition has a predominantly <i>elastic</i> response, whereas the more dilute compositions (consisting of the crystalline complex and an ion-containing amorphous phase) have a <i>viscoelastic</i> response and an ultraslow relaxation. Local polymer relaxation and ionic mobility are completely coupled. It is suggested that local ion jumps at subsegmental level are responsible for the measured conductivity

8OCB and 8CB Liquid Crystals Confined in Nanoporous Alumina: Effect of Confinement on the Structure and Dynamics

The effect of oxygen substitution is studied in two homologous compounds of <i>n</i>-cyanobiphenyls with <i>n</i> = 8 in the bulk and under confinement within self-ordered nanoporous alumina (AAO). Oxygen substitution in 8OCB increases the dipole moment and stabilizes the crystalline, smectic, and nematic phases to higher temperatures relative to 8CB. Within their smectic– <i>A</i> (<i>SmA</i>) phase both 8CB and 8OCB behave as weak viscoelastic solids with low shear moduli reflecting the underlying supramolecular defect structure. Dielectric spectroscopy assisted by DFT calculations identified strong dipolar associations within the isotropic phases characterized by a Kirkwood–Fröhlich interaction parameter, <i>g</i> ∼ 0.36. Dielectric spectroscopy further identified a slow process (∼ kHz) of low dielectric strength. The proximity of this process to the rheology time scale suggests as common origin a cooperative relaxation of the defect structure. Confinement alters the phase diagram by stabilizing certain crystalline phases and by reducing the <i>N</i>–<i>I</i> transition temperature in agreement with surface tension effects. However, the <i>N</i>–<i>I</i> transition seems to retain its first order character. Surface treatment with <i>n</i>-decyltrichlorosilane results in destabilization of the <i>SmA</i> phase at the expense of the <i>N</i> phase. This is consistent with a picture of surface anchored LC molecules at the pore walls that stabilize the nematic phase

Cyclic Topologies in Linear α,ω-Dihydroxy Polyisoprenes by Dielectric Spectroscopy

A series of mono- and di-functionalized polyisoprenes (PIs) bearing hydroxyl (OH−) end-group(s) with different molar masses ranging from 2 to 30 kg mol–1 were synthesized and studied by a combination of temperature- and pressure-dependent dielectric spectroscopy and rheology. In the di-functionalized PIs, the −OH end-group interactions result in a mixture of linear and cyclic configurations (up to 45% cyclic configurations for the lower molar masses). The formation of cyclic topologies due to increased H-bonding interactions restricted the backbone mobility and increased the glass temperature, Tg, especially for the lower molar masses. Moreover, an additional process (termed α*) was evidenced in the dielectric spectroscopy in the range between the segmental process and the global chain relaxation. It followed a Vogel–Fulcher–Tammann temperature dependence, freezing at a temperature in the vicinity of the liquid-to-glass temperature, being independent of molar mass. Its molecular origin was identified by employing the pressure sensitivity of the characteristic relaxation times and the pressure dependence of Tg. It reflects the relaxation of segments in the vicinity of the H-bonded groups. Overall, this study provided information on the impact of weakly associating polar end-groups (hydroxyl) on the molecular dynamics of type-A polymers. Furthermore, it suggested promising routes for designing polymers with a higher concentration (>50%) of cyclic topologies, for example, by employing (i) short chains with (ii) strongly associating end groups (stronger than the hydroxyl end-groups)

Dynamics of Unentangled <i>cis</i>-1,4-Polyisoprene Confined to Nanoporous Alumina

The dynamics of unentangled <i>cis</i>-1,4-polyisoprene confined within self-ordered nanoporous alumina (AAO) is studied as a function of molecular weight (5000–300 g/mol) and pore size (400–25 nm) with dielectric spectroscopy. The main effects are the pronounced broadening of both segmental and chain modes with decreasing AAO pore diameter. This suggests that the global chain relaxation is retarded on confinement. Remarkably, the distribution of relaxation times is broadened even within pores with size 50 times the unperturbed chain dimensions. The glass temperature is unaffected by confinement. These results are discussed in terms of confinement and adsorption effects. Confinement effects are negligible for the studied molecular weights. Chain adsorption, on the other hand, involves time and length scales distinctly different from the bulk that can account for the experimental findings

Molecular Dynamics and Viscoelastic Properties of the Biobased 1,4-Polymyrcene

We report the synthesis and dynamics of a series of polymyrcene homopolymers, all with identical microstructures (95% 1,4-units and 5% of 3,4-units), the latter by dielectric spectroscopy and rheology. Polymyrcene belongs to an important class of bio-based polymers (polyterpenes) with known members, the cis-1,4-polyisoprene and the cis-1,4-polyfarnesene. Polyterpenes constitute a class of type-A polymers where by architectural design, one can control the thickness of chains and henceforth the segmental and chain dynamics. The dielectric, rheology, and thermodynamic results showed a lower glass temperature in cis-1,4-polymyrcene as compared to cis-1,4-polyisoprene. A weak dependence of the segmental and longest normal mode on pressure revealed the dominant effect of the backbone. Furthermore, comparing polymyrcene with available literature data of polyisoprene and polyfarnesene, we report the effect of chain thickening on the viscoelastic properties. The plateau modulus, GN0, decreased, the entanglement molar mass increased (from 5 kg·mol–1 in cis-1,4-polyisoprene to 22 kg·mol–1 in cis-1,4-polymyrcene), and the packing length, p, increased (from 3.1 Å in cis-1,4-polyisoprene to 4.7 Å in cis-1,4-polymyrcene) as anticipated by chain thickening. The plateau modulus, GN0, followed the proposed empirical relation: GN0 = 0.00226kBT/p3, further reflecting the proportionality between the tube diameter and the packing length

Ionic Conductivity, Self-Assembly, and Viscoelasticity in Poly(styrene‑<i>b</i>‑ethylene oxide) Electrolytes Doped with LiTf

Diblock copolymers of poly­(styrene-<i>b</i>-ethylene oxide), PS-<i>b</i>-PEO, are employed together with lithium triflate (CF₃SO₃Li, LiTf) at several [EO]:[Li] ratios as solid polymer electrolytes. Their thermodynamic state, self-assembly, and viscoelastic properties are discussed in conjunction with the ionic conductivity. PS-<i>b</i>-PEO/LiTf differs from the well-investigated PS-<i>b</i>-PEO/LiTFSI system in that the electrolyte in the former binds intramolecularly to PEO chains. Microscopic and macroscopic parameters affecting ion transport are discussed. From a microscopic point of view different parameters were considered as potential regulators of ion transport: the characteristic domain spacing, <i>d</i>, the interfacial thickness, Δ, and the ratio of Δ/<i>d</i>. By comparing two block copolymer electrolytes (PS-<i>b</i>-PEO and PI-<i>b</i>-PEO) bearing the same conducting block (PEO) and the same electrolyte (LiTf) but in the presence of different interactions, among the microscopic parameters it is the domain spacing that appears to have the most decisive role in ionic conductivity. Ion conductivity in PS-<i>b</i>-PEO/LiTf exhibits a molecular weight dependence similar to that reported for the PS-<i>b</i>-PEO/LiTFSI system, however, with somewhat lower values reflecting anion size effects. Among the macroscopic factors that limit ionic conductivity, the possible preferential wetting of the electrodes by either of the constituent phases can lead to an orientation that effectively blocks ion transport. The viscoelastic properties of the block copolymer electrolytes differ substantially from the neat block copolymers. Li-ion coordination affects not only the PEO segments but also, surprisingly, the PS segments. An increase in PS glass temperature by ∼10 K is reported. In addition, the viscoelastic properties suggest the formation of transient structures in the molten complex

Segmental Dynamics in Multicyclic Polystyrenes

The segmental dynamics and the corresponding glass temperature, <i>T</i>_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 <i>T</i>_g with decreasing molecular weight for linear chains but only a moderate reduction for cyclic chains and this below a certain critical molecular weight (<i>M</i>_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 <i>T</i>_g, the dielectric strength, the distribution of relaxation times, and fragility. A nearly linear increase of <i>T</i>_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

Ion Size Approaching the Bjerrum Length in Solvents of Low Polarity by Dendritic Encapsulation

The Bjerrum length is approached in a low polarity solvent by encapsulating, both, a borate anion and a phosphonium cation in a rigid lipophilic dendrimer shell. In addition the cation size is varied by 34-fold. We thus obtain superweak ions with potential applications in catalytic processes

Order, Viscoelastic, and Dielectric Properties of Symmetric and Asymmetric Alkyl[1]benzothieno[3,2-b][1]benzothiophenes

The morphology, the viscoelastic, the dielectric properties and the dynamics of phase transformation are studied in symmetrically and asymmetrically substituted alkyl[1]­benzothieno­[3,2-<i>b</i>]­[1]­benzothiophenes (C₈-BTBT) by X-ray scattering, rheology, and dielectric spectroscopy. The interlayer spacing reflects the molecular and supramolecular ordering, respectively, in the symmetrically and asymmetrically substituted BTBTs. In the asymmetric BTBT, the core layer is double in size with a broader network of intermolecular interactions though the increased S–S contacts that is prerequisite for the development of high performance OFET devices. Two crystal states with elastic and viscoelastic responses were identified in the symmetric compound. In contrast, the SmA phase in the asymmetric compound is a viscoelastic solid. A path-dependent dielectric environment with a switchable dielectric permittivity was found in both compounds by cooling below 0 °C with possible implications to charge transport. The kinetics of phase transformation to the crystalline and SmA phases revealed a nucleation and growth mechanism with rates dominated by the low activation barriers