Charge Transport of Polyester Ether Ionomers in Unidirectional Silica Nanopores

Dielectric relaxation spectroscopy is employed to investigate charge transport properties of two polyester ether ionomers in the bulk state and when confined in unidirectional nanoporous membranes (average pore diameter = 7.5 nm). Under nanometric confinement in nonsilanized pores, the macroscopic transport quantities (dc conductivity and characteristic frequency rate) are lower by about 1.4 decades compared to the bulk. The remarkable decrease of transport quantities in nonsilanized nanoporous membranes can be quantitatively explained by considering the temperature dependence of the interfacial layer between the ionomer and the silica membrane surfaces. On the other hand, an enhancement of dc conductivity is observed when the surfaces of the pores are treated with a nonpolar organosilane. This effect becomes more pronounced at lower temperatures and is attributed to slight changes in molecular packing density caused by the two-dimensional geometrical constraint

Controlling Crystal Microstructure To Minimize Loss in Polymer Dielectrics

A model semicrystalline polymer, poly­(ethylene naphthalate) (PEN), was used to examine how morphological factors inhibit chain segment relaxations that contribute to dielectric loss. This was achieved by manipulating the extent of crystallization and the crystalline microstructure through a combination of annealing and uniaxial drawing and investigating the effects on dielectric performance. Varying crystallization conditions influenced the dynamic <i>T</i>_g and extent of rigid amorphous fraction formation but had only a moderate effect on loss magnitude. Film orientation, however, greatly reduced loss through strain-induced crystallization and the development of oriented amorphous mesophasic regions. Postdrawing annealing conditions were capable of further refining the crystal microstructure and, in turn, the dielectric properties. These findings demonstrate that the semicrystalline polymer morphology can have a very significant influence on amorphous chain relaxations that contribute to dielectric loss, and understanding how processing conditions affect morphology is critical to the rational design of polymer dielectrics

Effect of Thermal History on the Microstructure of a Poly(tetramethylene oxide)-Based Polyurea

The role of thermal history on the nanoscale segregated structure of a bulk polymerized polyurea containing oligomeric poly­(tetramethylene oxide) soft segments is investigated in the present study. Temperature-dependent unlike segment demixing was explored in two series of experiments: at constant heating (and cooling) rate and on annealing at selected elevated temperatures. Tapping mode atomic force microscopy on the as-polymerized polymer demonstrates that the polyurea hard segments self-assemble into a ribbon-like morphology that is generally preserved on annealing, although ribbon coarsening was observed at the highest annealing temperature. The results from the constant heating rate synchrotron X-ray scattering experiments demonstrate that the nanoscale structure begins to reorganize at temperatures as low as ∼70 °C, and the very significant changes in mean interdomain spacing observed at much higher temperatures are largely retained on returning to ambient conditions. Although there was surprisingly no detectable difference in the degree of hard/soft segment segregation in the longer time annealing experiments, changes in interdomain spacing were detected at the lowest annealing temperature (120 °C) used in this study. In combination with the findings from the synchrotron X-ray experiments, this demonstrates that domain reorganization is clearly both time and temperature dependent. The results from X-ray scattering and AFM experiments are also supported by those from FTIR spectroscopy and thermal analysis

Synthesis and Lithium Ion Conduction of Polysiloxane Single-Ion Conductors Containing Novel Weak-Binding Borates

Three borate monomers: lithium triphenylstyryl borate (B1), a variant with three ethylene oxides between the vinyl and the borate (B2) and a third with perfluorinated phenyl rings (B3) were synthesized and used to prepare polysiloxane ionomers based on cyclic carbonates via hydrosilylation. B1 ion content variations show maximum 25 °C conductivity at 8 mol %, reflecting a trade-off between carrier density and glass transition temperature (<i>T</i>_g) increase. Ethylene oxide spacers (B2) lower <i>T</i>_g, and increase the dielectric constant, both raising conductivity. Perfluorinating the four phenyl rings (B3) lowers the ion association energy, as anticipated by ab initio estimations. This increases conductivity, a direct result of 3 times higher measured carrier density. The ∼9 kJ/mol activation energy of simultaneously conducting ions is less than half that of ionomers with either sulfonate or bis­(trifluoromethanesulfonyl) imide anions, suggesting that ionomers with weak-binding borate anions may provide a pathway to useful single-ion Li⁺ conductors, if their <i>T</i>_g can be lowered

Introducing Large Counteranions Enhances the Elastic Modulus of Imidazolium-Based Polymerized Ionic Liquids

Polymerized ionic liquids (PILs) are believed to be ideal solid-state polymer electrolytes, and hence experimental and computational studies have been widely undertaken to understand the relationship between the chemical structure and mechanical/dielectric properties and the ionic conductivity of PILs. However, it is still a challenge to understand the effect of counterion ionic volume on the material properties of PILs. Herein, we demonstrate the effect of the ionic volume ratio of counteranions to side-chain cations on linear viscoelastic response using three imidazolium-based PILs with different counteranions. We show that the elastic modulus is significantly enhanced at temperatures higher than glass transition temperature once the ionic volume of the counteranion exceeds that of the side-chain cation. Our results provide an additional strategy to improve mechanical properties of PILs, while maintaining relatively high ionic conductivity

Segmental Dynamics of Ethylene Oxide-Containing Polymers with Diverse Backbone Chemistries

The dielectric response of seven nonionic ethylene oxide-containing polymers are investigated. Four different backbone chemistries are considered, including polymethacrylate, polyester ether, polyphosphazene, and polysiloxane. The chemistry of the backbone and the linking chemistry to incorporate ether oxygen (EO) groups dramatically affect polymer segmental dynamics. The <i>T</i>_g of the inorganic backbone polymers is ∼15 °C lower than that of the PEO homopolymer. Polysiloxanes exhibit the lowest <i>T</i>_g of −86 °C when attached with pendent −(CH₂–CH₂–O)₄–CH₃ groups. The strength of the alpha relaxation is the same for the hydrocarbon backbone polymers (Δε = 10), whereas the dielectric constants of inorganic polymers with short pendent groups is lower (Δε = 3). The difference in relaxation strengths is due to restricted motion of ether oxygens close to the backbone. This effect diminishes as the relative backbone concentration is decreased by increasing the pendent EO length. As pendent EO chain length is increased, the segmental relaxation broadens due to ether oxygens experiencing different local environments

Influence of Solvating Plasticizer on Ion Conduction of Polysiloxane Single-Ion Conductors

Lithium ion conduction is investigated for a polysiloxane-based single-ion conductor containing weak-binding borates and cyclic carbonate side chains, plasticized with poly­(ethylene glycol) (PEG). The addition of PEG increases the conductivity by up to 3 orders of magnitude compared to the host ionomer. A physical model of electrode polarization is used to separate ionic conductivity of the ionomers into number density of simultaneously conducting ions and their mobility. A reduction in <i>T</i>_g with increasing PEG content boosts ion mobility owing to an increase in polymer chain flexibility. Further, the PEG ether oxygens lower the activation energy of simultaneously conducting ions (from 14 to 8 kJ/mol), significantly increasing conducting ion content by 100X, suggesting that ion aggregates observed in the host ionomer are solvated by PEG. This directly reflects the disappearance of an ion aggregation peak observed in X-ray scattering, and an initial large increase in static dielectric constant (ε_<i>s</i>), upon addition of PEG, suggesting that ionic aggregation is significantly reduced by a small amount of PEG. Further dilution with lower dielectric constant PEG gradually reduces ε_<i>s</i>

Linear Viscoelasticity and Fourier Transform Infrared Spectroscopy of Polyether–Ester–Sulfonate Copolymer Ionomers

Fourier transform infrared spectroscopy (FTIR) and linear viscoelasticity (LVE) were used to characterize amorphous copolyester ionomers synthesized via condensation of sulfonated phthalates with mixtures of poly­(ethylene glycol) with <i>M</i> = 600 g/mol and poly­(tetramethylene glycol) with <i>M</i> = 650 g/mol. The copolymer ionomers exhibited microdomain separation, as confirmed in previous X-ray scattering measurements. Since PEO has superior ion solvating ability compared with PTMO, the ions near the interface reside preferentially in the PEO microdomain. FTIR measurements were used to quantify fractions of ions in different association states, in turn quantifying the fractions in the PEO-rich domains, in the PTMO-rich domains, and at the interface between these domains. FTIR shows that the structure of the interfacial ion aggregates is quite different for the copolymers with different counterions; at the interface Na⁺ aggregates into open string structures while Li⁺ aggregates into denser sheets of ions, as depicted schematically at the far right. Ionic conductivity is dominated by ions in the PEO domain, due to superior cation solvation by PEO; in the PTMO-rich microdomain both Na⁺ and Li⁺ form dense aggregates with of order 15 ion pairs. The temperature dependence of viscoelastic properties depends primarily on the PEO segmental dynamics, due to much higher <i>T</i>_g for the PEO-rich microdomains that are continuous at all copolymer compositions studied. Increasing the PTMO fraction increases the ionic association lifetime and delays the LVE terminal relaxation, creating an extended rubbery plateau, despite the fact that the chains are quite short

Molecular Mobility and Cation Conduction in Polyether–Ester–Sulfonate Copolymer Ionomers

Poly­(ethylene oxide) [PEO] ionomers are candidate materials for electrolytes in energy storage devices due to the ability of ether oxygen atoms to solvate cations. Copolyester ionomers are synthesized via condensation of sulfonated phthalates with glycol mixtures of PEO and poly­(tetramethylene oxide) [PTMO] to create random copolymer ionomers with nearly identical ion content and systematically varying solvation ability. Variation of the PEO/PTMO composition leads to changes in <i>T</i>_g, dielectric constant and ionic aggregation; each with consequences for ion transport. Dielectric spectroscopy is used to determine number density of conducting ions, their mobility, and extent of aggregation. Conductivity and mobility display Vogel temperature dependence and increase with PEO content; despite the lower <i>T</i>_g of PTMO. Conducting ion densities show Arrhenius temperature dependence and are nearly identical for all copolymer ionomers that contain PEO. SAXS confirms the extent of aggregation, corroborates the temperature response from dielectric measurements, and reveals microphase separation into a PTMO-rich microphase and a PEO-rich microphase that contains the majority of the ions. The trade-off between ion-solvation and low <i>T</i>_g in this study provides fundamental understanding of ionic aggregation and ion transport in polymer single-ion conductors

Polymerized Ionic Liquids: Correlation of Ionic Conductivity with Nanoscale Morphology and Counterion Volume

The impact of the chemical structure on ion transport, nanoscale morphology, and dynamics in polymerized imidazolium-based ionic liquids is investigated by broadband dielectric spectroscopy and X-ray scattering, complemented with atomistic molecular dynamics simulations. Anion volume is found to correlate strongly with <i>T</i>_g-independent ionic conductivities spanning more than 3 orders of magnitude. In addition, a systematic increase in alkyl side chain length results in about one decade decrease in <i>T</i>_g-independent ionic conductivity correlating with an increase in the characteristic backbone-to-backbone distances found from scattering and simulations. The quantitative comparison between ion sizes, morphology, and ionic conductivity underscores the need for polymerized ionic liquids with small counterions and short alkyl side chain length in order to obtain polymer electrolytes with higher ionic conductivity