Charge Transport and Glassy Dynamics in Blends Based on 1-Butyl-3-vinylbenzylimidazolium Bis(trifluoromethanesulfonyl)imide Ionic Liquid and the Corresponding Polymer.

Charge transport, diffusion properties, and glassy dynamics of blends of imidazolium-based ionic liquid (IL) and the corresponding polymer (polyIL) were examined by Pulsed-Field-Gradient Nuclear Magnetic Resonance (PFG-NMR) and rheology coupled with broadband dielectric spectroscopy (rheo-BDS). We found that the mechanical storage modulus (G') increases with an increasing amount of polyIL and G' is a factor of 10,000 higher for the polyIL compared to the monomer (GIL'= 7.5 Pa at 100 rad s-1 and 298 K). Furthermore, the ionic conductivity (σ0) of the IL is a factor 1000 higher than its value for the polymerized monomer with 3.4×10-4 S cm-1 at 298 K. Additionally, we found the Haven Ratio (HR) obtained through PFG-NMR and BDS measurements to be constant around a value of 1.4 for the IL and blends with 30 wt% and 70 wt% polyIL. These results show that blending of the components does not have a strong impact on the charge transport compared to the charge transport in the pure IL at room temperature, but blending results in substantial modifications of the mechanical properties. Furthermore, it is highlighted that the increase in σ0 might be attributed to the addition of a more mobile phase, which also possibly reduces ion-ion correlations in the polyIL.

Ion Dynamics of Monomeric Ionic Liquids Polymerized In Situ within Silica Nanopores.

Polymerized ionic liquids are a promising class of versatile solid-state electrolytes for applications ranging from electrochemical energy storage to flexible smart materials that remain limited by their relatively low ionic conductivities compared to conventional electrolytes. Here, we show that the in situ polymerization of the vinyl cationic monomer, 1-ethyl-3-vinylimidazolium with the bis(trifluoromethanesulfonyl)imide counteranion, under nanoconfinement within 7.5 ± 1.0 nm diameter nanopores results in a nearly 1000-fold enhancement in the ionic conductivity compared to the material polymerized in bulk. Using insights from broadband dielectric and Raman spectroscopic techniques, we attribute these results to the role of confinement on molecular conformations, ion coordination, and subsequently the ionic conductivity in the polymerized ionic liquid. These results contribute to the understanding of the dynamics of nanoconfined molecules and show that in situ polymerization under nanoscale geometric confinement is a promising path toward enhancing ion conductivity in polymer electrolytes.

Elucidating the impact of extreme nanoscale confinement on segmental and chain dynamics of unentangled poly(cis-1,4-isoprene).

Broadband dielectric spectroscopy is employed to probe dynamics in low molecular weight poly(cis-1,4-isoprene) (PI) confined in unidirectional silica nanopores with mean pore diameter, D, of 6.5 nm. Three molecular weights of PI (3, 7 and 10 kg/mol) were chosen such that the ratio of D to the polymer radius of gyration, Rg, is varied from 3.4, 2.3 to 1.9, respectively. It is found that the mean segmental relaxation rate remains bulk-like but an additional process arises at lower frequencies with increasing molecular weight (decreasing D/Rg. In contrast, the mean relaxation rates of the end-to-end dipole vector corresponding to chain dynamics are found to be slightly slower than that in the bulk for the systems approaching D/Rg ∼ 2, but faster than the bulk for the polymer with the largest molecular weight. The analysis of the spectral shapes of the chain relaxation suggests that the resulting dynamics of the 10kg/mol PI confined at length-scales close to that of the Rg are due to non-ideal chain conformations under confinement decreasing the chain relaxation times. The understanding of these faster chain dynamics of polymers under extreme geometrical confinement is necessary in designing nanodevices that contain polymeric materials within substrates approaching the molecular scale.

High Selective Mixed Membranes Based on Mesoporous MCM-41 and MCM-41-NH2 Particles in a Polysulfone Matrix.

The development of membrane technology for gas separation processes evolved with the fabrication of so-called mixed matrix membranes (MMMs) as an alternative to neat polymers, in order to improve the overall membrane effectiveness. Once the mixed matrix membranes are used, the gas separation properties of the porous materials used as fillers are combined with the economical processability and desirable mechanical properties of polymer matrix. Mixed mesoporous silica/polymer membranes with high CO2 and O2 permeability and selectivity were designed and prepared by incorporating MCM-41 particles into a polymer matrix. Ordered mesoporous silica MCM-41 with high surface confirmed by BET analysis were obtained and functionalized with amino groups. In order to obtain the mixed membranes, the mesoporous silica was embedded into the polysulfone matrix (PSF). Flat mixed matrix membranes with 5, 10, and 20 wt% MCM-41 and MCM-41-NH2 loadings have been prepared via the polymer solution casting method. The phase's interactions were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR) and thermogravimetry (TGA), while the gas separation performances were evaluated using pure gases (CO2, O2, N2). The MCM-41/PSF and MCM-41-NH2/PSF membranes exhibited increased permeabilities for O2 (between 1.2 and 1.7 Barrer) and CO2 (between 4.2 and 8.1 Barrer) compared to the neat membrane (0.8 Barrer). The loss of selectivity for the O2/N2 (between 6 and 8%) and CO2/N2 (between 25 and 41%) gas pairs was not significant compared with the pure membrane (8 and 39%, respectively). The MCM-41/PSF membranes were more selective for CO2/N2 than the O2/N2 pair, due to the size difference between CO2 and N2 molecules and to the condensability of CO2, leading to an increase of solubility. Stronger interactions have been noticed for MCM-41-NH2/PSF membranes due to the amino groups, with the selectivity increasing for both gas pairs compared with the MCM-41/PSF membranes.

Glassy dynamics predicted by mutual role of free and activation volumes.

Broadband Dielectric Spectroscopy (BDS) at elevated pressures and Positron Annihilation Lifetime Spectroscopy (PALS) are employed to elucidate the importance of the ratio of activation and free volumes during vitrification. We show that this ratio has a linear correlation with the structural relaxation of glass forming liquids in a wide temperature range hence engendering it as a vital input in the description of the dynamic glass transition.

Glassy dynamics of two poly(ethylene glycol) derivatives in the bulk and in nanometric confinement as reflected in its inter- and intra-molecular interactions.

The inter- and intra-molecular interactions as they evolve in the course of glassy solidification are studied by broadband dielectric-and Fourier-transform infrared-spectroscopy for oligomeric derivatives of poly(ethylene glycol) derivatives, namely, poly(ethylene glycol) phenyl ether acrylate and poly(ethylene glycol) dibenzoate in the bulk and under confinement in nanoporous silica having mean pore diameters 4, 6, and 8 nm, with native and silanized inner surfaces. Analyzing the spectral positions and the oscillator strengths of specific IR absorption bands and their temperature dependencies enables one to trace the changes in the intra-molecular potentials and to compare it with the dielectrically determined primarily inter-molecular dynamics. Special emphasis is given to the calorimetric glass transition temperature Tg and Tαß ≈ 1.25Tg, where characteristic changes in conformation appear, and the secondary ß-relaxation merges with the dynamic glass transition (α-relaxation). Furthermore, the impact of main chain conformations, inter- and intra-molecular hydrogen bonding, and nanometric confinement on the dynamic glass transition is unraveled.

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 Tg-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 Tg-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.

Environmental stress cracking performance of polyether and PDMS-based polyurethanes in an in vitro oxidation model.

Environmental stress cracking (ESC) was replicated in vitro on Optim™ (OPT) insulation, a polydimethylsiloxane-based polyurethane utilized clinically in cardiac leads, using a Zhao-type oxidation model. OPT performance was compared to that of two industry standard polyether urethanes: Pellethane® 80A (P80A), and Pellethane® 55D (P55D). Clinically relevant specimen configurations and strain states were utilized: low-voltage cardiac lead segments were held in a U-shape by placing them inside of vials. To study whether aging conditions impacted ESC formation, half of the samples were subjected to a pretreatment in human plasma for 7 days at 37°C; all samples were then aged in oxidative solutions containing 0.9% NaCl, 20% H2 O2 , and either 0 or 0.1M CoCl2 , with or without glass wool for 72 days at 37°C. Visual and SEM inspection revealed significant surface cracking consistent with ESC on all P80A and P55D samples. Sixteen of twenty P80A and 10/20 P55D samples also exhibited breaches. Seven of 20 OPT samples exhibited shallow surface cracking consistent with ESC. ATR-FTIR confirmed surface changes consistent with oxidation for all materials. The number average molecular weight decreased an average of 31% for OPT, 86% for P80A, and 56% for P55D samples. OPT outperformed P80A and P55D in this Zhao-type in vitro ESC model. An aging solution of 0.9% NaCl, 20% H2 O2 , and 0.1M CoCl2 , with glass wool provided the best combination of ESC replication and ease of use. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1544-1558, 2017.

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.

Limitations of predicting in vivo biostability of multiphase polyurethane elastomers using temperature-accelerated degradation testing.

Polyurethane biostability has been the subject of intense research since the failure of polyether polyurethane pacemaker leads in the 1980s. Accelerated in vitro testing has been used to isolate degradation mechanisms and predict clinical performance of biomaterials. However, validation that in vitro methods reproduce in vivo degradation is critical to the selection of appropriate tests. High temperature has been proposed as a method to accelerate degradation. However, correlation of such data to in vivo performance is poor for polyurethanes due to the impact of temperature on microstructure. In this study, we characterize the lack of correlation between hydrolytic degradation predicted using a high temperature aging model of a polydimethylsiloxane-based polyurethane and its in vivo performance. Most notably, the predicted molecular weight and tensile property changes from the accelerated aging study did not correlate with clinical explants subjected to human biological stresses in real time through 5 years. Further, DMTA, ATR-FTIR, and SAXS experiments on samples aged for 2 weeks in PBS indicated greater phase separation in samples aged at 85°C compared to those aged at 37°C and unaged controls. These results confirm that microstructural changes occur at high temperatures that do not occur at in vivo temperatures. In addition, water absorption studies demonstrated that water saturation levels increased significantly with temperature. This study highlights that the multiphase morphology of polyurethane precludes the use of temperature accelerated biodegradation for the prediction of clinical performance and provides critical information in designing appropriate in vitro tests for this class of materials.

How hydrogen bonds influence the mobility of imidazolium-based ionic liquids. A combined theoretical and experimental study of 1-n-butyl-3-methylimidazolium bromide.

The virtual laboratory allows for computer experiments that are not accessible via real experiments. In this work, three previously obtained charge sets were employed to study the influence of hydrogen bonding on imidazolium-based ionic liquids in molecular dynamics simulations. One set provides diffusion coefficients in agreement with the experiment and is therefore a good model for real-world systems. Comparison with the other sets indicates hydrogen bonding to influence structure and dynamics differently. Furthermore, in one case the total charge was increased and in another decreased by 0.1 e. Both the most acidic proton as well as the corresponding carbon atom were artificially set to zero, sequentially and simultaneously. In the final setup a negative charge was placed on the proton in order to introduce a barrier for the anion to contact the cation via this most acidic hydrogen atom. The following observations were made: changing the hydrogen bonding ability strongly influences the structure while the dynamic properties, such as diffusion and viscosity, are only weakly changed. However, the introduction of larger alterations (stronger hydrogen bonding and antihydrogen bonding) also strongly influences the diffusion coefficients. The dynamics of the hydrogen bond, ion pairing, and the ion cage are all affected by the level of hydrogen bonding. A change in total charges predominantly influences transport properties rather than structure. For ion cage dynamics with respect to transport porperties, we find a good correlation and a weak or no correlation for the ion pair or the hydrogen bond dynamics, respectively. Nevertheless, the hydrogen bond does influence ion cage dynamics. Therefore, we confirm that ionic liquids rather consist of loosely interacting counterions than of discrete ion pairs. Hydrogen bonding affects the properties only in a secondary or indirect manner.

Correlation between polarity parameters and dielectric properties of [Na][TOTO]--a sodium ionic liquid.

The ionic liquid (IL) [Na][TOTO], with sodium as the cation and an oligoethercarboxylate as the anion, shows properties that differ significantly from conventional ionic liquids, like imidazolium salts. Its polarity, determined in the temperature range of (293-333) K from measurements of Reichardt's E value and the Kamlet-Taft parameters is extraordinarily low and matches the dielectric constant extrapolated from frequency dependent complex permittivity measurements. The dielectric spectra also reveal split dynamics with the dominating slow mode probably associated with the reorientation of -COO(-)...Na(+) ion pairs and the fast mode arising from the flexibility of the oligoethylene moieties of the anion. This assignment, supported by the derived effective dipole moments, argues against our previous hypothesis [O. Zech et al., Chem.-Eur. J., 2009, 15, 1341-1345] that salts, such as [Na][TOTO], have low melting points because of cation complexation in a pseudo crown ether-like fashion and resulting "intra-molecular" charge neutralization. The present data rule out a rigid chelate-like complex as the dominating species. Considering the present findings together with the viscosities of [TOTO](-) salts, a crosslinked structure of the ionic liquid with strong -COO(-)...Na(+) interactions and relatively weaker forces between cations and ether oxygen groups of the anions appears to be plausible.

Charge transport and diffusion of ionic liquids in nanoporous silica membranes.

Charge transport in 1-hexyl-3-methylimidazolium hexafluorophosphate ionic liquid in oxidized nanoporous silicon membranes is investigated in a wide frequency and temperature range by a combination of Broadband Dielectric Spectroscopy (BDS) and Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR). By applying the Einstein-Smoluchowski relations to the dielectric spectra, diffusion coefficient is obtained in quantitative agreement with independent PFG NMR measurements. More than 10-fold systematic decrease in the effective diffusion coefficient from the bulk value is observed in hydrophilic silica nanopores. A model assuming a reduced mobility at the pore-matrix interface is shown to provide a quantitative explanation for the remarkable decrease of effective transport quantities (such as diffusion coefficient, dc conductivity and consequently, the dielectric loss) of the ionic liquid in non-silanized membranes. This approach is supported by the observation that silanization of porous silica membranes results in a significant increase of the effective diffusion coefficient, which approaches the value for the bulk liquid.