April 26, 2018
April 11, 2018
April 10, 2018
March 5-9, 2018
Ion Transport and Interfacial Dynamics in Disordered Block Copolymers of Ammonium-Based Polymerized Ionic Liquids
Macromolecules, 2018, 51(9), pp 3477-3486
A series of diblock copolymers bearing a polymerized ionic liquid (polyIL) block (poly(N-(methacryloyloxy)ethyl-N,N-dimethyl-N-ethylammonium bis(trifluoromethylsulfonyl)imide)) and a noncharged block (poly(methyl methacrylate) (PMMA)) or poly(n-butyl methacrylate) (PBuMA)) were studied using differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and broadband dielectric spectroscopy (BDS) to probe the effect of ion concentration on the morphology and ion transport in these polyelectrolytes. Two majority PMMA block copolymers, having mole ratios of the polyIL of 0.19 and 0.22, exhibited evidence of aggregation indicated by interfacial polarization in the dielectric spectra. The 0.19 mole ratio sample also displayed two distinct glass transitions by DSC. The SAXS measurements showed that no long-range order was present in these samples. The ionic conductivity of these samples were lower than the polyIL homopolymer due to hindered ion transport at the aggregate boundaries. Copolymers with majority polyIL blocks were found to exhibit disorder based on SAXS and DSC measurements. Furthermore, at a mole fraction of 0.91 of the polyIL the ionic conductivity was enhanced by a factor of ca. 1.5 with respect to the polyIL homopolymer, with a similar increase observed for the static dielectric permittivity. The effective number density and mobility of the ions were calculated for these systems from BDS and WAXS data, indicating that the enhancement of the ionic conductivity corresponds to an increase in the density of mobile charge carriers. The higher effective number density of charge carriers correlates with increased static dielectric permittivity, suggesting that ion pair dissociation is the likely mechanism behind the observed enhancement of ion transport. This study showcases the wealth of information that can be obtained from a combination of complementary experimental techniques.
Associating imidazoles: Elucidating the correlation between the static dielectric permittivity and proton conductivity
Physical Review Letters, 2018, 120, 136001
Broadband dielectric spectroscopy is employed to investigate the impact of supramolecular structure on charge transport and dynamics in hydrogen-bonded 2-ethyl-4-methylimidazole and 4-methylimidazol. Detailed analyses reveal (i) an inverse relationship between the average supramolecular chain length and proton conductivity and (ii) no direct correlation between the static dielectric permittivity and proton conductivity in imidazoles. These findings raise fundamental questions regarding the widespread notion that extended supramolecular hydrogen-bonded networks facilitate proton conduction in hydrogen bonding materials.
Dynamic and structural evidence of mesoscopic aggregation in phosphonium ionic liquids
The Journal of Chemical Physics, 2017, 147, 234504
Mesoscopic aggregation in aprotic ionic liquids due to the microphase separation of polar and non-polar components is expected to correlate strongly with the physicochemical properties of ionic liquids and therefore their potential applications. The most commonly cited experimental evidence of such aggregation is the observation of a low-q pre-peak in the x-ray and neutron scattering profiles, attributed to the polarity alternation of polar and apolar phases. In this work, a homologous series of phosphonium ionic liquids with the bis(trifluoromethylsulfonyl)imide anion and systematically varying alkyl chain lengths on the phosphonium cation are investigated by small and wide-angle x-ray scattering, dynamic-mechanical spectroscopy, and broadband dielectric spectroscopy. A comparison of the real space correlation distance corresponding to the pre-peak and the presence or absence of the slow sub-? dielectric relaxation previously associated with the motion of mesoscale aggregates reveals a disruption of mesoscale aggregates with increasing symmetry of the quaternary phosphonium cation. These findings contribute to the broader understanding of the interplay of molecular structures, mesoscale aggregation, and physicochemical properties in aprotic ionic liquids..