July 26, 2018
July 13, 2018
April 26, 2018
April 11, 2018
Impact of Molecular Architecture on Dynamics of Miktoarm Star Copolymers
Macromolecules, 2018, 51(14), 5401-5408.
Broadband dielectric spectroscopy (BDS) is used to probe the chain and segmental dynamics of A2B2 and AB2 miktoarm star copolymers based on polystyrene (PS, A block) and polyisoprene (PI, B block) that display lamellar morphologies as determined using small-angle X-ray scattering (SAXS). While no changes in the distribution of PI segmental relaxation times are observed with variation of the molecular architecture, an unexpected increase in the normalized PI chain relaxation intensity is realized for AB2 miktoarm star copolymers as well as a change in the distribution of chain relaxation rates as compared to A2B2 and AB diblock copolymer systems. This result is attributed to asymmetry in the molecular architecture near the junction point, which affects the osmotic constraint of the tethered PI chains within the interfacial region of the lamellae. The results highlight the importance of macromolecular design on fundamental chain dynamics in phase-separated thermoplastic elastomers.
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.