The γ relaxation in polymers containing ether linkages: conformational dynamics in the amorphous phase for a series of polybibenzoates containing oxyethylene spacers

Conductivity-Relaxation Relations in Nanocomposite Polymer Electrolytes Containing Ionic Liquid

In this study, we have used nanocomposite polymer electrolytes, consisting of poly(ethylene oxide) (PEO), δ-Al₂O₃ nanoparticles, and lithium bis(trifluoromethanesolfonyl)imide (LiTFSI) salt (with 4 wt % δ-Al₂O₃ and PEO:Li ratios of 16:1 and 8:1), and added different amounts of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesolfonyl)imide (BMITFSI). The aim was to elucidate whether the ionic liquid is able to dissociate the Li-ions from the ether oxygens and thereby decouple the ionic conductivity from the segmental polymer dynamics. The results from DSC and dielectric spectroscopy show that the ionic liquid speeds up both the segmental polymer dynamics and the motion of the Li⁺ ions. However, a close comparison between the structural (α) relaxation process, given by the segmental polymer dynamics, and the ionic conductivity shows that the motion of the Li⁺ ions decouples from the segmental polymer dynamics at higher concentrations of the ionic liquid (≥20 wt %) and instead becomes more related to the viscosity of the ionic liquid. This decoupling increases with decreasing temperature. In addition to the structural α-relaxation, two more local relaxation processes, denoted β and γ, are observed. The β-relaxation becomes slightly faster at the highest concentration of the ionic liquid (at least for the lower salt concentration), whereas the γ-relaxation is unaffected by the ionic liquid, over the whole concentration range 0-40 wt %.

Dielectric relaxation properties of carboxylic acid-terminated n-alkyl monolayers tethered to Si(1 1 1): dynamics of dipoles and gauche defects

Molecular-level insights into the organization and dynamics of n-alkyl monolayers covalently bonded to Si(1 1 1) were gained from admittance measurements of dipolar relaxation in rectifying Hg|| HOOC-C10H(25-n) Si junctions performed as a function of applied voltage and temperature. A collective behavior of dipole dynamics is inferred from the non-Debye asymmetric relaxation peak shape and strong coupling of the dipole relaxation path with some bending vibrations of the n-alkyl OML (multi-excitation entropy model). A variety of relaxation mechanisms is observed in the frequency range (0.1 Hz-10 MHz) with different dependence of relaxation frequency and dipolar strength on measurement temperature and applied voltage. Their microscopic origin is discussed by comparing the activation energy of relaxation frequency with previous molecular mechanics calculations of saddle point energy barriers for structural defects such as gauche conformations or chain kinks in n-alkanes assemblies. Gauche conformations organized in pairs (kinks) have vanishing relaxation strength below an order-disorder transition temperature T(D) =  175 K and their probability strongly increases with applied reverse voltage, above T(D). The presence of hydrogen bonds between terminal carboxylic acid functionalities is inferred from a comparison with a similar junction bearing a low density of carboxylic acid end groups. This temperature-dependent hydrogen-bond network provides some additional stiffness against external electrostatic stress, as deduced from the rather weak sensitivity of relaxation frequencies to applied bias voltage.

Mechanical and Transport Properties of Poly(propylene-co-1-heptene) Copolymers and Their Dependence on Monoclinic and/or Mesomorphic Polymorphs

Poly(propylene-co-1-heptene) copolymers have been evaluated in a wide 1-heptene content range. Two different polymorphs can be observed in this interval under the processing conditions imposed: monoclinic crystallites and mesomorphic entities. The unique presence of one or the other lattice at specific content or the coexistence of both of them at the intermediate composition interval is a key factor affecting some of the exhibited properties. The influence is observed in characteristics mainly dependent on either crystalline structure, like rigidity, or the amorphous regions, as location and intensity of glass transition. To get a deeper understanding, the transition from mesophase to monoclinic crystallites has also been studied by DSC and by WAXS and SAXS experiments using synchrotron radiation. It is observed that this transformation is only partial and just involves one-third of the initial ordered mesomorphic entities. Permeability to different gases is mainly dependent on amorphous content and hindrance that monoclinic or mesomorphic entities impose on those disordered regions.