Large-Scale Diffusion of Entangled Polymers along Nanochannels

Changes in large-scale polymer diffusivity along interfaces, arising from transient surface contacts at the nanometer scale, are not well understood. Using proton pulsed-gradient NMR, we here study the equilibrium micrometer-scale self-diffusion of poly­(butadiene) chains along ∼100 μm long, 20 and 60 nm wide channels in alumina, which is a system without confinement-related changes in segmental relaxation time. Unlike previous reports on nonequilibrium start-up diffusion normal to an interface or into particulate nanocomposites, we find a reduction of the diffusivity that appears to depend only upon the pore diameter but not on the molecular weight in a range between 2 and 24 kg/mol. We rationalize this by a simple volume-average model for the monomeric friction coefficient, which suggests a 10-fold surface-enhanced friction on the scale of a single molecular layer. Further support is provided by applying our model to the analysis of published data on large-scale diffusion in thin films

Diffusion Coefficients from ¹³C PGSE NMR MeasurementsFluorine-Free Ionic Liquids with the DCTA^– Anion

Pulsed-field gradient spin–echo (PGSE) NMR is a widely used method for the determination of molecular and ionic self-diffusion coefficients. The analysis has thus far been limited largely to ¹H, ⁷Li, ¹⁹F, and ³¹P nuclei. This limitation handicaps the analysis of materials without these nuclei or for which these nuclei are insufficient for complete characterization. This is demonstrated with a class of ionic liquids (or ILs) based on the nonfluorinated anion 4,5-dicarbonitrile-1,2,3-triazole (DCTA^–). It is demonstrated here that ¹³C-PGSE NMR can be used to both verify the diffusion coefficients obtained from other nuclei, as well as characterize materials that lack commonly scrutinized nuclei  all without the need for specialized NMR methods

High-Temperature Ionic-Conducting Material: Advanced Structure and Improved Performance

A new composite proton-conducting material based on the association of an ionic liquid and a porous polymer support was prepared with the aim of applying it as an electrolyte in a proton exchange membrane fuel cell (PEMFC) at elevated temperature (130 °C). The porous support was made from a high glass-transition temperature polymer (<i>T</i>g) by using the vapor-induced phase separation (VIPS) method in conditions leading to highly interconnected porous films. The ionic liquid tested was obtained by the reaction of a sulfonic acid with a tertiary amine and presents enough high-temperature stability to be used at elevated temperatures. Composite samples were prepared by immersing pieces of porous film in the ionic liquids under test. The porous support was characterized by scanning electron microscopy (SEM), gas permeation, and thermogravimetric analysis (TGA) tests, and the composite samples were characterized by mechanical and proton-conduction measurements. At 130 °C, this new material exhibits proton conductivity (20 mS cm^–1) below, but very close to, that of the pure ionic liquid (31 mS cm^–1) and presents, up to at least 150 °C, a storage modulus exceeding 200 MPa. This is very promising considering the PEMFC applications