3 research outputs found
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 <sup>13</sup>C PGSE NMR MeasurementsFluorine-Free Ionic Liquids with the DCTA<sup>–</sup> 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 <sup>1</sup>H, <sup>7</sup>Li, <sup>19</sup>F, and <sup>31</sup>P 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<sup>–</sup>). It is demonstrated here that <sup>13</sup>C-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<sup>–1</sup>) below, but very close to, that of the
pure ionic liquid (31 mS cm<sup>–1</sup>) and presents, up
to at least 150 °C, a storage modulus exceeding 200 MPa. This
is very promising considering the PEMFC applications