3 research outputs found
The Role of Architecture in the Melt-State Self-Assembly of (Polystyrene)<sub>star</sub>-<i>b</i>‑(Polyisoprene)<sub>linear</sub>-<i>b</i>‑(Polystyrene)<sub>star</sub> Pom-Pom Triblock Copolymers
Using
a unique one-pot convergent anionic polymerization strategy,
18 (polystyrene)<sub>star</sub>-<i>b</i>-(polyisoprene)<sub>linear</sub>-<i>b</i>-(polystyrene)<sub>star</sub> (S<sub><i>n</i></sub>IS<sub><i>n</i></sub>) pom-pom
triblock copolymers were synthesized varying a range of architectural
parameters including PS arm molecular weight (<i>M</i><sub>n,star</sub>), the number of arms contained in the star (<i>n</i>), and the PI midblock molecular weight (<i>M</i><sub>n,PI</sub>). A selected series of five of these 18, in which <i>M</i><sub>n,star</sub> was held approximately constant between
14.3 and 16.5 kDa, but with the numbers of arms in the star and PI
midblock molecular weight varied, were selected for detailed characterization
using rheology, AFM, and SAXS. The five selected all shared PS as
the minority component, with star volume fractions (<i>f</i><sub>PS</sub>) varying between 0.11 and 0.22. All samples showed
clear phase separation, with three of the five adopting a highly ordered
hexagonal packing of cylinders (HPC) confirmed through SAXS and AFM.
The remaining two systems were limited to liquid-like packing of cylindrical
domains (LLP). Longer midblock molecular weights and increased numbers
of arms in the star both showed a propensity to hinder formation of
a highly ordered hexagonal lattice. Increasing the number of arms
in the star also favored transitions to a disordered phase at lower
temperatures when overall S<sub><i>n</i></sub>IS<sub><i>n</i></sub> molecular weight was held constant. The behavioral
trends identified suggest interfacial packing frustration plays a
prominent role in determining the ability of the system to develop
highly ordered periodic structures. The chain crowding produced by
the PS star architecture intrinsically favors interfacial curvature
toward the majority PI component, contrary to that intrinsically favored
by the block composition alone. In the two systems in which the frustration
was architecturally most severe (largest <i>n</i> of 7.1,
highest <i>M</i><sub>n,PI</sub> of 191 kDa), evolution of
a hexagonal lattice could not be induced, even after significant thermal
annealing. The pom-pom architecture itself also appears to have a
significant impact on entanglement relaxation dynamics, with development
of HPC morphologies only possible at elevated temperatures
Polyethylene-Based Block Copolymers for Anion Exchange Membranes
Block copolymer membranes with a
semicrystalline polyethylene component
were prepared by anionic polymerization and postpolymerization functionalization
reactions. Polybutadiene-<i>b</i>-polyÂ(4-methylstyrene)
(PB-<i>b</i>-P4MS) precursors with four different block
compositions and 92–95% 1,4-content in the polybutadiene block
were produced by living anionic polymerization in a nonpolar solvent.
The polybutadiene block was subsequently hydrogenated to prepare a
polyethylene block, and the hydrogenated block copolymers were then
brominated at the arylmethyl group of the P4MS block. Subsequent quaternization
reaction with trimethylamine led to the anion exchange membranes.
The degree of crystallinity in the polyethylene block was determined
by differential scanning calorimetry to be approximately 24–27%.
The postpolymerization modification reactions were examined by <sup>1</sup>H NMR and IR spectroscopy. The amount of quaternary ammonium
groups was quantified by ion exchange capacity (IEC) measurements.
Membranes with IEC’s ranging from 1.17 to 1.92 mmol/g were
prepared. The IEC was varied by changing the relative amount of P4MS
in the precursor block copolymer, and water uptake and ionic conductivity
were found to increase with increasing IEC. Small-angle X-ray scattering
(SAXS) experiments and transmission electron microscopy (TEM) showed
phase-separated, bicontinuous structures at all compositions. Materials
with higher IEC show improved ionic conductivity as well as lower
activation energy of ion conduction compared to less functionalized
membranes. The hydroxide conductivity of the block copolymer membrane
with an IEC of 1.92 mmol/g reached 73 mS/cm at 60 °C in water.
Tensile measurements indicated excellent mechanical properties of
the semicrystalline membranes for potential use as alkaline fuel cell
membrane materials
Understanding Anion, Water, and Methanol Transport in a Polyethylene‑<i>b</i>‑poly(vinylbenzyl trimethylammonium) Copolymer Anion-Exchange Membrane for Electrochemical Applications
Herein, we report
the anion and water transport properties of an
anion-exchange membrane (AEM) comprising a block copolymer of polyethylene
and polyÂ(vinylbenzyl trimethylammonium) (PE-<i>b</i>-PVBTMA)
with an ion-exchange capacity (IEC) of 1.08 mequiv/g. The conductivity
varied little among the anions CO<sub>3</sub><sup>2–</sup>,
HCO<sub>3</sub><sup>–</sup>, and F<sup>–</sup>, with
a value of <i>E</i><sub>a</sub> ≈ 20 kJ/mol and a
maximum fluoride conductivity of 34 mS/cm at 90 °C and 95% relative
humidity. The Br<sup>–</sup> conductivity showed a transition
at 60 °C. Pulsed gradient stimulated spin echo nuclear magnetic
resonance (PGSE NMR) experiments showed that water diffusion in this
AEM is heterogeneous and is affected by the anion present, being fastest
in the presence of F<sup>–</sup>. We determined the methanol
self-diffusion in this membrane and observed that it is lower than
that in Nafion 117, because of the lower water uptake. This article
reports the first measurements of <sup>13</sup>C-labeled bicarbonate
self-diffusion in an AEM using PGSE NMR spectrometry, which was found
to be significantly slower than F<sup>–</sup> self-diffusion.
Back-calculation of the bicarbonate conductivity using the Nernst–Einstein
equation gave a value that was significantly lower than the measured
value, implying that bicarbonate transport involves OH<sup>–</sup> in the transport mechanism. Fourier transform infrared spectroscopy,
PGSE NMR spectrometry, and small-angle X-ray scattering (SAXS) indicated
the presence of different types of waters present in the membrane
at different length scales. The SAXS data indicated that there is
a water-rich region within the hydrophilic domains of the polymer
that has a temperature dependence in intensity at 95% relative humidity
(RH)