The Role of Architecture in the Melt-State Self-Assembly of (Polystyrene)_star-<i>b</i>‑(Polyisoprene)_linear-<i>b</i>‑(Polystyrene)_star Pom-Pom Triblock Copolymers

Using a unique one-pot convergent anionic polymerization strategy, 18 (polystyrene)_star-<i>b</i>-(polyisoprene)_linear-<i>b</i>-(polystyrene)_star (S_<i>n</i>IS_<i>n</i>) pom-pom triblock copolymers were synthesized varying a range of architectural parameters including PS arm molecular weight (<i>M</i>_n,star), the number of arms contained in the star (<i>n</i>), and the PI midblock molecular weight (<i>M</i>_n,PI). A selected series of five of these 18, in which <i>M</i>_n,star 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>_PS) 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_<i>n</i>IS_<i>n</i> 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>_n,PI 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>-poly­(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 ¹H 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 poly­(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 CO₃^2–, HCO₃^–, and F^–, with a value of <i>E</i>_a ≈ 20 kJ/mol and a maximum fluoride conductivity of 34 mS/cm at 90 °C and 95% relative humidity. The Br^– 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^–. 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 ¹³C-labeled bicarbonate self-diffusion in an AEM using PGSE NMR spectrometry, which was found to be significantly slower than F^– 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^– 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)