The structure and properties of segmented block copolymer films of aromatic polyimide (PI) and poly(ethylene glycol) (PEG) doped with an ionic liquid are studied for potential polymer electrolyte membrane applications for fuel cells. Poly(amic acid) precursors of PI‐PEG copolymers of 4,4′‐(hexafluoroisopropylidene) diphthalic anhydride, 4,4′‐(1,3‐phenylenedioxy) dianiline, and bis(3‐aminopropyl) terminated PEG (Mn ≈ 1500) are synthesized and then thermally imidized in membrane films, followed by swelling in ethylammonium nitrate (EAN) ionic liquid. The small‐angle X‐ray scattering results from the EAN‐doped PI‐PEG copolymer films show disordered bicontinuous phase‐separated nanostructures described by Teubner–Strey theory, with the interface fractal dimension determined from the Porod equation. Thermal annealing of the EAN‐doped membranes at 100–140 °C results in increased correlation lengths and smoother interfaces of the bicontinuous nanostructures. Such improved nanostructures lead to the increased ionic conductivity by two to five times with the maximum conductivity of 210 mS cm−1 at 60 °C and 70% RH, much greater (nearly fivefold) than that of Nafion films, while maintaining the mechanical stability possibly up to 140 °C. Moreover, the investigation of the disordered bicontinuous phase‐separated nanostructure of EAN‐doped PI‐PEG copolymer membranes is highly relevant to understanding the nanostructures of hydrated Nafion membranes and segmented block copolymers in general.

An understanding of the structure and properties of polymer electrolyte systems can be crucial to a variety of different applications. The current work performs a study of the composition, structure and properties of poly(ethylene glycol) (PEG)-aromatic polyimide systems incorporating ionic liquids that are relevant to several applications especially fuel cell membranes. Composition was varied through using different aromatic dianhydrides, aromatic diamines and in some cases synthesis solvent. Properties were characterized using Fourier transform infrared spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, small-angle x-ray scattering, electrochemical impedance spectroscopy and cyclic voltamme-try. By varying solvent, aromatic regularity and expected rigidity can be tuned, impacting average conductivity by 30%. Varying the aromatic diamine can influence the length scale and amount of aromatic regularity, which can ultimately affect the conductivity by a factor of four. The maximum conductivity reached was 83 mS/cm at 80 C and 70 %RH.

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Random copolymers play an important role in a range of soft materials applications and biological phenomena. An individual monomer is typically a single chemical unit whose length is comparable to or less than a Kuhn length, resulting in a monomer segment that is structurally rigid at length scales of a segregated domain. Previous work on random copolymer phase segregation addresses the impact of correlations between the chemical identities along the chains for flexible polymers. In these works, a single monomer unit is effectively a large polymer block that behaves as a random walk without conformational correlation associated with semiflexibility. In our work, we develop a model of semiflexible random copolymers using the wormlike chain model to capture conformational correlation of the polymer chains. To address the thermodynamics of microphase segregation and the structure of the segregated domains, we develop a random phase approximation up to quartic order in density fluctuations that leverages our exact results for the statistical behavior of the wormlike chain model. In this work, we focus on the quadratic-order expansion of the free energy, which provides the mean-field spinodal of the homogeneous phase. We explore the impact of conformational and chemical correlations on the formation of inhomogeneous microphases at the spinodal point. We show that the onset of phase segregation and the correlation length of domains are extremely sensitive to chain rigidity. ■ INTRODUCTIONBlock copolymers have attracted attention in numerous applications because of their ability to self-assemble into nanostructured morphologies. 1−3 Extensive theoretical and experimental studies have predicted and characterized the ordered morphologies of block copolymers with well-defined chemical architecture such as diblock and triblock copolymers. 4−7 In contrast, the phase behavior of multiblock copolymers is less well understood. In particular, theoretical, computational, and experimental studies on random copolymers, whose monomer chemical identities are stochastically arranged along the polymers, are far less prevalent. 8−11 Commercial products such as high impact polystyrene (HIPS), styrene−butadiene rubber (SBR), and Nafion are blends of macromolecules with randomly distributed chemical repeat units. 12,13 In these materials, the mesoscale morphology greatly influences their mechanical and transport properties. Recently, there has been interest in making new soft materials by introducing chemical stochasticity into block copolymers, such as tapered diblock copolymers 14 and gradient copolymers, 15−17 due to their ability to relieve packing frustration during self-assembly. Biology also needs to overcome random variability to achieve self-assembly in macromolecular environments in scenarios such as protein folding and chromatin condensation. 18−20 Despite the abundance of random copolymers in various industrial applications and biological systems, the understanding of their phase behavior and structure− property relationship ...

Current fuel cell technology demands improvements for widespread use, and novel polymer materials may be able to achieve the necessary enhancements. This work inspects the composition, structure, and properties of poly(ethylene glycol) (PEG)-aromatic polyimide systems aimed at polymer electrolyte membrane applications, as PEG is a known ion conductor and aromatic polyimides are quite stable. Liquid electrolytes were incorporated into the polymers through soaking to achieve ionic conductivity. By varying polyimide and liquid electrolyte, the polymers were analyzed for their structure and conductivity. Fourier transform infrared spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, small-angle X-ray scattering, electrochemical impedance spectroscopy, and cyclic voltammetry were used as characterization tools. Electrolyte identity impacts liquid uptake and conductivity. Polyimide identity can influence the size and variability of the doped polymer structure, which ultimately can change conductivity by up to 28%, with the maximum conductivity being 102 mS/cm at 80 C and 70% RH.