Anisotropic Ionic Conductivity in Block Copolymer Membranes by Magnetic Field Alignment

Abstract

The self-assembly of diblock copolymers provides a convenient route to the formation of mechanically robust films with precise and tunable periodic arrangements of two physically demixed but chemically linked polymeric materials. Chemoselective transport membranes may be realized from such films by selective partitioning of an active species into one of the polymer domains. Here, lithium ions were selectively sequestered within the poly(ethylene oxide) block of a liquid crystalline diblock copolymer to form polymer electrolyte membranes. Optimization of the membrane conductivity mandates alignment of self-assembled structures such that conduction occurs via direct as opposed to tortuous transport between exterior surfaces. We show here that magnetic fields can be used in a very simple and scalable manner to produce highly aligned hexagonally packed cylindrical microdomains in such membranes over macroscopic areas. We systematically explore the dependence of the ionic conductivity of the membrane on both temperature and magnetic field strength. A surprising order of magnitude increase in conductivity relative to the nonaligned case is found in films aligned at the highest magnetic field strengths, 6 T. The conductivity of field aligned samples shows a nonmonotonic dependence on temperature, with a marked decrease on heating in the proximity of the order-disorder transition of the system before increasing again at elevated temperatures. The data suggest that domain-confined transport in hexagonally packed cylindrical systems differs markedly in anisotropy by comparison with lamellar systems.

Protonation-Induced Microphase Separation in Thin Films of a Polyelectrolyte-Hydrophilic Diblock Copolymer

“…4 Recent research in this area has also focused on block copolymers because they can be designed to combine one noninteracting block which supplies mechanical stability and one solubilizing block which provides conductive pathways. 5 In many cases, the morphologies of these composite materials have had profound effects on their ion transport properties, 6 making morphology control of central importance. For ion-containing polymers in nonaqueous environments, ion incorporation can be used to improve the mechanical properties of plastics by forming ionic cross-links resulting from the phase separation of ion-rich and ion-poor domains within a material.…”

Ion Conduction in Microphase-Separated Block Copolymer Electrolytes

“…It is interesting to note that PSS-block-PMB showed a decrease in conductivity when aligned by shear and electric fields suggesting a nontrivial relationship between conductivity and alignment. In contrast, films aligned by magnetic fields were reported to achieve an enhancement in ion conductivity of A =10, 17 which exceeds predictions based on effective medium theory (EMT) that suggest a maximum of A = 2 for polymer blends with heterogeneous structure. 43 The theoretical maximum of A = 2 is achieved from reductions in tortuosity and path length for an already percolated network.…”

“…17 The theoretical conductivity of a LiClO 4 /PEO based BCE with ideal alignment may be predicted using a geometric model that simply determines the ionic conductivity based on the volume fraction of the conducting phase (ϕ c ), the morphological contribution of the microphase separated BCE (f st ), and the known ionic conductivity of the corresponding homopolymer (σ homopolymer ):…”

Polymer electrolyte membranes with exceptional conductivity anisotropy via holographic polymerization

“…Anisotropic ion conducting films were first reported in 1995 in Nafion films after melt processing, where an anisotropy of 1.4 was reported. [7] Since then, applying external mechanical, [7e10] electrical [10] or magnetic fields, [11,12] solvent casting methods, [13] electrospraying block-co-polymers (BCPs), [14] liquid crystals polymer templating, [9,15e17] and tailoring crystalline polymer morphology have been demonstrated to increase ion flux anisotropy in polymer electrolyte films; however, there remains hurdles for each of these techniques to effectively direct ion transport, both from processing and device standpoints. Conductivity anisotropies of up to 10e12 can be achieved with single-step solution casting, [13] and higher anisotropies can be demonstrated via secondary processing techniques: 20 for mechanical stretching, [18] 75 for melt pressing diblock-copolymers, [10] and approaching 1000 for liquid crystalline BCPs aligned under an external 6 T magnetic field.…”

“…Conductivity anisotropies of up to 10e12 can be achieved with single-step solution casting, [13] and higher anisotropies can be demonstrated via secondary processing techniques: 20 for mechanical stretching, [18] 75 for melt pressing diblock-copolymers, [10] and approaching 1000 for liquid crystalline BCPs aligned under an external 6 T magnetic field. [11] Of particular interest for this work is anisotropic conducting behavior in solid polymer electrolyte membranes (PEMs) for fuel cell and battery applications. PEMs with high conductivity and high mechanical integrity are critical for energy related applications including solid state batteries and fuel cells.…”