The proton exchange membrane (PEM) is the centerpiece of low temperature fuel cell which offers means to directly convert chemical fuels to electrical energy. To be suitable for both vehicular and stationary applications, the PEM must exhibit several properties including high proton conductivity, sufficient mechanical properties, low fuel permeability, and hydrolytic stability. In addition to these, the production and processing of the membrane material must be cost-effective. Nafion®, a perfluorinated sulfonic acid polymer, is the state-of-art, industrial standard for the development of new membranes. However, Nafion is expensive, vulnerable to fuel crossover, and, due to dehydration, it is unsuitable to high temperature operation. Over the past decade, an immense amount of research has been coordinated to develop alternatives to Nafion including for example polybenzimidazoles, poly(arylene ether sulfones), crosslinked sulfonated poly(ether ether ketones), and sulfonated polyimides. In particular, sulfonated polyimides (SPIs) are attractive due to their good mechanical properties and excellent thermal stability.

Proton transport through sulfonated membranes depends largely on the state of hydration and the ionomer's corresponding morphology. For fluorinated materials, it is believed that water clusters form upon swelling, and, at high enough water content, they become interconnected, creating pathways for proton transport. Studies on sulfonated poly(etherketones) suggest a similar mechanism may be true for rigid-rod ionomers. In an effort to influence the ordering and distribution of ionic clusters in rigid-rod SPIs, we synthesized a new class of SPIs that contain polysiloxane segments. The incompatibility between the polyimide and polysiloxane phase is intended to drive phase segregation at the same length scale as ionic cluster ordering. Microphase segregation of incompatible siloxane domains is encouraged to create ion-rich channels that facilitate proton conduction.

Here we report on the synthesis and characterization of several SPI-polysiloxane copolymers. Incorporation of siloxane segments was found to be inhibited by the presence of ion-containing monomers such as diaminobiphenyldisulfonic acids. Samples were synthesized with ion exchange capacities ranging from 0-2.15 meq/g. Samples with higher ion content were more soluble (in aprotic solvents like NMP) and could be processed into films. Samples with siloxane contents greater than 3% adhered strongly to glass surfaces. Water mass-uptake experiments, conducted at controlled RH conditions, showed, that the presence of hydrophobic siloxane groups did not significantly inhibit water uptake. This result suggests that siloxane groups are expelled from the polyimide phase. TEM conducted on Ag+ exchanged films, and small angle X-ray scattering experiments were applied to study the materials' microstructure. Both techniques provided evidence of microphase-segregation into ion-rich and ion-poor domains. Samples containing siloxane segments did not exhibit X-ray correlation peaks, however, their morphology in the dry state could be imaged using electron microscopy. The siloxane segments enabled larger ionic domains to form (50 – 100 nm) while stabilizing a shorter length-scale periodicity (~ 5-8 nm). Proton conductivity of membranes was studied using a 4-point probe fuel cell coupled to an impedance analyzer. The membranes containing siloxane groups showed similar conductivities to their parent SPI membranes—all measured conductivities approached that of Nafion at 90 % RH. The fact that the dry-state morphology strongly influenced by siloxane groups, yet proton conductivity in the hydrated state is not, hints that the phase-segregated morphology may undergo an order-to-disorder transition upon hydration. Current efforts are directed at other routes to impart nano-structure into these SPI membranes.