Crystallization-Arrested Viscoelastic Phase Separation in Semiconducting Polymer Gels

Through a combination of rheological characterization and temperature-variable imaging methods, a novel gelation pathway in dilute solutions of a semiconducting polymer to achieve interconnected, crystalline networks with hierarchical porosity is reported. Upon rapid cooling, solutions of regioregular poly(3-hexylthiophene) in ortho-dichlorobenzene formed thermoreversible gels. Confocal microscopy revealed cooling-induced structural rearrangement to progress through viscoelastic phase separation (VPS), which arrested prematurely during the formation of micron-sized solvent-rich "holes"due to interchain crystallization. Cryogen-based scanning electron microscopy uncovered an interfibrillar network exhibiting nanosized pores. These networks formed to equal gel strengths when a third component, either small molecule phenyl-C61-butyric acid methyl ester or noncrystallizing regiorandom, poly(3-hexylthiophene), was added to the solution. Organic solar cells deposited with active layers from phase-separated solutions displayed 45% higher efficiency compared to reference cells. The demonstrated ability to arrest VPS enables control over the morphology of porous materials for applications ranging from membrane filtration to plastic foam manufacturing.