Solution-state nanoconfined aggregation and microstructure evolution in blends of conjugated polymers and elastomers

Emerging wearable health monitoring technologies require conformable and stretchable devices. Polymer semiconductors composed of π-conjugated polymer aggregates in an elastomeric matrix are remarkable in their ability to provide both high stretchability and enhanced charge transport. Understanding their film formation process is critical in improving charge transport, imparting added functionalities, and advancing large-scale production of high-performing polymer electronic devices. Here, using a poly-thieno[3,2-b]thiophene-diketopyrrolopyrrole (DPPTT): polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) blend as a model system, electron tomography of the blend reveals the presence of bundles of conjugated polymer nanofibers spanning the thickness of the films. High-resolution cryogenic electron microscopy (cryo-EM) of solution and thin films reveals that the nanoconfined DPPTT nanofibers in blends are composed of the aligned DPPTT 1D aggregates present in solution. In contrast, neat DPPTT solutions and thin films contain irregular crystalline domains with random orientations. In situ grazing incidence wide-angle X-ray scattering (GIWAXS) studies reveal that DPPTT crystallization commences earlier in blends compared to neat films. Combining observations from both in situ ultraviolet-visible spectroscopy, in situ GIWAXS and cryo-EM reveal that 1D aggregates in blend solution bundle and align into interconnected larger fibers that are nanoconfined in the SEBS matrix. This morphology is desirable for efficient charge transport and good mechanical strength. In contrast, neat DPPTT films contain randomly oriented smaller aggregates with an increased fraction of disordered domains. Overall, our work provides critical insights on the impact of solution composition and processing conditions on thin film morphology for achieving multifunctional high-performing electronic polymer composites.