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J Phys Chem B. 2016 Jul 7;120(26):6215-24. doi: 10.1021/acs.jpcb.6b02202. Epub 2016 Apr 29.

Structure and Conductivity of Semiconducting Polymer Hydrogels.

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Department of Chemistry and Biochemistry, UCLA , Los Angeles, California 90095-1569, United States.
Department of Microbiology, Immunology & Molecular Genetics, and Biomedical Engineering Program, UCLA , 609 Charles E. Young Drive South, Los Angeles, California 90095, United States.
California NanoSystems Institute, UCLA , 570 Westwood Plaza, Los Angeles, California 90095, United States.
Chemical and Nanoscience Center, National Renewable Energy Laboratory , 15013 Denver West Parkway, Golden, Colorado 80401, United States.


Poly(fluorene-alt-thiophene) (PFT) is a conjugated polyelectrolyte that self-assembles into rod-like micelles in water, with the conjugated polymer backbone running along the length of the micelle. At modest concentrations (∼10 mg/mL in aqueous solutions), PFT forms hydrogels, and this work focuses on understanding the structure and intermolecular interactions in those gel networks. The network structure can be directly visualized using cryo electron microscopy. Oscillatory rheology studies further tell us about connectivity within the gel network, and the data are consistent with a picture where polymer chains bridge between micelles to hold the network together. Addition of tetrahydrofuran (THF) to the gels breaks those connections, but once the THF is removed, the gel becomes stronger than it was before, presumably due to the creation of a more interconnected nanoscale architecture. Small polymer oligomers can also passivate the bridging polymer chains, breaking connections between micelles and dramatically weakening the hydrogel network. Fits to solution-phase small-angle X-ray scattering data using a Dammin bead model support the hypothesis of a bridging connection between PFT micelles, even in dilute aqueous solutions. Finally, time-resolved microwave conductivity measurements on dried samples show an increase in carrier mobility after THF annealing of the PFT gel, likely due to increased connectivity within the polymer network.

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