Resolving the Molecular Origin of Mechanical Relaxations in Donor-Acceptor Polymer Semiconductors

The thermomechanical behavior of polymer semiconductors plays an important role in the processing, morphology, and stability of organic electronic devices. However, donor-acceptor-based copolymers exhibit complex thermal relaxation behavior that is not well understood. This study uses dynamic mechanical analysis (DMA) to probe thermal relaxations of a systematic set of polymers based around the benzodithiophene (BDT) moiety. The loss tangent curves are resolved by fitting Gaussian functions to assign and distinguish different relaxations. Three prominent transitions are observed that correspond to: i) localized relaxations driven primarily by the side chains (gamma), ii) relaxations along the polymer backbone (beta), and iii) relaxations associated with aggregates (alpha). The side chains are found to play a clear role in dictating T-gamma, and that mixing the side chain chemistry of the monomer to include alkyl and oligo(ethylene glycol) moieties results in splitting the gamma-relaxation. The beta relaxations are shown to be associated with backbone elements along with the monomer. In addition, through processing, it is shown that the alpha-relaxation is due to aggregate formation. Finally, it is demonstrated that the thermal relaxation behavior correlates well with the stress-strain behavior of the polymers, including hysteresis and permanent set in cyclically stretched films.