Morphological Understanding of the Effect of the Elastomer’s Molecular Weight in Conjugated Polymer/ Elastomer Blends

Flexible and stretchable electronics are of great interest given their wide range of uses, such as bioelectronics. Solution processable conjugated polymers are great candidates for the creation of flexible and stretchable electronics due to their ability to be fabricated into devices at low cost and at a large scale through different processing techniques, such as roll-to-roll printing. The semi-crystalline nature of most state-of-the-art conjugated polymer semiconductors limits their use by inhibiting their mechanical flexibility. To promote improved mechanical properties, previous studies have shown promising results by blending conjugated polymers with elastomers to produce flexible and stretchable devices while maintaining and even promoting charge transport. However, there is still not a systematic understanding of the phase behavior of such composite films and how it may be affected by different processing conditions and polymer structures. This study focuses on gaining further understanding of the elastomer’s molecular weight on the overall film morphology. The molecular weight of the elastomer was chosen given its known impact on the thermodynamics of phase separation in a two-polymer system. Diketopyrrolopyrrole (DPP)-based conjugated polymer was blended with various molecular weights of polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) in solution. Composite solutions were spincast and the morphologies of these films were studied through the use of optical (UV-Vis) spectroscopy, grazing incidence wide angle x-ray scattering (GIWAXS), and atomic force microscopy (AFM) to gain a better understanding of the aggregation and planarization of the polymer backbone, crystallinity, and phase separation. The aggregation and planarization of the polymer backbone of the DPP increases with decreasing SEBS molecular weight. The effects of the SEBS’ molecular weight on the final film morphology and phase separation will be discussed in addition to their effect on the electronic properties of the material. This work will allow for the creation of a more systematic method to creating thin films with specific morphologies.