Branched Polyethylene as a Plasticizing Additive to Modulate the Mechanical Properties of π-Conjugated Polymers

A new approach for improving the mechanical properties of semiconducting polymers was established via physical combination of a diketopyrrolopyrrole-based conjugated polymer with a low-molecular-weight branched polyethylene (BPE). The influence of the BPE additive on the stretchability and mechanical properties of the conjugated polymer was studied at different scales, using various characterization techniques, including atomic force microscopy, UV-vis spectroscopy, and grazing incidence X-ray diffraction. At the micron scale, the BPE additive acts as a plasticizer and significantly reduces Young's modulus of the conjugated polymer and increases the crack onset strain, reaching a maximum of a 75% strain elongation when 90 wt % of BPE is blended with the conjugated polymer. The introduction of BPE to the blended systems decreases the crack propagation of polymer thin films, making them softer and more ductile, with Young's modulus of 112 MPa at 25 wt % of BPE before thermal annealing. At the nanoscale, the improvement of stretchability is shown by the reduction of the crack size under a 100% strain, going from 3100 to 600 nm at 0 and 90 wt % of BPE, respectively. The results obtained in this investigation confirm that an improvement in the mechanical properties and a modulation of the solid-state morphology of the semiconducting materials can be enabled by the physical mixing of conjugated polymers with a nontoxic, low-molecular-weight branched polyethylene, particularly favorable for the solution deposition of organic semiconductors.