Recent studies on conjugated semiconducting polymers have begun to shed light on how the polymer repeat unit and macromolecular structure play important roles in the determination of the ultimate properties of the materials. So far, the majority of researchers have focused on using the structure of the polymer systems to maximize the electrical properties of these materials for use in organic electronics. Such electronics have much promise for new organic display technologies, solid-state lighting, electrochromic, solar cells, and flexible and stretchable electronics. However, very little focus has been placed on understanding the impact of the structure of the polymers on their mechanical properties. In terms of mechanical properties, the functional backbone and side chains impact the long-range ordering and the development of semi-crystalline phases that greatly influence mechanical properties such as modulus and fracture toughness of the materials.
In this work, we will study the impact of structural ordering on the mechanical properties of polythiophene-based conjugated polymers including mechanical modulus, onset cracking strain, and fracture toughness. Materials such as P3HT and PBDTTT will be deposited from solution on soft substrates for onset crack strain studies under tensile loading, and deposited on Si for fracture toughness and modulus experiments. Based on the molecular side chains and processing conditions, these films will form amorphous or crystalline phases, which will be studied through X-ray diffraction and AFM analysis. Using a micromechanical test frame, we will strain the films with in-situ visualization and measure the onset crack strain to determine how structural ordering in the polymers impacts the failure strain. Mechanical modulus will be probed using nanoindentation and fracture toughness using the double cantilever beam method. Atomistic modeling will be performed to analyze the impact of the polymer backbone and molecular side chains on force/deformation responses in the conjugated polymers. Data analytics will be performed to provide additional understanding of the influence of molecular structure and processing on the expected mechanical properties of these systems. The goal is the development of films that can withstand 15% mechanical strain with fracture toughness greater than 10 J/m2. These experiments will yield insight into how processing and structural ordering in conductive polymers impacts their mechanical properties and potential for use in flexible electronic devices.