Understanding the Ultrafast Crystallization Kinetics of Donor-Acceptor Semiconducting Polymers.

The crystallization of donor-acceptor semiconducting polymers occurs on subsecond time scales at elevated temperatures (>300 °C), preventing a rigorous kinetic description using conventional experimental approaches. Here, we use a single-step fast scanning calorimetry (FSC) methodology that enables investigating the fast isothermal crystallization kinetics of advanced donor-acceptor semiconducting polymers such as PTQ10 and D18. This approach overcomes the intrinsic limitations of the indirect "by-step" method and provides reliable kinetic data within tenths of a second. Furthermore, we demonstrate that a modified version of the Malkin model, which we also introduce here, successfully captures the entire kinetic evolution and provides important information about how crystallization develops in these polymers. For example, we observe two well-differentiated crystallization regimes as a function of the crystallization temperature. A newly introduced geometrical parameter of the kinetic model reveals that the distinct crystallization kinetics result in different crystalline morphologies, which were further confirmed by X-ray diffraction, melting analysis, and electron microscopy. Our analysis suggests that crystallization proceeds via a nearly instantaneous formation of a dense population of nanoscopic crystallites and is effectively governed by a single dominant kinetic step that must therefore be strongly linked to nucleation. However, self-nucleation experiments reveal that this process does not conform to the classical nucleation behavior typically observed in semicrystalline polymers. These findings allow establishing a new framework for the understanding and control of the solid-state microstructure of semicrystalline polymers.