Unraveling the Influence of the Preexisting Molecular Order on the Crystallization of Semiconducting Semicrystalline Poly(9,9-di-n-octylfluorenyl-2,7-diyl (PFO).

Abstract

Understanding the complex crystallization process of semiconducting polymers is key for the advance of organic electronic technologies as the optoelectronic properties of these materials are intimately connected to their solid-state microstructure. These polymers often have semirigid backbones and flexible side chains, which results in a strong tendency to organize/order in the liquid state. Therefore, crystallization of these materials frequently occurs from liquid states that exhibit-at least partial-molecular order. However, the impact of the preexisting molecular order on the crystallization process of semiconducting polymers- indeed, of any polymer-remained hitherto unknown. This study uses fast scanning calorimetry (FSC) to probe the crystallization kinetics of poly(9,9-di-n-octylfluorenyl-2,7-diyl (PFO) from both an isotropic disordered melt state (ISO state) and a liquid-crystalline ordered state (NEM state). Our results demonstrate that the preexisting molecular order has a profound impact on the crystallization of PFO. More specifically, it favors the formation of effective crystal nucleation centers, speeding up the crystallization kinetics at the early stages of phase transformation. However, samples crystallized from the NEM state require longer times to reach full crystallization (during the secondary crystallization stage) compared to those crystallized from the ISO state, likely suggesting that the preexisting molecular order slows down the advance in the latest stages of the crystallization, that is, those governed by molecular diffusion. The fitting of the data with the Avrami model reveals different crystallization mechanisms, which ultimately result in a distinct semicrystalline morphology and photoluminescence properties. Therefore, this work highlights the importance of understanding the interrelationships between processing, structure, and properties of polymer semiconductors and opens the door for performing fundamental investigations via newly developed FSC methodologies of such materials that otherwise are not possible with conventional techniques.