Substrate-induced crystallization of the conjugated polymer TQ1: Theoretical prediction, nucleation and growth style identification through single chain resolved structures and crystallization kinetics analysis

Abstract

For both the applications and fundamental sciences of polymers, crystallization of the polymer layer adsorbing directly on solid substrate (adlayer) is crucially important, but its complicate mechanisms and kinetics remain disputable. The evidences at molecular level are great valuable for clarifying these discussions. The substrate-induced crystallization of the glassy adlayer of conjugated polymer TQ1 was predicted through the density functional theory (DFT) calculations and monitored through the molecular-level observations of scanning tunneling microscopy (STM). The STM micrographs disclose the nucleation and growth styles of TQ1 lamella including nucleation through the adjustment of local part of polymer chains, lamella widening through the chain attachment to the growing front and thickening through the gradual linearization of TQ1 backbones which causes the wedge shape of TQ1 lamellae, termination of the lamella thickening by the chain folding or extra free volume because of the existence of short chains. The DFT calculations further reveal that the energy barrier for lamella thickening comes mainly from the conformation changing of quinoxaline groups. Furthermore, the crystallization kinetics were unveiled in detail by the changing tendency of crystallinity, lamella thickness and width. At the initial stage, crystallization is very efficient and its kinetics can be rationalized with the Arrhenius equation. The crystallization process in this period is dominated by the surface diffusion of TQ1 chains to the growing front of lamella. After this stage, crystallization experiences a slow fluctuation increment to the maximum value then begins to decline, which may be caused by the multiple physical processes and cannot be rationalized simply with one model currently. The results here provide direct molecular-level evidences for understanding the complicate crystallization processes of polymer adlayers and fundamental supports for developing high performance polymer electronics.