Interface-Induced Crystallization in Polymers: From Model Systems to Functional Semiconducting Polymers

Crystallization is often initiated at interfaces. Understanding the physical process underlying interface-induced crystallization is of fundamental interest and is relevant for many material applications. Interface-induced crystallization of liquids can occur either by heterogeneous nucleation or by the equilibrium phenomenon of prefreezing. First, we present a combined theoretical and experimental study of the effect of substrate-material interactions on the thermodynamics of prefreezing [1-3] and on the kinetics of heterogeneous nucleation in model polymers on various substrates [4]. Second, the knowledge gained about interface-induced crystallization is used to elucidate the role of interfaces for crystal orientation in films of conjugated polymers, which is important for device performance. Using polythiophenes as model conjugated polymers, we show that different crystal orientations can be formed at the interfaces to a substrate and vacuum as a result of two competing interfacial interactions. Our results demonstrate that increasing the polarity of polythiophene side chains influences the interactions at the interfaces, resulting in a change of crystal orientations [5]. Thus, we disclose the crucial role of interfacial interactions for crystallization kinetics, thin film morphology, and control of molecular orientation in films of model and semiconducting polymers.