(581a) Towards Molecular Design of Conjugated Polymers: Glass Transition, Liquid Crystal Phases, and Entanglements

Polymers are poised to play an important role in various emerging optoelectronic applications, because they can combine chemical versatility, flexibility, stretchability, and mechanical toughness with dielectric or semiconducting properties. Nevertheless, in order to exploit their full potential, a clear description of how the structure, morphology, and macroscopic properties of polymers are interrelated is needed. We propose that the starting point for understanding conjugated polymers includes a description of chain conformations and phase behavior; unfortunately, further efforts to measure these crucial characteristics are needed. Predictions and measurements of the persistence length of various conjugated polymers have significantly refined our intuition of the chain stiffness, and have led to predictions of the nematic coupling parameter and nematic-to-isotropic transitions. We show that the consequence of stiff backbones is a ubiquitous alignment layer near interfaces. Rheological measurements have led to refined estimates of the entanglement molecular weight and the glass transition temperature of both poly(3-alkylthiophenes) and push-pull copolymers, leading to new ways of thinking about how crystallites are interconnected within semicrystalline structures. For example, our work suggests that liquid crystalline order is ubiquitous in conjugated polymers, and we have therefore developed an analytical description of how charge conduction depends on molecular weight for nematic polymers that agrees with experimental data. Current efforts continue to refine our knowledge of chain conformations and phase behavior and the factors that influence these properties, thereby enabling the design of novel optoelectronic materials based on conjugated polymers.