Cristina Greco1 Kurt Kremer1 Kostas Daoulas1

1, Max Planck Institute for Polymer Research, Mainz, , Germany

Due to their ease of processability and mechanical flexibility, conjugated polymers are attractive materials for organic electronics. Though substantial progress has been made in enhancing their charge transport properties, clear design guidelines are still difficult to establish. A fundamental reason resides in the intimate connection between charge transport and morphology [1]. Morphologies of conjugated polymers are complex and diverse, and thus hard to predict and characterize. In addition, identifying the structural features that are most crucial for charge transport is not trivial. One interesting recent observation is that, to achieve high mobilities, perfect lamellar order is not necessary [2]. Morphologies with only partial lamellar order (“lamellar-like”) can either originate from the intrinsic properties of the polymer, as exemplified by the microstructures of some aggregating materials [2] and by the smectic mesophases of liquid-crystalline polymers [3], or can be induced by processing, e.g. with chain aligning techniques [4].
Here we present a simple model that enables the study of morphologies with lamellar-like order, at device-relevant length scales. Nonbonded interactions responsible for biaxiality of chain orientation and for stacking are described by anisotropic soft potentials, constructed in a top-down manner on the basis of symmetry considerations. Bonded interactions are introduced bottom-up, to capture the conformational characteristics of the polymer under study. Considering polyalkylthiophenes as a test system, we perform Monte Carlo simulations of chains of various lengths, starting from different initial configurations. Lamellar-like morphologies are obtained, either as mono- or polydomains. The type of lamellar-like order realized is characterized by computing 2D scattering patterns, which can be easily compared with experimental GIWAXS data. We conclude that the obtained morphologies correspond to a lamellar-like smectic mesophase, an organization that was also reported in experiments [3]. We analyze conformational properties, quantifying in a simple way the length of conjugated segments and identifying connectivity pathways between the lamellae. In perspective, atomistic detail can be reintroduced via backmapping, allowing for prediction of charge transport [5].
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[3] Z. Wu, A. Petzold, T. Henze, T. Thurn-Albrecht, R. H. Lohwasser, M. Sommer, M. Thelakkat, Macromolecules 2010, 43, 4646.
[4] L. Hartmann, K. Tremel, S. Uttiya, E. Crossland, S. Ludwigs, N. Kayunkid, C. Vergnat, M. Brinkmann, Adv. Funct. Mater. 2011, 21, 4047.
[5] P. Gemünden, C. Poelking, K. Kremer, K. Daoulas, D. Andrienko, Macromol. Rapid Commun. 2015, 36, 1047.