Model Experiments for the Crystallization of Conjugated Polymers

G. Reiter1, F.M. Keheze1, D. Raithel2, R. Hildner2, D.Schiefer3, M. Sommer3

1Physikalisches Institut, Universität Freiburg, Freiburg, Germany
2Experimentalphysik IV, University of Bayreuth, Bayreuth, Germany.
3Institut für Makromolekulare Chemie, Universität Freiburg, Freiburg, Germany

Much insight into crystallization of long chain polymers can be gained by studying mono-lamellar single crystals. Because of the kinetically determined lamellar thickness and the corresponding variations in melting temperature, polymer crystals allow for self-seeding, i.e., crystals can be re-grown from a melt, which contains a few thermodynamically stable remnants of pre-existing crystals acting as seeds. Employing such a self-seeding approach, we demonstrated that large single crystals can be grown even from long poly(3-hexylthiophene) (P3HT) chains, with a control over the number density, size, and internal structure of these crystals exhibiting monoclinic form II with interdigitated hexyl side groups [1].

We also studied bulky substituted poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT) where weakened interchain interactions lead to a much lower melting temperature as compared to P3HT [2]. Using absorption and photoluminescence spectroscopy and a comparison with calorimetric measurements, we followed in-situ melting and recrystallization processes.

Well-ordered crystalline structures of semiconducting polymers exhibiting the same molecular conformation over many length scales represent valuable model systems for exploring relations between structure and optoelectronic properties.

[1] Khosrow Rahimi, et al., Angew. Chem. 124, 11293 –11297 (2012).
[2] Yingying Wang, et al., ACS MacroLett. 3, 881-885 (2014).