Spatial Orientation and Order of Molecular Subunits in Films of Organic Semiconductors

A. M. Anton and F. Kremer

Peter Debye Institute for Soft Matter Physics, Leipzig University, Linnéstr. 5, 04013 Leipzig

Conjugated donor/acceptor copolymers have obtained significant attention due to their soft matter properties combined with semiconducting characteristics, which allows for their operation in organic field effect transistors or solar cells, for instance [1]. Because the devices’ macroscopic properties arise from the materials’ molecular organization, a detailed understanding of the microscopic structure is essential for targeted developments. In order to shed light on the spatial orientation and order in thin films of P(NDI2OD-T2) the technique of Infrared Transition Moment Orientational Analysis (IR-TMOA) is employed. Therefore, the absorbance of structure-specific bands depending on the inclination of the sample and on the polarization of the IR light is evaluated [2,3]. This enables to determine the tensor of absorption separately for the respective molecular moieties as well as to deduce the orientation of atomistic planes defined through the polymer subunits, relative to the substrate and hence relative to each other. We found that the solvent used for spin coating (chlorobenzene or a chloronaphthalene:xylene mixture) determines the alignment of the T2 part (either face on or edge on), whereas the NDI unit is not affected. On the other hand, the inclination of the NDI plane is well retained for diverse sample thicknesses in between nano- and micrometers.

[1] A. C. Arias, J. D. MacKenzie, I. McCulloch, J. Rivnay, A. Salleo, Chem. Rev. 110, 3 (2010). (link)
[2] A. M. Anton, R. Steyrleuthner, W. Kossack, D. Neher, F. Kremer, JACS 137, 6034 (2008). (link)
[3] A. M. Anton, R. Steyrleuthner, W. Kossack, D. Neher, F. Kremer, Macromolecules 49, 1798 (2016). (link)

Large area three-dimensional polarization control in P(VDF-TrFE) polymer films on graphite

R. Roth1, M. Koch2, J. Schaab2, M. Lilienblum2, T. Thurn-Albrecht1, and K. Dörr1

1Institute of Physics, Martin Luther University Halle-Wittenberg, 06099 Halle, Germany
2Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 1-5/10,
8093 Zürich, Switzerland

Ferroelectric polymers are attractive candidates for functional layers in electronic devices like non-volatile memories, piezo- and magnetoelectric sensors, and capacitor-based high speed energy storage devices. Unfortunately, such thin films often reveal low di- and piezoelectric responses due to reduced crystalline and electrical dipole order, leading to compensation effects and low effective electric performance. One of the best characterized and often applied ferroelectric polymers is poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)). We will present results on micron-sized domains with three dimensional ferroelectric polarization alignment in P(VDF-TrFE) films on graphite. The ferroelectric domains have been achieved by a combined procedure of electrical poling and mechanical annealing with an atomic force microscopy tip. They show strongly increased lateral and vertical piezoresponse compared to the as-prepared film and can be deliberately written and switched.

Crystallization-Driven Reversible Actuation in Cross-Linked Poly(ε-caprolactone)

O. Dolynchuk1, I. Kolesov2, D. Jehnichen3, U. Reuter3, H.-J. Radusch4, and J.-U. Sommer3

1Institute of Physics, Martin Luther University Halle-Wittenberg, Von-Danckelmann-Platz 3, 06120 Halle, Germany
2Interdisciplinary Center for Transfer-oriented Research, Martin Luther University Halle-Wittenberg, 06099 Halle, Germany
3Leibniz-Institut für Polymerforschung Dresden e.V., 01069 Dresden, Germany
4Center of Engineering Sciences, Martin Luther University Halle-Wittenberg, 06099 Halle, Germany

Crystallization of the pre-deformed polymer network under constant load can result in a non-trivial macroscopic elongation accompanied by network stiffening, which is reversible upon melting. Such actuation, known as the reversible shape-memory effect (rSME), is in focus due to fundamental interest of underlying molecular mechanisms and numerous potential applications. The rSME was studied in cross-linked linear poly(ε-caprolactone) (PCL) under various constant loads [1]. A striking rSME under stress-free conditions was found in PCL with the highest obtained cross-link density. The viscoelastic and thermal properties of the material as well as size and orientation of the crystals formed in PCL networks under and without load were investigated. As concluded, the directed growth of crystals is the origin of the reversible actuation in both loaded and free-standing PCL. The mean field approach was employed to calculate the free energy change during non-isothermal crystallization of PCL networks under load, whereby the possible morphology and orientation of crystals were analyzed. The analytical results on the nanocrystalline structure along with fitting curves of the temperature dependent strain, which were obtained by modeling the SME in PCL under load, are in good accordance with experimental findings.

[1] O. Dolynchuk, I. Kolesov, D. Jehnichen, U. Reuter, H.-J. Radusch, J.-U. Sommer, Macromolecules 50, 3841 (2017). (link)

Development of improved materials from poly(lactic acid) with the aid of plasticizers and crystallization-promoting gelators

M. Colaers1, W. Thielemans2, and B. Goderis1

1Polymer Chemistry and Materials, KU Leuven, 3001 Heverlee, Belgium
2Chemical Engineering, Campus Kulak Kortrijk, 8500 Kortrijk, Belgium

Polylactic acid (PLA) is a bio-based polymer which might become an alternative for petroleum-based plastics such as polypropylene and polystyrene. However, at present, the properties of PLA do not meet the requirements for a number of applications. The challenge to be addressed is to develop transparent PLA with an increased heat distortion temperature, balanced stiffness and toughness and increased barrier properties. The method explored to reach this multi-dimensional goal is to blend PLA with combinations of plasticizers (for reducing the brittleness) and crystallization-promoting gelators (to increase the crystallinity). Upon cooling, the gelator forms a fine fibrillary network, which nucleates the PLA crystallization. The resulting, small-sized crystal aggregates limit the scattering of visible light and enhance transparency, which is desired for packaging purposes. An increased crystallinity is required to enhance the barrier properties and increase the heat distortion temperature.

The semicrystalline morphology and efficiency of the gelator fibrillary network depends on the cooling conditions and the addition of plasticizers. Both aspects are addressed using Differential Scanning Calorimetry, optical microscopy and time-resolved synchrotron SAXS/WAXD.