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.

Multi-Shape Memory Effect of Columnar Side-Chain Liquid Crystalline Polymer

R. Y. Zhao, S. Yang, and E. Q. Chen

Department of Polymer Science and Engineering and Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China

Recently, we are interested in side-chain liquid crystalline (LC) polymers bearing the hemiphasmid side-chain that contains a rod-like mesogen linked with a half-disk end group [1-3]. We found that they could self-organize into the hexagonal and/or rectangular columnar LC phase when the size of flexible tails on the half-disk was properly chosen and the dimension of columnar lattice could approach to 10 nm easily. It is identified that the supramolecular column in the columnar phase shall contain several chains (e.g., ~5 chains) laterally associated together rather than a single chain. This “multi-chain column” provides a new type of physical crosslinking. Namely, within the confined space of the column the backbones and pendant groups of the polymer can get entangled. Using hemiphasmid side-chain LC polynorbornene as the example [4], we demonstrate that such physical crosslinks can be rather robust, giving the polymer with the typical properties of thermal plastic elastomer. Furthermore, taking the physical crosslinks to define the permanent shape and the LC formation to fix the temporary shape, we realized the side-chain polynorbornene with excellent shape memory effect. For the dual shape memory, both the shape fixity (Rf) and shape recovery (Rr) are admirably high (approaching 100%), even when a large strain of 600% was applied. Benefited from a broad LC transition, the polymer can present the high-strain multi-shape memory effect, exampled by its triple- and quadruple-shape memory with high Rf and Rr at each step.

[1] J. F. Zheng, X. Liu, X. F. Chen, X. K. Ren, S. Yang, E. Q. Chen, ACS Macro. Lett. 1, 641 (2012). (link)
[2] X. Q. Liu, J. Wang, S. Yang, E. Q. Chen, ACS Macro Lett. 3, 834 (2014). (link)
[3] Y. S. Xu, D. Shi, J. Gu, Z. Lei, H. L. Xie, T. P. Zhao, S. Yang, E. Q. Chen, Polym. Chem. 7, 462 (2016). (link)
[4] R. Y. Zhao, T. P. Zhao, X. Q. Jiang, X. Liu, D. Shi, C. Y. Liu, S. Yang, E. Q. Chen, Adv. Mater. DOI 10.1002/adma.201605908 (2017) (link)

Highly oriented and crystalline semi-conducting and conducting polymer films prepared by high-temperature rubbing

Martin Brinkmann (1), Amer Hamidi-Sakr (1), Laure Biniek (1), Patrick Lévêque (2), Jean-Louis Bantignies (3),  David Maurin (3), Nicolas Leclerc (4)

(1) Université de Strasbourg, CNRS, ICS UPR22, F67000 Strasbourg, France
(2) Université de Strasbourg, CNRS, ENGEES, INSA, ICube UMR 7357, F-67000 Strasbourg, France
(3) Université de Montpellier, Laboratoire Charles Coulomb, F34095 Montpellier, France
(4) Université de Strasbourg, CNRS, ICPEES, UMR 7515, F67000 Strasbourg, France

This contribution focuses on recent advances in growth control and oriented crystallization of semi-conducting and conducting polymers. Particular emphasis will be given to the progress made in high-temperature rubbing of such polymers. This effective large scale alignment method can orient a large palette of polymer semiconductors (PSCs) with n- or p-type character without the use of an alignment substrate. The concurrent roles of the polymer molecular weight distribution and the rubbing temperature (TR) on the in-plane orientation have been rationalized for P3HT and PBTTT. Continue reading Highly oriented and crystalline semi-conducting and conducting polymer films prepared by high-temperature rubbing

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]. Continue reading Model Experiments for the Crystallization of Conjugated Polymers

Kinetics of crystallization in a model poly(thiophene)

Alberto Salleo

Materials Science and Engineering Department
Stanford University, Stanford CA 94305

Semicrystalline conjugated polymers have attracted much interest as disruptive materials for flexible, low-cost and printed electronics. Indeed, these polymers can be used as semiconductors in thin-film transistors, light-emitting diodes, solar cells and sensors. Furthermore, they have recently been made in stretchable forms. From the materials perspective, it has been known for decades that their electronic performance, as measured by carrier mobility, is very strongly dependent on the film microstructure. One of the goals of this field is to learn how to manipulate the microstructure through processing. In spite of this recognized fundamental need, very little is known about the crystallization processes in these polymers, which are crucial in microstructure formation. We used a model poly(thiophene), poly(3-hexyl-ethyl-thiophene)-(P3EHT)- to perform an in-depth, multi-technique study of crystallization kinetics and its effect on charge transport. Continue reading Kinetics of crystallization in a model poly(thiophene)