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Adam Kiersnowski2 1 Dorota Chlebosz3 Krzysztof Janus1

2, Mechanics and Composite Materials, Leibniz Institute for Polymer Research, Dresden, , Germany
1, Wroclaw University of Science and Technology, Wroclaw, , Poland
3, Max Planck Institute for Polymer Research, Mainz, , Germany

Electroactive and semiconducting polymer materials attract attention because of their potential applications in e.g. sensors, actuators or energy harvesting, which are crucial in development of the wearable electronic devices. In order to take advantage of such materials it is necessary to control the charge generation and charge carrier transport through their volume. The crystal phases play key roles here: crystallinity and crystal sizes as well as polymorphism and crystal orientation have crucial influence on electric properties of electronic devices. Control over the materials performance can be achieved by controlling crystallization from the length scales characteristic of crystal unit cells up to microdomain morphology.1,2
In this work we showcase the crystallization in hybrid blends based on two semicrystalline polymers: poly(3-hexylthiophene) (P3HT): the p-type semiconductor, and poly(vinylidene fluoride) (PVDF) with remarkable piezo- and ferroelectricity. Despite dissimilar in terms of the architecture of their main chains, these polymers have an important thing in common: crystallinity-driven electrical properties. Typically, PVDF and P3HT are used in the form of films or fibers being active parts of the devices. Polymorphism and orientation of crystals in such films can be controlled during their fabrication. In the case of the solution-based processing of pure polymers, the crystallinity can be controlled by tuning polymer-solvent interactions, aggregation of macromolecules in solution, and solvent evaporation rate.2 In the case of melt-processing, the crystallinity depends mainly on the cooling regime whereas the orientation of crystals is controlled by machine-induced shearing forces or mechanical deformation after the processing.3
Blending of either PVDF or P3HT with other materials typically leads to heterogeneous systems, where the crystallinity is additionally driven by interfacial phenomena like heterogeneous nucleation or epitaxy. These together with the aforementioned effects are particularly important in nucleating the preferred polymorphs. In PVDF the ferro- or piezoelectricity are observed only for polar crystal polymorphs, i.e. the crystal forms where the unit cells are non-centrosymmetric, as in the case of Form I or Form III resulting from e.g. nucleation by e.g. silver nanoparticles. In addition, nanoparticles with high aspect ratios such as nanoplatelets of organoclays have an ability to “direct” the diffusion of the polymer chains towards the crystal growth zones, which allows formation of the oriented PVDF crystals.4 Formation of the oriented crystals of P3HT can also be driven by anisotropic nanoparticles, such as needle-like nanocrystals of perylene diimides or graphene nanoribbons.5 In the case of P3HT, however, the formation of oriented crystals results from the specific interactions between the nanofibers and the polymer.6
The orientation of the polymer crystals can be further enhanced by thermally stimulated diffusion of polymer macromolecules to the crystal growth zones, which can be achieved by e.g. local laser heating. For this purpose we have developed Laser-Assisted Zone Crystallization technique (LAZEC) enabling solution crystallization of the polymers and other organic materials under controlled thermal conditions. Application of the LAZEC in crystallization of the blends with finely tuned composition enables a large-scale formation of continuous films with controlled polymorphism and spatial orientation of polymer crystals.

The work was supported by National Sci. Centre Poland (NCN) through the grants UMO-2016/22/E/ST5/00472 and UMO-2017/25/B/ST5/02869

References:

1) Zhang G. et al; Energy & Environmental Science 11, 2018, 2046
2) Zhao K. et al; ACS Applied Materials & Interfaces 8, 2016, 19649
3) Martin J. et al; Materials Horizons 4, 2017, 408
4) Kiersnowski A. et al; Langmuir, accept. 2018
5) ElGemayel M. et al; Nanoscale 6, 2014, 6301
6) Chlebosz D. et al; Dyes and Pigments 140, 2017, 491

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