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Molecular Engineering for Mechanically Resilient and Stretchable Electronic Polymers and Composites

Establishing the design criteria for elasticity and ductility in conjugated polymers and composites by analysis of the structural determinants of the mechanical properties.

The ability to predict the mechanical properties of organic semiconductors is of critical importance for roll-to-roll production and thermomechanical reliability of organic electronic devices. This research describes the use of coarse-grained molecular dynamics simulations to predict the density, tensile modulus, Poisson ratio, and glass transition temperature for poly(3-hexylthiophene) (P3HT) and its blend with C60. In particular, it is shown that the resolution of the coarse-grained model has a strong effect on the predicted properties.

Visualization of coarse-grained molecular dynamics simulations on polymer:fullerene bulk heterojunction films. The plot shows the stiffening of C60 fullerene (anti-plasticization) on the mechanical properties of P3HT simulated with a three-site coarse-grained model.

It was found that a one-site model, in which each 3-hexylthiophene unit is represented by one coarse-grained bead, predicts significantly inaccurate values of density and tensile modulus. In contrast, a three-site model, with one coarse-grained bead for the thiophene ring and two for the hexyl chain, predicts values that are very close to experimental measurements (density = 0.955 g cm–3, tensile modulus = 1.23 GPa, Poisson ratio = 0.35, and glass transition temperature = 290 K). The model also correctly predicts the strain-induced alignment of chain, as well as the vitrification of P3HT by C60 and the corresponding increase in the tensile modulus (tensile modulus = 1.92 GPa, glass transition temperature = 310 K).

Schematic drawing indicating a library of donor-acceptor low-bandgap conjugated polymers comprising 13 different acceptors and 9 different donors. The mechanical properties were measured and the structural determinants of stiffness and brittleness were elucidated.

Although extension of the model to poly(3-alkylthiophenes) (P3ATs) containing side chains longer than hexyl groups—nonyl (N) and dodecyl (DD) groups—correctly predicts the trend of decreasing modulus with increasing length of the side chain measured experimentally, obtaining absolute agreement for P3NT and P3DDT could not be accomplished by a straightforward extension of the three-site coarse-grained model, indicating limited transferability of such models. Nevertheless, the accurate values obtained for P3HT and P3HT:C60 blends suggest that coarse graining is a valuable approach for predicting the thermomechanical properties of organic semiconductors of similar or more complex architectures.

The mechanical properties of low-bandgap polymers are important for the long-term survivability of roll-to-roll processed organic electronic devices. Such devices — e.g., solar cells, displays, and thin-film transistors — must survive the rigors of roll-to-roll coating and also thermal and mechanical forces in the outdoor environment and in stretchable and ultra-flexible form factors. This research measured the stiffness (tensile modulus), ductility (crack-onset strain), or both, of a combinatorial library of 51 low-bandgap polymers.

The purpose of this study was to systematically screen a library of low-bandgap polymers to better understand the connection between molecular structures and mechanical properties, in order to design conjugated polymers that permit mechanical robustness and even extreme deformability. While one of the principal conclusions of these experiments is that the structure of an isolated molecule only partially determines the mechanical properties — another important co-determinant is the packing structure — some general trends can be identified. Fused rings tend to increase the modulus and decrease the ductility. Branched side chains have the opposite effect. Despite the rigidity of the molecular structure, the most deformable films can be surprisingly compliant (modulus ≥ 150 MPa) and ductile (crack-onset strain " 68%). The project concluded by proposing a new composite merit factor that combines the power conversion efficiency in a fully solution processed device obtained via roll and roll-to-roll coating and printing, and the mechanical deformability toward the goal of producing modules that are both efficient and mechanically stable.

This work was done by Darren J. Lipomi of the University of California, San Diego, for the Air Force Research Laboratory. AFRL-0278

Topics:
Aerospace Composite materials Composites Defense Electronic Components Electronic equipment Electronics Electronics & Computers Glass Materials Materials properties Mechanical Components Polymers Production Reliability Research and development Semiconductors Semiconductors & ICs Tensile strength
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Molecular Engineering for Mechanically Resilient and Stretchable Electronic Polymers and Composites

(reference AFRL-0278) is currently available for download from the TSP library.

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    Aerospace & Defense Technology Magazine

    This article first appeared in the December, 2019 issue of Aerospace & Defense Technology Magazine (Vol. 4 No. 7).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document is a final report for the AFOSR Young Investigator Program Award (Contract Number: FA9550-13-1-0156) authored by Prof. Darren J. Lipomi from the University of California, San Diego. The report covers the research conducted over a three-year period from April 1, 2013, to March 31, 2016, focusing on the development of mechanically resilient and stretchable electronic polymers and composites.

    The primary objective of the project was to establish design criteria for introducing elasticity and ductility into high-performance conjugated (semiconducting) polymers and polymer-small molecule composites. This was achieved through a rigorous analysis of the structural determinants that influence the mechanical properties of these materials. The research aimed to enhance the performance of electronic devices by improving the mechanical characteristics of the materials used.

    Throughout the project, Prof. Lipomi's laboratory published a total of 24 papers, reflecting significant contributions to the field of organic materials chemistry. The report summarizes key results from the final year of funding, highlighting advancements in understanding how to manipulate the mechanical properties of polymers to achieve desired levels of flexibility and strength.

    The report includes essential details such as the report date (May 18, 2016), the type of report (final), and the specific dates covered by the research. It also provides information about the performing organization, the program officer (Dr. Charles Lee), and the distribution approval for public release.

    In addition to the technical findings, the report emphasizes the importance of this research in the context of developing next-generation electronic materials that can withstand mechanical stress while maintaining their functionality. The insights gained from this project are expected to have broad implications for various applications, including flexible electronics, wearable devices, and other technologies that require materials with both electronic and mechanical resilience.

    Overall, this final report encapsulates a significant research endeavor aimed at advancing the field of electronic polymers, providing a foundation for future innovations in material science and engineering.

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