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.

December 1, 2019

Air Force Research Laboratory, Arlington, Virginia

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