title: Comparison of Methods for Determining the Mechanical Properties of Semiconducting Polymer Films for Stretchable Electronics.
abstract: This paper describes a comparison of two characterization techniques for determining the mechanical properties of thin-film organic semiconductors for applications in soft electronics. In the first method, the film is supported by water (film-on-water, FOW), and a stress-strain curve is obtained using a direct tensile test. In the second method, the film is supported by an elastomer (film-on-elastomer, FOE), and is subjected to three tests to reconstruct the key features of the stress-strain curve: the buckling test (tensile modulus), the onset of buckling (yield point), and the crack-onset strain (strain at fracture). The specimens used for the comparison are four poly(3-hexylthiophene) (P3HT) samples of increasing molecular weight (Mn= 15, 40, 63, and 80 kDa). The methods produced qualitatively similar results for mechanical properties including the tensile modulus, the yield point, and the strain at fracture. The agreement was not quantitative because of differences in mode of loading (tension vs compression), strain rate, and processing between the two methods. Experimental results are corroborated by coarse-grained molecular dynamics simulations, which lead to the conclusion that in low molecular weight samples (Mn= 15 kDa), fracture occurs by chain pullout. Conversely, in high molecular weight samples (Mn> 25 kDa), entanglements concentrate the stress to few chains; this concentration is consistent with chain scission as the dominant mode of fracture. Our results provide a basis for comparing mechanical properties that have been measured by these two techniques, and provide mechanistic insight into fracture modes in this class of materials.
authors.affiliation: Department of NanoEngineering, University of California , San Diego 9500 Gilman Drive, Mail Code 0448, La Jolla, California 92093-0448, United States.
authors.forename: Daniel
authors.initials: D
authors.lastname: Rodriquez
authors.affiliation: Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST) , Yuseong-gu, Daejeon 34141, Korea.
authors.forename: Jae-Han
authors.initials: JH
authors.lastname: Kim
authors.affiliation: Department of NanoEngineering, University of California , San Diego 9500 Gilman Drive, Mail Code 0448, La Jolla, California 92093-0448, United States.
authors.forename: Samuel E
authors.initials: SE
authors.lastname: Root
authors.affiliation: Department of Chemistry and Centre for Plastic Electronics, Imperial College London , Exhibition Rd, London, SW7 2AZ, United Kingdom.
authors.forename: Zhuping
authors.initials: Z
authors.lastname: Fei
authors.affiliation: Department of Chemistry and Centre for Plastic Electronics, Imperial College London , Exhibition Rd, London, SW7 2AZ, United Kingdom.
authors.forename: Pierre
authors.initials: P
authors.lastname: Boufflet
authors.affiliation: Department of Chemistry and Centre for Plastic Electronics, Imperial College London , Exhibition Rd, London, SW7 2AZ, United Kingdom.
authors.forename: Martin
authors.identifier: http://orcid.org/0000-0001-6879-5020
authors.initials: M
authors.lastname: Heeney
authors.affiliation: Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST) , Yuseong-gu, Daejeon 34141, Korea.
authors.forename: Taek-Soo
authors.identifier: http://orcid.org/0000-0002-2825-7778
authors.initials: TS
authors.lastname: Kim
authors.affiliation: Department of NanoEngineering, University of California , San Diego 9500 Gilman Drive, Mail Code 0448, La Jolla, California 92093-0448, United States.
authors.forename: Darren J
authors.identifier: http://orcid.org/0000-0002-5808-7765
authors.initials: DJ
authors.lastname: Lipomi
keywords: P3HT; buckling-based metrology; film on elastomer; film on water; mechanical properties; organic semiconductors; stretchable electronics; thin films