Quantifying the Fracture Behavior of Brittle and Ductile Thin Films of Semiconducting Polymers

MA Alkhadra and SE Root and KM Hilby and D Rodriquez and F Sugiyama and DJ Lipomi, CHEMISTRY OF MATERIALS, 29, 10139-10149 (2017).

DOI: 10.1021/acs.chemmater.7b03922

One of the primary complications in characterizing the mechanical properties of thin films of semiconducting polymers for flexible electronics is the diverse range of fracture behavior that these materials exhibit. Although the mechanisms of fracture are well understood for brittle polymers, they are underexplored for ductile polymers. Experimentally, fracture can be characterized by observing the propagation of cracks and voids in an elongated film. For brittle polymers, we find that films bifurcate in such a way that the crack density increases linearly with applied strain (R-2 >= 0.91) at small strains. Linear regression is used to estimate the fracture strength and strain at fracture of each material using an existing methodology. For the case of ductile polymers, however, we find that diamond-shaped microvoids, which originate at pinholes and defects within the film, propagate with an aspect ratio that increases linearly with applied strain (R-2 >= 0.98). We define the rate of change of the aspect ratio of a microvoid with respect to applied strain as the "microvoid- propagation number." This dimensionless film parameter, previously unreported, is a useful measure of ductility in thin films supported by an elastomer. To explore the significance of this parameter, we correlate the microvoid-propagation number with nominal ductility using several ductile polymer films of approximately equal thickness. Since the fracture of a film supported by a substrate depends on the elastic mismatch, we study the effect of this mismatch on the propagation of microvoids and observe that the microvoid-propagation number increases with increasing elastic mismatch. Moreover, we find that thicker films exhibit greater resistance to the propagation of fracture. We hypothesize that this behavior may be attributed to a larger volume of the plastic zone and a higher density of entanglements. To understand how the intrinsic mechanical properties of a film influence the fracture behavior on a substrate, we perform tensile tests of notched and unnotched films floated on the surface of water. We find a linear correlation (R-2 = 0.99) between the logarithm of the microvoid- propagation number and the fracture stress obtained from tensile tests of unnotched films.

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