An extensive family of semi-random polymers was prepared via Stille polycondensation with varying contents of alkyl spacers incorporated into the polymer backbone to serve as a break in conjugation. This family was investigated to determine the effect of alkyl spacer length and percent incorporation on the optical, electronic, and mechanical properties. The optical bandgap was found to steadily increase from 1.53 to 1.70 eV as the amount of spacer was increased from 10 mol percent to 40 mol percent while the length of the spacer had little to no effect. In space charge limited current (SCLC) carrier mobility measurements, hole mobility was found to decrease as the amount of spacer increased but was found to steadily increase as the length of the spacer was increased from 6 to 10 carbons. Mechanical properties were observed by film-on-elastomer and film-on-water measurements, with low elastic moduli and high ductility attributed both to the break in conjugation as well as the semi-random structure of the polymer backbone. Measurements of the mechanical properties using the buckling method revealed elastic moduli between 0.14 and 1.3 GPa, and several polymers, when bonded to an elastomeric substrate, could be stretched beyond 80% strain. These polymers were further tested as free-standing films by obtaining a pull test on the surface of water, where we obtained tensile moduli between 0.13 and 0.75 GPa. These results indicate that semi-random polymers with conjugation-break spacers are promising candidates for further study in flexible electronics.

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.

Conjugation-break spacers (CBS) have been shown to enhance the mechanical properties of conjugated polymers. In particular, incorporation of CBS units into semi-random polymers has revealed high ductility and low elastic moduli, attributed to the combined influence of the CBS units and the semi-random architecture. To further elucidate the structure–property relationships in these polymers, two new families of semi-random polymers are reported here. In the first, poly­(3-hexylthiophene)-based semi-random polymers incorporating diketo­pyrrolopyrrole (DPP) units were synthesized in which CBS units with 4–10 carbons were incorporated from 10 to 40% with an equivalent content of 2-decyltetradecyl-DPP (dtdDPP) to overcome solubility limitations previously observed with 2-ethylhexyl-DPP (ehDPP). These polymers had much higher solubility and could attain higher molecular weights, formed films with high integrity, and displayed extraordinary mechanical properties, with elastic moduli as low as 5.45 MPa and fracture strains as high as 398%. In the second family, the content of ehDPP was held constant at 10%, while the CBS content was varied from 10 to 50% (with an eight-carbon spacer) to deconvolute the influence of CBS and DPP content on mechanical properties. Polymer solubility, molecular weight, and processability were not shown to improve dramatically relative to the previous generation of ehDPP polymers with matched DPP and CBS content, but the mechanical properties of this series were quite notable, with elastic moduli as low as 4.08 MPa, an increase in toughness, and fracture strains as high as 432%. The extraordinary mechanical properties exhibited by these polymers can serve as a guide in the judicious selection of monomers and backbone architectures in the future synthesis of semiconducting polymers for flexible electronic applications.