This paper describes the synthesis and characterization of a class of highly stretchable and degradable semiconducting polymers. These materials are multi-block copolymers (BCPs) in which the semiconducting blocks are based on the diketopyrrolopyrrole (DPP) unit flanked by furan rings and the insulating blocks are poly(ε-caprolactone) (PCL). The combination of stiff conjugated segments with flexible aliphatic polyesters produces materials that can be stretched >100%. Remarkably, BCPs containing up to 90 wt% of insulating PCL have the same field-effect mobility as the pure semiconductor. Spectroscopic (ultraviolet-visible absorption) and morphological (atomic force microscopic) evidence suggests that the semiconducting blocks form aggregated and percolated structures with increasing content of the insulating PCL. Both PDPP and PCL segments in the BCPs degrade under simulated physiological conditions. Such materials could find use in wearable, implantable, and disposable electronic devices.

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.

This paper describes effects of the flexibility, length, and branching of side chains on the mechanical properties of low-bandgap semiconducting polymers. The backbones of the polymer chains comprise a diketopyrrolopyrrole (DPP) motif flanked by two furan rings and copolymerized by Stille polycondensation with thiophene (DPP2FT). The side chains of the DPP fall into three categories: linear alkyl (C8, C14, or C16), branched alkyl (ethylhexyl, EH, or hexyldecyl, HD), and linear oligo(ethylene oxide) (EO3, EO4, or EO5). Polymers bearing C8 and C14 side chains are obtained in low yields and thus not pursued. Thermal, mechanical, and electronic properties are plotted against the number of carbon and oxygen atoms in the side chain. We obtain consistent trends in the thermal and mechanical properties for branched alkyl and linear oligo(ethylene oxide) side chains. For example, the glass transition temperature (Tg) and elastic modulus decrease with increasing number of carbon and oxygen atoms, whereas the crack-onset strain increases. Among polymers with side chains of 16 carbon and oxygen atoms (C16, HD, and EO5), C16 exhibits the highest Tg and the greatest susceptibility to fracture. Hole mobility, as measured in thin-film transistors, appears to be a poor predictor of electronic performance for polymers blended with [60]PCBM in bulk heterojunction (BHJ) solar cells. For example, while EO3 and EO4 exhibit the lowest mobilities (< 10–2 cm2 V–1 s–1) in thin-film transistors, solar cells made using these materials performed the best (efficiency > 2.6%) in unoptimized devices. Conversely, C16 exhibits the highest mobility (≈ 0.2 cm2 V–1 s–1) but produces poor solar cells (efficiency < 0.01%). We attribute the lack of correlation between mobility and power conversion efficiency to unfavorable morphology in the BHJ solar cells. Given the desirable properties measured for EO3 and EO4, the use of flexible oligo(ethylene oxide) side chains is a successful strategy to impart mechanical deformability to organic solar cells, without sacrificing electronic performance.

The regiospecific radical polymerization of vinyl methacrylate (VMA), a divinyl compound with two different radically polymerizable double bonds, was investigated in the presence of Lewis acids, such as SnCl 4 , Et n AlCl 3-n , and R n Al(ODBP) 3-n [R ) Me, Et, i-Bu; ODBP ) 2,6-di-tert-butylphenoxy]. The polymerization with R,R-azobis(isobutyronitrile) (AIBN) in the presence of aluminum-based Lewis acids gave soluble polymers, in which the methacryloyl group was selectively polymerized, while insoluble polymer gels were obtained without Lewis acids or with SnCl 4 . When using monoalkylaluminum bulky diphenoxides [RAl(ODBP) 2 ], the highly regiospecific radical polymerization significantly proceeded with a high conversion (>85%) to give soluble polymers with quantitatively remaining vinyl ester pendants and a relatively high molecular weight (M n g 10 4 ). The copolymerization of VMA with methyl methacrylate in the presence of EtAl(ODBP) 2 induced the simultaneous consumption of both monomers at almost the same rate to directly give soluble random pendant-functionalized copolymers with vinyl ester moieties. The NMR analysis of the mixture of VMA and EtAl(ODBP) 2 showed a 1:1 complexation with a relatively high association constant (K assn ) 59.6 M -1 ) via the coordination of the aluminum Lewis acid to the carbonyl moiety. The ruthenium-catalyzed living radical polymerization of VMA was also possible in the presence of EtAl(ODBP) 2 and an iodide initiator to give soluble polymers with controlled molecular weights.

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