Controlling Healing and Toughness in Polyurethanes by Branch-Mediated Tube Dilation.

In this work, we propose the use of regular branching of polyurethanes as a way to regulate chain dynamics and govern crystallization in highly dense hydrogen-bonded systems. As a result, robust and healable polyurethanes can be obtained. To this end, we synthesized a range of aliphatic propane diol derivatives with alkyl branches ranging from butyl (C4) to octadecanyl (C18). The series of brush polyurethanes was synthesized by polyaddition of the diols and hexamethylene diisocyanate. Polyurethanes with very short (C < 4) and very long (C = 18) brush lengths did not lead to any significant healing due to crystallization. An intermediate amorphous regime appears for polymers with middle branch lengths (C = 4 to 8) showing a fine control of material toughness. For these systems, the side chain length regulates tube dilation, and significant macroscopic healing of cut samples was observed and studied in detail using melt rheology and tensile testing. Despite the high healing degrees observed immediately after repair, it was found that samples with medium to long length brushes lost their interfacial strength at the healed site after being heated to the healing temperature for some time after the optimal time to reach full healing. Dedicated testing suggests that annealed samples, while keeping initial tackiness, are not able to completely heal the cut interface. We attribute such behavior to annealing-induced interfacial crystallization promoted by the aliphatic branches. Interestingly, no such loss of healing due to annealing was observed for samples synthesized with C4 and C7 diols, which is identified as the optimal healing regime. These results point at the positive effect of branching on healing, provided that a critical chain length is not surpassed, as well as the need to study healing behavior long after the optimal healing times.

Diketopyrrolopyrroles as acceptor materials in organic photovoltaics.

In the search of new electron acceptor, n-type materials for organic solar cells that combine a strong absorption over a broad range with good electrical characteristics, the use of diketopyrrolopyrrole (DPP) derivatives with low reduction potentials is explored. A series of small molecule DPP derivatives is presented and the compounds are tested as electron acceptors in combination with poly(3-hexylthiophene) (P3HT) as the donor material. Working photovoltaic devices are obtained that show a photoresponse in the wavelength region where the DPP molecules absorb. The best device shows a power conversion efficiency of 0.31% in simulated solar light, with a photon-to-electron conversion efficiency of ∼10% up to 700 nm. The efficiency seems to be limited by the coarse morphology of the blend.

Poly(diketopyrrolopyrrole-terthiophene) for ambipolar logic and photovoltaics.

A new semiconducting polymer, PDPP3T, with alternating diketopyrrolopyrrole and terthiophene units is presented. PDPP3T has a small band gap of 1.3 eV and exhibits nearly balanced hole and electron mobilities of 0.04 and 0.01 cm(2) V(-1) s(-1), respectively, in field-effect transistors (FETs). By the combination of two identical ambipolar transistors, an inverter was constructed that exhibits a gain of approximately 30. When PDPP3T was combined with [60]PCBM or [70]PCBM in a 1:2 weight ratio, photovoltaic cells were made that provide a photoresponse up to 900 nm and an AM1.5 power conversion efficiency of 3.8 or 4.7%, respectively. In contrast to the almost constant FET mobility, the efficiency of the photovoltaic cells was found to be strongly dependent on the molecular weight of PDPP3T and the use of diiodooctane as a processing agent.