Impact of Peptide Length and Solution Conditions on Tetrameric Coiled Coil Formation.

Unlike naturally derived peptides, computationally designed sequences offer programmed self-assembly and charge display. Herein, new tetrameric, coiled coil-forming peptides were computationally designed ranging from 8 to 29 amino acids in length. Experimental investigations revealed that only the sequences having three or more heptads (i.e., 21 or more amino acids) exhibited coiled coil behavior. The shortest stable coiled coil sequence had a melting temperature (Tm) of approximately 58 ± 1 °C, making it ideal for thermoreversible assembly over moderate temperatures. Effects of pH and monovalent salt were examined, revealing structural stability over a pH range of 4 to 11 and an enhancement in Tm with the addition of salt. The incorporation of the coiled coil as a hydrogel cross-linker results in a thermally and mechanically reversible hydrogel. A subsequent demonstration of the hydrogel printed through a syringe illustrated one of many potential uses from 3D printing to injectable hydrogel drug delivery.

Tuning of the elastic modulus of a soft polythiophene through molecular doping.

Molecular doping of a polythiophene with oligoethylene glycol side chains is found to strongly modulate not only the electrical but also the mechanical properties of the polymer. An oxidation level of up to 18% results in an electrical conductivity of more than 52 S cm-1 and at the same time significantly enhances the elastic modulus from 8 to more than 200 MPa and toughness from 0.5 to 5.1 MJ m-3. These changes arise because molecular doping strongly influences the glass transition temperature Tg and the degree of π-stacking of the polymer, as indicated by both X-ray diffraction and molecular dynamics simulations. Surprisingly, a comparison of doped materials containing mono- or dianions reveals that - for a comparable oxidation level - the presence of multivalent counterions has little effect on the stiffness. Evidently, molecular doping is a powerful tool that can be used for the design of mechanically robust conducting materials, which may find use within the field of flexible and stretchable electronics.

Glass transition temperature from the chemical structure of conjugated polymers.

The glass transition temperature (Tg) is a key property that dictates the applicability of conjugated polymers. The Tg demarks the transition into a brittle glassy state, making its accurate prediction for conjugated polymers crucial for the design of soft, stretchable, or flexible electronics. Here we show that a single adjustable parameter can be used to build a relationship between the Tg and the molecular structure of 32 semiflexible (mostly conjugated) polymers that differ drastically in aromatic backbone and alkyl side chain chemistry. An effective mobility value, ζ, is calculated using an assigned atomic mobility value within each repeat unit. The only adjustable parameter in the calculation of ζ is the ratio of mobility between conjugated and non-conjugated atoms. We show that ζ correlates strongly to the Tg, and that this simple method predicts the Tg with a root-mean-square error of 13 °C for conjugated polymers with alkyl side chains.

Thermal Fluctuations Lead to Cumulative Disorder and Enhance Charge Transport in Conjugated Polymers.

All conjugated polymers examined to date exhibit significant cumulative lattice disorder, although the origin of this disorder remains unclear. Using atomistic molecular dynamics (MD) simulations, the detailed structures for single crystals of a commonly studied conjugated polymer, poly(3-hexylthiophene-2,5-diyl) (P3HT) are obtained. It is shown that thermal fluctuations of thiophene rings lead to cumulative disorder of the lattice with an effective paracrystallinity of about 0.05 in the π-π stacking direction. The thermal-fluctuation-induced lattice disorder can in turn limit the apparent coherence length that can be observed in diffraction experiments. Calculating mobilities from simulated crystal structures demonstrates that thermal-fluctuation-induced lattice disorder even enhances charge transport in P3HT. The mean inter-chain charge transfer integral is enhanced with increasing cumulative lattice disorder, which in turn leads to pathways for fast charge transport through crystals.