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This Perspective outlines our current understanding of molecular gels composed of short and ultrashort peptides over the past 20 years. We discuss in detail the state of the art regarding self-assembly mechanisms, structure, thermal stability, and kinetics of fibril and/or network formation. Emphasis is put on the importance of the combined use of spectroscopy and rheology for characterizing and validating self-assembly models. While a range of peptide chemistries are reviewed, we focus our discussion on a unique new class of ultrashort peptide gelators, denoted GxG peptides (x: guest residue), which are capable of forming self-assembled fibril networks. The storage moduli of GxG gels are tunable up to 100 kPa depending on concentration, pH, and/or cosolvent. The sheet structures of the fibrils differ from canonical β-sheets. When appropriate, each section highlights opportunities for additional research and technologies that would further our understanding.

Upon deprotonation of its imidazole group at ∼pH 6, the unblocked tripeptide glycylhistidylglycine (GHG) self-assembles into very long crystalline fibrils on a 10-1000 μm scale which are capable of forming a volume spanning network, that is, hydrogel. The critical peptide concentration for self-assembly at a pH of 6 lies between 50 and 60 mM. The fraction of peptides that self-assemble into fibrils depends on the concentration of deprotonated GHG. While IR spectra seem to indicate the formation of fibrils with standard amyloid fibril β-sheet structures, vibrational circular dichroism spectra show a strongly enhanced amide I' signal, suggesting that the formed fibrils exhibit significant chirality. The fibril chirality appears to be a function of peptide concentration. Rheological measurements reveal that the rate of gelation is concentration-dependent and that there is an optimum gel strength at intermediate peptide concentrations of ca. 175 mM. This paper outlines the unique properties of the GHG gel phase which is underlain by a surprisingly dense fibril network with an exceptionally strong modulus that make them potential additives for biomedical applications.

Clay-reinforced nylon-6 nanocomposites (NCs)-characterized by the full exfoliation of the nanoreinforcement-were introduced in the marketplace in the 1990s. Herein, we demonstrate, for the first time, that Ti3C2T z MXene can be incorporated into nylon-6 to synthesize melt-processable nanocomposites with excellent water barrier properties (94% reduction in water vapor permeation). To intercalate the e-caprolactam monomer between the MXene multilayers, the latter were first treated with 12-aminolauric acid, a low-cost, nontoxic, biodegradable, and long shelf life compound. Upon heating to 250 °C, in the presence of 6-aminocaproic acid, in situ polymerization occurred, yielding melt-processable nylon-6/MXene NCs that were, in turn, studied by thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, scanning and transmission electron microscopy, infrared spectroscopy, and dynamic vapor sorption analysis. Using the latter, moisture-sorption isotherms of a neat and a 1.9 vol % NC, at 60 °C, were fit to the Guggenheim, Anderson, and de Boer equation. Solubility, permeation, and diffusion coefficients of water through the NCs were measured as a function of temperature and found to be the lowest ever reported for nylon-6, despite the fact that, at ∼1.9 and 5.0 vol %, the MXene loads were relatively low. This record low diffusivity is ascribed to the very large aspect ratios-500 to 1000-of Ti3C2T z flakes and their dispersion. The water permeation rate is a factor of 5 lower than the best reported in the much more mature nylon/clay field, suggesting lower values can be achieved with further optimization. Lastly infrared spectroscopy spectra of neat and NC samples suggest the surface terminations of the 12-Ti3C2T z flakes bind with nylon-6, limiting water adsorption sites, resulting in reduced solubility in the NC films.

We recently showed that two systems have the same nonlinear flow dynamics if they have (1) the same number of entanglements, Z, (2) the same number of Kuhn segments between entanglements, Ne, and (3) a single monomeric friction reduction between all species. Because differences in polymer chemistry result in different values of (1)–(3), these results show that the likelihood of two melts having the same nonlinear behavior is improbable. This work determines the dependence of the nonlinear extensional flow behavior on Ne for three polymer melts: polystyrene, poly(methyl methacrylate), and poly(tert-butylstyrene). We show that polymer melts with the same Z have almost identical scaled linear viscoelasticity. Extensional rheology at constant strain rate shows strain hardening depends strongly on the value of Ne. More specifically, our data suggest a power-law dependence of steady state stress on the Weissenberg number which increases with increasing Ne.

Understanding the rheological properties of two-dimensional (2D) materials in suspension is critical for the development of various solution processing and manufacturing techniques. 2D carbides and nitrides (MXenes) constitute one of the largest families of 2D materials with >20 synthesized compositions and applications already ranging from energy storage to medicine to optoelectronics. However, in spite of a report on clay-like behavior, not much is known about their rheological response. In this study, rheological behavior of single- and multilayer Ti3C2Tx in aqueous dispersions was investigated. Viscous and viscoelastic properties of MXene dispersions were studied over a variety of concentrations from colloidal dispersions to high loading slurries, showing that a multilayer MXene suspension with up to 70 wt % can exhibit flowability. Processing guidelines for the fabrication of MXene films, coatings, and fibers have been established based on the rheological properties. Surprisingly, high viscosity was ...

We confirm the observation from Ianniruberto and Marrucci [ Macromolecules 2013, 46, 267-275 ] that entangled melts of branched polystyrenes behave like linear polystyrenes in the steady state of fast extensional flow, by measuring a linear, an asymmetric star, and a symmetric star polystyrene with the same span molecular weight (180 kg/mol). We show that all three melts reach the same extensional steady-state viscosity in fast extensional flow (faster than the inverse Rouse time). We further measure stress relaxation following steady extensional flow for the three melts. We show that initially they relax in a similar way, most likely via arm retraction, at short time, but behave differently at long time due to both the length of the arm and the branch point. The terminal relaxation is described by a Doi and Edwards based model, i.e., considering pure orientational relaxation.

We compare the linear and nonlinear rheological response of three entangled polystyrene solutions with the same concentration of polymer, but diluted using different solvents. The three solutions have exactly the same physical tube model parameters when normalized to the same time scale. Although the three solutions behave identically in small amplitude oscillatory shear flow, they behave markedly different in large strain extensional flow. The experimental observations presented here directly demonstrate that the tube model and its governing parameters are insufficient to describe the nonlinear rheological behavior of entangled polymer solutions. We introduce a new hypothesis that relates the observed nonlinear behavior of three different polymer solutions to the existence of nematic interactions between polymer-solvent and polymer-polymer molecules.

We compare viscoelastic properties of several polystyrene solutions and melts with the same number of entanglements. It is demonstrated that the modulus and time can be shifted such that the linear viscoelastic properties are identical provided the number of entanglements are identical. However the nonlinear properties in strong extensional flow are different with polymer solutions showing markedly stronger extensional hardening than the corresponding melts. While increased chain extensibility for solutions may provide part of the explanation, it is demonstrated that other mechanisms are needed for a full explanation for the differences between solutions and melts.

Particle-stabilized emulsions and foams offer a number of advantages over traditional surfactant-stabilized systems, most notably a greater stability against coalescence and coarsening. Nanoparticles are often less effective than micrometer-scale colloidal particles as stabilizers, but nanoparticles grafted with polymers can be particularly effective emulsifiers, stabilizing emulsions for long times at very low concentrations. In this work, we characterize the long-time and dynamic interfacial tension reduction by polymer-grafted nanoparticles adsorbing from suspension and the corresponding dilatational moduli for both xylene-water and air-water interfaces. The dilatational moduli at both types of interfaces are measured by a forced sinusoidal oscillation of the interface. Surface tension measurements at the air-water interface are interpreted with the aid of independent ellipsometry measurements of surface excess concentrations. The results suggest that the ability of polymer-grafted nanoparticles to produce significant surface and interfacial tension reductions and dilatational moduli at very low surface coverage is a key factor underlying their ability to stabilize Pickering emulsions at extremely low concentrations.