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PCL nanofiber crystallinity is significantly increased in all orientations after annealing.

Low molecular weight gelators (LMWGs) are the subject of intense research for a range of biomedical and engineering applications. Peptides are a special class of LMWG, which offer infinite sequence possibilities and, therefore, engineered properties. This work examines the propensity of the GxG peptide family, where x denotes a guest residue, to self-assemble into fibril networks

Hypothesis Understanding interfacial phenomena at elevated pressure is crucial to the design of a variety of processes, modeling important systems, and interpreting interfacial thermodynamics. While many previous studies have offered insight into these areas, current techniques have inherent drawbacks that limit equilibrium measurements. Experiments In this work, we adapt the ambient microtensiometer of Alvarez and co-workers into a high pressure microtensiometer (HPMT) capable of experimentally quantifying a wide range of interfacial phenomena at elevated pressures. Particularly, the HPMT uses a microscale spherical interface pinned to the tip of a capillary to directly measure surface tension via the Laplace equation. The stream of microscale bubbles used to pressurize the system ensures quick saturation of the bulk phases prior to conducting measurements. The HPMT is validated by measuring the surface tension of air–water as a function of pressure. We then measure the surface tension of CO2 vapor and water as a function of pressure, finding lower equilibrium surface tension values than originally reported in the literature. Findings This work both introduces further development of a useful experimental technique for probing interfacial phenomena at elevated pressures and demonstrates the importance of establishing bulk equilibrium to measure surface tension. The true equilibrium state of the CO2-water surface has a lower tension than previously reported. We hypothesize that this discrepancy is likely due to the long diffusion timescales required to ensure saturation of the bulk fluids using traditional tensiometry. Thus we argue that previously reported elevated pressure measurements were performed at non-equilibrium conditions, putting to rest a long standing discrepancy in the literature. Our measurements establish an equilibrium pressure isotherm for the pure CO2-water surface that will be essential in analyzing surfactant transport at elevated pressures.

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.

Talcum powder is recognized as the leading drug for pleurodesis, a treatment of choice for malignant pleural effusions. Recently, it was shown that hydrogel foam delivery systems significantly enhanced the number of adhesions between the chest wall and the lung in a New Zealand rabbit model due to the sol-gel transition. However, many questions still remain regarding the cause of improved efficacy, such as: (1) Would only hydrogel foams improve the efficacy of talc pleurodesis? (2) Is it possible to achieve the same efficacy of hydrogels using non-hydrogel foams? 3) What are the physicochemical properties that can be correlated to the efficacy of talc pleurodesis? In this study, we use non-hydrogel foam formulations to determine the efficacy of pleurodesis. Foam stability and rheology of the formulations were correlated to adhesion formation. The results clearly suggest a correlation of pleurodesis efficacy to the viscosity and modulus of the foam delivery system.

Pitch-based carbon fibers are of considerable interest as high-performance materials. There are reports over the last several decades detailing (i) methods of improving pitch-based carbon fiber performance, and (ii) reducing the cost of production via novel processing techniques. However, there remain considerable challenges in producing high-performance pitch-based carbon fibers consistently on an industrial scale. This is arguably due to the difficulty of scaling the melt-spinning process to compensate for variability in pitch feedstock quality and a lack of understanding of processing-structure-performance relationships. This work focuses on the early stages of heat treatment (pyrolysis) of isotropic pitch and its effect on the chemical, thermal, and rheological properties of the pitch, which help determine its processability. More specifically, we quantify significant changes in chemical structure, Mw, Tg, Ts, and shear and extensional rheology as a function of pyrolysis time at 400 °C. The extensional rheology, in particular, shows that the ‘stretchability’ of the pitch samples strongly depends on pyrolysis severity, and is important for characterizing ‘drawability’. Using a novel analysis of the uniaxial stretching kinematics, we show an isothermal ‘drawability window’ that allows for the largest axial and radial Hencky strains at constant rate. We hypothesize that this extensional drawability window could facilitate the successful processing of pitch into high quality fiber, minimizing the trial-and-error approach currently used in the field.

It is well established that polymer chain architecture and the distribution of molecular weight play a key role in the flow behavior (processing) and performance of a given polymer material. Long-chain branching (LCB) in particular is known to strongly affect the processability and the material performance of polymers. Often branching is a result of the polymerization process and therefore must be quantified in every sample. We study four commercial high-density polyethylene (HDPE) samples with unknown degrees of polydispersity and LCB. We first use size-exclusion chromatography and linear shear rheology to identify differences in molecular weight, polydispersity, and LCB. Each material is then tested in constant rate and constant stress uniaxial extension using a filament stretching rheometer to quantify extensional viscosity and strain hardening. Correlations between nonlinear extensional rheology, LCB and polydispersity are discussed. We show that the combination of the van Gurp-Palmen plot and extensional rheology allows for a full characterization of the LCB fraction and their effect on extensional rheology.

The tripeptide glycyl-histidyl-glycine (GHG) self-assembles into long, crystalline fibrils forming a strong hydrogel (G'∼ 50 kPa) above a critical concentration of 40 mM upon the deprotonation of its imidazole group. Spectroscopic data reveal a mixture of helically twisted β-sheets and monomers to coexist in the gel phase.

Strong ionic interactions between macromolecular chains improve physical and mechanical properties beyond those of the base polymer. Associating groups strongly affect the rheological properties and therefore the polymer processing window. Typically, there is a trade-off between mechanical properties and processability of associating polymers. From a molecular design perspective, we are interested in defining the chemical structure parameters that independently control mechanical properties and melt processability. In this work, we measure the effect of association strength on the mechanical properties of the solid, and the viscoelastic properties of the melt for poly-(ethylene-co-methacrylic acid) and its zinc ionomer. The linear and non-linear solid mechanics are dependent on the strength of associations. The linear viscoelasticity and thermal rheological complexity of the melt are also strongly dependent on association strength. We examine the non-linear flow properties via uniaxial extensional rheology. We show that the processing window for a given associating polymer is bounded by three timescales, namely the association lifetime, the bare sticker lifetime, and the longest relaxation time measured via small amplitude oscillatory shear. The presence of entanglements strongly impacts the magnitude of the longest relaxation time and has a positive impact on the processing window. These findings should increase our ability to design associating polymers for given processes.

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.

This work revisits several open questions regarding the mechanisms of GAG fibril formation and structure as a function of temperature. The authors recently hypothesized that there is a solubility limit of GAG in ethanol/water that induces self-assembly. In other words, not all peptides can participate in fibrillization and some fraction is still soluble in solution. We show via FTIR spectroscopy that, indeed, free peptides are still present in solution after fibril formation, strongly supporting the solubility model. Furthermore, previous work showed GAG self-assembled into right-handed (phase I) or left-handed (phase II) chiral structures depending on temperature. In this study, we analyze the crystalline structure of phase I and II gels via WAXS and SAXS to compare their crystalline structures and order. Rheological measurements were used to investigate the response of the fibrillar network to temperature. They reveal that the ability of the peptide to self-assemble depends on the solubility at a given temperature and not on thermal history. Furthermore, the gel softening point, the linear viscoelastic gel microstructure, and relaxation spectrum are very similar between phase I and phase II. Overall, the temperature only affects the chirality of the fibrils and the formation kinetics.

This study examines the dynamic fracture behavior of high-performance thermosetting polymers for additive manufacturing (AM) to enable rapid production of lightweight and functional polymeric and composite materials that meet military requirements. We present a systematic investigation of the role of print orientation on the dynamic fracture response of a bis-GMA based AM polymer as a model material, using single edge notched tension (SENT) geometry. While most fracture criteria in predictive simulations use quasi-static values, fracture is innately dynamic and impulsively loaded cracks can have toughness values that differ by more than 20%. A unique long-bar striker apparatus is used to fire a striker at the opposite end of the notched and pre-cracked specimens in order to create a dominantly dynamic Mode-I (opening) fracture load. Digital Image Correlation (DIC) is used in conjunction with ultra high-speed imaging to capture the evolving displacement, and hence strain fields ahead of the crack tip. The elastodynamic solution accounting for inertial effects and wave interactions is optimized using a least squares fit to extract the evolving critical stress intensity factor (SIF) leading to initiation. Preliminary findings suggest that under impulsive loading, the print orientation does slightly affect the overall dynamic fracture toughness of the material.

A well-defined resin system is needed to serve as a benchmark for 3D printing of high-performance composites. This work describes the design and characterization of such a system that takes into account processability and performance considerations. The Grunberg&ndash, Nissan model for resin viscosity and the Fox equation for polymer Tg were used to determine proper monomer ratios. The target viscosity of the resin was below 500 cP, and the target final Tg of the cured polymer was 150 &deg, C based on tan-&delta, peak from dynamic mechanical analysis. A tri-component model resin system, termed DA-2 resin, was determined and fully characterized. The printed polymer exhibited good thermal properties and high mechanical strength after post-cure, but has a comparatively low fracture toughness. The model resin will be used in additive manufacturing of fiber reinforced composite materials as well as for understanding the fundamental processing&ndash, property relationships in light-based 3D printing.

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.

Hypothesis Drop shape analysis (DSA) techniques measure interfacial tension subject to error in image analysis and the optical system. While considerable efforts have been made to minimize image analysis errors, very little work has treated optical errors. There are two main sources of error when considering the optical system: the angle of misalignment and the choice of focal plane. Due to the convoluted nature of these sources, small angles of misalignment can lead to large errors in measured curvature. We demonstrate using microtensiometry the contributions of these sources to measured errors in radius, and, more importantly, deconvolute the effects of misalignment and focal plane. Our findings are expected to have broad implications on all optical techniques measuring interfacial curvature. Experiments A geometric model is developed to analytically determine the contributions of misalignment angle and choice of focal plane on measurement error for spherical cap interfaces. This work utilizes a microtensiometer to validate the geometric model and to quantify the effect of both sources of error. For the case of a microtensiometer, an empirical calibration is demonstrated that corrects for optical errors and drastically simplifies implementation. Findings The combination of geometric modeling and experimental results reveal a convoluted relationship between the true and measured interfacial radius as a function of the misalignment angle and choice of focal plane. The validated geometric model produces a full operating window that is strongly dependent on the capillary radius and spherical cap height. In all cases, the contribution of optical errors is minimized when the height of the spherical cap is equivalent to the capillary radius, i.e. a hemispherical interface. The understanding of these errors allow for correct measure of interfacial curvature and interfacial tension regardless of experimental setup. For the case of microtensiometry, this greatly decreases the time for experimental setup and increases experiential accuracy. In a broad sense, this work outlines the importance of optical errors in all DSA techniques. More specifically, these results have important implications for all microscale and microfluidic measurements of interface curvature.

Fundamental understanding of how processing affects composite battery electrode structure and performance is still lacking, especially for industry-relevant electrodes with low fractions of inactive material. This work combines rheology, electronic conductivity measurements, and battery rate capability tests to prove that short-range electronic contacts are more important to cathode rate capability than either ion transport or long-range electronic conductivity. LiNi0.33Mn0.33Co0.33O2, carbon black, and polyvinylidene difluoride in 1-methyl-2-pyrrolidinone represent a typical commercial electrode with

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 ...

A new type of chemically-crosslinked polymeric network was synthesized by grafting-through ring-opening metathesis polymerization (gt-ROMP) using well-defined norbornene functionalized macrocrosslinkers. These macromolecular grafted-through networks uniquely possessed two molecular-level dials for tuning network microstructure and mechanical properties. In particular, both facets of the crosslink density, i.e. network junction functionality and molecular weight between crosslinks, could be independently manipulated by varying the macrocrosslinker degree of polymerization or the gt-ROMP degree of polymerization, providing shear storage moduli ranging over an order of magnitude.

HYPOTHESIS Recently, we reported a three-dimensional phase diagram for the gelation of cationic tripeptide glycylalanylglycine (GAG) in water-ethanol mixtures. We showed that the gel strength reaches an optimum for a peptide concentration of 200 mM and ethanol/water mixtures with ca. 55-60 mol% ethanol. An increase of the ethanol fraction causes a substantial upshift of the gel's softening temperature which is indicative of a reduced peptide solubility. We expect the formation of long crystalline fibrils which form the sample spanning network of the gel phase to precede the gelation process and that the fibril microstructure depends on the rate and concentration of peptide. EXPERIMENTS We used UV circular dichroism (UVCD) spectroscopy to probe the kinetics of GAG fibril formation as a function of peptide concentration and ethanol fraction. We provide experimental evidence for the notion that the utilized CD signal reflects the three-dimensional assembly of peptides rather than a two-dimensional sheet structure. UVCD was also used to probe the melting of GAG fibrils with increasing temperature. FTIR and vibrational circular dichroism (VCD) spectroscopy were employed to characterize the structure of sheets with which the observed fibrils were formed. FINDINGS Fibrilization and gelation kinetics occur on a very similar time scale for very short gelation times (

Hypothesis The cationic tripeptide glycylalanylglycine (GAG) self-assembles into long, thick crystalline fibrils in an ethanol/water solution. At sufficiently high concentrations, the fibrils form a volume spanning hydrogel network. We report an extensive rheology and microscopy-based study regarding the self-assembly of GAG in ethanol/water solutions to understand the conditions for fibril formation as well as the thermal stability for future developments of this material. Experiments By systematically varying GAG concentration and ethanol fraction, we observe a two-dimensional fibril aggregate phase diagram. Microscopy studies shed light on the shape and size of fibrils as well as the macroscopic packing depending on conditions. The kinetics and evolution of the macroscopic fibril microstructure was investigated using rheology. Findings The mechanism of fibril formation is put into the context of a solubility framework, where ethanol reduces peptide solubility and induces self-assembly. The rate of fibril formation and strength of the gel can be controlled by peptide concentration and ethanol fraction. The faster rate of fibril formation leads to inhomogeneous packing of fibrils denoted by discrete dense fibril clusters. The solubility of the fibrils can be manipulated by temperature making the gel thermo-switchable, a property of interest for biomedical systems.

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 Ti

Typical commercial surface treatments for continuous carbon fibers are often unavailable for discontinuous fibers. As such, there is little variety of chopped fiber surfaces leading to non-ideal coating solutions which result in poor interfacial compatibility between fibers and a composite matrix. A method of applying a highly effective coating using a high throughput technique for chopped carbon fibers. The method provides the ability to tune both the coating thickness and chemical functionality using processing parameters. The coatings are evaluated using X-ray photoelectron spectroscopy (XPS) for uniformity and composition. Using this technique, thermoplastic composites are highlighted showing an increase in interfacial shear strength (IFSS) of 25 MPa. This process shows promise for increasing the throughput of surface treatment of chopped fiber on the industrial scale.

Cationic glycylalanylglycine (GAG) self-assembles into a gel in a 55 mol% ethanol/45 mol% water mixture. The gel exhibits a network of crystalline fibrils grown to lengths on a 10−4–10−5 m scale (Farrel et al., Soft Matter, 2016, 12, 6096–6110). Rheological data are indicative of a rather strong gel with storage moduli in the 10 kPa regime. Spectroscopic data revealed the existence of two gel phases; one forms below T = 15 °C (phase I) while the other one forms in a temperature range between 15 °C and the melting temperature of ca. 35 °C (phase II). We explored the reformation of the cationic GAG gel in 55 mol% ethanol/45 mol% water after thermal annealing by spectroscopic and rheological means. Our data reveal that even a short residence time of 5 minutes in the sol phase at 50 °C produced a delay of the gelation process and a gel of lesser strength. These observations suggest that the residence time at the annealing temperature can be used to adjust the strength of both gel phases. Our spectroscopic data show that the annealing process does not change the chirality of peptide fibrils in the two gel phases and that the initial aggregation state of the reformation process is by far more ordered for phase I than it is for phase II. In the gel phases of GAG/ethanol/water mixtures, ethanol seems to function as a sort of catalyst that enables the self-assembly of the peptide in spite of its low intrinsic propensity for aggregation.

Understanding electrical percolation and charging mechanisms in electrochemically active biphasic flowable electrodes is critical for enabling scalable deionization (desalination) and energy storage. Flowable electrodes are dynamic material systems which store charge (remove ions) and have the ability to flow. This flow process can induce structural changes in the underlying material arrangement and result in transient and non-uniform material properties. Carbon-based suspensions are opaque, multi-phase, and three dimensional, and thus prior characterization of the structural properties has been limited to indirect methods (electrochemical and rheology). Herein, a range of mixed electronic and ionically conducting suspensions are evaluated to determine their static structure, function, and properties, utilizing synchrotron radiation X-ray tomographic microscopy (SRXTM). The high brilliance of the synchrotron light enables deconvolution of the liquid and solid phases. Reconstruction of the solid phase reveals agglomeration cluster volumes between 10 μm3 and 103 μm3 (1 pL) for low loaded samples (5 wt% carbon). The largest agglomeration cluster in the low loaded sample (5 wt%) occupied only 3% of the reconstructed volume whereas samples loaded with 10 wt% activated carbon demonstrated electrically connected clusters that occupied 22% of the imaged region. The highly loaded samples (20 wt%) demonstrated clusters of the order of a microliter, which accounted for 63–85% of the imaged region. These results demonstrate a capability for discerning the structural properties of biphasic systems utilizing SRXTM techniques, and show that discontinuity in the carbon particle networks induces decreased material utilization in low-loaded flowable electrodes.

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.

The nonlinear rheology of three selected commercial low-density polyethylenes (LDPE) is measured in uniaxial extensional flow. The measurements are performed using three different devices including an extensional viscosity fixture (EVF), a homemade filament stretching rheometer (DTU-FSR) and a commercial filament stretching rheometer (VADER-1000). We show that the measurements from the EVF are limited by a maximum Hencky strain of 4, while the two filament stretching rheometers are able to probe the nonlinear behavior at larger Hencky strain values where the steady state is reached. With the capability of the filament stretching rheometers, we show that LDPEs with quite different linear viscoelastic properties can have very similar steady extensional viscosity. This points to the potential for independently controlling shear and extensional rheology in certain rate ranges.

A long standing goal in the interfacial thermodynamics community is free energies of adsorption and micellization in terms of molecular parameters. Historically, correlations for C i E j 's, a common class of nonionic surfactants, have been proposed between the number of ethylene glycol groups and/or the number of carbons and thermodynamic parameters, such as the critical micelle concentration (CMC), the equilibrium constant ( K ), and the maximum surface concentration ( Γ ∞ ). However, no such correlations to date are capable of capturing a large dataset of C i E j thermodynamic parameters. In this paper we propose the functional form of the critical micelle concentration and adsorption isotherm parameters in terms of molecular parameters for nonionic, C i E j surfactant chemistry. An extensive review of the literature is performed to generate interfacial thermodynamic parameters for a wide-range of C i E j chemistries. The data are used to parameterize and validate the molecular models. More specifically, we combine the well known linear dependence of CMC and K on the number of carbons, N C , with the theorized dependence on hydrophilic-lipophilic balance (HLB), represented as y phob , which is capable of collapsing all experimental data onto mastercurves. The resulting free energy models represent the first successful phenomenological theory which is able to collapse a broad range of experimental data for C i E j surfactants. Furthermore, the proposed models for the free energies of micellization and adsorption are capable of predicting the maximum surface concentration of C i E j surfactants at the air–water interface, thus providing a link between the physics of adsorption and self-assembly. This theory is a step towards engineering interfacial thermodynamics by chemical design which enables prediction of interfacial properties of novel surfactants without extrapolation. In addition, this paper summarizes the many years of experimental and theoretical understanding of surfactant structure property relationships for C i E j 's and the resulting theory quantitatively explains many hypotheses about their interfacial thermodynamics.

Traditional manufacturing of carbon fiber/polymer composite parts is restricted to non-complex geometries and remains expensive due to costly carbon fiber production, laborious processing steps, and generates 20–30% short fiber waste. We present an inexpensive method of aligning and consolidating short (waste) fibers into needle-like building blocks for composite part manufacturing that capitalizes on the anisotropy of short fibers. The method consists of mixing suspended short fibers with a secondary phase that induces self-assembly of the short fibers into an overlapping needle-like bundle with polymer binder distributed in the interstitial sites. The process can achieve high volume fraction of fiber comparable to that of high-performance composites. These self-assembled building blocks can be consolidated to form large scale parts, and they offer several advantages over traditional continuous fiber manufacturing, such as forming complex geometries, utilizing less expensive short fibers, simplifying processability, and increasing fiber recyclability.

High performance ultra high molecular weight polyethylene (UHMWPE) fibers are primarily composed of extended chain (shish) and lamellae (kebab) crystalline domains. Characterization of these architectures commonly utilizes x-ray scattering techniques, which rely upon models to transform from inverse scattering space to real space. Scattering models for lamellae and straight chain crystalline domains were developed and intended for use in ideal scattering scenarios; ie. purely extended chain or purely lamellae. Simultaneous implementation of both models with real (non-ideal) fibers takes for granted the validity of either model in the presence of the other crystalline feature. In this work, we utilize atomic force microscopy (AFM) to directly couple real space stiffness measurements to the analysis of scattering data. The validity of scattering models is tested against real space image analysis and conclusions are drawn regarding the analysis of scattering data to quantify crystalline domain sizes. We find that the lamellae long spacing estimated from scattering data is consistent with real space image analysis. Whereas significant differences are observed for the analysis of lamellae diameter. The characteristic size of extended chain crystalline domains is measured via the streak analysis method on both SAXS 2D scattering data and the Fast Fourier Transform (FFT) of AFM stiffness maps. The streak analysis results in comparable length scales when applied to SAXS and FFT AFM images. A comparison of the streak analysis with novel filtered AFM images suggest that the streak analysis is capturing continuity of straight chain crystalline domains.

Self-healing materials are prized for their ability to recover mechanical properties after damage. Supramolecular polymer networks have been demonstrated to have the ability to recover without the need for extraneous material components or the use of external stimuli. Surprisingly, there is little quantitative measure of self-healing dynamics and recovery. In this work, we develop a tool using a filament stretching rheometer to probe self-healing dynamics in creep and constant rate of extension. We experimentally determine the effect of process time scales, τ H and τ W , on the degree of recovery for two distinct supramolecular architectures. These results are put into the context of molecular time scales such as disengagement time, the Rouse time, and bond lifetime. We find that entangled polymers undergo sequential healing, whereby at short times, dynamics are dominated by entanglement recovery followed by recovery of associations. For an unentangled polymer, recovery is seemingly dominated by association dynamics. Our experimental results are put into the context of leading theoretical models. We also introduce the importance of the measurement flow time scale on perceived material recovery. These initial results and reliable experimental tools begin to construct a framework for measuring, understanding, and predicting recovery of self-healing soft materials.

The preceding Comment by Xu et al. (Phys. Rev. Lett. 122, 059803 (2019); arXiv:1808.05390) erroneously applies the entropic stress expression in our Letter (T.C. O'Connor et al., Phys. Rev. Lett. 121, 047801 (2018); arXiv:1806.09509) to transient stress. In addition, the authors only apply this expression at extreme extension rates where we clearly showed deviations from the entropic stress expression for steady-state extensional flow. Hence the surprisingly minor discrepancies noted in the Comment between observed and "predicted" stress are entirely expected and have no bearing on the discussion or conclusions in our Letter.

Most studies are focused on the final mechanical properties of the fiber and the processing window required to achieve high moduli and tensile strength. Several studies have alluded to the fact that the crystalline morphologies developed during gel spinning and post-drawing are very important in determining the final mechanical properties. However, it is surprising to know that no clear correlation exists between the crystalline structure and initial, evolving, and final mechanical properties. In an attempt to define structure-property relationships, we have developed novel tools to quantify the effect of processing on crystalline structure evolution. We examine through controlled gel-spinning and SAXS analysis the effect of flow kinematics on the development of crystalline structures. Direct correlations are made between polymer solution relaxation time, extension rates, crystallization time and gel-spun crystalline morphologies. We report direct evidence of flow induced crystallization, which approaches an asymptotic crystallization rate at high Weissenberg numbers. For Wi1, the crystalline structure is only slightly affected by equilibrium. For Wi1, the crystalline structure is highly anisotropic due to chain orientation/stretch during spinning. Fibers spun at different Weissenberg numbers are drawn to low draw ratios at constant temperature to measure the initial structure evolution. A qualitative SAXS analysis clearly shows similar evolution of different starting structures with the formation of more straight chain crystals upon drawing. However, there remain quantitative differences between the length of straight chain crystals and the size and distribution of lamellar domains depending on the starting structure.

Nonlinear extensional flows are common in polymer processing, but they remain challenging theoretically because dramatic stretching of chains deforms the entanglement network far from equilibrium. Here, we present coarse-grained simulations of extensional flows in entangled polymer melts for Rouse-Weissenberg numbers ${\mathrm{Wi}}_{R}=0.06--52$ and Hencky strains $\ensuremath{\epsilon}\ensuremath{\ge}6$. Simulations reproduce experimental trends in extensional viscosity with time, rate, and molecular weight. Studies of molecular structure reveal an elongation and thinning of the confining tube with increasing ${\mathrm{Wi}}_{R}$. The rising stress is quantitatively consistent with the decreasing entropy of chains at the equilibrium entanglement length. Molecular weight dependent trends in viscosity are related to a crossover from the Newtonian limit to a high rate limit that scales differently with chain length.

Hydrogels formed by polypeptides could be much-favored tools for drug delivery because their main ingredients are generally biodegradable. However, the gelation of peptides in aqueous solution generally requires a minimal length of the peptide as well as distinct sequences of hydrophilic and hydrophobic residues. The aggregation of short peptides like tripeptides, which are relatively cheap and offer a high degree of biodegradability, are generally thought to require a high hydrophobicity of their residues. We found that contrary to this expectation cationic glycylalanylglycine in 55 mol% ethanol/45 mol% water forms a gel below a melting temperature of ca. 36 °C. A pure hydrogel state can be obtained after allowing the ethanol component to evaporate. The gel phase consists of crystalline fibrils of several 100 μm, which form a sample-spanning network. Rheological data reveal a soft elastic solid gel. We investigated the kinetics of the various processes that lead to the final gel state of the ternary mixture by a unique combination of UV circular dichroism, infrared, vibrational circular dichroism (VCD) and rheological measurements. A mathematical analysis of our data show that gelation is preceded by the formation of peptide β-sheet like tapes or ribbons, which give rise to a significant enhancement of the amide I' VCD signal, and the subsequent formation of rather thick and long fibrils. The VCD signals indicate that the tapes exhibit a right-handed helicity at temperatures above 16 °C and a left-handed helicity below. The tapes'/ribbons' helicity change occurs at a temperature where the UVCD data reflect a relatively long nucleation process. The kinetics of gel formation probed by the storage and loss moduli are composed of a fast process that follows tape/ribbon/fibril formation and is clearly identifiable in a movie that shows the gelation process and a slow process that causes an additional gel stabilization. The rheological data indicate that left-handed fibrils observed at low temperatures form a more solid-like structure than their right-handed counterparts formed at higher temperatures. Taken together our data reveal GAG as an unexpected gelator, the formation of which is underlied by a set of distinguishable kinetic processes.

Concentrated solutions of nearly monodisperse poly(methyl methacrylate), PMMA-270k and PMMA-86k, in oligo(methyl methacrylate), MMA o-4k and MMA o-2k, investigated by Wingstrand et al. (Phys Rev Lett 115:078302, 2015) and Wingstrand (2015) do not follow the linear-viscoelastic scaling relations of monodisperse polystyrenes (PS) dissolved in oligomeric styrene (Wagner in Rheol Acta 53:765–777, 2014a, in Non-Newtonian Fluid Mech 222:121–131, 2014b; Wagner et al. in J Rheol 59:1113–1130, 2015). Rather, PMMA-270k shows an attractive interaction with MMA, in contrast to the interaction of PMMA-86k and MMA. This different behavior can be traced back to different tacticities of the two polymers. The attractive interaction of PMMA-270k with o-4k creates pseudo entanglements, which increase the interchain tube pressure, and therefore, the solution PMMA-270k/o-4k shows, as reported by Wingstrand et al. (Phys Rev Lett 115:078302, 2015), qualitatively a similar scaling of the elongational viscosity with $$ {\left(\dot{\varepsilon}{\tau}_R\right)}^{-1/2} $$ as observed for polystyrene melts. For the solution PMMA-270/o-2k, this effect is only seen at the highest elongation rates investigated. The elongational viscosity of PMMA-86k dissolved in oligomeric MMA is determined by the Rouse time of the melt, as in the case of polystyrene solutions.

Pendant bubble and drop devices are invaluable tools in understanding surfactant behavior at fluid–fluid interfaces. The simple instrumentation and analysis are used widely to determine adsorption isotherms, transport parameters, and interfacial rheology. However, much of the analysis performed is developed for planar interfaces. The application of a planar analysis to drops and bubbles (curved interfaces) can lead to erroneous and unphysical results. We revisit this analysis for a well-studied surfactant system at air–water interfaces over a wide range of curvatures as applied to both expansion/contraction experiments and interfacial elasticity measurements. The impact of curvature and transport on measured properties is quantified and compared to other scaling relationships in the literature. The results provide tools to design interfacial experiments for accurate determination of isotherm, transport and elastic properties.

The ability to predict transitions in the microstructure of mixed colloidal suspensions is of extreme interest and importance. The data presented here is specific to the case of battery electrode slurries whereby the carbon additive is reported to form strong colloidal gels. Using rheology, we have determined the effect of mixed particle systems on the critical gel transition ϕ gel . More specifically, we show that the introduction of a high volume fraction of large non-Brownian particles has little to no effect on ϕ gel . Although ϕ gel is unchanged, the larger particles do change the shape of the linear viscoelasticity and the nonlinear yielding behavior. There are interesting similarities to the nonlinear behavior of the colloidal gels with trends observed for colloidal glasses. A comparison of experimental data and the prediction from theory shows that the equation presented by Poon et al. is able to quantitatively predict the transition from a fluid state to a gel state.