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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Key properties of mixed cerium and zirconium microspheres prepared by the internal gelation process with previously boiled HMTA and urea

The internal gelation process using previously boiled hexamethylenetetramine-urea was used in exploratory study to produce CeO 2 –ZrO 2 microspheres, which can serve as a simulant for ceramic transuranic fuel particles and as a viable three-way catalyst. The calcined CeO 2 and ZrO 2 microspheres with Barrett–Joyner–Halenda (BJH) pore size and volumes of 8–10 nm and 0.19–0.20 mL/g, respectively, had many more surface cracks than their relatively smooth CeO 2 –ZrO 2 counterparts even though the BJH pore sizes and volumes of mixed oxide spheres were lower at 3 nm and 0.11–0.12 mL/g, respectively. The Brunauer–Emmett–Teller (BET) surface areas for the CeO 2 and ZrO 2 microspheres were 74 and 93 m2/g, respectively, and the BET surface areas for the mixed oxides were much greater at 155–158 m 2 /g, which should improve catalytic performance. Overall, the catalytic activity of each microsphere composition was confirmed through the oxidation of carbon monoxide.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Crosslinking and Gelation of Polymer Brushes and Free Polymer Chains in a Confined Space during Controlled Radical Polymerization-A Computer Simulation Study

The confinement effect on crosslinking and gelation during controlled/living copolymerization of a monovinyl and a divinyl monomer was studied by Monte Carlo computer simulation using the dynamic lattice liquid method. The simulated reactions took place in plane-parallel slits of various widths for various grafting densities (GDs) and various initial initiator/crosslinker ratios ([X] 0 /[Ini] 0 ). Monomers and crosslinkers were initially randomly distributed in the slit, but initiators were either anchored to one solid plane (polymer brushes) or randomly distributed (free chains). For brushes, the gel point (GP) depended on GD and [X] 0 /[Ini] 0 . The higher these parameters, the lower was the gel point. The gel point was also affected by the slit width. For free, nongrafted chains, the dependence of GP on slit width was negligible. For brushes, a nonuniform density of crosslinks was found, the highest density was close to the wall. This asymmetry was high at the gel point and decreased at higher conversions.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Gelation and Re-entrance in Mixtures of Soft Colloids and Linear Polymers of Equal Size

Liquid mixtures composed of colloidal particles and much smaller non-adsorbing linear homopolymers can undergo a gelation transition due to polymer-mediated depletion forces. We now show that the addition of linear polymers to suspensions of soft colloids having the same hydrodynamic size yields a liquid-togel-to-re-entrant liquid transition. In particular, the dynamic state diagram of 1,4-polybutadiene star-linear polymer mixtures was determined with the help of linear viscoelastic and small-angle Xray scattering experiments. While keeping the star polymers below their nominal overlap concentration, a gel was formed upon increasing the linear polymer content. Further addition of linear chains yielded a re-entrant liquid. This unexpected behavior was rationalized by the interplay of three possible phenomena: (i) depletion interactions, driven by the size disparity between the stars and the polymer length scale which is the mesh size of its entanglement network; (ii) colloidal deswelling due to the increased osmotic pressure exerted onto the stars; and (iii) a concomitant progressive suppression of the depletion efficiency on increasing the polymer concentration due to reduced mesh size, hence a smaller range of attraction. Our results unveil an exciting new way to tailor the flow of soft colloids and highlight a largely unexplored path to engineer soft colloidal mixtures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Polymers in molten inorganic salt hydrate phase change materials: solubility and gelation

Inorganic salt hydrates are of interest as phase change materials (PCMs) for thermal energy storage because of their unique properties, such as high latent heats of fusion, moderate melting temperatures, high volumetric energy storage densities, and enhanced thermal conductivities compared to their organic counterparts. Because of the low viscosities of molten salt hydrates, the leakage and settling of solids can easily occur during their application. Polymers are promising candidates for preventing these issues. In addition, the non-flammability, non-volatility, and nontoxicity of many molten salt hydrates make them attractive options as “green” solvents for polymers. However, solutions of polymers in molten salt hydrates have rarely been studied and are poorly understood. The highly ionic environments in these materials created due to a scarcity of hydration water for ions leads to strong ionic interactions, with ramifications for polymer solubility and chain expansion. This Perspective article aims to focus on polymer solutions and gels in inorganic salt hydrates by comparing these systems with existing knowledge of polymers in traditional ionic liquids and aqueous salt solutions. To provide insight into the states of ions and water in molten salt hydrates and the degree of hydration of polymers in these solvents, infrared spectroscopy experiments have been performed, and the data are correlated with the temperature-responsive gelation of poly(vinyl alcohol), one candidate material currently used for the shape stabilization of inorganic PCMs. Finally, the thermal properties of inorganic salt hydrates and ionic liquids are compared and strategies for the shape stabilization of inorganic salt hydrates using polymers, through the formation of physical gels, are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Dehydration-Driven Ion Aggregation and the Onset of Gelation in ZnCl2 Solution

A minimal model of ionic aggregation in concentrated ZnCl2 is developed, guided by molecular dynamics simulations with a machine-learned potential. It explicitly incorporates solvent-site depletion, correlated chloride binding, and allows for loops within Zn–Cl clusters. Dehydration drives two coordination-controlled structural changes: a crossover at Z=2 from predominantly isolated Zn-containing species to Cl-bridged clusters and the onset of gelation near Z≈3. The model reproduces the concentration-dependent trends observed in the molecular dynamics simulations with two fitted parameters, while the cluster-size distribution at the highest concentration is consistent with three-dimensional percolation scaling over the accessible range of cluster sizes.

25 ENERGY STORAGE↗
DOE OSTI2021

Structural evolution during gelation of pea and whey proteins envisaged by time-resolved ultra-small-angle x-ray scattering (USAXS)

Hydrogels from plant proteins commonly exhibit inferior gel strength compared to those from dairy proteins partially due to their distinct gel networks. How protein aggregates to form such networks in response to heat remains largely unknown. In this work, pea (PPI) and whey (WPI) protein isolate gels were produced at the same protein content and similar heating/cooling rate. The process was monitored using rheology, microscopy and in situ ultra-small-angle x-ray scattering (USAXS). Rheology showed an initial decrease in G' and G'' in PPI followed by a steady increase when the temperature surpassed ~60 °C whereas a much higher temperature (~80 °C) was required for WPI, both using 2 °C/min heating rate. Microscopy showed a coarse and heterogenous network in PPI, whereas for WPI, the network was finer and more continuous. In both gels, nano-sized spherical or ellipsoidal particles were present as the basic constituents. USAXS found individual protein was dominant in PPI or WPI solution at temperature below 57 °C. Their proportions decreased together with appearance of aggregates with an average R g of 9–10 nm in PPI and 6–7 nm in WPI at higher temperature. The size of the aggregates changed slightly during further heating and cooling, but their proportions increased. Power law exponents revealed the aggregates were mass fractals for WPI and PPI gels, and they became more compact during heating. Our findings suggested formation of primary aggregates in protein gel networks is a more organized process and provided theoretical guidance for production of high protein food gels with desirable texture.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Shear-Induced Gelation of Self-Yielding Active Networks

An enticing feature of active materials is the possibility of controlling macroscale rheological properties through the activity of the microscopic constituents. Using a unique combination of microscopy and rheology we study three dimensional microtubule-based active materials whose autonomous flows are powered by a continually rearranging connected network. We quantify the relationship between the microscopic dynamics and the bulk mechanical properties of these nonequilibrium networks. Experiments reveal a surprising nonmonotonic viscosity that strongly depends on the relative magnitude of the rate of internally generated activity and the externally applied shear. Here, a simple two-state mechanical model that accounts for both the solidlike and yielded fluidlike elements of the network accurately describes the rheological measurements.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Direct ink writing techniques for in situ gelation and solidification

Direct ink writing (DIW) is an extrusion-based 3D printing method which prints near ambient temperatures and has one of the broadest printable material selections among additive manufacturing techniques. However, DIW uses viscoelastic materials susceptible to collapse during printing. Overall, one promising route to improve the structural integrity of viscoelastic inks is using in situ curing methods to increase the yield strength of the printed structures after deposition. This review summarizes progress in three representative methods of in situ curing for DIW, including ultra-violet-induced crosslinking, rapid cure of reactive ingredients, and flash vaporization of a solution’s solvent to coagulate dissolved polymers.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Rheology and Gelation of Hyaluronic Acid/Chitosan Coacervates

Hyaluronic acid (HA) and chitosan (CHI) are biopolyelectrolytes which are interesting for both the medical and polymer physics communities due to their biocompatibility and semi-flexibility, respectively. In this work, we demonstrate by rheology experiments that the linear viscoelasticity of HA/CHI coacervates depends strongly on the molecular weight of the polymers. Moduli for coacervates were found significantly higher than those of individual HA and CHI physical gels. A remarkable 1.5-fold increase in moduli was noted when catechol-conjugated HA and CHI were used instead. This was attributed to the conversion of coacervates to chemical gels by oxidation of 3,4-dihydroxyphenylalanine (DOPA) groups in HA and CHI to di-DOPA crosslinks. These rheological results put HA/CHI coacervates in the category of strong candidates as injectable tissue scaffolds or medical adhesives.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Mucus-Inspired Tribology, a Sticky Yet Flowing Hydrogel

The mucus blanket can trap foreign particles before they enter the lungs, while at the same time, it flows up to remove these particles. This manifests the dual nature of mucus: sticky, on one hand, and fluid, on the other. Inspired by this function of mucus in the lungs, we designed a mucus simulant which emulates this dual nature. While many existing mucus simulants do not target bioadhesion particularly, poly(vinyl alcohol) (PVA)-based simulants make an exception. Despite their bioadhesion tendency, unlike mucus, they do not gelate. In this study, we added a physical cross-linking agent to PVA in order to add the gelation aspect and to better represent mucous properties. We show that the resultant mucus simulant develops into two regions: a highly sticky region near the surface of a foreign object (we used hydrophobized silicon to mimic the foreign object) and a fluid region far away from that surface. We show that the sticky part can slide past the less sticky part, while the foreign object is stuck to it. However, this mechanism changes with time. At short gelation times, this tendency to separate into two parts is enhanced and the foreign object remains stuck, while the rest of the gel flows. With time, the force required to allow the sticky part to slide over the fluid part is further reduced. However, if the gelation is allowed to proceed for even longer times without disturbance, the force required to slide the two parts past each other increases and the separation between the two parts is inhibited. The hydrogel becomes a sticky goo, which requires a higher force to move or unclog if placed in a duct (much like what happens with mucus in the tracheal duct). We explain the physics of our findings in terms of a competition between the tendency of the polymer to form a gel network and the tendency of the polymer to adsorb onto the foreign object.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗