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At least 379 records · Page 21

All sources NASA NTRS DOE OSTI

DOE OSTI2021

Design of nematic liquid crystals to control microscale dynamics

The dynamics of small particles, both living such as swimming bacteria and inanimate, such as colloidal spheres, has fascinated scientists for centuries. If one could learn how to control and streamline their chaotic motion, that would open technological opportunities in the transformation of stored or environmental energy into systematic motion, with applications in micro-robotics, transport of matter, guided morphogenesis. This review presents an approach to command microscale dynamics by replacing an isotropic medium with a liquid crystal. Orientational order and associated properties, such as elasticity, surface anchoring, and bulk anisotropy, enable new dynamic effects, ranging from the appearance and propagation of particle-like solitary waves to self-locomotion of an active droplet. By using photoalignment, the liquid crystal can be patterned into predesigned structures. In the presence of the electric field, these patterns enable the transport of solid and fluid particles through nonlinear electrokinetics rooted in anisotropy of conductivity and permittivity. Director patterns command the dynamics of swimming bacteria, guiding their trajectories, polarity of swimming, and distribution in space. This guidance is of a higher level of complexity than a simple following of the director by rod-like microorganisms. Namely, the director gradients mediate hydrodynamic interactions of bacteria to produce an active force and collective polar modes of swimming. The patterned director could also be engraved in a liquid crystal elastomer. When an elastomer coating is activated by heat or light, these patterns produce a deterministic surface topography. The director gradients define an activation force that shapes the elastomer in a manner similar to the active stresses triggering flows in active nematics. Finally, the patterned elastomer substrates could be used to define the orientation of cells in living tissues. The liquid-crystal guidance holds a major promise in achieving the goal of commanding microscale active flows.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Degradation Studies of Silicone Rubbers in Direct Contact with Corrosive Solids Under Compression

Silicone elastomers are polymer materials widely used within the complex, serving as cushions, adhesives, binders, and potting compounds. Excellent high-temperature resistance combined with low-temperature flexibility, high resistance to chemicals and long-term age-related stability, make silicones first-choice materials in many application areas. However, the siloxane bonds comprising the backbone of silicone elastomers are susceptible to hydrolysis when catalytic amounts of acid or base are present. Hydrolysis is a form of degradation that results in chain-cleavage and rearrangement reactions, which can manifest in changes to the physical response and mechanical properties of the elastomer. This task aims to assess and quantify the extent of degradation and/or corrosion of silicone elastomer materials when placed in direct compressive contact with an Arrhenius base under varying environments (oxidative/nonoxidative, relative humidity, temperature, time, etc.).

36 MATERIALS SCIENCE↗

NASA NTRS1972

Life prediction of expulsion bladders through fatigue test and fold strain analysis

Cycle life data are presented in terms of true maximum strain for four metals, two plastics, and two elastomers. The Coffin-Manson fatigue theory was applied for metals and plastics, and cut-growth fatigue theory for elastomers. The data are based on measurements made at room and elevated temperatures. It was found that double folds give rise to far severer folding strains than do simple folds. It was also found that, except for the elastomers, all the bladder materials develop surface cracks due to double folds after only one cycle. The findings indicate that metals, which are bets for premeation resistance, are worst for fatigue resistance, and vice versa for elastomers. The intermediate plastics were found to be unsatisfactory for both permeation and fatigue resistance for missions of extended duration.

Chu, H. N.↗

DOE OSTI2023

Elastomeric vitrimers from designer polyhydroxyalkanoates with recyclability and biodegradability

Cross-linked elastomers are stretchable materials that typically are not recyclable or biodegradable. Medium-chain-length polyhydroxyalkanoates (mcl-PHAs) are soft and ductile, making these bio-based polymers good candidates for biodegradable elastomers. Elasticity is commonly imparted by a cross-linked network structure, and covalent adaptable networks have emerged as a solution to prepare recyclable thermosets via triggered rearrangement of dynamic covalent bonds. Here, we develop biodegradable and recyclable elastomers by chemically installing the covalent adaptable network within biologically produced mcl-PHAs. Specifically, an engineered strain of Pseudomonas putida was used to produce mcl-PHAs containing pendent terminal alkenes as chemical handles for postfunctionalization. Thiol-ene chemistry was used to incorporate boronic ester (BE) cross-links, resulting in PHA-based vitrimers. mcl-PHAs cross-linked with BE at low density (<6 mole %) affords a soft, elastomeric material that demonstrates thermal reprocessability, biodegradability, and denetworking at end of life. The mechanical properties show potential for applications including adhesives and soft, biodegradable robotics and electronics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Material assembly from collective action of shape-changing polymers

Some animals form transient, responsive and solid-like ensembles through dynamic structural interactions. These ensembles demonstrate emergent responses such as spontaneous self-assembly, which are difficult to achieve in synthetic soft matter. In this work, we use shape-morphing units comprising responsive polymers to create solids that self-assemble, modulate their volume and disassemble on demand. The ensemble is composed of a responsive hydrogel, liquid crystal elastomer or semicrystalline polymer ribbons that reversibly bend or twist. The dispersions of these ribbons mechanically interlock, inducing reversible aggregation. The aggregated liquid crystal elastomer ribbons have a 12-fold increase in the yield stress compared with cooled dispersion and contract by 34% on heating. Ribbon type, concentration and shape dictate the aggregation and govern the global mechanical properties of the solid that forms. Coating liquid crystal elastomer ribbons with a liquid metal begets photoresponsive and electrically conductive aggregates, whereas seeding cells on hydrogel ribbons enables self-assembling three-dimensional scaffolds, providing a versatile platform for the design of dynamic materials.

36 MATERIALS SCIENCE↗

DOE OSTI2024

A multiscale anisotropic polymer network model coupled with phase field fracture

The study of polymers has continued to gain substantial attention due to their expanding range of applications, spanning essential engineering fields to emerging domains like stretchable electronics, soft robotics, and implantable sensors. These materials exhibit remarkable properties, primarily stemming from their intricate polymer chain network, which, in turn, increases the complexity of precisely modeling their behavior. Especially for modeling elastomers and their fracture behavior, accurately accounting for the deformations of the polymer chains is vital for predicting the rupture in highly stretched chains. Despite the importance, many robust multiscale continuum frameworks for modeling elastomer fracture tend to simplify network deformations by assuming uniform behavior among chains in all directions. Recognizing this limitation, our study proposes a multiscale fracture model that accounts for the anisotropic nature of elastomer network responses. At the microscale, damage in the chains is assumed to be driven by both the chain's entropy and the internal energy due to molecular bond distortions. In order to bridge the stretching in the chains to the macroscale deformation, we employ the maximal advance path constraint network model, inherently accommodating anisotropic network responses. As a result, chains oriented differently can be predicted to exhibit varying stretch and, consequently, different damage levels. To drive macroscale fracture based on damages in these chains, we utilize the micromorphic regularization theory, which involves the introduction of dual local-global damage variables at the macroscale. The macroscale local damage variable is obtained through the homogenization of the chain damage values, resulting in the prediction of an isotropic material response. The macroscale global damage variable is subjected to nonlocal effects and boundary conditions in a thermodynamically consistent phase field continuum formulation. Moreover, the total dissipation in the system is considered to be mainly due to the breaking of the molecular bonds at the microscale. To validate our model, we employ the double-edge notched tensile test as a benchmark, comparing simulation predictions with existing experimental data. Additionally, to enhance our understanding of the fracturing process, we conduct uniaxial tensile experiments on a square film made up of polydimethylsiloxane (PDMS) rubber embedded with a hole and notches and then compare our simulation predictions with the experimental observations. Furthermore, we visualize the evolution of stretch and damage values in chains oriented along different directions to assess the predictive capacity of the model. In conclusion, the results are also compared with another existing model to evaluate the utility of our model in accurately simulating the fracture behavior of rubber-like materials.

42 ENGINEERING↗

DOE OSTI2023

A statistical mechanics framework for polymer chain scission, based on the concepts of distorted bond potential and asymptotic matching

To design increasingly tough, resilient, and fatigue-resistant elastomers and hydrogels, the relationship between controllable network parameters at the molecular level (bond type, non-uniform chain length, entanglement density, etc.) to macroscopic quantities that govern damage and failure must be established. Many of the most successful constitutive models for elastomers have been rooted in statistical mechanical treatments of polymer chains. Typically, such constitutive models have used variants of the freely jointed chain model with rigid links. However, since the free energy state of a polymer chain is dominated by enthalpic bond distortion effects as the chain approaches its rupture point, bond extensibility ought to be accounted for if the model is intended to capture chain rupture. To that end, a new bond potential is supplemented to the freely jointed chain model (as derived in the u FJC framework of Buche and Silberstein (2021) and Buche et al. (2022)), which we have extended to yield a tractable, closed-form model of single chain behavior that should be amenable to continuum-level constitutive model development. Inspired by the asymptotically matched u FJC model response in both the low/intermediate chain force and high chain force regimes, a simple, quasi-polynomial bond potential energy function is derived. This bond potential exhibits harmonic behavior near the equilibrium state and anharmonic behavior for large bond stretches tending to a characteristic energy plateau (akin to the Lennard-Jones and Morse bond potentials). Using this bond potential, approximate yet highly-accurate analytical functions for bond stretch and chain force dependent upon chain stretch are established. Then, using this polymer chain model, a stochastic thermal fluctuation-driven chain rupture framework is developed. This framework is based upon a force-modified tilted bond potential that accounts for distortional bond potential energy, allowing for the derivation and subsequent calculation of the dissipated chain scission energy. Here, the cases of rate-dependent and rate-independent scission are accounted for throughout the rupture framework. The impact of Kuhn segment number on chain rupture behavior is also investigated. The model is fit to single chain mechanical response data collected from atomic force microscopy tensile tests for validation and to glean deeper insight into the molecular physics taking place. Due to their analytical nature, this polymer chain model and the associated rupture framework can in the future be implemented in finite element models accounting for fracture and fatigue in polydisperse elastomer networks.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Chain rigidity controlled aggregation ability and solid-state microstructures for efficient stretchable conjugated polymer films

Stretchable conjugated polymer films with good electrical performance under mechanical deformation are highly desirable for soft electronics. However, the mechanical and electrical properties of these films, particularly in conjugated polymer:elastomer blends, are not fully understood at molecular level. Here, this study explores the relationships among molecular structure, aggregation ability, film microstructure, and the electrical/mechanical properties of three diketopyrrolopyrrole-based conjugated polymers (P1, P2, P3) with decreasing backbone rigidity and their corresponding polymer:elastomer blends. The most flexible polymer P3 shows strong aggregation, which forms highly crystalline fibers to produce fragile neat film and produces large isolated crystallites restricting charge transport in blend film. As chain rigidity increases, the P1 and P2 polymers show weaker aggregation, and produce smaller crystallites in neat films with enhanced ductility. P1 and P2 based blend films display nanocrystallites polymer networks with dispersed elastomer domains. As a result, we achieved near-constant charge mobility before and after stretching under 50 % strain for P2-based blend films with well-controlled pathways for both charge transport and energy dissipation. This study demonstrates the critical role of backbone rigidity in regulating the properties of stretchable conjugated polymer films, paving the way for more reliable and deformable materials in soft electronics.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Impact of melt viscosity on filler dispersion in elastomeric nanocomposites

Compounding of commercial nanocomposites usually involves the addition of viscosity enhancers such as binder resins in ink jet inks, and paints. Contrary to this, plasticizers such as process oils are added to reduce the melt viscosity and ease processability of reinforced elastomers. Nanofillers such as silica and carbon black are typically added to reinforce rubber and enhance performance of automotive tire treads. Filler dispersion has traditionally been qualitatively (indirectly) assessed by measuring the properties of reinforced elastomers. While dispersion can be quantified by examining filler agglomeration through surface roughness measurements and microscopy, the size-scale dependence for these hierarchical fillers has usually been ignored. Here, we have recently devised a method to quantify nano-scale dispersion of fillers using Ultra small-angle X-ray scattering (USAXS) techniques. This method is advantageous since it directly links the controllable processing/compounding parameters such as the mixing speed, mixer geometry, residence time (or mixing duration), melt density, flow gap distance, and melt viscosity to nano-scale dispersion. While our previous studies have explored the impact of different processing parameters, this study specifically investigates the impact of melt viscosity on nano-scale filler dispersion in elastomer compounds. Commercially available polybutadienes with different Mooney viscosities were used in conjunction with different grades and amounts of process oils to modify the melt viscosity.

Carbon black↗

NASA NTRS1974

Control of elasticity in cast elastomeric shock/vibration isolators

Elasticity is determined by isolators physical dimensions and by type of elastomer used. Once elastomer is selected and cast between two concentric tubes of device, isolator elasticity will remain fixed. Isolators having same dimensions can be built to different elasticity requirements using same elastomer.

Owens, L.↗

NASA NTRS1973

Investigation into reversion of polyurethane encapsulants

The effect of high humidity (95% RH) at 60 C, 70 C, 85 C and 100 C on the solid-to-liquid reversion of polyurethane elastomers (used for potting electrical connectors and conformal coating printed circuit boards) was investigated. Hardness measurements were conducted on eleven elastomers to track reversion for a 101-day period. The primary purpose of the tests was to provide data to predict service life for the polyurethane elastomers. This was not accomplished as the hardness did not deteriorate rapidly enough at the lower test temperatures. The tests did determine that the potting and coating materials most widely used on the S-1C Program are susceptible to reversion but appear adequate for service in the S-1C environment.

Lynch, C. R.↗

NASA NTRS1982

Evaluation of shear mounted elastomeric damper

Viton-70 elastomeric shear mounted damper was built and tested on a T-55 power turbine spool in the rotor's high speed balancing rig. This application of a shear mounted elastomeric damper demonstrated for the first time, the feasibility of using elastomers as the primary rotor damping source in production turbine engine hardware. The shear damper design was selected because it was compatible with actual gas turbine engine radial space constraints, could accommodate both the radial and axial thrust loads present in gas turbine engines, and was capable of controlled axial preload. The shear damper was interchangeable with the production T-55 power turbine roller bearing support so that a direct comparison between the shear damper and the production support structure could be made. Test results show that the Viton-70 elastomer damper operated successfully and provided excellent control of both synchronous and nonsynchronous vibrations through all phases of testing up to the maximum rotor speed of 16,000 rpm. Excellent correlation between the predicted and experienced critical speeds, mode shapes and log decrements for the power turbine rotor and elastomer damper assembly was also achieved.

Zorzi, E.↗

NASA NTRS1986

Polyimide Film of Increased Tear Strength

High-temperature linear aromatic polyimide with improved resistance to tearing made by new process that incorporates elastomer into polyimide. Linear aromatic condensation polyimides are materials of prime choice for use as films and coatings on advanced spacecraft and aircraft where durability at temperatures in range of 200 to 300 degree C required. Elastomer-containing polyimide film with improved toughness proves useful for applications where resistance to tearing and long-term thermal stability necessary. Desired resistance to tearing achieved by careful control of amount and chemical composition of added elastomer.

St. Clair, A. K.↗

NASA NTRS1988

Factors affecting the sticking of insects on modified aircraft wings

Previous work showed that the total number of insects sticking to an aluminum surface was reduced by coating the aluminum surface with elastomers. Due to a large number of possible experimental errors, no correlation between the modulus of elasticity, the elastomer, and the total number of insects sticking to a given elastomer was obtained. One of the errors assumed to be introduced during the road test is a variable insect flux so the number of insects striking one surface might be different from that striking another sample. To eliminate this source of error, the road test used to collect insects was simulated in a laboratory by development of an insect impacting technique using a pipe and high pressure compressed air. The insects are accelerated by a compressed air gun to high velocities and are then impacted with a stationary target on which the sample is mounted. The velocity of an object exiting from the pipe was determined and further improvement of the technique was achieved to obtain a uniform air velocity distribution.

Yi, O.↗

NASA NTRS1987

Elastomeric binders for Li-SOCl2 cell carbon electrodes

Nonoptimized elastomer bonded carbon electrodes made with 100-percent compressed Gulf Acetylene Black have demonstrated performance comparable to that of optimized Teflon bonded carbon electrodes, made from the same carbon, when tested at 1-10 mA/sq cm, at 24 and -26 C. The enhanced performance of elastomer bonded carbon electrodes appears to be due to the more uniform utilization of the carbon electrode to store insoluble discharge products, as compared to Teflon bonded carbon electrodes. With even minimal optimization of elastomer bonded carbon electrodes, significant improvement in Li-SOCl2 cell performance can be expected.

Carter, B. J.↗

NASA NTRS1989

Making Reliable Large-Diameter O-Rings

Vacuum curing yields joint-free, voidless elastomer seals. Method for manufacturing large-diameter O-rings produces them in single pieces, without bonded joints. Reduces probability trapped gases form flaws. O-rings produced, having diameters as much as 144 in. (3.66m), reliable and of high quality. Nesting upper and lower halves of mold hold elastomer rings for curing. Oil flowing through upper and lower cavities heats elastomer to cure it.

Larsen, Glade L.↗

NASA NTRS2002

Studies on Stress-Strain Relationships of Polymeric Materials Used in Space Applications

A two-year research plan was undertaken in association with Polymers Branch, NASA Glenn Research Center, to carry out experimental and modeling work relating stress and strain behavior of polymeric materials, especially elastomers and vulcanized rubber. An experimental system based on MTS (Mechanical Testing and Simulation) A/T-4 test facility environment has been developed for a broader range of polymeric materials in addition to a design of laser compatible temperature control chamber for online measurements of various strains. Necessary material processing has been accomplished including rubber compounding and thermoplastic elastomer processing via injection molding. A broad suite of testing methodologies has been identified to reveal the complex non-linear mechanical behaviors of rubbery materials when subjected to complex modes of deformation. This suite of tests required the conceptualization, design and development of new specimen geometries, test fixtures, and test systems including development of a new laser based technique to measure large multi-axial deformations. Test data has been generated for some of these new fixtures and has revealed some complex coupling effects generated during multi-axial deformations. In addition, fundamental research has been conducted concerning the foundation principles of rubber thermodynamics and resulting theories of rubber elasticity. Studies have been completed on morphological properties of several thermoplastic elastomers. Finally, a series of steps have been identified to further advance the goals of NASA's ongoing effort.

Jana, Sadhan C.↗

NASA NTRS2004

Polymeric blends for sensor and actuation dual functionality

The invention described herein supplies a new class of electroactive polymeric blend materials which offer both sensing and actuation dual functionality. The blend comprises two components, one component having a sensing capability and the other component having an actuating capability. These components should be co-processable and coexisting in a phase separated blend system. Specifically, the materials are blends of a sensing component selected from the group consisting of ferroelectric, piezoelectric, pyroelectric and photoelectric polymers and an actuating component that responds to an electric field in terms of dimensional change. Said actuating component includes, but is not limited to, electrostrictive graft elastomers, dielectric electroactive elastomers, liquid crystal electroactive elastomers and field responsive polymeric gels. The sensor functionality and actuation functionality are designed by tailoring the relative fraction of the two components. The temperature dependence of the piezoelectric response and the mechanical toughness of the dual functional blends are also tailored by the composition adjustment.

St. Clair, Terry L.↗