Thermal and particulate radiation effects on potting polymers
Clear potting polymers, investigating temperature and ionizing particle irradiation effects on properties
Beatty, M. E., III↗
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Nanoparticle Filtration in a RTM Processed Epoxy/Carbon Fiber Composite
Several epoxy matrix composite panels were fabricated by resin transfer molding (RTM) E862/W resin onto a triaxially braided carbon fiber pre-form. Nanoparticles including carbon nanofiber, synthetic clay, and functionalized graphite were dispersed in the E862 matrix, and the extent of particle filtration during processing was characterized. Nanoparticle dispersion in the resin flashing on both the inlet and outlet edges of the panel was compared by TEM. Variation in physical properties such as Tg and moisture absorption throughout the panel were also characterized. All nanoparticle filled panels showed a decrease in Tg along the resin flow path across the panel, indicating nanoparticle filtration, however there was little change in moisture absorption. This works illustrates the need to obtain good nano-particle dispersion in the matrix resin to prevent particle agglomeration and hence particle filtration in the resultant polymer matrix composites (PMC).
Miller, Sandi G.↗
Stretchable hydrophobic materials and methods for making the same
Hydrophobic materials, processes for their production, and uses thereof are described. The materials can be made with silica or polytetrafluoroethylene particles embedded into a liquid polymer. The hydrophobic materials are stretchable.
36 MATERIALS SCIENCE↗
Tunable negative coefficient thermal expansion materials and composites
The present disclosure is directed to variable composition ceramics. Zr (2−x) Hf (x) WP 2 O 12 and Hf 2 WP 2 O 12 exhibit large negative thermal expansion that is linear over a large temperature range up to at least 900° C., These new ceramic material particles may be mixed with polymers to make a composite suitable for use in backsheets for photovoltaic modules or in other applications. The thermal expansion coefficient of the composite can be tailored to match that of the solar cell in order to reduce stress resulting from daily thermal cycling.
Gordon, Margaret Ellen↗
Modelling the interlayer bond strength of 3D printed concrete with surface moisture
Highlights: • An analytical model for interlayer adhesion of 3DPC is proposed based on the amount of surface moisture on the interlayer. • For this mix, the model accurately (RMSE=2.5%) predicts the reduction in interlayer adhesion from 30% to 50%. • Internal curing with superabsorbent polymers increases the interlayer adhesion by 10% and flexural strength by 19%. • SAPs also improve the initial thixotropy by 49% and buildability by 25% compared to the reference. Providing additional water to the hydrating cementitious particles is essential to achieve the optimal mechanical performance of the low w/b concrete mixes preferred for 3D printing. This study incorporates superabsorbent polymers (SAP) and additional water in 3D printed concrete (3DPC) to promote the hydration process through delayed internal water release. The study shows that a retentive SAP modifies the rheological development by absorbing the pore fluid for a short period after printing. The absorption-induced stiffening increases the thixotropy and buildability by 49% and 25%, respectively. A retentive SAP increases the flexural strength and interlayer adhesion by 19% and 10%, respectively. This is due to the internal water release that promotes hydration. Evaporation of the interlayer moisture during the pass time has the opposite effect— evaporation reduces the interlayer adhesion. Based on this assumption, an analytical model is proposed. The model accounts for the pass time, bleeding, and the environmental evaporation rate to estimate the surface moisture and predict the lack of interlayer adhesion. In this study, the model accurately (RMSE = 2.5%) predicted an interlayer adhesion reduction from 30% to 50%. The interlayer adhesion results of other studies could also be predicted.
36 MATERIALS SCIENCE↗
Self-Assembled Vesicles from Mixed Brush Nanoparticles in Solution
The self-assembly of binary polymer-grafted nanoparticles (NPs) in a selective solvent is investigated using coarse-grained simulations. Simulations are performed using theoretically informed Langevin dynamics (TILD), a particle-based method that employs a particle-to-mesh scheme to efficiently calculate the nonbonded interactions. Here, the particles are densely grafted with two immiscible polymers, A and B, that are permanently bound to the NP either at random grafting sites (random-grafted) or with all the A chains on one hemisphere of the NP and all the B chains on the other hemisphere (Janus-grafted). For NPs with random grafting, the polymers phase-separate on the surface of the NP to form Janus-type structures in dilute solution, even though some of the chains have to stretch around the particle to form the Janus structure. When the solvent quality is sufficiently poor for the solvophobic chains, the binary grafted NPs assemble into various structures, including double-walled vesicles. In particular, vesicles are formed when the solvophilic volume fraction is between 0.2 and 0.3, in a similar range to that required for vesicle formation in diblock copolymers in a selective solvent. For mixed-grafted NPs, there is considerable variation in the structure of each individual NP, but nevertheless, these NPs form ordered vesicles, similar to those formed by Janus-grafted NPs.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
The intact capture of hypervelocity dust particles using underdense foams
The impact of a hypervelocity projectile (greater than 3 km/s) is a process that subjects both the impactor and the impacted material to a large transient pressure distribution. The resultant stresses cause a large degree of fragmentation, melting, vaporization, and ionization (for normal densities). The pressure regime magnitude, however, is directly related to the density relationship between the projectile and target materials. As a consequence, a high-density impactor on a low-density target will experience the lowest level of damage. Historically, there have been three different approaches toward achieving the lowest possible target density. The first employs a projectile impinging on a foil or film of moderate density, but whose thickness is much less than the particle diameter. This results in the particle experiencing a pressure transient with both a short duration and a greatly reduced destructive effect. A succession of these films, spaced to allow nondestructive energy dissipation between impacts, will reduce the impactor's kinetic energy without allowing its internal energy to rise to the point where destruction of the projectile mass will occur. An added advantage to this method is that it yields the possibility of regions within the captured particle where a minimum of thermal modification has taken place. Polymer foams have been employed as the primary method of capturing particles with minimum degradation. The manufacture of extremely low bulk density materials is usually achieved by the introduction of voids into the material base. It must be noted, however, that a foam structure only has a true bulk density of the mixture at sizes much larger than the cell size, since for impact processes this is of paramount importance. The scale at which the bulk density must still be close to that of the mixture is approximately equal to the impactor. When this density criterion is met, shock pressures during impact are minimized, which in turn maximizes the probability of survival for the impacting particle. The primary objectives of the experiment are to (1) Examine the morphology of primary and secondary hypervelocity impact craters. Primary attention will be paid to craters caused by ejecta during hypervelocity impacts of different substrates. (2) Determine the size distribution of ejecta by means of witness plates and collect fragments of ejecta from craters by means of momentum-sensitive mcropore foam. (3) Assess the directionality of the flux by means of penetration-hole alignment of thin films placed above the cells. (4) Capture intact the particles that perforated the thin film and entered the cell. Capture media consisted of both previously flight-tested micropore foams and aerogel. The foams had different latent heats of fusion and, accordingly, will capture particles over a range of momenta. Aerogel was incorporated into the cells to determine the minimum diameter than can be captured intact.
Maag, Carl R.↗
A diffusion–deformation model with damage for polymer undergoing rapid decompression failure
Green hydrogen is emerging as one of the most promising alternatives to replace fossil fuels. While hydrogen gas has a good energy density by weight, its poor energy density by volume requires it to be stored under high pressure for commercial use. The hydrogen infrastructure developed to handle this high pressure hydrogen contains multiple components made of polymers as sealing agents. Although polymers do not react chemically with hydrogen gas, they undergo a mechanical failure when high pressure hydrogen gas is suddenly depressurized. This phenomenon known as 'rapid decompression failure' occurs due to the diffusion of hydrogen through polymer and getting trapped inside pre-existing cavities or voids. Here, in this paper, a continuum mechanics-based coupled diffusion-deformation-damage model was developed to predict the hydrogen distribution, stress distribution, and damage propagation inside the polymer while it undergoes rapid decompression failure. The polymer was modeled as a hyperelastic material because it represents the nonlinear material response observed in uniaxial tensile tests perfectly. The effects of hydrogen diffusivity, pre-existing cavity size, cavity location, applied hydrogen pressure, and depressurization rate on damage initiation were studied. It was found that the coefficient of diffusion plays an important role in damage initiation and damage was mostly concentrated in the inside areas rather than near the surface. Experiments were conducted with EPDM polymer which agreed well with the predicted trends using the given model. The effect of adding carbon black and silica filler particles and plasticizer to the pure EPDM polymer was also studied. It was found that damage during RDF decreases with the addition of fillers, but increases with the addition of the plasticizer. Finally, the damage evolution in the presence of two cavities was also studied, and was found that the interaction of stress fields around the cavities alters the damage occurring during RDF.
36 MATERIALS SCIENCE↗
Evaluating Cryo–TEM Reconstruction Accuracy of Self–Assembled Polymer Nanostructures
Cryogenic transmission electron microscopy (cryo–TEM) combined with single particle analysis (SPA) is an emerging imaging approach for soft materials. However, the accuracy of SPA–reconstructed nanostructures, particularly those formed by synthetic polymers, remains uncertain due to potential packing heterogeneity of the nanostructures. In this study, the combination of molecular dynamics (MD) simulations and image simulations is utilized to validate the accuracy of cryo–TEM 3D reconstructions of self–assembled polypeptoid fibril nanostructures. Using CryoSPARC software, image simulations, 2D classifications, ab initio reconstructions, and homogenous refinements are performed. By comparing the results with atomic models, the recovery of molecular details is assessed, heterogeneous structures are identified, and the influence of extraction location on the reconstructions is evaluated. In conclusion, these findings confirm the fidelity of single particle analysis in accurately resolving complex structural characteristics and heterogeneous structures, exhibiting its potential as a valuable tool for detailed structural analysis of synthetic polymers and soft materials.
36 MATERIALS SCIENCE↗
Studies of Particle Sedimentation by Novel Scattering Techniques
The four-year grant began May 1, 1996 (5-1-96 to 4-30-00, $100,000/year). We have finished 4 major research projects and published 10 papers during this grant period. An important aspect of this research has been the education of students at graduate and undergraduate levels. They have been fully involved in the research described below: 1. Polymer-induced depletion interaction in colloid-polymer mixtures. 2. Colloidal sedimentation in polymer solutions. 3. Velocity fluctuations in particle sedimentation. New laser light scattering techniques for velocity difference measurements.
Tong, Penger↗
Understanding the origin of softness in structurally tailored and engineered macromolecular (STEM) gels: A DPD study
Structurally tailored and engineered macromolecular (STEM) gels are polymer networks containing a primary network (the “STEM-0” gel) and side chains that are anchored to this network (forming the “STEM-1” gel). The ability to vary the features of the side chains provides a robust means of tailoring the macroscopic properties of the material. We use dissipative particle dynamics (DPD) simulations to determine the mechanical response of STEM-1 gels to uniaxial compression for various values of the side chain length, n sc . While the Young’s modulus of the material is significantly decreased with increasing n sc (at a fixed density of grafted side chains), above a certain saturation value, increasing n sc does not lead to any further softening of the sample. Using the simulations, we calculate the relevant stresses, which are related to the Young’s modulus and the number of entanglements in the network. We show that the backbone chains become more spread out upon addition of sidechains, leading to a decrease in the physical entanglements between backbone segments. This observation accounts for the decrease of stress in the backbone network and the softening of the networks through the addition of sidechains. For long chains, however, the number density of physical crosslinks between side chains is independent of side chain length; this observation explains the observed stress saturation in the STEM-1 gels. Our approach allows us to correlate the molecular architecture of the gels to the resultant macroscopic mechanical behavior and provide guidelines for fabricating STEM gels with well-defined mechanical properties, which allow the materials to be used for a variety of applications.
36 MATERIALS SCIENCE↗
Shock compression of additively manufactured high solids loaded polymer composites with aligned porosity
The effects of aligned porosity on the shock-compression response of additively manufactured (AM) composite structures with collinear and log-cabin geometries are investigated. The composites contain two inorganic and two organic particle populations and a UV-curable polymer binder. X-ray phase contrast imaging (X-PCI) is used as an in-material diagnostic in plate-impact experiments performed at the Dynamic Compression Sector of the Advanced Photon Source at the Argonne National Laboratory. Macroscale shock and particle velocities and microscale pore collapse velocities are measured from X-PCI images obtained from samples impacted in various orientations relative to the AM build directions. The linear shock and particle velocity relation for the structures with collinear build geometry shows directional differences, with the build direction exhibiting the highest slope and the filament print direction having the shallowest slope. Composites with log-cabin geometry, due to their slightly lower porosity and smaller sized pores, show higher shock velocities than the collinear structures at the same particle velocity. Microscale directionality effects are also observed, with the pore collapse velocity dependent on the pore configuration. Specifically, the pore collapse velocity is highest for pores elongated along the impact direction which is the case for the colinear structures impacted in the filament print direction, which is consistent with the directionality trend observed in the data for the shock and particle velocity equation of state. The pore collapse velocity approaches ∼80 % of the shock velocity and is several times the average bulk particle velocity, suggesting a highly hydrodynamic mechanism of pore collapse. Overall, both the macroscale response as measured by the shock and particle velocity relationship, and the microscale response as measured by the pore collapse velocity, are found to be anisotropic and dependent on the AM composite structure.
Wagner, K. B. [Georgia Institute of Technology, At↗
Neutron Reflectometry Reveals Diffusion in Contrast-Matched Brush Particle Bilayers
A material system for performing layer-spread experiments on brush particle bilayers is presented and used to determine the diffusion constant of brush particles in the melt state. Selective deuteration of the core and shell of organo-silica nanoparticles grafted with poly(methyl methacrylate) was used to match the scattering length density of the core and the polymer canopy layer. This subdued the scattering of particle cores (i.e., formfactor scattering) and enabled the analysis of the interdiffusion kinetics using neutron reflectivity. For low molecular grafts, i.e., grafts with a molecular weight below the entanglement limit, the interdiffusion kinetics revealed both a sub- and Fickian diffusion regime. The former was attributed to the local dynamics that was constrained by the slow-moving cores of neighboring brush particles that acted as long-lived physical cross-links. No transition to Fickian diffusion was observed for entangled systems, even at prolonged annealing times. This suggested a higher level of kinetic restraint in entangled brush particle melts as compared to, for example, star polymers with a comparable chain length for which Fickian diffusion has been reported under similar conditions.
Diffusion, Polymer Grafted Nanoparticles, Neutron ↗
Method of Making an Electroactive Sensing/Actuating Material for Carbon Nanotube Polymer Composite
An electroactive sensing or actuating material comprises a composite made from a polymer with polarizable moieties and an effective amount of carbon nanotubes incorporated in the polymer for a predetermined electromechanical operation of the composite when such composite is affected by an external stimulus. In another embodiment, the composite comprises a, third component of micro -sized to nano-sized particles of an electroactive ceramic that is also incorporated in the polymer matrix. The method for making the three-phase composite comprises either incorporating the carbon nanotubes in the polymer matrix before incorporation of the particles of ceramic or mixing the carbon nanotubes and particles of ceramic together in a solution before incorporation in the polymer matrix.
Ounaies, Zoubeida↗
Preparation of Mechanically Robust, Macroscopic Low-Density Gold Foams by Highly Efficient Electroless Gold Plating
Mechanically robust, macroscopic-sized low-density gold foams with unique morphology of hollow gold shells were prepared by the synthesis of gold-coated polystyrene coreshell particles followed by thermal removal of the polymer core. Here, we reported a simple, highly efficient electroless gold plating procedure to form ultrathin, conformal gold coatings on the surface of polystyrene particles. A modified, scalable casting approach to form uniform, large monoliths (up to 1 cm diameter) and a baking condition to minimize undesirable densification of final foams were also developed. To demonstrate the mechanical stability and integrity of the Au foams, a representative monolithic sample (~0.9 g/ cm3 ) was successfully cut and shaped into a gold foam “washer” by a series of precision machining and processing. A depth-sensing indentation test (nanoindentation) as well as characterizations on other useful properties such as surface areas and electrical conductivity of the Au foams were carried out to understand the mechanical behavior and potential applicability.
Electroless Gold Plating↗
Huygens/ACP: An instrument for aerosols chemical composition measurements
The Aerosol Collector Pyrolyzer (ACP) which will be used aboard the Huygens probe is introduced. The scientific objectives, scientific rationale, and a technical description of the ACP are given. The ACP will sample the aerosols of the Titan's atmosphere in two regions, 160 to 60 and 60 to 25 km, respectively. In the first region, it will collect aerosol particles absorbed by a complex core (organic polymers) coated with absorbed or condensed molecules. The second collect will sample larger aerosols and the N2-CH4 droplets. In each sampling, the particles are collected on a metallic filter through which the atmospheric gas is pumped. The filter is then inserted into an oven connected to the Gas Chromatograph Mass Spectrometer (GCMS) Experiment which analyzes the gases produced during the heating of the samples. For each sample it will be possible to bring information on the nature and abundance of the volatile components contained in the particles and/or the composition of the photochemical core.
Israel, Guy↗
Design Principles for Using Amphiphilic Polymers To Create Microporous Water
Aqueous dispersions of microporous nanocrystals with dry, gas-accessible pores—referred to as “microporous water”—enable high densities of gas molecules to be transported through water. For many applications of microporous water, generalizable strategies are required to functionalize the external surface of microporous particles to control their dispersibility, stability, and interactions with other solution-phase components—including catalysts, proteins, and cells—while retaining as much of their internal pore volume as possible. Here, in this work, we establish design principles for the noncovalent surface functionalization of hydrophobic metal–organic frameworks with amphiphilic polymers that render the particles dispersible in water and enhance their hydrolytic stability. Specifically, we show that block co-polymers with persistence lengths that exceed the micropore aperture size of zeolitic imidazolate frameworks (ZIFs) can dramatically enhance ZIF particle dispersibility and stability while preserving porosity and >80% of the theoretical O 2 carrying capacity. Moreover, enhancements in hydrolytic stability are greatest when the polymer can form strong bonds to exposed metal sites on the external particle surface. More broadly, our insights provide guidelines for controlling the interface between polymers and metal–organic framework particles in aqueous environments to augment the properties of microporous water.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Sensing/actuating materials made from carbon nanotube polymer composites and methods for making same
An electroactive sensing or actuating material comprises a composite made from a polymer with polarizable moieties and an effective amount of carbon nanotubes incorporated in the polymer for a predetermined electromechanical operation of the composite when such composite is affected by an external stimulus. In another embodiment, the composite comprises a third component of micro-sized to nano-sized particles of an electroactive ceramic that is also incorporated in the polymer matrix. The method for making the three-phase composite comprises either incorporating the carbon nanotubes in the polymer matrix before incorporation of the particles of ceramic or mixing the carbon nanotubes and particles of ceramic together in a solution before incorporation in the polymer matrix.
Ounaies, Zoubeida↗