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Mays, Jimmy

Publications and source records attributed to Mays, Jimmy.

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Failure behavior after stepwise uniaxial extension of entangled polymer melts

This paper studies how stepwise extension of various well-entangled polymer melts produce mechanical/structural breakdowns during stress relaxation. Depending on how stepwise extension is imposed on five different styrene-butadiene random copolymers, two different forms of specimen failure are observed. When a step extension is produced with a low Hencky rate or to a low strain below some thresholds, the sample breaks up rather sharply after an appreciable period of induction during which the stress relaxes quiescently. After step extension, the sample draws and undergoes unsustainable necking due to shear yielding, if the step extension is produced with a Hencky rate higher than the Rouse relaxation rate and the magnitude is beyond a Hencky strain of 1.5. Moreover, introduction of long-chain branching suppresses the elastic breakup, postponing it to Hencky strains beyond 2.5. The clearly identifiable characteristics of the elastic yielding may be understood in terms of some speculative interpretations. Finally, more convincing explanations have yet to come from future computer experiments that hopefully the present work is able to motivate.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Atomic force microscopy of thermally renatured xanthan with low molar mass

Here, this work presents a three-step synthetic procedure to obtain a hypergrafted copolymer composed of a glassy backbone with flexible branched pendant segments. The desired hypergrafted structure was obtained by using a polyfunctional macroinitiator, linear poly(styreneco- 4-hydroxystyrene), to yield polystyrene-graft-hyperbranched polyglycidol with randomly placed branch junctions. Atomic force microscopy, dynamic light scattering, and viscometry probed the aggregation and viscometric behavior of the polymer in DMF and DMF LiBr solutions. The polymer exhibited polyelectrolyte behavior demonstrated by a large increase in the reduced viscosity prior to neutralization with LiBr salt. Additionally, conformational changes were observed by dynamic light scattering in both the average aggregate size and aggregate population with the addition of LiBr salt.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Synthesis and Characterization of Graft Copolymers Poly(isoprene-g-styrene) of High Molecular Weight by a Combination of Anionic Polymerization and Emulsion Polymerization

In this study, high molecular weight “comb-shaped” graft copolymers, poly(isoprene-g-styrene), with polyisoprene as the backbone and polystyrene as side chains, were synthesized via free radical emulsion polymerization by copolymerization of isoprene with a polystyrene macromonomer synthesized using anionic polymerization. A small amount of toluene was used in order to successfully disperse the macromonomer. Both a redox and thermal initiation system were used in the emulsion polymerization, and the latex particle size and distribution were investigated by dynamic light scattering. The structural characteristics of the macromonomer and comb graft copolymers were investigated through use of size exclusion chromatography, spectroscopy, microscopy, thermal analysis, and rheology. While the macromonomer was successfully copolymerized to obtain the desired multigraft copolymers, small amounts of unreacted macromonomer remained in the products, reflecting its reduced reactivity due to steric effects. Nevertheless, the multigraft copolymers obtained were very high in molecular weight (5–12 × 10 5 g/mol) and up to 10 branches per chain, on average, could be incorporated. A material incorporating 29 wt% polystyrene exhibits a disordered microphase separated morphology and elastomeric properties. As a result, these materials show promise as new, highly tunable, and potentially low cost thermoplastic elastomers.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Poly(styrene-graft-hyperbranched polyglycidol): synthesis and solution behavior of a hyperbranched polyelectrolyte

This work presents a three-step synthetic procedure to obtain a hypergrafted copolymer composed of a glassy backbone with flexible branched pendant segments. The desired hypergrafted structure was obtained by using a polyfunctional macroinitiator, linear poly(styreneco- 4-hydroxystyrene), to yield polystyrene-graft-hyperbranched polyglycidol with randomly placed branch junctions. Atomic force microscopy, dynamic light scattering, and viscometry probed the aggregation and viscometric behavior of the polymer in DMF and DMF LiBr solutions. The polymer exhibited polyelectrolyte behavior demonstrated by a large increase in the reduced viscosity prior to neutralization with LiBr salt. Additionally, conformational changes were observed by dynamic light scattering in both the average aggregate size and aggregate population with the addition of LiBr salt.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Synthesis and Characterization of Comb and Centipede Multigraft Copolymers P n BA- g -PS with High Molecular Weight Using Miniemulsion Polymerization

For this study, comb and centipede multigraft copolymers, poly(n-butyl acrylate)-g-polystyrene (PnBA-g-PS) with PnBA backbones and PS side chains, were synthesized via high-vacuum anionic polymerization and miniemulsion polymerization. Single-tailed and double-tailed PS macromonomers were synthesized by anionic polymerization and Steglich esterification. Subsequently, the copolymerization of each macromonomer and nBA was carried out in miniemulsion, and multigraft copolymers were obtained. The latex particles of multigraft copolymers were characterized using dynamic light scattering. The molecular weights of macromonomers and multigraft copolymers were analyzed by size exclusion chromatography. Moreover, the molecular weights and structures of macromonomers were investigated by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and 1H nuclear magnetic resonance spectroscopy. The weight contents of PS in comb and centipede multigraft copolymers were calculated by 1H nuclear magnetic resonance spectroscopy. The thermal properties of multigraft copolymers were characterized by thermogravimetric analysis and differential scanning calorimetry. The microphase separation of multigraft copolymers was observed by atomic force microscopy and transmission electronic microscopy. Rheological measurements showed that comb and centipede multigraft copolymers have elastic properties when the weight content of PS side chains is 26–32 wt %. Centipede multigraft copolymers possess better elastic properties than comb multigraft copolymers with the similar weight content of PS. In conclusion, these findings are similar to previous results on poly(isoprene-g-polystyrene) comb and centipede copolymers made by anionic polymerization.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Macroscopic Properties of Restacked, Redox-Liquid Exfoliated Graphite and Graphite Mimics Produced in Bulk Quantities

The excellent properties exhibited by monolayer graphene have spurred the development of exfoliation techniques using bulk graphite to produce large quantities of pristine monolayer sheets. Development of simple chemistry to exfoliate and intercalate graphite and graphite mimics in large quantities is required for numerous applications. To determine the macroscopic behavior of restacked, exfoliated bulk materials, a systematic approach is presented using a simple, redox-liquid sonication process along to obtain large quantities of 2D and 3D hexagonally layered graphite, molybdenum disulfi de, and boron nitride, which are subsequently characterized to observe chemical and structural changes. For MoS 2 sonicated with the antioxidant sodium bisulfi te, results from Raman spectroscopy, X-ray diffraction, and electron microscopy indicate the presence of distorted phases from different polymorphs, and apparent nanotube structures in the bulk, restacked powder. Furthermore, using thermograviemtric analysis, the antioxidant enhances the resistance to oxidative degradation of MoS 2 , upon thermal treatment up to 900 C. The addition of the ionic antioxidant decreased dispersion stability in non-polar solvent, suggesting decreased compatibility with non-polar systems. Using simple chemical methods, the ability to generate tailored multidimensional layered materials with unique macroscopic properties is critical for numerous applications, including electrical devices, reinforced polymer composites, lithium ion capacitors, and chemical sensing.

Srivastava, Vikram K↗
DOE OSTI2014

High Yield Synthesis of Mesoscopic Conductive and Dispersible Carbon Nanostructures via Ultrasonication of Commercial Precursors

The need to produce large quantities of graphenic materials displaying excellent conductivity, thermal resistance, and tunable properties for industrial applications has spurred interest in new techniques for exfoliating graphite. In this paper, sonication-assisted exfoliation of graphitic precursors in the presence of chloroform is shown to produce chemically and structurally unique exfoliated graphitic materials in high yields. These exfoliated graphites, referred to as mesographite and mesographene, respectively, exhibit unique properties which depend on the number of layers and exfoliation conditions. Structural characterization of mesographene reveals the presence of nanoscale two-dimensional graphene layers, and threedimensional carbon nanostructures sandwiched between layers, similar to those found in ball-milled and intercalated graphites. The conductivities of mesographite and mesographene are 2700 and 2000 S/m, respectively, indicating high conductivity despite flake damage. Optical absorption measurements of mesographite sonicated in various solvents showed significant changes in dispersion characteristics, and also indicated significant changes to mesoscopic colloidal behavior. A mechanism for functionalization and formation of capped carbon nanostructures is proposed by integrating the chemical and structural characterization in relation to the various carbon structures observed by electron microscopy. Composites based on common polymers were prepared by solution processing, and changes in thermal properties indicate improved dispersion of mesographite in polar polymers.

Srivastava, Vikram K↗
DOE OSTI2013

Electric Field Induced Selective Disordering in Lamellar Block Copolymers

External electric fields align nanostructured block copolymers by either rotation of grains or nucleation and growth depending on how strongly the chemically distinct block copolymer components are segregated. In close vicinity to the orderdisorder transition, theory and simulations suggest a third mechanism: selective disordering. We present a time-resolved small-angle X-ray scattering study that demonstrates how an electric field can indeed selectively disintegrate ill-aligned lamellae in a lyotropic block copolymer solution, while lamellae with interfaces oriented parallel to the applied field prevail. The present study adds an additional mechanism to the experimentally corroborated suite of mechanistic pathways, by which nanostructured block copolymers can align with an electric field. Our results further unveil the benefit of electric field assisted annealing for mitigating orientational disorder and topological defects in block copolymer mesophases, both in close vicinity to the orderdisorder transition and well below it.

Ruppel, Markus A↗
DOE OSTI2013

Synthesis, Characterization (Molecular-Morphological) and Theoretical Morphology Predictions of Poly(cyclohexadiene) Containing Linear Triblock Terpolymers

The synthesis via anionic polymerization of six linear triblock terpolymers with various sequences of blocks such as PS (polystyrene), PB (polybutadiene), PI (polyisoprene) and PCHD (poly(1,3-cyclohexadiene)) is reported. The synthesis of the terpolymers was accomplished by the use of anionic polymerization with high vacuum techniques and sequential monomer addition. Molecular characterization of the samples was performed via size exclusion chromatography and membrane osmometry to measure polydispersity indices and the number-average molecular weights, respectively. Proton nuclear magnetic resonance spectroscopy was adopted to verify the type of microstructure for the polydienes as well as to calculate the molar composition. Structural characterization was performed via transmission electron microscopy and small angle X-ray scattering and several morphologies were observed including one which has not been reported previously. Real-space self-consistent field theory (SCFT) without a priori knowledge about the symmetry of the periodic structures was used to elucidate the thermodynamics of the synthesized triblock copolymers.

Kumar, Rajeev↗
DOE OSTI2013

Effect of Macromolecular Architecture on the Morphology of Polystyrene Polyisoprene Block Copolymers

The impact of block connectivity on the morphologies of four block copolymers of varying architecture containing polystyrene (PS) and polyisoprene (PI) has been studied. The volume fraction of PS and molecular weight are held constant while varying the architecture from a linear PS-PI diblock copolymer to three different miktoarm star architectures: PS2PI, PSPI2, and PS2PI2. Morphologies of the PS2PI and PSPI2 miktoarm stars are different from those observed for the linear copolymer and dependent on the connectivity of the copolymer blocks. The change in morphology with connectivity indicates that combining two chains at a junction point leads to chain crowding, where subsequent excluded volume effects drive the change in morphology for each sample. The PS2PI2 miktoarm star exhibits the same morphology as the linear diblock but with a reduction in the size of the domains. The extent of the decrease in domain size indicates that chain stretching impacts the formation of this morphology. Experimentally observed morphologies for different chain architectures are generally consistent with three-dimensional self-consistent field theory simulations, taking into account conformational asymmetry and experimental uncertainty in the copolymer composition. Furthermore, these results generally agree with analytical theory predictions that account for architectural and conformational asymmetry.

Kumar, Rajeev↗
DOE OSTI2013

Disk-cylinder and disk-sphere nanoparticles from block copolymer blend solution construction

Researchers strive to produce nanoparticles with complexity in composition and structure. Although traditional spherical, cylindrical and membranous, or planar, nanostructures are ubiquitous, scientists seek more complicated geometries for potential functionality. Here we report the simple solution construction of multigeometry nanoparticles, disk-sphere and diskcylinder, through a straightforward, molecular-level, blending strategy with binary mixtures of block copolymers. The multigeometry nanoparticles contain disk geometry in the core with either spherical patches along the disk periphery in the case of disk-sphere particles or cylindrical edges and handles in the case of the disk-cylinder particles. The portions of different geometry in the same nanoparticles contain different core block chemistry, thus also defining multicompartments in the nanoparticles. Although the block copolymers chosen for the blends are important for the definition of the final hybrid particles, the control of the kinetic pathway of assembly is critical for successful multigeometry particle construction.

Zhu, Jiahua↗
DOE OSTI2012

Ternary behavior and systematic nanoscale manipulation of domain structures in P3HT/PCBM/P3HT-b-PEO films

Nanophase separation plays a critical role in the performance of donor acceptor based organic photovoltaic (OPV) devices. Although post-fabrication annealing is often used to enhance OPV efficiency, the ability to exert precise control over phase separated domains and connectivity remains elusive. In this work, we use a diblock copolymer to systematically manipulate the domain sizes of an organic solar cell active layer at the nanoscale. More specifically, a poly(3-hexylthiophene)-bpoly( ethylene oxide) (P3HT-b-PEO) diblock copolymer with a low polydispersity index (PDI 1.3) is added to a binary blend of P3HT and 6,6-phenyl C61-butyric acid methyl ester (PCBM) at different concentrations (0 20 wt%). Energy-filtered TEM (EFTEM) results suggest systematic changes of P3HT distribution as a function of block copolymer compatibilizer concentration and thermal annealing. X-ray scattering and microscopy techniques are used to show that prior to annealing, active layer domain sizes do not change substantially as compatibilizer is added; however after thermal annealing, the domain sizes are significantly reduced as the amount of P3HT-b-PEO compatibilizer increases. The impact of compatibilizer is further rationalized through quantum density functional theory calculations. Overall, this work demonstrates the possibility of block copolymers to systematically manipulate the nanoscale domain-structure of blends used for organic photovoltaic devices. Furthermore, if coupled with efficient charge transport and collection (through judicious choice of block copolymer type and composition), this approach may contribute to further optimization of OPV devices.

14 SOLAR ENERGY↗