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Sokolov, Alexei P

Publications and source records attributed to Sokolov, Alexei P.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Untangleing the effects of chain rigidity on the structure and dynamics of strongly adsorbed polymer melts

Here, we present a detailed analysis of coarse-grained molecular dynamics simulations of semiflexible polymer melts in contact with a strongly adsorbing substrate. We have characterized the segments in the interfacial layer by counting the number of trains, loops, tails and unadsorbed segments. For more rigid chains, a tail and an adsorbed segment (a train) dominate while loops are more prevalent in more flexible chains. The tails exhibit a non-uniformly stretched conformation akin to the polydispersed pseudobrush envisioned by Guiselin. To probe the dynamics of the segments we computed the layer z-resolved intermediate coherent collective dynamics structure factor, S(q, t, z), mean-square displacement of segments, and the 2nd Legendre polynomial of the time-autocorrelation of unit bond vectors, 2 [n i (t,z)•n i (0,z)]>. Our results show that segmental dynamics is slower for stiffer chains and there is a strong correlation between the structure and dynamics in the interfacial layer. There is no glassy layer, and the slowing down in dynamics of stiffer chains in the adsorbed region can be attributed to the densification and the more persistent layering of segments.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Quantum effects in the dynamics of deeply supercooled water

In spite of its simple chemical structure, water remains one of the most puzzling liquids with many anomalies at low temperatures. Combining neutron scattering and dielectric relaxation spectroscopy, we show that quantum fluctuations are not negligible in deeply supercooled water. Our dielectric measurements reveal the anomalously weak temperature dependence of structural relaxation in vapor-deposited water close to the glass transition temperature T g ~136K. We demonstrate that this anomalous behavior can be explained well by quantum effects. In conclusion, these results have significant implications for our understanding of water dynamics.

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

Room temperature reduction of multilayer graphene oxide film on a copper substrate: Penetration and participation of coper phase in redox reactions.

A self-reduction of graphene oxide (GO) at room temperature after prolonged storage on a copper substrate is evidenced by decrease of oxygen content and a dramatic, 6 orders in magnitude, increase in dc conductivity. Experiments revealed that the stored GO film contains copper hydroxide phase embedded in the reduced GO structure.

Voylov, Dmitry N↗
DOE OSTI2014

Dynamics at the Polymer/Nanoparticle Interface in Poly(2-vinylpyridine)/Silica Nanocomposites.

The static and dynamic properties of poly(2-vinylpyridine)/silica nanocomposites are investigated by temperature modulated differential scanning calorimetry, broadband dielectric spectroscopy (BDS), small-angle X-ray scattering (SAXS), and transmission electron microscopy. Both BDS and SAXS detect the existence of an interfacial polymer layer on the surface of nanoparticles. The results show that whereas the calorimetric glass transition temperature varies only weakly with nanoparticle loading, the segmental mobility of the polymer interfacial layer is slower than the bulk polymer by 2 orders of magnitude. Detailed analysis of BDS and SAXS data reveal that the interfacial layer has a thickness of 4 6 nm irrespective of the nanoparticle concentration. These results demonstrate that in contrast to some recent articles on polymer nanocomposites, the interfacial polymer layer is by no means a dead layer . However, its existence might provide some explanation for controversies surrounding the dynamics of polymer nanocomposites.

Holt, Adam P↗
DOE OSTI2014

Carbon nanomaterial produced by microwave exfoliation of graphite oxide: new insights.

We present detailed characterization of graphene-like material obtained through microwave stimulated exfoliation of graphite oxide (GO). Properties of this material were studied by multiple techniques including, among others, X-ray photoelectron spectroscopy, mass-spectroscopy, infrared and Raman spectroscopy, scanning electron microscopy and broadband dielectric spectroscopy. Specific surface area and volume of microwave exfoliated graphite oxide reached 600 m2 g1 and 6 cm3 g1, respectively. It is shown that during such an explosive reduction process the sample emits CO2, CO and H2O and, in some cases, SO2 gases. The resulting reduced material exhibits IR spectra similar to that of graphite and a dc-conductivity of 0.12 S cm1.

Shulga, Y. M.↗
DOE OSTI2013

Rheological Study of Mutarotation of Fructose in Anhydrous State

Rheological measurement was employed to study the mutarotation of D-fructose in anhydrous state. By monitoring the evolution of shear viscosity with time, rate constants for mutarotation were estimated, and two different stages of this reaction were identified. One of the mutarotation stages is rapid and has a low activation energy, whereas the other is much slower and has a much higher activation energy. Possible conversions corresponding to these two phases are discussed. This work demonstrates that, in addition to the routine techniques such polarimetry and gas liquid chromatography, rheological measurement can be used as an alternative method to continuously monitor the mutarotation of sugars.

Wang, Yangyang↗
DOE OSTI2013

Dynamics of Hydration Water in Sugars and Peptides Solutions

We analyzed solute and solvent dynamics of sugars and peptides aqueous solutions using extended epolarized light scattering (EDLS) and broadband dielectric spectroscopies (BDS). Spectra measured with both techniques reveal the same mechanism of rotational diffusion of peptides molecules. In the case of sugars, this solute reorientational relaxation can be isolated by EDLS measurements, whereas its ontribution to the dielectric spectra is almost negligible. In the presented analysis, we characterize the hydration water in terms of hydration number and retardation ratio between relaxation times of hydration and bulk water. Both techniques provide similar estimates of . The retardation imposed on the hydration water by sugars is 3.3 1.3 and involves only water molecules hydrogen-bonded (HB) to solutes ( 3 water molecules per sugar OH-group). In contrast, polar peptides cause longer range erturbations beyond the first hydration shell, and between 2.8 and 8, increasing with the number of chemical groups engaged in HB formation. We demonstrate that chemical heterogeneity and specific HB interactions play a crucial role in hydration dynamics around polar solutes. The obtained results help to disentangle the role of excluded volume and enthalpic contributions in dynamics of hydration water at the interface with biological molecules.

Perticaroli, Stefania↗
DOE OSTI2013

Chain and Segmental Dynamics of Poly(2-vinlylpyridine)

The influence of different oxide nanoparticles on segmental and chain dynamics of poly(2-vinylpyridine) is investigated by broadband dielectric spectroscopy (BDS), dynamic mechanical measurements, and calorimetry. While significant changes in rheological properties are observed with increasing nanoparticle loading, only weak effects are found in the glass transition temperature, the segmental mobility, and fragility. These results are discussed in the context of recent controversies in studies of polymer nanocomposites and polymer thin films.

Holt, Adam P↗
DOE OSTI2013

Effects of backbone rigidity on the local structure and dynamics in polymer melts and glasses

Frustration in chain packing has been proposed to play an important role in thermodynamic and dynamic properties of polymeric melts and glasses. Based on a quantitative analysis using Voronoi tessellations and large scale molecular dynamics simulations of flexible and semi-flexible polymers, we demonstrate that the rigid polymer chains have higher averaged Voronoi polyhedral volumes and significantly wider distribution of the volume due to frustration in the chain packing. Using these results, we discuss the advantage of the rigid polymers for possible enhancement of transport properties, e.g. for enhancing ionic conductivity in solid polymer electrolytes.

Kumar, Rajeev↗
DOE OSTI2013

Solvent effects on proteinfast dynamics: iplications for biopreservation

In the context of biopreservation, we study the influence of water, glycerol and trehalose on the ps ns dynamics of lyzosyme using neutron scattering. Results indicate that the choice of bioprotectant depends on the storage temperature; glycerol is the most effective for low temperatures and trehalose for high temperatures.

Garcia Sakai, V↗
DOE OSTI2012

Role of methyl groups in dynamics and evolution of biomolecules

Recent studies have discovered strong differences between the dynamics of nucleic acids (RNA and DNA) and proteins, especially at low hydration and low temperatures. This difference is caused primarily by dynamics of methyl groups that are abundant in proteins, but are absent or very rare in RNA and DNA. In this paper, we present a hypothesis regarding the role of methyl groups as intrinsic plasticizers in proteins and their evolutionary selection to facilitate protein dynamics and activity. We demonstrate the profound effect methyl groups have on protein dynamics relative to nucleic acid dynamics, and note the apparent correlation of methyl group content in protein classes and their need for molecular flexibility. Moreover, we note the fastest methyl groups of some enzymes appear around dynamical centers such as hinges or active sites. Methyl groups are also of tremendous importance from a ydrophobicity/folding/entropy perspective. These significant roles, however, complement our hypothesis rather than preclude the recognition of methyl groups in the dynamics and evolution of biomolecules.

Nickels, Jonathan D↗