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Salerno, K. Michael

Publications and source records attributed to Salerno, K. Michael.

5 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Viscoelastic Response of Dispersed Entangled Polymer Melts

Any polymer synthesis routes result in molecular weight dispersity DAI, depending on the polymerization mechanism. The lowest dispersity polymers are those made by anionic and atom-transfer radical polymerization, which exhibit relatively narrow distributions DM=g,„1A172 — 1.02-1.04. This small divergence from monodispersity results in significant number of molecules that differ in their molecular weight from the average and their impact on the viscoelastic response remains an open question. Here the effects of low dispersity on stress relaxation and shear viscosity of entangled melts are studied using a coarse-grained model for polyethylene. Polymer melts with chain length dispersity set to follow a Schulz-Zimm distribution of Dm = 1.0 — 1.16 for an entangled polyethylene melt using molecular dynamic simulations. The systems were studied to times up to 800 ,us which is beyond the terminal time. These systems are compared to those with binary and ternary distributions. The stress relaxation functions were extracted from the Green-Kubo relation and from stress-strain relaxation following a uniaxial extension. We find on the entanglement time scale, both the mean squared displacement and the stress relaxation are independent of D m . The tube diameter and the rubbery plateau for entangled dynamics do not depend on D m . At longer times, the terminal relaxation time decreases as the faster motion of the shorter chains results in constraint release for the longer chains. Probing this low dispersity regime opens the way to evaluate the degree of dispersity that affects mechanical properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Mechanics of Gold Nanoparticle Superlattices at High Hydrostatic Pressures

Pressure-driven assembly of ligand-grafted gold nanoparticle superlattices is a promising approach for fabricating gold nanostructures, such as nanowires and nanosheets. Optimizing this fabrication method will require extending our understanding of superlattice mechanics to regimes of high pressures. We use molecular dynamics simulations to characterize the response of alkanethiol-grafted gold nanoparticle superlattices to applied hydrostatic pressures up to 15 GPa. At low applied pressures, intrinsic voids govern the mechanics of compaction. As applied pressures increase, the void collapse and ligand compression depend significantly on the ligand length. These microstructural observations correlate directly with trends in bulk modulus and elastic constants. For short ligands, core–core contact between gold nanoparticles is observed at high pressures, which augurs irreversible response and eventual sintering. In conclusion, this presintering behavior was unexpected under hydrostatic loading and is observed only for the shortest ligands.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of shape and friction on the packing and flow of granular materials

Here, the packing and flow of aspherical frictional particles are studied using discrete element simulations. Particles are superballs with shape |x| s + |y| s + |z| s = 1 that varies from sphere (s = 2) to cube (s = ∞), constructed with an overlapping-sphere model. Both packing fraction, Φ, and coordination number, z, decrease monotonically with microscopic friction μ, for all shapes. However, this decrease is more dramatic for larger s due to a reduction in the fraction of face-face contacts with increasing friction. For flowing grains, the dynamic friction ~μ —the ratio of shear to normal stresses—depends on shape, microscopic friction, and inertial number I . For all shapes, ˜μ grows from its quasistatic value ˜μ 0 as (˜μ - ˜μ 0 ) = dI α , with different universal behavior for frictional and frictionless shapes. For frictionless shapes the exponent α ≈ 0.5 and prefactor d ≈ 5˜μ 0 while for frictional shapes α ≈ 1 and d varies only slightly. Lastly, the results highlight that the flow exponents are universal and are consistent for all the shapes simulated here.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Chain Length Dispersity on the Mobility of Entangled Polymers

While nearly all theoretical and computational studies of entangled polymer melts have focused on uniform samples, polymer synthesis routes always result in some dispersity, albeit narrow, of distribution of molecular weights ( Ð M = M w / M n ~ 1.02 – 1.04 ). Here, the effects of dispersity on chain mobility are studied for entangled, disperse melts using a coarse-grained model for polyethylene. Polymer melts with chain lengths set to follow a Schulz-Zimm distribution for the same average M w = 36 kg / mol with Ð M = 1.0 to 1.16, were studied for times of 600 – 800 μ s using molecular dynamics simulations. This time frame is longer than the time required to reach the diffusive regime. We find that dispersity in this range does not affect the entanglement time or tube diameter. However, while there is negligible difference in the average mobility of chains for the uniform distribution Ð M = 1.0 and Ð M = 1.02 , the shortest chains move significantly faster than the longest ones offering a constraint release pathway for the melts for larger Ð M .

36 MATERIALS SCIENCE↗