1,023 research outputs found
Molecular Dynamics Simulation Study of Nonconcatenated Ring Polymers in a Melt: I. Statics
Molecular dynamics simulations were conducted to investigate the structural
properties of melts of nonconcatenated ring polymers and compared to melts of
linear polymers. The longest rings were composed of N=1600 monomers per chain
which corresponds to roughly 57 entanglement lengths for comparable linear
polymers. For the rings, the radius of gyration squared was found to scale as N
to the 4/5 power for an intermediate regime and N to the 2/3 power for the
larger rings indicating an overall conformation of a crumpled globule. However,
almost all beads of the rings are "surface beads" interacting with beads of
other rings, a result also in agreement with a primitive path analysis
performed in the following paper (DOI: 10.1063/1.3587138). Details of the
internal conformational properties of the ring and linear polymers as well as
their packing are analyzed and compared to current theoretical models.Comment: 15 pages, 14 figure
Degradation and healing in a generalized neo-Hookean solid due to infusion of a fluid
The mechanical response and load bearing capacity of high performance polymer
composites changes due to diffusion of a fluid, temperature, oxidation or the
extent of the deformation. Hence, there is a need to study the response of
bodies under such degradation mechanisms. In this paper, we study the effect of
degradation and healing due to the diffusion of a fluid on the response of a
solid which prior to the diffusion can be described by the generalized
neo-Hookean model. We show that a generalized neo-Hookean solid - which behaves
like an elastic body (i.e., it does not produce entropy) within a purely
mechanical context - creeps and stress relaxes when infused with a fluid and
behaves like a body whose material properties are time dependent. We
specifically investigate the torsion of a generalized neo-Hookean circular
cylindrical annulus infused with a fluid. The equations of equilibrium for a
generalized neo-Hookean solid are solved together with the convection-diffusion
equation for the fluid concentration. Different boundary conditions for the
fluid concentration are also considered. We also solve the problem for the case
when the diffusivity of the fluid depends on the deformation of the generalized
neo-Hookean solid.Comment: 24 pages, 10 figures, submitted to Mechanics of Time-dependent
Material
Elastic models for the non-Arrhenius viscosity of glass-forming liquids
This paper first reviews the shoving model for the non-Arrhenius viscosity of
viscous liquids. According to this model the main contribution to the
activation energy of a flow event is the energy needed for molecules to shove
aside the surrounding, an energy which is proportional to the instantaneous
shear modulus of the liquid. Data are presented supporting the model. It is
shown that the fractional Debye-Stokes-Einstein relation, that quantitatively
expresses the frequently observed decoupling of, e.g., conductivity from
viscous flow, may be understood within the model. The paper goes on to review
several related explanations for the non-Arrhenius viscosity. Most of these are
also "elastic models," i.e., they express the viscosity activation energy in
terms of short-time elastic properties of the liquid. Finally, two new
arguments for elastic models are given, a general solid-state defect argument
and an Occam's razor type argument
Disulfides – Effective radical generators for flame retardancy of polypropylene
The potential of thirteen aliphatic, aromatic, thiuram and heterocyclic substituted organic disulfide derivatives of the general formula R-S-S-R’ as a new group of halogen-free flame retardants (FR) for polypropylene films have been investigated. According to DIN 4102-1 standard ignitibility test, for the first time it has been demonstrated that many of the disulfides alone can effectively provide flame retardancy and self-extinguishing properties to polypropylene (PP) films at already very low concentrations of 0.5 wt%. In an effort to elucidate the mechanism of the thermal decomposition of disulfide derivatives the fragmentation patterns of the evolved gases from a thermogravimetric analyzer (TGA) have been analyzed by simultaneous mass spectrometry (MS) and Fourier transform infrared spectrometry (FTIR). The main decomposition products initiated by homolytic scission of the S-S bond and/or scission of the C-S bond were identified as thiols, aliphatic and aromatic hydrocarbons, isothiocyanates (depending on the disulfide structures) with further evolution of elemental sulfur and sulfur dioxide at temperatures of above 300 oC and 450 oC, respectively. Based on this preliminary study, we have shown that disulfides represented by e.g. diphenyl disulfide (1), 5,5'-dithiobis(2-nitrobenzoic acid) (2), bis(1-phenyl-1H-tetrazol-5yl)-disulfide (4), 2-bisbenzothiazole-2,2′-disulfide (6) and N,N-dithiobis-(phtalimide) (10) constitute a new halogen-free family of additives for flame retarding of polypropylene
Influence of Thermally Induced Chemorheological Changes on the Inflation of Spherical Elastomeric Membranes
When an elastomeric material is deformed and subjected to temperatures above some chemorheological value T cr (near 100°C for natural rubber), its macromolecular structure undergoes time and temperature dependent chemical changes. The process continues until the temperature decreases below T cr . Compared to the virgin material, the new material system has modified properties (often a reduced stiffness) and permanent set on removal of the applied load. A recently proposed constitutive theory is used to study the influence of chemorheological changes on the inflation of an initially isotropic spherical rubber membrane. The membrane is inflated while at a temperature below T cr . We then look at the pressure response assuming the sphere's radius is held fixed while the temperature is increased above T cr for a period of time and then returned to its original value. The inflation pressure during this process is expressed in terms of the temperature, representing entropic stiffening of the elastomer, and a time dependent property that represents the kinetics of the chemorheological change in the elastomer. When the membrane has been returned to its original temperature, it is shown to have a permanent set and a modified pressure-inflated radius relation. Their dependence on the initial inflated radius, material properties and kinetics of chemorheological change is studied when the underlying elastomeric networks are neo-Hookean or Mooney–Rivlin.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42688/1/10659_2005_Article_9020.pd
Influence of thermally induced chemorheological changes on the torsion of elastomeric circular cylinders
When an elastomeric material is deformed and subjected to temperatures above some characteristic value T cr (near 100 ∘ C for natural rubber), its macromolecular structure undergoes time and temperature-dependent chemical changes. The process continues until the temperature decreases below T cr . Compared to the virgin material, the new material system has modified properties (reduced stiffness) and permanent set on removal of the applied load.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46097/1/161_2006_Article_9.pd
Test of a simple and flexible S8 model molecule in alpha-s8 crystals
Alpha S8 is the most stable crystalline form, at ambient pressure and
temperature (STP), of elemental sulfur. In this paper we analyze the zero
pressure low temperature part of the phase diagram of this crystal, in order to
test a simple and flexible model molecule. The calculations consist in a series
of molecular dynamics (MD) simulations, performed in the constant pressure-
constant temperature ensemble. Our calculations show that this model, that
gives good results for three crystalline phases at STP and T>~300 K, fails at
low temperatures, predicting a structural phase transition at 200 K where there
should be none.Comment: 6 pages, 4 figures, submitted to Chem. Phys. Lett, a figure change
Effect of internal friction on transformation twin dynamics in SrxBa1-xSnO3 perovskite
The dynamics of transformation twins in SrxBa1-xSnO3 (x=0.6,0.8) perovskite
has been studied by dynamical mechanical analysis in three-point bend geometry.
This material undergoes phase transitions from orthorhombic to tetragonal and
cubic structures on heating. The mechanical loss signatures of the
transformation twins include relaxation and frequency-independent peaks in the
orthorhombic and tetragonal phases, with no observed energy dissipation in the
cubic phase. The macroscopic shape, orientation and relative displacements of
twin walls have been calculated from bending and anisotropy energies. The
mechanical loss angle and distribution of relaxation time are discussed in term
of bending modes of domain walls.Comment: 20 pages, 4 figure
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