Filler-induced deformations of amorphous polyethylene chains. The effects of the deformations on elastomeric properties, and some comparisons with experiments (original) (raw)
Related papers
Polymer, 2004
Reinforcing effects in an amorphous polyethylene matrix were estimated for spherical filler particles arranged either on a cubic lattice or randomly in space. Attention was first focused on the effects of the type of arrangement of the particles on the microscopic properties of the polymer chains. Specifically, Monte Carlo rotational isomeric state (MC-RIS) simulations were carried out to predict the effects of the volumes excluded by the filler particles on the configurational distribution functions of the chains, and from these distributions the elastomeric properties of the composites. The calculations were carried out for a range of particle sizes and particle volume fractions. As expected, filler inclusions are found to increase the non-Gaussian behavior of the chains. The results were compared with those from smallangle neutron scattering (SANS) experiments. In the case of arrangement on a cubic lattice, chains dimensions were always found to decrease. In the randomly-dispersed filler arrangements, there were significant increases in chain dimensions relative to the unfilled system in some instances, and the changes were in excellent agreement with the SANS results. The present simulations thus give further encouragement to interpretations of chain deformations in filled systems in terms of volume exclusion effects from the nanoparticle inclusions, including their dispersions and arrangements within polymer matrices.
2000
Reinforcement of elastomers is modeled using Monte Carlo simulations on rotational isomeric state chains, to characterize their spatial configurations in the vicinity of filler particles. The resulting filler-perturbed distributions of the chain end-to-end distances are in agreement with experimental results gotten by neutron scattering. The use of these distributions in a standard molecular theory of rubberlike elasticity produces stress-strain isotherms for elongation that are consistent with available experimental results.
Polymer
Although the ®ller particles typically used to reinforce elastomers are at least approximately spherical, prolate (needle-shaped) or oblate (disc-shaped) particles have been used in some cases. The fact that anisotropic structures and properties can be obtained in these cases has encouraged a number of experimental and theoretical investigations. The present study extends some earlier Monte Carlo simulations on prolate particles in an amorphous polyethylene matrix, but now focuses on oblate particles. The particles were placed on a cubic lattice, and were oriented in a way consistent with their orientation in composites that were the subject of an experimental investigation by one of the authors. Rotational isomeric state representations of the chains were then generated to model the elastomeric network in the presence of the ®ller particles. The chain end-to-end distributions were found to be non-Gaussian, and to depend signi®cantly on the excluded volumes of the particles. The particle-induced deformations of the network chains were consistent with results of some other relevant simulations and with recent neutron scattering results. Speci®cally, the chain dimensions were found to decrease with increase in the axial ratios characterizing the oblate shapes. As anticipated, the chain dimensions became anisotropic, with signi®cant differences parallel and perpendicular to the direction of the particle axes. In general, the network chains tended to adopt more compressed con®gurations relative to those of prolate particles having equivalent sizes and aspect ratios. Use of these distributions in a standard molecular model for rubberlike elasticity gave values of the elongation moduli, and these were found to depend on the sizes, number, and axial ratios of the particles, as expected. In particular, the reinforcement from the oblate particles was found to be greatest in the plane of the particles, and the changes were in at least qualitative agreement with the corresponding experimental results.
Some Simulations on Filler Reinforcement in Elastomers
Molecular Crystals and Liquid Crystals, 2002
This review illustrates how elastomer reinforcement can be modeled using Monte Carlo simulations on rotational isomeric state chains to characterize their spatial configurations in the vicinity of filler particles. The results are distributions of the chain end-to-end distances as perturbed by this excluded-volume effect, and the results obtained are in agreement with experimental results gotten by neutron scattering. The use of these distributions in standard molecular theories of rubberlike elasticity then produces stress-strain isotherms suitable for comparison with those in elongation experiments. Such simulations have now been carried out for elastomeric matrices reinforced by spherical filler particles (either on a cubic lattice or randomly dispersed), or by prolate or oblate particles on cubic lattices (either with their axes oriented or randomized). The simulated mechanical properties are consistent with experimental results available at the present time, and should provide encouragement and guidance for additional simulations and experiments.
Journal of Polymer Science Part B: Polymer Physics, 1996
Monte Carlo computer simulations were carried out on filled networks of poly(dimethylsiloxane) (PDMS), which were modeled as composites of crosslinked chains and randomly arranged spherical filler particles. The primary concern of the investigation was the effect of the excluded volume of these particles on the elastomeric properties of the polymers. Calculations were carried out for PDMS chains with different molecular masses between crosslinks, and for filler particles with different sizes and a t various volume percentages. Distributions of end-to-end vectors for both unfilled and filled networks were obtained using Monte Carlo simulations based on rotational isomeric state (RIS) theory. More extended configurations, with a higher end-to-end distance, were observed for networks filled with smaller particles. The nominal stress f * and the modulus or reduced nominal stress [f*] were calculated from the distributions of end-to-end vectors using the Mark-Curro approach. Relatively small filler particles were found to increase the non-Gaussian behavior and to increase the normalized moduli above the reference value of unity. Temperature effects on the stress were also investigated. 0 1996 John Wiley & Sons, Inc. Keywords: elastomers excluded volume effect fillers filler particle size moduli non-Gaussian effects poly(dimethylsi1oxane) reinforcement rotational isomeric state (RIS) theory
Molecular deformation of polyethylene
Materials Chemistry and Physics, 1987
The tensile modulus E of a polyethylene (PE) chain in the extended all-trans confzrmation and of a chain with the kink conformational defect has been calculated by the molecular mechanics method with Boyd's parametrization.
Analysis of structural changes during plastic deformations of amorphous polyethylene
2012
Molecular dynamics (MD) simulations have been used to analyze yielding and stress-softening processes during stepped simple tensile loading of bulk amorphous polyethylene (PE) at temperatures (Tdef) well below the glass transition temperature (Tg). Specimens formed by 20 linear chains of 1000 beads each (2× 104 coarse grained-CH2-units), with energetics described by a united atom potential, were deformed at Tdef= 100K.
Modeling the response of filled elastomers at finite strains by rigid-rod networks
Archive of Applied Mechanics (Ingenieur Archiv), 2002
A constitutive model is developed for the isothermal response of particle-reinforced elastomers at ®nite strains. An amorphous rubbery polymer is treated as a network of long chains bridged to permanent junctions. A strand between two neighboring junctions is thought of as a sequence of rigid segments connected by bonds. In the stress-free state, a bond may be in one of two stable conformations:¯exed and extended. The mechanical energy of a bond in the¯exed conformation is treated as a quadratic function of the local strain, whereas that of a bond in the extended conformation is neglected. An explicit expression is developed for the free energy of a network. Stress±strain relations and kinetic equations for the concentrations of bonds in various conformations are derived using the laws of thermodynamics. In the case of small strains, these relations are reduced to the constitutive equation for the standard viscoelastic solid. At ®nite strains, the governing equations are determined by four adjustable parameters which are found by ®tting experimental data in uniaxial tensile, compressive and cyclic tests. Fair agreement is demonstrated between the observations for several ®lled and un®lled rubbery polymers and the results of numerical simulation. We discuss the effects of the straining state, ®ller content, crosslink density and temperature on the adjustable constants.
Single chain elasticity and thermoelasticity of polyethylene
The Journal of Chemical Physics, 2002
Single-chain elasticity of polyethylene at θ point up to 90% of stretching with respect to its contour length is computed by Monte-Carlo simulation of an atomistic model in continuous space. The elasticity law together with the free-energy and the internal energy variations with stretching are found to be very well represented by the wormlike chain model up to 65% of the chain elongation, provided the persistence length is treated as a temperature dependent parameter. Beyond this value of elongation simple ideal chain models are not able to describe the Monte Carlo data in a thermodynamic consistent way. This study reinforces