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ABSTRACT: In this research, molecular dynamics (MD) simulations were used to study the transport properties of small gas molecules in
the butadiene-styrene copolymer (SBR). The condensed-phase optimized molecular potentials for atomistic simulation studies
(COMPASS) force field was applied. The diffusion coefficients were obtained from MD (NVT ensemble) and the relationship between
gas permeability; the chemical structure and free volume of butadiene-styrene copolymer were investigated. The results indicated
that the diffusion coefficient of oxygen declined with increasing styrene content. The fraction of free volume (FFV) in butadiene-styrene
copolymer was calculated. It was concluded that diffusion coefficient increased as the FFV increases, which is in accordance
with the analysis of the small molecular hop through the free volume in polymer matrix. Subsequently, the glass transition
temperatures of these copolymers were calculated by MD. The result showed that the glass transition temperature increased
with increasing styrene content in polymer.
Keywordsmolecular dynamics-SBR-diffusion coefficient-fraction of free volume-glass transition temperature
Frontiers of Chemical Engineering in China 04/2012; 4(3):257-262.
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ABSTRACT: By employing an idealized model of a polymer network and filler, we have investigated the stress-strain behavior by tuning the filler loading and polymer-filler interaction in a broad range. The simulated results indicate that there actually exists an optimal filler volume fraction (between 23% and 32%) for elastomer reinforcement with attractive polymer-filler interaction. To realize this reinforcement, the rubber-filler interaction should be slightly stronger than the rubber-rubber interaction, while excessive chemical couplings are harmful to mechanical properties. Meanwhile, our simulated results qualitatively reproduce the experimental data of Bokobza. By introducing enough chemical coupling between the rubber and the filler, an upturn in the modulus at large deformation is observed in the Mooney-Rivlin plot, attributed to the limited chain extensibility at large deformation. Particularly, the filler dispersion state in the polymer networks is also characterized in detail. It is the first demonstration via simulation that the reinforcement mechanism stems from the nanoparticle-induced chain alignment and orientation, as well as the limited extensibility of chain bridges formed between neighboring nanoparticles at large deformation. The former is influenced by the filler amount, filler size and filler-rubber interaction, and the latter becomes more obvious by strengthening the physical and chemical interactions between the rubber and the filler. Remarkably, the reason for no obvious reinforcing effect in filled glassy or semi-crystalline matrices is also demonstrated. It is expected that this preliminary study of nanoparticle-induced mechanical reinforcement will provide a solid basis for further insightful investigation of polymer reinforcement.
Physical Chemistry Chemical Physics 11/2010; 13(2):518-29. · 3.57 Impact Factor
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ABSTRACT: In this study, the conductive silicone rubber composites filled with nickel-coated graphite (NCG) have been prepared, and their morphology structure, electrical conductivity, electromagnetic interference shielding efficiency (EMI SE), and mechanical properties have been investigated with reference to the NCG filler loading. The mechanical strength of NCG particle was poor that it can be easily ground into smaller particle during the mixing process if the shear force during mixing is large enough. The electrical conductivity of the composites existed an obvious threshold value with the variation of the loading amount of the conductive filler. EMI SE of the composites increases with the decrease of the volume electrical resistivity. The Payne effect can be used to characterize the intensity of the three-dimensional conductive network structure in silicone rubber matrix, and the difference of storage modulus in the low and high shear strain has good linear correlation with the electrical conductivity. So, the electrical conductivity and EMI SE can be estimated by means of the difference of storage modulus obtained from rubber process analysis test. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010
Journal of Applied Polymer Science 03/2010; 115(5):2710 - 2717. · 1.29 Impact Factor
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ABSTRACT: Nano-strengthening by employing nanoparticles is necessary for high-efficiency strengthening of elastomers, which has already been validated by numerous researches and industrial applications, but the underlying mechanism is still an open challenge. In this work, we mainly focus our attention on studying the variation of the tensile strength of nanofilled elastomers by gradually increasing the filler content, within a low loading range. Interestingly, the percolation phenomenon is observed in the relationship between the tensile strength and the filler loading, which shares some similarities with the percolation phenomenon occurring in rubber toughened plastics. That is, as the loading of nanofillers (carbon black, zinc oxide) increases, the tensile strength of rubber nanocomposites (SBR, EPDM) increases slowly at first, then increases abruptly and finally levels off. Meanwhile, the bigger the particle size, the higher the filler content at the percolation point, and the lower the corresponding tensile strength of rubber nanocomposites. The concept of a critical particle-particle distance (CPD) is proposed to explain the observed percolation phenomenon. It is suggested that rubber strengthening through nanoparticles is attributed to the formation of stretched straight polymer chains between neighbor particles, induced by the slippage of adsorbed polymer chains on the filler surface during tension. Meanwhile, the factors to govern this CPD and the critical minimum particle size (CMPS) figured out in this work are both discussed and analyzed in detail. Within the framework of this percolation phenomenon, this paper also clearly answers two important and intriguing issues: (1) why is it necessary and essential to strengthen elastomers through nanofillers; (2) why does it need enough loading of nanofillers to effectively strengthen elastomers. Moreover, on the basis of the percolation phenomenon, we give out some guidance for reinforcement design of rubbery materials: the interfacial interactions between rubber and fillers cannot be complete chemical bonding, and partial physical absorption of macromolecular chains on the filler surface is necessary, otherwise the formation of stretched straight chains would be seriously hindered. There should exist such an optimum crosslinking density for a certain filler reinforced rubber system, and as well an optimum filler loading for rubber strengthening. Additionally, the different percolation behaviors of Young's modulus, the tensile strength and the electrical conductivity are compared and analyzed in our work. Lastly, molecular simulation indicates that it is not possible to strengthen glassy or hard polymer matrices by incorporating spherical nanoparticles. In general, by providing substantial experimental data and detailed analyses, this work is believed to promote the fundamental understanding of rubber reinforcement, as well provide better guidance for the design of high-performance and multi-functional rubber nanocomposites.
Physical Chemistry Chemical Physics 03/2010; 12(12):3014-30. · 3.57 Impact Factor
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ABSTRACT: On the basis of an idealized model of an elastomer, we use molecular dynamics simulations to explore the effects of pressure on the glass transition, structure, and dynamics of the model elastomer. The simulated results indicate that with the pressure increasing, the glass transition temperature T(g) increases while the glass transition strength decreases, which is in accordance with the experimental result from Colucci et al. [J. Polym. Sci., B: Polym. Phys. 35, 1561 (1997)] For the structure of the elastomer, it is found that the intramolecular packing remains nearly unchanged over the pressure range studied, also validated by the independence of the chain size and shape on the pressure, while the intermolecular distribution exhibits a more efficient packing effect at high pressures. By analyzing the end-to-end vector correlation and incoherent intermediate dynamic structure factor, which are well fitted by a stretched exponential Kohlrauch-William-Watts (KWW) function, we observe that the time-pressure superposition principle (TPSP) takes effect at the chain length scale, while at the segmental length scale the TPSP does not completely hold, attributed to the enhanced dynamic heterogeneity with the pressure increasing, which is evidenced by the beta values in stretched exponential fitting over the pressure range studied. Extracting the characteristic relaxation time from the KWW function, and then plotting the logarithm of the characteristic relaxation time versus the pressure, we observe a good linear relationship and find that the pressure exerts nearly the same effect on the relaxation behavior at both the segmental and chain length scales. This point is further validated by almost the same dependence of the alpha-relaxation time for three representative q wave vectors, indicating that the segmental and chain relaxations of the elastomer are influenced similarly by the pressure variation and the same physical processes are responsible for relaxation at the probed length scales. The calculated activation volume is independent of pressure at fixed temperature but increases with the temperature decreasing at fixed pressure. Finally, the pressure effect on the stress autocorrelation function is also examined, and a more difficult trend for stress relaxation and dissipation of the elastomer at high pressure is found. It is expected that all these simulated results would shed some light on the relevant experimental and theoretical studies.
The Journal of chemical physics 11/2008; 129(15):154905. · 3.09 Impact Factor
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ABSTRACT: Polyacrylonitrile-based carbon fibers, embedded with single-walled carbon nanotubes have been prepared by the electrospinning technique. The as-spun nanofibers were hot-stretched in an oven to enhance the orientation and crystallinity which has been confirmed by X-ray diffraction and DSC etc. With the hot-stretched process and the introduction of SWNTs, the mechanical properties of PAN nanofibers such as the modulus and tensile strength will be enhanced correspondingly. In addition, the electrical conductivities of the PAN/SWNTs nanofiber composites were also enhanced. It was concluded that the hot-stretched nanofibers and the PAN/SWNTs nanofiber composites can be used as a potential precursor to produce high-performance carbon nanocomposites.
Materials & Design.
