Effects of pressure on structure and dynamics of model elastomers: A molecular dynamics study

J Liu and SZ Wu and DP Cao and LQ Zhang, JOURNAL OF CHEMICAL PHYSICS, 129, 154905 (2008).

DOI: 10.1063/1.2996009

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 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. (c) 2008 American Institute of Physics. DOI: 10.1063/1.2996009

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