**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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