Fractional Coupling of Primary and Johari-Goldstein Relaxations in a Model Polymer

A polymer model exhibiting heterogeneous Johari-Goldstein (JG) secondary relaxation is studied by extensive molecular-dynamics simulations of states with different temperature and pressure. Time-temperature-pressure superposition of the primary (segmental) relaxation is evidenced. The time scales of the primary and the JG relaxations are found to be highly correlated according to a power law. The finding agrees with key predictions of the Coupling Model (CM) accounting for the decay in a correlation function due to the relaxation and diffusion of interacting systems. Nonetheless, the exponent of the power law, even if it is found in the range predicted by CM (0 < 1), deviates from the expected one. It is suggested that the deviation could depend on the particular relaxation process involved in the correlation function and the heterogeneity of the JG process.

Automated summary

Extensive molecular-dynamics simulations were used to study a polymer model exhibiting heterogeneous Johari-Goldstein secondary relaxation. The time-temperature-pressure superposition of the primary relaxation was confirmed. The time scales of the primary and JG relaxations were found to be highly correlated, in agreement with the predictions of the Coupling Model. However, the exponent of the power law deviated from the expected value, potentially due to the specific relaxation process and the heterogeneity of the JG process.