Multiple character of non-monotonic size-dependence for relaxation dynamics in polymer-particle and binary mixtures
E. Mahmoudinezhad Zirdehi, T. Voigtmann, F. Varnik.
Adding plasticizers is a well-known procedure to reduce the glass transition temperature in polymers. It has been recently shown that this effect shows a non-monotonic dependence on the size of additive molecules [The Journal of Chemical Physics 150 (2019) 024903]. In this work, we demonstrate that, as the size of the additive molecules is changed at fixed concentration, multiple extrema emerge in the dependence of the system's relaxation time on the size ratio. The effect occurs on all relevant length scales including single monomer dynamics, decay of Rouse modes and relaxation of the chain's end-to-end vector. A qualitatively similar trend is found within mode-coupling theoretical results for a binary hard-sphere (HS) mixture. An interpretation of the effect in terms of local packing efficiency and coupling between the dynamics of minority and majority species is provided.
The end-to-end autocorrelation function of polymer chains versus time for the present FENE-polymer model containing a number concentration of 20% additive molecules. Different curves correspond to different size ratios delta. The data show a clear signature of a non-monotonic dependence of the relaxation rate on delta. The inset highlights this fact by showing the corresponding relaxation time versus delta.