Average and Local T_g Shifts of Plasticized PVC from Simulations

The effects of plasticizers on the dynamics of polymer/plasticizer blends are practically important and theoretically interesting. Although it is well-known that plasticizers alter the glass transition of polymers, a microscopic view of the interplay between polymers and plasticizers and consequently the T_g shifts is not completely clear. In this work, we perform atomistic simulations on plasticized poly(vinyl chloride) (PVC) plasticized by varying concentrations of di(2-ethylhexyl) phthalate (DEHP). After validating the model by comparing simulated thermodynamic and mechanical properties with experimental values, we obtain both average and local T_g shifts from short-time segmental dynamics from data collapse. We find the average T_g determined from fast dynamics decreases substantially with increasing plasticizer concentration, consistent with volumetric T_g obtained from the temperature dependence of density. Furthermore, the local T_g reduction of PVC similarly obtained decays exponentially as a function of the distance from the nearest plasticizer molecule. Interestingly, the local T_g shifts seem to follow a simple additive rule, which can be described as the sum of the local T_g shifts in the dilute limit resulting from each plasticizer independently.