Average and Local Tg Shifts in Plasticized PVC from Simulations

The effects of small molecules on the dynamics of polymer/plasticizer blends are of high engineering importance and theoretical interests. Although it is well known that plasticisers can alter the glass transition of polymers, the interplay between polymers and plasticizers on the molecular scale and the resulting Tg shifts are not fully understood. In this work, we perform simulations on di-(2-ethylhexyl) phthalate (DEHP) plasticized polyvinyl chloride (PVC) using OPLS-AA force field. After validating the model by comparing simulated thermodynamic and mechanical properties with experimental values, we obtain both average and local Tg shifts from short-time segmental dynamics by collapsing the data. We find the average Tg determined from fast dynamics decreases substantially with increasing plasticizer concentration, consistent with volumetric Tg referred from the temperature dependence of density. Moreover, the local Tg reduction of PVC decays exponentially as a function of the distance from the nearest DEHP. Interestingly, for all the concentrations studied, the local Tg reduction can be described by a simple additive rule—it is approximately the sum of the distance-dependent local Tg shifts arising from surrounding plasticizers in the dilute limit.