An atomistic evaluation of the compatibility and plasticization efficacy of phthalates in poly(vinyl chloride)

Abstract

Using full-atom molecular simulation, we report the first systematic investigation of common phthalate plasticizers for PVC. A multistep model generation and equilibration protocol are proposed for amorphous polymer–plasticizer mixtures, from which statistically robust prediction of materials properties is achieved. Plasticizer performance is evaluated with our molecular models, which considers both their plasticization efficacy and thermodynamic compatibility with the host polymer. Effects of the alkyl side chain configuration in these phthalates are systematically discussed. The results agree well with all known experimental observations. In addition to the size of the alkyl chains, their branching configuration is another factor affecting the phthalate compatibility with PVC. Relaxation of the alkyl side chains is found to be the limiting step in the diffusion of phthalates in PVC, making it a key design parameter for better migration resistance. With the addition of plasticizers, the dynamics of PVC backbones remain the same in the short-time relaxation process, but an earlier onset of the cooperative motion between molecules allows it to enter the long-time diffusive regime earlier. The main outcomes of this study include (1) a molecular modeling protocol validated with commonly used phthalates, which can be used to predict the performance of alternative plasticizers, and (2) molecular insight that can better inform the molecular design of new plasticizers. As a side outcome, we also report a nontrivial chain-length dependence of the cohesive energy and solubility parameter of long-chain polymers, which is an important consideration in the calculation of these quantities using molecular simulation.