Plasticizing polymers with small-molecule additives: a not so simple picture revealed by a simple molecular model
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Processing and manufacturing of many polymer products require the addition of plasticizers for tuning the flow properties of the melt as well as the thermo-mechanical properties of the product (lower Tg and improved material flexibility and ductility). These effects are often collectively described as plasticizers reducing the inter-chain friction and ”softening” the material. However, a detailed molecular mechanism has not been fully revealed. We use molecular simulation to show that these effects are often not monotonically correlated. Most notably, additives that are better at reducing Tg may not be as effective at reducing the material stiffness. Indeed, our simulation shows that by simply changing the size of plasticizers, the Tg and Young’s modulus vary in opposite directions. In-depth analysis of the free volume distribution and molecular mobility indicates that plasticizers have different effects on the dynamics at different length and time scales. Ongoing research focuses on the plasticizer effects on polymer viscoelasticity. Findings of this study reveal the rich complexity of the plasticization phenomena. In particular, the notion of ”plasticization” is indeed a collection of a wide range of physical phenomena that are only loosely correlated at best.