How do the addition of plasticizers influence phase separation in polymer blends, specifically regarding their mechanical properties?

Plasticizers are small molecules that fit between polymer chains, lowering the glass‑transition temperature and making the chains more mobile. This increased mobility reduces the interfacial tension between the components of a blend, so the polymers stay more mixed and the phase‑separated domains become smaller or disappear. As a result, the blend behaves more uniformly, giving it a softer, more flexible feel while still maintaining some toughness. For example, adding a plasticizer to a PVC–rubber blend keeps the rubber droplets from coalescing, so the material becomes pliable without losing its ability to resist cracking. Thus, plasticizers improve mechanical properties by suppressing large‑scale phase separation and promoting a finer, more stable morphology.