How do plasticizers affect the morphological features of polymers, and which technique is most suitable for observing these changes at a microscale?

Plasticizers are small molecules that slip between polymer chains, making them more flexible and reducing the glass‑transition temperature; this loosening of the chains often turns a rigid, crystalline material into a softer, more amorphous one. Because the chains can move more freely, the polymer’s surface may become smoother and the internal phase boundaries can blur, creating larger, less defined domains. To see these subtle changes, optical or scanning electron microscopy (SEM) can’t resolve the chain packing, so atomic force microscopy (AFM) or transmission electron microscopy (TEM) are the best choices; TEM, in particular, can show the reduced crystallite size and increased amorphous regions at the nanoscale. For example, adding a plasticizer to a poly(vinyl chloride) sample will shrink its crystalline lamellae, and a TEM image will reveal thinner, more diffuse lamellae compared to the untreated sample. Thus, plasticizers soften polymers by loosening chain packing, and TEM is the most suitable technique to observe these microscale morphological changes.