Effect of Plasticizers on Crystallization of Polyesters and Mesomorphic Phase Formation in Plasticized Poly(Trimethylene Terephthalate)

This study examines the impact of two structurally distinct plasticizers, poly(ethylene glycol) dimethyl ether (P) (linear) and dioctyl phthalate (D) (cyclic), on the crystallization of polyesters (PEN, PET, PTT, and PBT). The Continuous Cooling Transformation (CCT) method is used to simulate real processing conditions. The solidification curve (density versus cooling rate) shows that PEN and PET are slow-crystallizing polymers, crystallizing at low cooling rates of 0.1 K/s and 2 K/s, respectively. Adding 5 wt.% of P and D plasticizers to PEN and PET shifted the density drop from 0.1 K/s to around 7 K/s and from 2 K/s to 40 K/s, respectively. In P-plasticized (5 wt.%) PTT, the crystalline-to-amorphous transition occurred at a higher cooling rate (60 K/s), which is ten times higher than that of pure PTT (∼6 K/s). Unexpectedly, in P-plasticized PTT, the amorphous density plateau was significantly higher than expected, indicating the presence of a mesomorphic phase even at higher cooling rates of 1000 K/s. For PBT, a faster-crystallizing polymer, the plasticizing effect was minimal, demonstrating a threshold behavior for plasticization as we transition from a slow-crystallizing, stiffer material to a faster-crystallizing, softer one.