Application of Tammann’s Two-Stage Crystal Nuclei Development Method for Analysis of the Thermal Stability of Homogeneous Crystal Nuclei of Poly(ethylene terephthalate)

Tammann’s two-stage crystal nuclei development method has been applied to gain information about the kinetics of formation and the stability of homogeneous crystal nuclei in poly­(ethylene terephthalate). The method requires rapid cooling of the relaxed melt to different nucleation temperatures close to the glass transition, in order to allow isothermal formation of homogeneous nuclei. Nuclei formation is then probed by fast heating of the system after predefined nucleation times to the growth-stage temperature where, according to the classical nucleation theory, nuclei grow to crystals connected with a measurable change of latent heat. For polymers, this picture does not hold. Homogeneous nuclei melt during the transfer from the temperature of their formation to the growth temperature as was proven by analysis of the melting behavior of even more stable crystals grown at the nucleation temperature. Though melting on heating, it is proposed that remnants of both nuclei and crystals, due to incomplete dissolution, allow very rapid reestablishing of slightly larger and more stable nuclei. This way, similar to the melting–recrystallization–remelting mechanism of polymer crystals of low stability on heating, the initially formed nuclei keep their overcritical size up to temperatures 100 °C, or even higher, above the nucleation temperature by continuous increase of their size.