Advanced evaluation methods of isothermal crystallization kinetics with consideration of incubation time and changes in the crystallization mechanism

• Influence of incubation and transient period on crystallization kinetics. • Discussion the limits of the classical KJMA evaluation. • The time-independent kinetic equation for measurement of the incubation time. • Introduction of the derivative evaluation method to avoid entropy errors. The study of crystallization using various scanning calorimetry techniques is often used to investigate the solidification processes of melts or the crystallization behavior of glassy materials. The classic Kolmogorov-Johnson-Mehl-Avrami (KJMA) approach is usually applied for this purpose. Conventional evaluation methods using the linearized KJMA equation or direct fitting can in some cases lead to larger errors. These include crystallization processes with unknown incubation times, rapid crystallization with greater uncertainties regarding crystallinity and time due to transition effects, overlapping crystallization processes, crystallization with changing mechanisms, or incomplete measurements. In such cases, alternative evaluation methods such as the time-independent and derivative methods are more robust. These methods are presented here and discussed using the example of unfilled polypropylene (PP) and PP highly filled with carbon nanotubes (CNT). The measurements were performed using conventional differential scanning calorimeter (DSC). As expected, PP exhibits heterogeneous nucleated three-dimensional growth of the crystalline superstructure in the temperature range investigated. The CNT-filled PP exhibits heterogeneous, nucleated two-dimensional growth in the early stage of crystallization, which corresponds to the formation of a shish-kebab structure. Interestingly, the filled material shows a longer incubation time, which is probably caused by the formation of equidistant nuclei on the surface of the nanotubes.