Abstract:
Copyright © 2020 American Chemical Society. Homogeneous crystal nucleation in polyamide 66 (PA 66) was studied by a combination of atomic-force microscopy (AFM) and fast-scanning chip calorimetry (FSC), with a specific experimental setup allowing precise correlation of AFM images and FSC heating curves. PA 66 was subjected to Tammann's two-stage crystal nuclei development method, including quenching the relaxed melt to the nuclei-formation temperature of 310 K, annealing for different times to allow crystal nucleation, supplemented by an analysis of nuclei growth to crystals by heating the sample. With the number of crystals/nuclei detected by AFM imaging and by the variation of the nucleation time, a steady-state nucleation rate of (2.3 ± 0.2) × 1018 m-3 s-1 (2.3 μm-3 s-1) at 310 K has been determined. Comparing the total enthalpy of melting of the crystals, obtained by FSC, with the number of crystals, observed by AFM, yields the enthalpy of melting/formation of one single crystal of (5.2 ± 0.5) × 10-13 J at the specific growth conditions applied. Application of this approach, that is, correlating transition enthalpies with nuclei numbers, for analysis of crystal nucleation in a wide range of temperatures between 310 and 375 K yielded a maximum homogeneous nucleation rate close to 1020 m-3 s-1 (100 μm-3 s-1) at around 350 K. The present study offers a qualitatively new approach of analyzing the kinetics of homogeneous nucleation of polymers. In addition, it suggests employing the specific AFM-FSC setup as a valuable tool for direct observation of crystal-specific enthalpies of crystallization.
