Experimental validation of the modified Avrami model for non-isothermal crystallization conditions

Abstract

The modified Avrami model was found to accurately predict the induction time, maximum phase volume and dimensionality of crystal growth for stearic acid containing molecules when the experimental method employed measures as a function of phase volume. Four methods were examined to validate the model including: Fourier transformed infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), small deformation rheology and polarized light microscopy (PLM). PLM and FT-IR were able to detect the nucleation event prior to DSC and rheology. FT-IR and PLM provided the most accurate data due to the similarities between the experimental and fitted induction times (x₀), maximal phase change (y_max) and the Avrami exponent (n). Further, the Avrami exponent, obtained from FT-IR, was sensitive to both the mode of nucleation and the dimensionality of crystal growth. Therefore, the apparent rate constants (k_app) obtained by FT-IR and PLM are useful in providing further insights into the kinetics of non-isothermal crystallization. The calculated apparent rate constants suggest a diffusion limited crystallization at slow cooling rates (i.e., below 5–7 °C min⁻¹) and at cooling rates greater than 5–7 °C min⁻¹, the incorporation of the gelator molecules onto the crystal lattice becomes limited by the reaction rate constant.