A molecular insight into the nature of crystallographic mismatches in self-assembled fibrillar networks under non-isothermal crystallization conditions

Abstract

The lengths of the 12-hydroxystearic acid (12HSA) fibers are influenced by crystallographic mismatches resulting from the incorporation of 12HSA monomers into the crystal lattice in an imperfect manner. On a molecular level, this can be differentiated using synchrotron Fourier transform infrared (FTIR) spectroscopy by monitoring the change in area of the 1700 cm⁻¹ and 3200 cm⁻¹ peaks, corresponding, respectively, to the dimerization of the carboxylic acid groups and hydroxyl non-covalent interactions, during crystallization. The crystallographic mismatch is attributed to a plateau in the apparent rate constant for the dimerization of the carboxylic acid head groups while the hydroxyl interactions linearly increase as a function of cooling rate (ϕ). The rate constant for hydroxyl interactions linearly increases as a function of cooling rate while a plateau is observed for the rate of dimerization at cooling rates between 5 and 7 °C min⁻¹. At cooling rates greater than 5 to 7 °C min⁻¹ 12HSA monomers do not effectively dimerize before being incorporated into the crystal lattice causing crystal imperfections impeding linear epitaxial crystal growth and produces branched fibers. At slow cooling rates (i.e., less than 5 to 7 °C min⁻¹), long fibers are produced with a fractal dimension between 0.95 and 1.05 and for rapid cooling rates (i.e., greater than 5 to 7 °C min⁻¹) short branched fibers are produced with a fractal dimension between 1.15 and 1.32.