Balancing Melt Solubility and Morphology in Epitaxial Nucleation: The Case of Nicotinic Acid and Poly(hydroxybutyrate-co-hydroxyhexanoate)

Nicotinic acid was evaluated as a melt-soluble nucleator for a poly(hydroxy-butyrate-co-hydroxy-hexanoate) (PHBHHx) copolymer for melt-processing applications. A series of nicotinic acid concentrations (1-5 wt %) were analyzed to determine the best concentration for overall nucleation performance. 2 w/w% nicotinic acid was found to be the optimal concentration, successfully crystallizing PHBHHx at a peak temperature of 73 ? under nonisothermal conditions. When extrusion is performed at 150 ?, 2 w/w% of nicotinic acid occupies an optimal concentration within the polymer where all the nicotinic acid is dissolved in the PHBHHx matrix, which is not the case at higher concentrations. 2 w/w% also recrystallizes rapidly to produce many fine, needle-like crystals that are highly active toward PHBHHx nucleation, which is not observed with other concentrations. Powder X-ray diffraction (PXRD) analysis of the differing nicotinic acid crystals determined that the (110) and (120) faces are likely responsible for nucleation. Gel permeation chromatography (GPC) analysis revealed a modest degradation of molecular weight, likely due to the E1cb degradation mechanism common in PHAs.