Toward a More Comprehensive Understanding on the Structure Evolution and Assembly Formation of a Bisamide Nucleating Agent in Polypropylene Melt

Understanding the "real" structure of a nucleating agent (NA) inducing polymer crystallization is the basis for establishing the structure–nucleation performance relationship. N,N′-Dicyclohexylterephthalamide (DCHT) is an important β-NA for isotactic polypropylene (iPP) and widely reported to form granular, needle, and dendritic assemblies under different conditions. However, the factors determining the structure evolution and the molecular mechanism of assembly formation are still not very clear. In this work, the formation of diverse DCHT assemblies in the iPP melt at different concentrations was systematically investigated, and a possible mechanism was proposed, where the ultimate morphology of NA may be determined by its dissociation state, local molecule distribution, and diffusion ability in the matrix. When the majority of NAs remained in the hydrogen bond-associated state at the final heating temperature, DCHT retained the granular morphology upon cooling. When the minority of NAs existed in the associated form and most of them dissociated into "free" molecules after heating, DCHT molecules with strong diffusion ability could be rapidly reassembled at high temperature attracted by hydrogen bonds, forming needle structures, and the length of needles became longer with decreasing relative amount of associated NAs in total. When DCHT completely dissociated upon heating, the intermolecular hydrogen bonds could be reformed when cooling to a lower temperature, connecting molecules to form dendritic structures affected by relatively weak diffusion ability, and the branching degree may be dependent on the distribution uniformity of "free" molecules.