Exploring supramolecular self-assembly of a bisamide nucleating agent in polypropylene melt: The roles of hydrogen bond and molecular conformation

Nucleating agents (NA) is a kind of the most important additives in polymer industry. The phenomena of NA self-assembling/crystallizing into various supramolecular aggregations in the polymer melt and showing different nucleation effects have been widely reported. However, the process of the self-assembly formation, which is essential for understanding the exact nucleating mechanism of NAs, is not well elucidated. In this work, the self-assembly process of a bisamide NA, N, N'-dicyclohexylter-ephthalamide (DCTH), in polypropylene (iPP) matrix was systematically investigated. The Fourier transform infrared spectroscopy (FTIR) results reveal that the self-assembly of DCTH is related to the dissociation/association of hydrogen bonds and the conformational change of molecules. Upon heating, the hydrogen-bond dissociation generates the free DCTH molecules in iPP melt and provides space for the conformational rotation of cyclohexyl groups toward co-plane direction with the aromatic core plane; while upon cooling, the hydrogen-bond association triggers the self-assembly/recrystallization of free DCTH molecules into supramolecular aggregations and the conformational rotation makes hydrogen bonds stronger. By exploring the microenvironments of individual chemical groups of DCTH and the interactions between DCTH and iPP matrix, we demonstrate the physical mechanism of how the NA self-assembles into supramolecular crystals at molecular level. (C) 2016 Elsevier Ltd. All rights reserved.