Glass transition analysis of model metallosupramolecular polyesters bearing pendant pyridine ligands with a controlled ligand–ligand distance

Abstract

Supramolecular materials formed via metal-ligand coordination, so-called metallosupramolecular materials, have attracted attention due to their ability to modify a broad range of physical properties depending on the association strength at the coordination bond. Here, we demonstrate the correlation between the glass transition properties and the ligand distance by utilizing amorphous polyesters (denoted as PE-Py) with pendant pyridine ligands arranged homogeneously along the chain. The ligand distance is treated as the pyridine group equivalent molecular weight (MPy) and takes values of 900, 750, 550, and 420. The metal salt ZnCl2 forms coordination bonds with the pyridine ligands, generating a metallosupramolecular network. The fraction of coordinated pyridine is finely tuned by the mole ratio between the pyridine ligand and ZnCl2 ([Zn2+]/[Py]) while adhering to the stoichiometric rule. DSC measurements reveal that the variation in the glass transition temperature (Tg) with increasing [Zn2+]/[Py] is closely correlated with 1/MPy. Temperature-ramp rheological measurements are also performed, revealing that the apparent activation energy (Ea) of segmental motion has a quasi-linear relationship with 1/MPy. Thus, the present study demonstrates that the degree of restriction for segmental motion is very systematically strengthened as the ligand distance decreases, highlighting the importance of ligand distance in physical property control of metallosupramolecular materials. We here demonstrate the importance of ligand–ligand distance on physical property tuning of metallosupramolecular materials by utilizing model-like polyesters bearing pyridine ligands at the side groups with a controlled ligand-distance. The ligand distance is treated as the pyridine group equivalent molecular weight (MPy) and takes values of 900, 750, 550, and 420. The metal salt ZnCl2 forms coordination bonds with the pyridine ligands, generating a metallosupramolecular network. DSC and rheological measurements revealed that the glass transition temperatures and apparent activation energy (Ea) of segmental motion changes in a close correlation with 1/MPy.