Polar small molecule regulation of epoxy networks: Free volume filling, hydrogen bonding, and catalytic effects

Abstract

According to classical polymer physics, free volume is a critical factor influencing the mechanical behavior of epoxy-based coatings. In this study, three polar small molecules—Tetramethylene sulfone (TMS), Diphenyl sulfone (DS), and Succinimide (SI)—were incorporated into the epoxy network through molecular-level design to regulate the structure of network and enhance coating performance. By filling free volume and promoting hydrogen bonding, these additives improved both modulus and toughness of the coatings. The mechanisms underlying these effects were systematically elucidated using positron annihilation lifetime spectroscopy (PALS), molecular dynamics simulations, and DFT calculations. SI exhibited catalytic activity during the curing process, resulting in increased network heterogeneity, whereas TMS achieved an effective balance between strength and toughness, thereby improving impact resistance. The densified microstructure and strengthened intermolecular interactions imparted by polar small molecules endow the epoxy coatings with superior mechanical integrity under extreme mechanical environments. This work provides valuable insights into the structure–property relationship of epoxy coatings and offers a new strategy for designing high-performance protective coating systems.