Comparing Surface and Bulk Curing Processes of an Epoxy Vitrimer

Abstract

We used atomic force microscopy-based infrared spectroscopy (AFM-IR) and nanomechanical mapping (AFM-NM) to image the surface of a vitrimer, specifically dicarboxylic acid-cured diglycidyl ether of bisphenol A (DGEBA), to assess the curing process of a surface layer and compared this to the process in the bulk. We identified the β-hydroxy esters with various functionalities that are the key to form the cross-links for a system, including difunctional DGEBA and carboxylic acids. The IR peaks of the carbonyl group in generated ester groups are distinguished clearly from those in acids, allowing us to quantitatively assess the curing process at the surface and in the bulk. The initial curing at the surface exhibits a gradual cross-linking and is found to be lower than a rapid cross-linking in the bulk due to a relatively lower concentration of the β-hydroxy esters with high functionalities. This curing process leads to a smaller chemically and mechanically heterogeneous nanostructure at the surface relative to the bulk. After multiple reprocessings, a substantial number of esters lacking dynamic exchange capability form in the bulk, which decrease the flowability and reprocessability of the vitrimers and therefore the mechanical properties.

One-Pot Preparation of Double-Network Epoxy Vitrimers with High Performance, Recyclability, and Two-Stage Stress Relaxation

“…Siloxane and disulfide bonds have been widely used to fabricate elastomers and self-healing polymeric materials. , The exchange reactions of siloxane bonds generally occur in the presence of basic catalysts, such as 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD). TBD promotes the exchange reactions of siloxane bonds by binding to hydroxyl groups and facilitating their transfer, thus enabling reversible bond exchange between TBD and the cross-linkers. ,, In contrast, the exchange reactions of disulfide covalent bonds do not require a catalyst due to their inherently weak nature, allowing these bonds to undergo reversible exchange reactions even under mild conditions. ,, Following our previous studies in developing vitrimers and understanding their surface and interface properties, − in this work, we fabricated a novel double-network epoxy vitrimer using two distinct amine curing agents, and each contains a dynamic covalent bond: an aliphatic amine with a siloxane bond and an aromatic amine with a disulfide bond. When a controlled two-stage curing process is employed, the markedly different reactivities of aliphatic amine and aromatic amine with epoxy allow for sequential cross-linked network formation, thereby leading to the development of a double-network.…”