Composition processing property relationship of vitrimers Based on polyethyleneimine

The composition–processing–property relationship of imine containing reversible covalent bond containing polymers (RCBPs) was studied using an innovative (regarding this type of polymer) combination of characterization methods. This combination led, for the first time, to a classification of the various chemical reactions occurring during the processing of imine based RCBP, affecting their final chemical structure and properties. RCBPs, based on PIs (polyimines), were synthesized from polyethyleneimine (PEI) and terephthalaldehyde (TPA) in the presence of ethanol. The chemical composition and derived thermal and mechanical properties were investigated. Material characterization using ATR-IR, 13C-solid state NMR, swell tests, and a novel illustrative model based on the Flory–Rehner equation led to the conclusion that at least 80% of the secondary amine groups reacted to form a cross-linking network, along with 100% of the primary amines, and that during 20 min compression molding at 220 °C under 2.7 MPa, a post-curing reaction occurs. 13C-SSNMR and ATR-IR indicated that the chemical structure is determined not only by the reactants but also by the elevated temperature processing conditions. During heating, three mechanisms affect the final composition–transimination, reduction (back into amine groups) and oxidation (to oxaziridine and amide groups which are almost negligible). The reduction/oxidation processes were found to dominate the mechanical properties. The vitrification temperature (Tv), which is characteristic of RCBPs, is dependent on the thermal history, with a higher Tv resulting from longer compression molding times. The mechanical properties are affected by the processing temperatures, changing from high-performance to very low strength. Under specific processing conditions, the imine cross-linking bonds are reduced/oxidized, which causes the formation of more polar but lower energy bond cross-linking, resulting in lower tensile strength and a change in the hydrophobicity of the polymer, which may be used in future applications.