Mechanistic insights into ortho-blocked and ortho-free vitrimeric polybenzoxazines incorporating dynamic Schiff linkages for closed-loop recyclability

Abstract

Vitrimers are an important breakthrough in the field of sustainable materials, particularly within the context of plastic waste management. These polymers can be designed with reversible linkages, which allows them to undergo bond exchange processes without compromising the integrity of the material. Hence, comprehending the correlation between the phenolic precursor used in benzoxazine monomer synthesis and subsequent vitrimeric properties obtained in polybenzoxazine networks can aid us in maximizing the potential of these reusable networks. This work presents an investigation using an ortho-blocked phenol (vanillin, 4-hydroxy-3-methoxybenzaldehyde) and an ortho-free phenol (4-hydroxybenzaldehyde) as aldehyde precursors, together with p-phenylenediamine as an amine precursor, facilitating the formation of dynamic imine networks. Before the formation of the imine network, phenolic groups were utilized for the formation of benzoxazines using stearylamine. The imine-containing benzoxazine monomers were subsequently subjected to further heating to induce polymerization. The inclusion of a methoxy group in a vanillin-based polybenzoxazine (PVBI) results in a reduced crosslinking density of 414 kJ mol⁻¹. In contrast, the polybenzoxazine (PHBI) synthesized using 4-hydroxybenzaldehyde exhibited a significantly higher crosslinking density (4842 kJ mol⁻¹). At a temperature of 25 °C, the storage modulus of PVBI was 115 ± 5 MPa, while the storage modulus of PHBI was 356 ± 5 MPa. The rheological properties demonstrate that a decrease in crosslinking density leads to a substantial reduction in relaxation time. This is because a lower crosslinking density allows higher chain mobility, facilitating exchange kinetics. Due to the topological reconfiguration of the structure induced by the exchange reaction between imine bonds, PVBI demonstrated remarkable malleability, having a stress relaxation time of less than 4 seconds, while PHBI had a stress relaxation time of 200 seconds at 120 °C. The activation energy determined from stress relaxation plots was 51 kJ mol⁻¹ for PVBI and 68 kJ mol⁻¹ for PHBI. The reprocessing studies conducted under hot-pressing showed that the mechanical properties, such as storage modulus and T_g, remained consistent even after three remolding cycles. Furthermore, incorporating imine bonds in the polybenzoxazine matrix facilitated a closed-loop recycling procedure where the material can be broken down into smaller fragments and then reassembled. After conducting recycling investigations, it was shown that 85% of the storage modulus was preserved in PVBI vitrimeric samples, indicating that these samples exhibit a significant degree of recyclability within a closed-loop system. The reprocessed and chemically recycled vitrimeric materials demonstrate significant preservation of thermal and mechanical properties.