Abstract :
[en] Covalently crosslinked polymer systems, otherwise known as thermosets, are well suited to many applications within the field of polymer science. This is due to their mechanical and thermal integrity, frequently optimal optical performance, and capacity to be utilized in a diverse range of high-tech and biomedical applications. The thermosetting class of covalently cross-linked polymers represents an important and versatile category of materials, employed in a wide range of applications including coatings, adhesives, biomedical materials, and structural composites. The presence of covalent crosslinks within the material results in a predominantly elastic response to strain deformations. These materials are not typically reconfigurable, recyclable, or reprocessable, and they possess a constrained capacity to modify their stress state, topology, or structure
permanently. A novel approach to crosslinked polymers has recently emerged, namely that of covalent adaptable networks (CANs). This involves the formation of covalently crosslinked networks in which triggerable, reversible chemical structures persist throughout the network. Several factors, such as changes in temperature can trigger these reversible covalent bonds. Upon the application of the aforementioned stimulus, instead of a transient alteration in shape, the CANstructure responds by undergoing a permanent adjustment in its structure. The result is the material reequilibrating to its new state and condition. In this thesis, the potential of benzoxazine networks as a foundational element in the CAN formulation was investigated.
Polybenzoxazines represent one of the few new polymers that have been successfully com
mercialized over the past four decades. Polybenzoxazines exhibit excellent stiffness, low
flammability, and high-temperature performance, coupled with minimal impact from moisture, chemicals, and other corrosive materials. This renders them ideal for utilization in extreme environments. Although research has concentrated on the creation of benzoxazine resins with enhanced flexibility, ductility, and elevated glass transition temperature (Tg), few studies have focused on the production of benzoxazine CANs. However, these studies do not examine the structural/relaxation impact of these CANs. Moreover, none of the aforementioned studies have conclusively demonstrated the intrinsic vitrimeric character of polybenzoxazine systems.
Two distinct approaches have been pursued. Firstly, an investigation was conducted into
the influence of the structure of a transesterified CAN benzoxazine, derived from a bio-based reference molecule obtained from phloretic acid and ethylene glycol. Subsequently, the thermal and thermomechanical consequences of each structural alteration were examined, and the progression of its relaxation was monitored. The results obtained demonstrated that the presence of phenol groups alone ensured transesterification, and that the reaction rate was not influenced by the presence of additional alcohol functions. Furthermore, the presence of additional ester functions or their orientations exerts negligible influence on the transesterification rate.
Secondly, a new generation of bio-based benzoxazine CANs was synthesized without any
additional moieties. The dynamic nature of the N,O acetal bond obtained after polymerization of benzoxazine precursors was demonstrated by a stress relaxation test. In addition, the study confirmed the ability of polybenzoxazine to be reprocessed. These novel polybenzoxazine CANs, synthesized with the assistance of tyrosol and a long aliphatic diamine, have been utilized in the fabrication of high-performance natural composites and anti-corrosion coatings. Tests were carried out on natural composites, which demonstrated good affinity between the fiber and the resin. This resulted in mechanical properties that were superior to those of composites
manufactured from alternative natural fibers. Concurrently, the protective properties of coatings applied to magnesium alloys were demonstrated to be effective against a saline environment, with these properties being maintained after 35 days of exposure.
