adding FeCl3 into polybenzoxazines. In this approach, the molecular design concept utilizes two different attractions for self-healing ability such as strong inter- and intra-molecular hydrogen bonding and metal-ligand interactions of Fe3+–phenolic oxygen–tertiary amine ligation. Accordingly, self-standing films of Poly(Si-Bz)/FeCl3 were prepared at low curing temperatures and by this way inner hydrogen-bonding interactions were not disturbed and utilized effectively for self-healing applications. The healing of these films were tested by cutting them into two pieces and then cut edges were firmly placed to maintain efficient contact with applying slight pressure by using glass slides. The film was self-healed after 24 h of contact, and it maintained its flexibility after self-repairing as seen in the visual image (Figure 2.7). The healing efficiency of cut-healed sample was calculated by tensile measurements as ca. 61%.
Interestingly, Poly(Si-Bz)/FeCl3 films also exhibited shape recovery (SR) property besides self-healing ability, which is another important feature of smart polymers. In order to demonstrate the shape memory property, curled or spiral shapes were given to these films by using a pin or fastener and then heated at ca. 90°C in water. The heated films cooled to room temperature rapidly to fix curled/spiral shapes. These samples were unwrapped by hand and deformation recovered rapidly after removal of force and the films returned back to their fixed shapes in 5−7 s (Figure 2.8).
Similar experiments were performed for the films without containing Fe3+ salt and SMP behavior could not be observed. Therefore, in the light of blank experiments, it was concluded that SMP behavior of Poly(Si-Bz)/FeCl3 films may stem from dynamic metal ligand interactions between Fe3+ and oxygen/nitrogen atoms polybenzoxazine chain (Scheme 2.13). Possibly, the metal complex exhibits reversible weak bonding at 90–100 °C, but during rapid cooling to room temperature the given shape fixes due to reformation of stronger metal−ligand attractions. Generally, in SMP systems hard and soft segments are required and besides dynamic metal−ligand binding, polysiloxane chains act as soft segments and polybenzoxazine provides the hard segment. Accordingly, it can be deduced that polysiloxane chains and metal−ligand interactions form the “switch” and hard polybenzoxazine chains generate the “fix points” in this SMP.
Figure 2.7 The images of cured Poly(Si-Bz)/FeCl3 film after healing. Arrow indicates the contact edges.
Figure 2.8 Images of shape recovery behavior of cured Poly(Si-Bz)/FeCl3 film.
Scheme 2.13 Metal–ligand bond breakage and reforming at certain temperature changes.
2.4 Conclusion
In this chapter, self-healing and shape memory systems based on benzoxazines are reviewed. The results demonstrate that it is possible design various different smart materials by using benzoxazines. In general, polybenzoxazines with soft segments such as poly(propylene oxide)s (Jeffamines) can be exhibit self-healing ability with different levels of success. The healing mechanisms can be autonomous or stimuli induced types depending on the design of polybenzoxazines. Especially, in terms of autonomous healing systems, the hydrogen bonds of polybenzoxazines can be exploited. However, from the practical point of view, the hydrogen bonding in polybenzoxazines must augmented for successful results by additives or designing special benzoxazines. Moreover, by mixing benzoxazine resins with suitable polymers shape memory polymers can be obtained easily after a simple curing process. Soft polymers such as polysiloxanes, epoxies, poly(-caprolactone)s were proven to be useful for this purpose. These polymers act as “switch” and polybenzoxazine as hard segments provide the “fix points” in SMPs. As stated in the introduction, the design flexibility of benzoxazine chemistry is vast and just by switching between phenols and primary amines, many different benzoxazines with designed properties can be synthesized. Actually, the potential of benzoxazine chemistry in designing smart materials has not been recognized widely. However, the pioneering studies in this line clearly reveal that there will be intensive studies to develop new smart materials by using benzoxazine resins. Particularly, different healing strategies or SMPs based on benzoxazine resins would be one of the major interest in both academia and industry.
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