mobility of the polybenzoxazine chains was sufficient to ship BisBDiox molecules to the damaged zone for healing reactions to take place. The amount of healing could not be monitored by classical methods, however the damaged zone was monitored by using AFM (Figure 2.3) and complete healing was observed especially in the case of THF vapor assistance. AFM results clearly reveal that using BisBDiox as self-healing agent is a convenient strategy due to its ability to react with nucleophiles and compatibility with phenolics.
Figure 2.3 AFM pictures of PPO-Benz/BisBDiox before (a, c) and after irradiation (b, d).
In another approach, ester formation on polybenzoxazines were also used to obtain self-healable polybenzoxazines. In contrast to previous method, the healing is based on ester exchange reactions. For the purpose, succinic anhydride was admixed with bisphenol F derived benzoxazine prior to curing. The ring-opening reaction generates free phenolic OH groups that are available to react with anhydride at high temperature. By this way, the polybenzoxazine contains both phenolic OH and carboxylic groups in the network (Scheme 2.8) [60].
Therefore, the network structure is suitable for possible transesterification reactions that can be used for healing the damaged material. It was shown that such a structure can behave like thermoplastic polymer at ca. 140 °C with a transesterification catalyst, zinc acetate (Zn(Ac)2), and exhibit self-healing even after several healing cycles. An optical microscope was used to demonstrate the autonomic repairing capability of these thermosets. Accordingly, a cut-healed sample was presented in Figure 2.4a. Initially, a cut approximately 65.8 μm wide was generated on the surface of the material. As seen in Figure 2.4, the damaged area decreased reasonably, and diameter of the damaged zone reduced to ca. 6.1 μm after a certain time. Moreover, the damage was almost healed after heating the material at 140°C for 40 min. The ability of repeated recovery was also tested via mechanical measurements of the present thermoset. The storage modulus (Eʹ) of specimen decreased from ca. 2.9 to 2.1 GPa after the first damage as demonstrated in Fig. 4b. However, the storage modulus almost reached to its original value (2.86 GPa) after heating. According to storage modulus measurements, the healing efficiencies (η) of the specimen was 99, 92 and 89% after first, second, and third repairing cycles, respectively. Moreover, a control experiment was also conducted to understand the catalytic effect of Zn(Ac)2 for the proposed transesterification reaction. And in contrast to catalyzed samples, Eʹ values decreased linearly after each damage and the samples without Zn(Ac)2 could not be recovered effectively.
Scheme 2.8 Synthesis of polybenzoxazine-succinic anhydride thermoset.
Figure 2.4 (a) Images showing the healing on the surface of sample film (b) Storage modulus for damage and healing cycles, (c) A simplified illustration showing transesterifications for healing (Copyright: https://creativecommons.org/licenses/by/4.0/).
In conclusion, based on the strategy of ester exchange reactions, a novel self-healable polybenzoxazine system was presented by using simple chemical such as succinic anhydride and bisphenol F based commercially available benzoxazine monomer. The results clearly show that it is possible to obtain a bulk-state self-healing in polybenzoxazines through dynamic ester bonds and other anhydrides can also be used for a similar purpose.
As known, classical polybenzoxazines are regarded as a non-healable polymer, because the Ar–CH2–N bridge and chemical bonds on the aromatic moiety of the network structure are irreversible. However, those linkages are not unique occurred during ring-opening polymerization, besides Mannich bridges depending on the structure of benzoxazibe and curing conditions other structures can be generated. Previous studies showed that, reversibility of N−CH2−R (R: S, N or phenoxy type O) chemical bonds generated via thiol/benzoxazine and amine/benzoxazine reactions can exhibit reversibility at a certain degree. Because, N−CH2−R bonds are more labile and a reversible bond cleavage/reformation can take place at relatively low temperatures when compared with N−CH2−Ar Mannich bridge. The dynamics of N−CH2−R bonds is high, and therefore, polybenzoxazines containing N−CH2−R bonds can be healable or recyclable. To confirm this assumption, polybenzoxazines must be synthesized with dominant N−CH2−O (phenoxy type) linkages in their network structure. As well known, under certain conditions such as Lewis acid catalyst and low curing temperatures force the ring-opening polymerization to generate more N−CH2−O than N−CH2−Ar bonds. Moreover, by blocking the ortho-position of a benzoxazine monomer similar results can be obtained and possible reversibility based on phenoxy type N−CH2−O bonding can form a self-healable polybenzoxazines without complicated monomer or macromonomer designs. This possibility was investigated by curing benzoxazines with functional groups at ortho-, meta- and para- positions. After curing those benzoxazines at 210 °C for 4 h, the obtained polybenzoxazines were compressed at 180 °C under 5 MPa for 20 min. Polybenzoxazines from ortho-functional monomers (Scheme 2.9) exhibited healing even after crushing into pieces, which is unusual for classical polybenzoxazines [61]. The deformation and healing behaviors of the ortho-functional polybenzoxazines (Scheme 2.9) were found to be quite similar according to experiments. Further molding studies for ortho-functional polybenzoxazines also revealed that the finely chopped fragments of these polybenzoxazines can even be remolded successfully into the different shapes by using molds after compressing under 5 MPa, at ca. 180 °C for 2 h. Higher pressures were much beneficial since the amount defects in the molded samples were reduced. The recycled sample from ortho-difunctional benzoxazine showed 48 MPa of tensile strength with a self-healing efficiency more than 90%. In summary, this work proved that phenoxy type N−CH2−O bonding is a dynamic covalent bond and can be generated on polybenzoxazine by using an ortho-blocked benzoxazine monomer. Moreover, the amount of phenoxy structures can be increased by controlling the curing temperature. The exchange reactions between N−CH2−O (phenoxy) bonds is the major cause that facilitates both healing and recycling of the polybenzoxazines.
Scheme 2.9 The structures of ortho functional mono and dibenzoxazines and their curing to give phenoxy linkage containing polybenzoxazines.
2.3 Benzoxazine Resins for Shape Memory Applications
Among smart materials shape memory polymers (SMPs) have high potential in material chemistry since SMPs can shift between different shapes and return back to their initial shape upon stimulus [62, 63]. Generally, these materials exhibit SMP property by temperature change using infra-red light, a heater or simply warm water [64–66]. Due their transition between shapes via stimulus, SMPs have various application
