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In polymer and material science, controlling the molecular structure of polymers/materials and resulting properties has been an ongoing challenge. Among various approaches, the syntheses of responsive polymers are important in this context since their properties can be controlled by either stimulus or self-intervention of these polymers. Materials that can respond temperature, light, pH, mechanical deformation, etc. are named as smart materials and many studies have been reported about them. Among these materials, especially, self-repairing polymers gained much attention considering the opportunity to extend the lifetime by fixing damage during usage without human intervention. Excluding capsule based approaches [33], self-healing polymers can be categorized into two subbranches as “autonomous” and “stimuli-responsive” healing materials [34–37]. Stimuli-responsive materials require a triggering event and conveyed energy for repairing the damage such as electricity, thermal energy, light, and ultrasound [38–40]. By this approach, successive self-healable materials were designed based on reversible chemical reactions for multiple healings. In example, [2 + 2], [3 + 2], [4 + 2], [4 + 4] cycloadditions were used extensively [41–43]. In the case autonomic self-healing materials supramolecular attractions such as hydrogen bonding were utilized in their designs and besides chemical also physical cross-linking were established through supramolecular interaction that govern the entanglements of polymer chains. However, in both approaches, the primary mechanism can be simplified as molecular diffusions, flow of molecules interconnected with segmental mobility of polymer chains by a stimulus [44–48]. In brief, for a self-healable material a dynamic network is required with suitable functional groups capable of performing healing reactions or interactions.

As stated previously, benzoxazine chemistry has a vast design capacity to append purpose oriented functional groups to resulting polybenzoxazines. Moreover, a unique property of polybenzoxazines is related to the intra/intermolecular hydrogen bonding that contributes many interesting properties of these polymers. Those hydrogen bonding are also useful in designing materials when considering supramolecular attractions for the final material [49–51]. Thus, both design flexibility and extensive amount of hydrogen bonding makes polybenzoxazines good candidates to use as self-healable phenolics with careful molecular designs. According to the stated background, for example, polyether chains are flexible and could be appropriate in benzoxazine design for the purpose of obtaining a self-healable phenolic network. Hence, curable benzoxazine macro-monomers were synthesized from jeffamines (polyethers with primary amines at the chain ends) and a coumarine (Scheme 2.3). The obtained end-chain coumarine-benzoxazine macro-monomers were oily, which is an advantage to prepare films in Teflon molds. The molded macro-monomers were cured at ca. 180 °C to obtain cross-linked but soft/flexible polybenzoxazine films [52].

In this system, the coumarine functionality was selected to act as reversible reaction site based on light triggered [2 + 2] dimerization between wavelengths 300 and 350 nm. The long-chain jeffamine moieties provided the indispensable mobility for the system to ease the molecular collusions of coumarines. The self-healing efficiencies (η) of these films were calculated by comparing fracture toughness of the pristine and healed samples. The η value was found to be ca. 10% for the cut and healed specimen, but for cracked then healed specimen η was ca. 44%. Although these results are not quite sufficient for a real-life application, the system can be improved and this study is the first report about intrinsically self-healable polybenzoxazines.

In contrast to self-healing materials relied on bond reformations by external stimulus, healing with self-intervention of polymer by supramolecular attractions like hydrogen bonding are autonomous and can perform multiple healing cycles upon damage. In this context, autonomic self-healable polybenzoxazines can be obtained by carefully arranging the hydrogen bonds that naturally present on polybenzoxazines and the soft segments in these networks. Accordingly, jeffamine based main-chain polybenzoxazine precursors (PPO-Benz) were synthesized and mixed with a carboxylic acid group containing monofunctional-benzoxazine (Benz-COOH) in molds with various mass percentages prior to curing [53]. The obtained films were flexible and the material contained more hydrogen bonds compared to pristine polybenzoxazine due to excess phenolic –OH and –COOH functionalities originating from Benz-COOH (Scheme 2.4).

Scheme 2.3 Synthesis of end-chain coumarine functional benzoxazine macromonomers.

Films were prepared with different mass ratios of PPO-Benz and Benz-COOH by using solvent casting method. These films then cured at 180 °C before healing tests. Typically, the films were cut into two pieces and kept in contact for 12 h at room temperature. Then, the healing efficiencies of the healed films were found by tensile tests by calculating their toughness recovery. The extent of healing was found to be related to the added amount of Benz-COOH in the films. The cut films were able to restore themselves to certain degrees of healing (Figure 2.1). For example, healing efficiency was calculated as 96% for the polybenzoxazine film with 10% Benz-COOH but only 26% for the sample bearing 2.5% Benz-COOH. The findings clearly reveal that the number of hydrogen bonds is the major effect on recovery and the presence of Benz-COOH in curing formulations, as extra hydrogen bonding source, augments the self-mending ability.

In another strategy, supramolecular attractions and S–S bond cleavagereformations were used to design recyclable and self-healable polybenzoxazines. In this approach, low cost chemicals were converted to self-healable materials in only 30 min with a simple process. This strategy relies on inverse vulcanization of benzoxazines, which is simply performed via mixing benzoxazine monomer or polybenzoxazine prepolymers with elemental sulfur at ca. 180 °C [54]. According to the proposed reaction mechanisms, the reaction proceeds over a radical mediated process which was named as sulfur radical transfer and coupling (SRTC) (Scheme 2.5) [55].

Scheme 2.4 Synthesis of polybenzoxazine with augmented hydrogen bonds.

Figure 2.1 The images of cut-healed PPO-Benz/Benz-COOH films.

Main-chain polybenzoxazine from poly(propylene oxide) (PPO-Benz), a di-ally functional benzoxazine monomer (B-ala) and sulfur was heated up to 185 °C (Scheme 2.6) for self-healable material fabrication [56]. The obtained crosslinked polymeric films was recycled and healed up to 5 cycles by heating (Figure 2.2). As observed from tensile tests, brittleness of the films increased and the toughness decreased after each thermal (180 °C) healing. The stress value of the sample was measured 877 kPa after the 1st and 2,007 kPa after 5th healings. Conversely, elongation at break reduced gradually from ca. 80% to ca. 20% after 5 healing cycles. Such toughness loss indicates the reduction of chain mobility after healings. Because S–S bond cleavage and reformation reactions in each cycle shortens the polysulfide chains. The rigidity and crosslinking density of the material increased due to these short chains and self-healing ability after a certain cycle number eventually reduced. Although healing of this system is limited, the results demonstrate that it is possible to use elemental sulfur with benzoxazines to produce recyclable and self-healable poly (benzoxazines-co-sulfide)s.

Obviously, previous studies revealed that benzoxazines with poly(propylene oxide)s has high potential to design different self-healing materials. In this line, for self-healable polybenzoxazines, PPO-benz were used for ring-opening polymerization and subsequent ketene formation by light for healing reactions. The oxoketenes were generated over a bisdioxinone (BisBDiox) molecule which was admixed in the benzoxazine precursor [57, 58]. Upon irradiation ca. 300 nm at room temperature, bisdioxinones ring-open and cleave to produce oxoketene and side-product ketone. And then the ketene immediately reacts with phenolic –OH of polybenzoxazine that occurred during curing and esters eventually form (Scheme 2.7) [59]. By this way, light triggered crosslinking was achieved and healing on surface was achieved.

Scheme 2.5 Simplified mechanism of inverse vulcanization reaction of a benzoxazine monomer and elemental sulfur to produce a poly(benzoxazine-co-sulfide).

Scheme 2.6 Synthesis of recyclable and self-healable poly(benzoxazines-co-sulfide).

Figure 2.2 Stress–Strain (%) behavior of 5 times chopped and healed PPOBenz40%–B-ala40%–S20%.

Scheme 2.7 Curing and subsequent light induced healing of PPO-Benz/BisBDiox system.

A typical self-healable film was prepared by dissolving PPO-Benz and BisBDiox (5% w/w) in tetrahydrofuran (THF) and then this solution was used to cast a thin layer on silicon wafers by spin coating technique. After drying, the obtained films had a thickness of ca. 300 nm. Then, these films were cured gradually by using an open-air oven at 180 °C and at 200 °C for ~30 min. To test healing, initially, the surface was scratched by using an atomic force microscopy (AFM) and its nano-indentation diamond tip. All the films were monitored by AFM in tapping mode before light exposure. Then, the films were exposed to light between 300 and 350 nm for 10 h under THF vapor at room temperature to trigger oxoketene formation. During irradiation, BisBDiox acted as bridging agent and polybenzoxazines chemically bind to each other over ester linkages and efficient healing on surface was achieved. The segmental 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.