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
