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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 possibilities in many different areas including electronics, packaging, textiles, transportation, medication. Hence, the challenge for designing such materials with common polymers offering cost advantages is a continuous interest. Moreover, several new functional groups are incorporated in these systems to obtain side-benefits along with shape memory. In this manner, epoxies and related polymers have attracted attention to be used as SMPs, because epoxies exhibit low T_g, high tolerance against deformation and low recovery stress. Therefore, epoxies have high potential to possess excellent shape memory performance. On the other hand, pristine epoxy polymers had practical application due to requirement of high strength and stiffness besides shape memory property. Thus, addition of benzoxazine monomers into epoxies may generate SMP thermosets having better properties. The compatibility of benzoxazine and epoxies was found to be high and epoxy thermosets were produced successfully. It was reported that benzoxazine resins acted as curing compounds for epoxies and copolymers with improved properties were obtained compared to pristine epoxy polymers due to the synergism between epoxies and benzoxazine resins [67–70]. Polybenzoxazine-epoxy based SMPs were fabricated by Rimdusit et al. and those SMPs exhibited better thermal stability and mechanical performance [71]. Polybenzoxazine-epoxy SMPs were synthesized by using a bisbenzoxazine derived from bisphenol A and aniline (BA-a), two different epoxies EPON 826, NGDE and Jeffamine D230 (as curative) (Scheme 2.10). All these ingredients were mixed in a mold and melted at 70 °C to obtain a homogeneous mixture prior to curing in an open-air oven. Mechanical properties, shape recovery capacity and thermal properties of the obtained SMPs were characterized (Figure 2.5). Accordingly, all of the deformed SMP samples are completely recovered to their original shapes after a few minutes suggesting a good shape recovery performance. However, increasing the amount of BA-a content in epoxy-benzoxazine mixture affected the shape recovery time in a negative manner. Because, polybenzoxazine moiety is hard and naturally would increase the rigidity of the materials, resulting in a slow shape memory response.

Scheme 2.10 Synthesis of polybenzoxazine-epoxy SMP.

Another SMP based on poly(ε-caprolactone) (PCL) and polybenzoxazine was reported as alternative to epoxy systems [72, 73]. Hydroxyl (PCL-OH) and tosyl (PCL-OTs) terminated two different types of PCLs were used with difunctional benzoxazine monomer to prepare SMPs. The curing studies revealed that polybenzoxazines could not bind to hydroxyl terminated PCL, however, polybenzoxazines were able to bind tosyl terminated PCL by substitution reaction between phenolic –OH and tosyl groups (Scheme 2.11). By this way polybenzoxazines were covalently incorporated to PCL chains and thermally triggered shape memory behavior with high shape recovery (S_r) and fixity (S_f) were obtained for these copolymer thermostes. Unbound PCL-polyenzoxazines (PCL-(OH₂)/benzoxazine) was shown to be brittle, heterogeneous in morphology, and therefore, lacking of shape memory properties. Owing to the fact that PCL and polybenzoxazines are not interconnected, the pure polybenzoxazine zones in the matrix can be considered as the reason of the brittleness. Because polybenzoxazines are known to have high crosslinking density with strong intraand inter-hydrogen bonds.

Figure 2.5 Photographs of polybenzoxazine-epoxy samples showing original (a), temporary (b) and recovered (c) shapes. Copyright: CC BY-NC-SA license: https://creativecommons.org/licenses/by-nc-sa/3.0/).

Scheme 2.11 Synthesis of crosslinked PCL-benzoxazine.

Shape memory properties of all PCL-polybenzoxazines, PCL-(OTs₂)/ benzoxazines, were studied by bending tests both at ambient temperature and above the phase transition temperatures predetermined by dynamic mechanical analysis. Accordingly, unlike PCL-(OH₂)/benzoxazine, PCL(OTs₂)/benzoxazine samples softened at 100 °C and bending was much easier for high PCL containing copolymers (Figure 2.6). Thus, increasing PCL content in PCL-(OTs₂)/benzoxazine improved both S_f and S_r values and among all samples for PCL-(OTs₂)-80/benzoxazine-20 (80:20, w/w) was almost 100% even after several cycles. According to gel content analysis, this sample had the largest amount of polybenzoxazine bonded with PCL and the largest amount of free PCL.

In summary, interconnected PCL and polybenzoxazine chains generated hard and soft segments required for a typical SMP system. The amount of PCL was found to be crucial for the proposed system because PCL endowed strong fixing abilities via molecular interactions between the polybenzoxazine and PCL chains. Besides, increasing PCL content augmented the bending ability of the copolymer at temperatures above PCLs melting temperature.

In another approach, polybenzoxazines with both intrinsic self-healing and shape memory properties was reported. In order to synthesize this smart material initially the benzoxazine was obtained via reacting formaldehyde, bisphenol A and bis(3-aminopropyl) terminated polydimethylsiloxane (Scheme 2.12) [74]. By this way, a main-chain polybezoxazine was obtained having film formation ability and essential chain dynamics due to the polydimethylsiloxane segments. The crosslinked films of main-chain polybenzoxazine precursor (Poly(Si-Bz)) containing 2% FeCl₃ by mass were prepared in Teflon molds and could be cured at relatively low temperatures (100–120 °C). Because, Lewis acids are good catalysts to reduce the ring-opening polymerization of benzoxazines, these compounds can even trigger ring-opening reaction at room temperature [15, 75].

Figure 2.6 Images for PCL/benzoxazine materials; Bending tests of PCL-(OH₂)/ benzoxazine samples failed at 100 °C (top), shape recovery of PCL-(OTs₂)/benzoxazine samples (bottom). (Elsevier Ltd. License number: 4790661063144).

Scheme 2.12 Synthesis of Poly(Si-Bz) from bis(3-aminopropyl) terminated polydimethylsiloxane (PolySi).

During the studies about the effect of Lewis acid catalysts, it was well established that the CH₂–N–CH₂ bridges and phenolic –OH groups of polybenzoxazines can bind metal ions by using nitrogen and oxygen atoms [76–79]. For example, a coordination system was demonstrated between polybenzoxazine and Hg²⁺ ions and the use of polybenzoxazines for environmental issues such as heavy metal extraction from water was reported, previously [80, 81]. As known, similar metal–ligand interactions has also been shown to be beneficial for the fabrication of self-healing materials [82–86]. For example, efficient self-healing was observed for polymers having 2,6-pyridinedicarboxamide groups. In these polymers, adjacent strong and weak metal ion binding sites were located in a single ligand to generate highly dynamic metal–ligand interactions [87]. Similarly, the metal binding ability of polybenzoxazine was exploited and Fe³⁺ ligated polybenzoxazines were prepared for both self-repairing and shape memory applications simply by 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.