expected that there will be intensive studies to synthesize benzoxazine based self-healing, shape memory materials. In this chapter, an overview about polybenzoxazines and their current progress about the synthesis of benzoxazine-based materials for these unconventional uses are summarized.
Keywords: Polybenzoxazines, benzoxazine resins, ring opening polymerization, smart phenolic resins
2.1 Introduction
Although there are many different types of thermosets that were synthesized as high performance polymers during the last century, phenolic resins, as the first man-made plastic, have been continuously used in industry for more than 100 years. As a reflection of this interest, many papers and books were published and conference activities were successfully performed about these resins. The worldwide production of classical phenolic resins, resole and novolac, has reached to a very large volume such as 5 million tons per year and the growth of the production is parallel to the global economic growth rate [1–3]. Definitively, these resins will survive in the second century of their existence due to the interest in global markets. Accordingly, novel applications have already emerged related to phenolic resins such as porous materials by the carbonization of phenolic foams, fiber reinforced phenolics, modern composites for aircrafts, blast protection materials, phenolic resins based adhesives for honeycomb sandwich panels and more [4]. On the other hand, resole and novolac synthesis requires formaldehyde (Scheme 2.1) and the emission problem of this molecule resulted in regulations for phenolic industry. Especially, in wood products formaldehyde residues expose a serious concern of health and environment, therefore, efforts to reduce formaldehyde emissions in these resins is one of the major trend. Moreover, phenolic resins are cured either by using acids or bases before usage and the curing rate of classical phenolic resins are relatively slow compared to other resins and, besides formaldehyde emission, formation of other by-products is a fact during curing [5]. Hence, these problems gave rise to new research and development projects for phenolic resins with the anticipation of more reactivity and less emissions.
Scheme 2.1 Synthesis and representative structures for Novolac and Resole.
The benzoxazine type resins emerged with high potential as a solution for drawbacks of phenolic resins where they have not succeeded sufficiently. Polybenzoxazines (PBZs), a kind of polyaminophenol, are the curing product of 1,3-benzoxazines and are considered as a contender to novolak due to the structural resemblance (Scheme 2.2). Hence, most of the PBZs have properties both similar and also unique that are unseen in novolac resins [6]. They generally exhibit high glass transition temperatures (Tg), char yield, flame resistance, low water absorption, and good mechanical performance [6–12]. In addition, benzoxazines do not exhibit an important shrinkage during curing due to the ring-opening polymerization character [11]. As well known, shrinkage is a problem for many other curable monomer types such as acrylates. Another important superiority of benzoxazines is related to their reactive nature, and thus, by-product release is much lower compared to classical phenolic resins. Typical polymerization of a benzoxazine monomer takes place between 160 and 250 °C without any catalyst or curative (Scheme 2.2) [13–16]. The polymerization of benzoxazines proceed over a cationic ring-opening pathway, and therefore, polymerization temperatures can be reduced by using acidic compounds as catalyst [17–21]. Another advantage of benzoxazine resin chemistry is related to the synthetic simplicity of benzoxazine monomer synthesis. Any phenol and a primary amine, which can tolerate the synthesis conditions, with 2 moles of formaldehyde can yield a benzoxazine monomer (Scheme 2.2) [22–31]. Accordingly, benzoxazine chemistry has a vast design flexibility due to the presence of many different commercially available phenols and amines. The huge molecular diversity of benzoxazines unleashes many possibilities for unconventional uses ranging from self-healing to shape-memory material applications [32].
Scheme 2.2 Synthesis of a 1,3-benzoxazine and production of a polybenzoxazine therefrom.
2.2 Self-Healable Polybenzoxazines
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
