Tarek Agag

Various Approaches for Main-Chain Type Benzoxazine Polymers

Publisher Summary Main-chain type benzoxazine polymers (MCBPs) as a new class of polybenzoxazines are very widely used in the recent years due to their unique properties as thermoplastic linear polymers that can undergo thermoset conversion when they are further polymerized and cross-linked. MCBPs are unique polymers with a thermoplastic/ thermoset crossover nature attributed to the presence of cross-linkable sites in the polymer backbone, while the uncross-linked polymer itself has properties comparable to those of many thermoplastics. The cross-linking mechanism for such polymers as a function of various processing parameters is of great importance. MCBPs have an edge over cross-linked polymers derived from monomeric benzoxazine. MCBPs are developed as a new generation of polybenzoxazine precursors. The properties of MCBPs derived from the polymeric Mannich condensation are much better compared to those of the monomeric type dibenzoxazines. This chapter argues that the polycondensation via Mannich reaction approach has some disadvantages. The resultant MCBPs has relatively low molecular weight. This is because it is difficult to maintain the stoichiometry during the condensation reaction due to the many side reactions and branching that are caused by the partial ring-opening of the benzoxazine structure. Additionally, the poor solubility of some aromatic diamines and bisphenols in solvents used for MCBP synthesis limits specific molecular design or achievement of higher molecular weight. To overcome these difficulties, benzoxazine monomers with reactive end groups other than phenols and amines are used in copolymerization reactions with other functional groups (e.g., ethynyl, propargyl, allyl, alcohol, and carboxyl) to form linear polymers with benzoxazine rings as repeating units in the main chain.

Thermal Properties Enhancement of Polybenzoxazines: The Role of Additional Non-Benzoxazine Polymerizable Groups

Publisher Summary Benzoxazine research is developing rapidly because of the many potential applications of the materials. While future applications might span across many fields, currently, commercial applications of this new class of thermosetting polymers are just beginning to be implemented in the electronic and aerospace industries. Molecular design flexibility is illustrated by the different functionalities present in the monomers. Benzoxazine polymerizes via a cationic ring-opening mechanism and is studied extensively. The polymerization of benzoxazine typically takes place through thermally accelerated ring-opening polymerization; a process that occurs without purposefully added initiators and/or catalysts or producing byproducts. Polymerizable groups in the benzoxazine monomer are a viable method for significantly improving thermal stability of the cross-linked polybenzoxazines. Increased thermal stability is achieved due to a higher cross-linking density in the polymer matrix. Anchoring the dangling side groups that are susceptible to earlier thermal degradation helps increase the degradation temperature. Extremely thermally stable engineering materials can be created while maintaining many advantages of benzoxazine resin. Benzoxazine resin has excellent thermal stability for an organic polymeric material, processing similar thermal properties to phenolic resins and polyimides. The thermal stability and degradation patterns for many types of polybenzoxazines have been studied with great interest for applications that require thermal resistance.

Effect of Neighboring Groups on Enhancing Benzoxazine Autocatalytic Polymerization

Publisher Summary The unique features of polybenzoxazines make them attractive for a wide range of applications. The characteristics of polybenzoxazines such as excellent dimensional stability, lower surface energy, higher Tg, and lower moisture absorption are attributed to the stable intramolecular hydrogen bonding in their network structure. The features make polybenzoxazines excellent candidates for the applications that require near zero shrinkage and high dimensional stability. Polybenzoxazines are formed by the autocatalytic polymerization of benzoxazine monomers. Polybenzoxazines are well known as a new class of thermoset polymers used for high performance applications. They provide the characteristic properties found in phenolic resins, such as high thermal stability, excellent electrical properties, good mechanical properties, and better flame retardancy. Furthermore, they have the advantages of requiring no harsh initiators or catalysts for polymerization and of forming no byproduct upon polymerization. This chapter highlights the different approaches to the synthesis of benzoxazine monomers polymerized at a lower temperature by the autocatalytic effect on the ring-opening benzoxazine polymerization. The various approaches are mostly based on the introduction of electron-withdrawing or electron-donating groups, or acidic groups that can activate the ring opening or stabilize the intermediates. In particular, we have focused on the DSC thermograms to find out the exothermic peaks of the monomers under study. From the systematic studies of the substituted benzoxazine monomers, carboxylic acid and hydroxyl containing benzoxazine monomers, it was clearly demonstrated that all the aforementioned systems represent successful approaches to improving the autocatalytic ring opening of the benzoxazine monomers.

Polybenzoxazine-Clay Nanocomposites

Publisher Summary Polybenzoxazines exhibit the characteristics properties of traditional phenolic resins such as high thermal stability and flame retardancy. In contrast to phenolic resins, they require no harsh catalysts for polymerization, release no byproducts, and have excellent dimensional stability because of near-zero volume shrinkage or expansion upon curing. The monomers can be polymerized to polybenzoxazines through thermally activated ring-opening polymerization in the absence of catalysts. Motivated by the unique features of polybenzoxazine as a new thermosetting polymer, tremendous research efforts are done to study the incorporation of thermoplastic and thermoset polymers, fibers, inorganics, and nanoparticles into polybenzoxazines, to produce novel materials with high potential for commercial applications. Thus, a variety of novel polybenzoxazine-based materials of excellent toughness, enhanced thermal stability, low polymerization temperature, better processability, and many other desirable properties are developed. As novel structural materials, polybenzoxazine-clay Nanocomposites offer a major opportunity for creating a large number of new materials with useful combinations of properties. The dispersion of the organoclay into the polybenzoxazine matrix in nanoscale to form an intercalated or exfoliated nanocomposite depends on a number of factors. The size of the intercalating agent and its ease of ion exchange are important to provide sufficient nonpolar environment in the interlayer volume of clay to accommodate benzoxazine monomers. The thermal stability of the intercalating agent of MMT should be higher than the maximum temperature required for the polymerization of benzoxazine to avoid the degradation of the intercalating agent.

Main-Chain Type Benzoxazine Oligomers: A New Concept for Easy Processable High Performance Polybenzoxazines

Publisher Summary Main-chain type benzoxazine oligomers (MCBOs) are similar to MCBPs in terms of the chemical structure in which benzoxazine is the repeating unit in the backbone. The concept of MCBO is developed to minimize the melt processing problems associated with the use of MCBPs; however, there is no problem in solution processing of MCBPs as a number of commonly available solvents dissolve those MCBPs. MCBOs are synthesized using the similar methodology of MCBPs via Mannich base polycondensation. However, the stoichiometry is manipulated in a way to control the molecular weight to obtain polymers with rather smaller molecular weights that satisfy the required viscosity characteristics for a specific application. This chapter explains that polybenzoxazines have shown to have a number of attractive properties including high glass transition temperature, superb thermal stability, excellent mechanical properties, and a wide molecular design flexibility. Furthermore, benzoxazine chemistry possesses a number of inherent advantages in comparison to traditional phenolic resin, such as no release of volatiles during cure and no added catalyst to initiate polymerization. Polybenzoxazines derived from monomeric benzoxazines are usually brittle due to various types of chain end groups, which lead to termination of the chain propagation by the intramolecular six-membered ring hydrogen bonding. To minimize the formation of unwanted chain end groups, another class of polybenzoxazine precursors, known as MCBP, was developed. MCBP refers to linear polymers containing benzoxazine moieties as repeating units in the main chain.

Polybenzoxazine/Polyimide Alloys

Publisher Summary This chapter describes the polybenzoxazine-PI alloys prepared by the thermal curing of BA-a in the presence of various PI components, such as poly (amide acid), soluble PI, or poly (imide-siloxane). The thermal and mechanical properties of the polymer alloys were noticeably improved by the introduction of PI components. Polybenzoxazines have not only the characteristics of the traditional phenolic resins, such as excellent thermal properties and flame retardance, but also unique characteristics such as molecular design flexibility, low melt viscosities, near-zero shrinkage upon polymerization, low moisture absorption, and low dielectric constant. Polybenzoxazines overcome the disadvantages of the traditional phenolic resins without sacrificing the advantages of those resins, which leads to a novel and promising candidate for high performance thermosetting resins. However, polybenzoxazines have some shortcomings. It illustrates Polyimide (PI), which is known as one of the most important thermally stable polymers and is widely used as a suitable component of a polymer alloy due to the high thermal and mechanical properties, and the high molecular design flexibility. It describes the preparation and properties of high performance polymer alloys based on polybenzoxazine and PI. PIs are in general not soluble in organic solvents. Therefore, typical polybenzoxazine-PI alloys are prepared by blending BA-a as a benzoxazine monomer with poly (amic acid) (PAA) as a processable precursor of PI, followed by film casting and thermal treatment up to 240 °C for the ring-opening polymerization of the benzoxazine and imidization of PAA.