124 Research Article Received: 1 March 2011 Revised: 3 May 2011 Accepted: 30 May 2011 Published online in Wiley Online Library: 15 September 2011 (wileyonlinelibrary.com) DOI 10.1002/pi.3156 Flexible polybenzoxazine thermosets containing pendent aliphatic chains Tarek Agag, ∗ Ahmed Akelah, Ahmed Rehab and Salwa Mostafa Abstract The rich chemistry of polybenzoxazines allows a wide range of molecular structure design by using appropriate starting materials. A new class of polybenzoxazines has been developed from benzoxazine monomers containing pendent long aliphatic chains. The monomers have been synthesized by the reaction of phenol or bisphenol A with two different long-chain aliphatic amines. The chemical structure of the monomers was confirmed by 1 H nuclear magnetic resonance and Fourier transform infrared spectroscopy. The polymerization behavior of the monomers studied by differential scanning calorimetry shows exothermic peaks due to the ring-opening polymerization of benzoxazine monomers centered at 247 – 255 ◦ C. Dynamic mechanical analysis indicated that the glass transition temperatures T g were in the range 81–92 ◦ C. The thermal stability of the polymers was also examined by thermogravimetric analysis, demonstrating that the weight loss temperatures decreased in comparison with that of traditional polybenzoxazine. c  2011 Society of Chemical Industry Keywords: polybenzoxazine; long-chain aliphatic amine; easy processing; low T g INTRODUCTION For any new polymer, the balance between mechanical properties, via combination of toughness and stiffness, and thermal stability is required for practical advanced applications. Benzoxazine resin is a class of thermosetting polymers that has attracted much attention because of its outstanding physical and mechanical properties as well as its unique molecular flexibility design. 1 Besides possessing the typical characteristics of traditional phenolic resins, polyben- zoxazines as phenolic resin possess some unique features over traditional phenolic resin, which include low water absorption and near zero volumetric expansion or shrinkage upon curing. Further- more, polybenzoxazines are characterized by their cost effective- ness since they can be produced from inexpensive raw starting materials, including phenols, primary amines and formaldehyde. The polymerization takes place through thermally activated ring opening of the cyclic benzoxazine structure in the absence of a catalyst or initiator and without generating any by-products. 2 The polymerization can also be achieved at lower temperature by us- ing cationic initiators, resulting in phenoxy-type thermosets, which can be rearranged to produce phenolic-type polybenzoxazine. 4–6 Various studies have been done for enhancement of the properties of polybenzoxazine such as processability, flame re- tardancy and toughness. 3 A noticeable improvement in ther- mal and mechanical properties has been achieved by hy- bridization of polybenzoxazines with inorganic nanoparticles such as clays, 7–10 transition metals, 11 metal oxide, 12 carbon nanotubes 13 and polyhedral oligomeric silsesquioxane. 14,15 The blending of polybenzoxazines with other polymers such as poly(imide-siloxane), 16 epoxy, 17 bismaleimide, 18 polysiloxane, 19 polyimide 20 and poly(ε-caprolactone) 21 has also been studied. The incorporation of additional non-benzoxazine crosslinking sites into the benzoxazine structure has been reported, such as benzoxazine containing phthalonitrile, 22 propargyl, 23 maleimide, 24 ethynyl, 25 diacetylene 26 and allyl. 27 This approach led to polybenzoxazine thermosets with outstanding thermal properties such as a high glass transition temperature T g , a high storage modulus and a high char yield. Brittleness is a common problem in aromatic polybenzoxazines, similar to other thermosetting polymers. 28 Many studies have been reported aiming at toughness improvement of polyben- zoxazines, and generally there are two reported approaches. The first is the classical approach of toughness enhancement in which elastomeric or thermoplastic polymers have been used. Some examples of this approach include the use of polyurethane, 28–31 amine-terminated butadiene acrylonitrile 32 and polysiloxanes 16,19 as the toughening agent. The second approach is the designing of inherently tough polybenzoxazines by preparing benzoxazine monomers containing aliphatic soft segments to afford tough polybenzoxazines. 33–35 In the current study, we report on the preparation of some novel benzoxazine monomers containing pendent long aliphatic chains. The characterization and polymer- ization of this novel class of benzoxazine monomers as well as the properties of the thermosets are discussed. EXPERIMENTAL Materials Bisphenol A, stearylamine and dodecylamine were obtained from Sigma-Aldrich (St. Louis, USA). Paraformaldehyde was obtained ∗ Correspondence to: Tarek Agag, Department of Macromolecular Science and Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106-7202, USA. E-mail: taa16@case.edu Polymer Research Group, Chemistry Department, Faculty of Science, Tanta University, Tanta 31527, Egypt Polym Int 2012; 61: 124–128 www.soci.org c  2011 Society of Chemical Industry