Functional high performance polybenzoxazines and their properties

Phenolic resins are leading the polymer market due to their widespread applications such as adhesives, structural applications materials in aerospace, printed circuit boards, conductive polymer elements, and encapsulation materials for electronic applications. Phenolic resins, acrylates, bismaleimides, polyesters, epoxy resins and isocyanate polymers are the best known members of the thermosets. High processing temperatures and void formation during curing are the main challenges must be overcome regarding application of these materials. Lately, benzoxazine based phenolic resins has been developed and attracted significant attention as a novel type of phenolic resin. Polybenzoxazines have various outstanding properties, including good thermal stability, high glass transition temperature, high char yield, no need of catalysts for curing, near-zero volume changes during curing, low moisture absorption and no volatile release. Recently, owing to the design flexibility of benzoxazine and related polymeric benzoxazine precursors, various smart materials were synthesized by innovative strategies including self-healing materials, electrochemically activated smart coatings, smart sorbents for heavy metals, hydrophobic surface applications and porous polybenzoxazine resins. Taking account of the unique advantages of these strategies and attractive characteristics of polybenzoxazines, in this thesis, we focused on the combination of various functional groups with benzoxazine precursors and then investigated the resulted products properties. In the first part of the thesis, a self-healing strategy for poly(propylene oxide)s bearing benzoxazine units (PPO-Benz) through supramolecular attractions is described. Poly(propylene oxide) bisamine (PPO) with a molecular wieght of 2000 Da were reacted with formaldehyde and bisphenol A to yield desired PPO-Benz with 12360 Da. The cross-linked polymer films were then prepared by solvent casting of suitable compositions of PPO-Benz and carboxlic acid containing benzoxazine monomer (Carb-Benz) in chloroform followed by thermal ring opening reaction of benzoxazine groups at 200 °C. Thermal curing and thermal stability of the film and final products were investigated. It was demonstrated that the self-healing capacity of the films were improved by employing Carb-Benz in the formulation. Figure 1 : Self-healable film preparation from carboxylic acid containing benzoxazine monomer and PPO-Benz. In the second part of the thesis, a novel strategy to obtain sulfur rich polybenzoxazine copolymers by reacting allyl functional benzoxazine (BA-ala) and elemental sulfur was described. Simultaneous inverse vulcanization and ring-opening reactions of benzoxazine generated soluble copolymers in specific feed ratios. Parameters such as monomer structure and feed ratios on the polymerization were studied. The thermal stability of the copolymers was investigated and compared to that of polybenzoxazines derived from neat BA-ala by using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The surface properties of the materials as examined by scanning electron microscope (SEM) confirmed that elemental sulfur and benzoxazine copolymers can be produced without a phase separation at micrometer level. Moreover, a sponge like insoluble macroporous polybenzoxazine networks was obtained at the 20%-wt feed ratio of sulfur. Figure 2 : A novel strategy to obtain sulfur rich polybenzoxazine copolymers by reacting allyl functional benzoxazine (BA-ala) and elemental sulfur. Finally, side-chain benzoxazine functional polybutadienes was synthesized by photoinduced hydrogen abstraction process. First, photosensitive benzoxazine compounds possessing both chromophoric carbonyl and hydrogen donating sites in the structure were synthesized using vanillin or 4-hydroxybenzophenone in the conventional benzoxazine synthesis. Irradiation of neat polybutadiene (PB) in the presence of the corresponding benzoxazines, namely benzophenone benzoxazine (BPh-ptol) and vanillin benzoxazine (Van-a) under 300–350 nm light gave PBs with approximately 4-5 benzoxazine units per chain. Successful modification was confirmed by the spectral and thermal investigations. It is shown that benzoxazine modified PBs undergo thermally activated curing in the absence of any catalyst forming polybutadiene thermoset with high char yield. Figure 3 : Photoactive benzoxazines having both chromophoric carbonyl and hydrogen donating sites were synthesized using vanillin or 4-hydroxybenzophenone by conventional benzoxazine synthesis methodology.