Study On Preparation, Structure And Properties Of Nanostructured Thermosets Of Polybenzoxazine Resin

Polybenzoxazines (PBa) are a class of novel phenolic resins, which possess excellent thermal, mechanical properties and electrical properties. The thermosets can be prepared via thermally activated ring-opening polymerization of benzoxazine monomers and no hardeners are required involving the curing process without producing small-molecule reactants . Owing to the unique chemistry of polymerization together with their excellent properties, PBa thermosets are becoming the promising alternatives to some traditional thermosets such as epoxies, phenolic resins, bismaleimides. Monomers of PBa thermosets can be readily synthesized via Mannich condensation among phenols, formaldehyde and primary amines either in solution or in melt state。An important strategy for making polymer materials with combined properties is to prepare block copolymers consisting of well-defined blocks via facile approaches.Moreover,with the formation of nano-structure the properties of polymer materials can be greatly improved. It is proposed that the nanostructures in thermosets can be alternatively created while the block copolymers can be in situ formed during curing reaction. Under this circumstance, the efficient reaction of the polymeric modifiers with the precursors of benzoxazines is required to suppress the occurrence of microscopic phase separation.In this thesis, we designed and prepared the main-chain benzoxazine muti-block copolymer and benzoxazine-terminal PPO and PEO via Mannich condensation which were used to prepare nanostructured thermosets of benzoxazine resins based on different mechanisms. 1 A thermally-crosslinkable multiblock copolymer consisting of main-chain polybenzoxazine and poly(propylene oxide) subchains was synthesized via Mannich polycondensation among bisphenol A (BPA), 4,4'-diaminodiphenylmethane (DDM), amino-terminated poly(propylene oxide) (NH₂-PPO-NH₂) and paraformaldehyde. The multiblock copolymer was incorporated into polybenzoxazine to access the nanostructured PBa thermosets. The morphology of the nanostructured PBa thermosets were investigated by means of atomic force microscopy (AFM) and small angle X-ray scattering (SAXS). The formation of the nanostructures in the thermosetting composites has been interpreted on the basis of the mechanism of reaction-induced microphase separation in terms of the difference in miscibility of PPO and ph subchains. Owing to the big difference in thermal stability between thermosetting polybenzoxazine matrix and poly(propylene oxide) nanodomains, the nanostructured thermosets were subjected to the pyrolysis at moderate temperatures to remove poly(propylene oxide) component, to access the nanoporous polybenzoxazine thermosets. The nanoporosity of the resulting PBa thermosets was investigated by means of Fourier transform infrared spectroscopy (FTIR) and field-emission scanning electronic microscopy (FESEM). 2 We prepared a ba-Terminated PPO and PEO polymer via Mannich polycondensation, then incorporate them into benzoxazine resins to obtain the nanostructured thermosets and the nanostructures were characterized by means of atomic force microscopy (AFM), small-angle X-ray scattering. The formation of the nanostructures has been interpreted on the basis of the occurrence of the reaction of terminal groups of ba-PPO-ba and ba-PEO-ba with ba and this reaction could results in formation of block copolymer composed of PPO or PEO and PBa. Due to the presence of the block copolymer in situ produced, the phase separation of PPO or PEO induced by reaction was confined to the nanometer scale. The glass transition behavior of the nanostructured thermosets were investigated by means of differential scanning calorimetry (DSC) and the thermal stability of nanostructured thermosets was investigated by means of thermogravimetric analysis (TGA). In contrast, at the same time wo prepared benzoxazine resins containing PPO and PEG of the same molecule.With the help of scanning electronic microscopy (SEM) we found that benzoxazine resins containing PPO and PEG were macro-phased.