Design Of Cyclotriphosphazene-Based High Performance Benzoxazine Resins And Their Properties

Benzoxazine rensins are developed class of high-performance thermosetting resinsinstead of traditional phenolic-formaldehyde resin in high-tech field. These thermosettingresins not only possess all the advantages of traditional phenolic resins such as excellentmechanical, thermal properties, dimentional stability and high chemical resistivity, butalso have unique advantages of near-zero shrinkage upon curing, thermal and flameretardant, low water absorption and remarkable molecular design fiexibility. Besides,corresponding monomers can be conveniently designed and generated, so the funcatioanalpolybenzoxazines can be easily gained and satisfied for different applications. On theother hand, cyclotriphosphazene (CP) derivatives are a typical class of organic-inorganiccompounds with a planar non-delocalized cyclic ring consisting of alternating N and Patoms. Six functional groups can be attached onto a CP ring. Due to the versatility ofcyclotriphosphazene chemistry, high stability and biocompatibility of the CP ring allowsfor a wide range of functional groups to be attached onto CP, and corresponding CP-basedderivatives can be applied in many fields. So that, combining the CP and benzoxazinetogether by chemical bonding is an interesting method to obtain high-performancebenzoxazine monomer and polybenzoxazine. Similarly, synthesizing a linear polymerprecursor with benzoxazine moieties and CP in main chain is also a good way to gethigh-performance polybenzoxazine. Besides, the performances of thermal stability andmechanic property for traditional benzoxazine and epoxy resins can also be improved byco-polymerizing with CP-base benzoxazines.A series of organic-inorganic hybrid benzoxazine monomers based oncyclotriphosphazene (CP) have been synthesized, which possess variable number oforganic benzoxazine moieties and biphenyl groups distributed on the inorganic ring ofcyclotriphosphazene. The structures of the monomers were confirmed by ¹H NMR, ¹³C NMR, ³¹P NMR and elemental analysis. The ring-opening polymerization behaviors of thenew benzoxazine monomers were investigated by Fourier transform infrared spectroscopy(FT-IR) and differential scanning calorimetry (DSC). The thermal property and mechanicperformance of the thermoset polymer were also evaluated by thermal gravimetricanalyzer (TGA) and DMA, respectively. Besides, the fracture surfaces ofco-polybenzoxazines samples were also observed by scanning electron.The polymerization activity of the monomers relied on the number of benzoxazinemoiety and biphenyl group, the monomers with six benzoxazine moieties (HBOz) waseasier to polymerize than the others, while the monomer with two benzoxazine moietiesand two biphenyl groups (BBOz) was the hardest one to polymerize. Due to the highcrosslinking nature containing rigid and stable inorganic CP ring and biphenyl groups inthe networks, the polybenzoxazines resulting from the as-prepared monomers showeddifferent thermal stability, mechanic property and humidity resistance. Thepolybenzoxazine PTBOz resulting from TBOz which contain four benzoxazine moietiesand one biphenyl group showed the highest thermal stability and glass transitiontemperature (T_g). The onset decomposition temperature of PTBOz was about 440 , thechar yield at 850 (Y_c) was 69.6%, and corresponding T_g was as high as 254. WhilePHBOz which polymerized from HBOz got the lowest thermal stability and T_g, it startedto be thermal degradation at about 400, and its Y_c and T_g were 66.9% and 152,respectively. Meanwhile, PBBOz exhibited the highest humidity resistance, and PHBOzgot the lowest, but all of the polybenzoxazines showed high humidity resistance, and thebalance water uptakes were lower than 1wt%.Based on the CP-diamine, two routes were attempted to synthesize the linear polymerprecursor with benzoxazine moieties and CP rings in the main chain. Comparing totraditional method A, higher molecular weight can be gained by method B, and themolecular weight was about two times higher than that synthesized by method A.However, the polymer precursor synthesized with method A could undergo ring-opening polymerization at lower temperature than that using method B. No matter what, both thepolybenzoxazines resulting from the polymer precursors showed excellent thermalstability.The performances of traditional benzoxazine were improved by co-polymerizing withCP-based benzoxazines. The CP-based benzoxazines could decrease the polymerizationtemperature of traditional benzoxazines. Due to the different dimensional crosslinkingstructures of the CP-based benzoxazines under ring-opening polymerization, the CP-basedbenzoxazines showed different degree of improvement on the thermal stability andmechanic properties for the co-polybenzoxazines. However, the Y_c of allco-polybenzoxazines was improved at least 36%, even higher to 60% for some samplesafter adding 20wt% CP-based benzoxazines. What's more, the T_g of theco-polybenzoxazine raised at least 10 by adding 20wt% CP-based benzoxazines.Similarly, the Y_c and T_g of epoxy/CP-based benzoxazine co-polymers were alsoimproved. The Y_c of the co-polymer was improved at least 500%, and corresponding T_gwas improved about 10 ??. However, there was an appearance of microphase separationbetween epoxy resin and CP-based polybenzoxazine, the onset decomposition temperaturedecreased with the increasing content of CP-based benzoxazines; Meantime, there waslittle change to the storage modulus of the epoxy co-polymerizing with CP-basedbenzoxazines.