Synthesis And Cure Kinetics Of Main-chain Fluorene Benzoxazines

Benzoxazine polymers could be deemed as an alternative to traditional resin because ofthe similar main chain structures. Benzoxazines provide tremendous freedom in moleculardesigning which can modify products based on the structure-property relationships.Furthermore, they have demonstrated various attractive properties such as no releaseby-products during curing, low water absorption, high glass transition temperature (Tg),near-zero volumetric change upon curing, high char yield, good mechanical and dielectricproperties. Nowadays, the high performance benzoxazines have been widely used in the fieldof microelectronics and aerospace industries, etc.In this paper, three kinds of novel2,7-diamino-9,9-dialkyl-fluorene was synthesized inthree step. First, alkylation reaetions and nitration of fluorene were conducted to get2,7-dianitro-9,9-dialkyl-fluorene. And then,2,7-diamino-9,9-dialkyl-fluorene were obtainedby catalytic hydrogenation of the dinitro compound using hydrazine hydrate and Pd/C catalystin refluxing ethanol. The total yield was60%.2,7-dihydroxyl-fluorene was got from2,7-dihydroxy-9-fluorenone by Huang-Minlon reduction.Three kinds of diamine-based main-chain benzoxazine monomers were synthesizedusing2,7-dianitro-9,9-dialkyl-fluorene, formaldehyde, and phenols as starting materials bytwo-step method. The yield was75~80%. Then three kinds of diphenolic-based main-chainbenzoxazine monomers were synthesized using2,7-dihydroxyl-fluorene, paraformaldehyde,and phenol as starting materials by one-step method. The yield was78~81%. The structure ofmonomers was identified via Fourier transform infrared (FT-IR), proton and carbon nuclearmagnetic resonances (1H NMR and13C NMR). The polymerization behavior, curing kineticsand thermal stability of benzoxazine were measured using differential scanning calorimetry(DSC), FT-IR, thermogravimetric analysis (TGA).Its curing kinetics and curing reaction were investigated under non-isothermaldifferential scanning calorimetry (DSC) at different heating rates (5,10,15, and20℃/min).Two different methods (Ozawa and Kissinger) were employed to research the cure kinetics ofthese polybenzoxazine. The isoconversion method (Starink) provided the change of activationenergy Ea values as a function of the degree of conversion. The autocatalytic kinetic modelwas found to be the best description of the investigated curing reactions which curing process contains chemical kinetic control and diffusion control. Non-isothermal DSC curves obtainedfrom the experimental data showed a good agreement with that theoretically calculated.Macromolecules fluorenne group that was introduced in the main chain ofpolybenzoxazine, greatly improves the rigidity of the polymer chain, with the increase in thecrosslink density，the molecular weight of the polymer is increased, the bulk density increasedtherefore. the bis-aminetype main chain fluorenyl group polybenzoxazine resin showingexcellent thermal performance, far higher than the same type of bisphenol A-type benzoxazineresin, bisphenol type benzoxazine resins and bis-amine-type benzoxazine resin. In addition,with more aromatic rings in the monomer structure, the polymer have more greater thermalperformance.