Synthesis, Curing And Thermal Decomposition Kinetics Study Of Butylamine-based Fluorene-containing Benzoxazine

Benzoxazine polymers could be deemed as an alternative to traditional phenolics becauseof the similar main chain structures. Benzoxazines provide tremendous freedom in moleculardesign which can modify products based on the structure-property relationships. Furthermore,they have demonstrated various attractive properties such as no release by-products duringcuring, low water absorption, high glass transition temperature (Tg), near-zero volumetricchange upon curing, high char yield, good mechanical and dielectric properties. Recently,with the rapid development of microelectronics and aerospace industries, the demand of highperformance benzoxazines have increased tremendously in this fields.Benzoxazine monomers are synthesized by solvent method,9,9’-bis(4-hydroxyphenyl)-fluorene (BHPF), formaldehyde and butyl amine (n-butylamine, sec-butylamine, tert-butyl-amine and iso-butylamine) as raw materials in this paper. The chemical structures ofbenzoxazine monomers are characterized by Fourier transform infrared (FT-IR) spectroscopy,proton and carbon nuclear magnetic resonances (1H NMR and13C NMR). The polymerizationbehavior, curing kinetics, polymer network structure, thermal performance, thermaldecomposition kinetics and the mechanical performance of blending resins forbutylamine-based fluorene-containing benzoxazines are investigated by differential scanningcalorimetry (DSC), dynamic mechanical analyzer (DMA), FT-IR and thermogravimetricanalysis (TGA).Curing kinetics of butylamine-based fluorene-containing benzoxazines are researched bythe non-isothermal and isothermal DSC methods. Curing behaviors of benzoxazines by theisothermal method are the characteristic of autocatalyzed reactions which curing processcontains chemical kinetic control and diffusion control. A good fit of the experimental datahas been obtained in the early stage, but the deviations have been observed in the later stage,particularly near vitrification when the reaction is primarily diffusion-controlled bycomparing between the experimental datas and autocatalytic model datas. In order to describethe whole curing process, the diffusion control mechanism must be taken into account toformulate a model to describe the behavior of the reaction evolution in the later stage. Thediffusion-modified model improves significantly from the kinetic model and shows goodagreement with the experimental data. The thermal decomposition kinetics and mechanisms of fluorene-containing benzoxazine and hybird systems are researched.The curing reaction of fluorene-based benzoxazine followes the thermal ring-openingpolymerization of1,3-benzoxazines and the chains are connected by Mannich bridge,intermolecular and intramolecular hydrogen bonding, in addition, fluorenyl has excellentthermal performance. In the end, butylamine-based fluorene-containing benzoxazines havethe high glass-transition temperature (Tg) and the good thermal stability. The thermalperformance of the branched structure benzoxazine is better than the linear chain. The Tgvalue of the t-butylamine-based fluorene-containing benzoxazine (B-t-bbf) is250oC, thethermal decomposition temperatures of the weight loss5%and10%are206and270oC,respectively. The char yield is36%in800oC. The performances of B-t-bbf are superior ton-butylamine-based fluorene-containing benzoxazine (B-n-bbf) and aromatic-basedbenzoxazines. The reason may be related with the crosslinking density and the molecularstructure of the polymers. The free volume of the aliphatic-based benzoxazines is smaller thanthe aromatic-based benzoxazines. The free volume of B-t-bbf is smaller than the otherbutylamine-based benzoxazines, however, the thermal performance is the most optimum.BHPF has a structure of four phenyl rings connected to a quaternary carbon leads tosevere rotational hindrance of the phenyl groups. The stiff, bulky cardo moiety must arisefrom the higher rigidity of fluorene skeleton in the chain backbone. The toughness propertiesof flourene-based benzoxazine resins are modified due to the introduction of epoxy resins andpolyurethanes. The results lead to the enhancement of crosslinking density, thermaldecomposition temperature and thermal stability of the polymers, decrease the Tg. The charyield is increased with the concentrations of benzoxazine in the blending resins. Furthermore,these composites show a good processability, which can be hardly achieved by otherhigh-performance composites.