Synthesis, Polymerization Behavior And Thermal Properties Of Fluorene Diamine-based Benzoxazines

In this paper, a novel ether-containing fluorene diamine,9,9-bis(4-(4-aminophenoxy)phenyl)fluorene (BAOFL), was synthesized in two step. First,9,9-bis[4-(4-nitrophenoxy)phenyl] fluorene was prepared by nucleophilic aromatic halogen displacement ofp-chloronitrobenzene with9,9-bis(4-hydroxyphenyl)fluorene in the presence of potassiumcarbonate in DMF. And then, ether-containing fluorene diamine monomer was obtained bycatalytic hydrogenation of the dinitro compound using hydrazine hydrate and Pd/C catalyst inrefluxing ethanol. The total yield was77%. Ten benzoxazine monomers were synthesizedusing9,9-bis(4-aminophenyl)fluorene (BPF) or BAOFL, paraformaldehyde, and phenolsincluding phenol, o-cresol, m-cresol, p-cresol,2,4-xylenol, and2,3,5-trimethylphenol asstarting materials by one-step method. The yield was72-83%. The structure of monomers wasidentified via Fourier transform infrared (FT-IR), proton and carbon nuclear magneticresonances (1H NMR and13C NMR). The polymerization behavior, curing kinetics, polymernetwork structure, and thermal stability of benzoxazine were measured using differentialscanning calorimetry (DSC), dynamic themomechanical analyzer (DMA), FT-IR,1H NMRand thermogravimetric analysis (TGA).The curing reaction of fluorene diamine-based benzoxazines follows the thermalring-opening polymerization of1,3-benzoxazines, and the chains are connected by Mannichbridge, intermolecular and intramolecular hydrogen bonding. The results by the non-isothermal methods show that the curing behaviors of fluorene diamine-based benzoxazinesdisplay the characteristic of autocatalyzed reaction. The activation energy values calculated byKissinger and Ozawa methods are around101~133kJ/mol for fluorene-based diaminebenzoxazine monomers. Meanwhile, different reaction system of dynamic parameters islargely difference. Autocatalytic model displays a good agreement with the experimentalresults over the whole range of cure, and can be used to describe and predict the curingreaction of benzoxazine except for2,4-dimethyl phenol-based benzoxazine.Comparing with bisphenol flourene type polybenzoxazines, the fluorene diamine-basedpolybenzoxazines display high thermal decomposition temperature, high glass transitiontemperature (Tg), and good thermal mechanical properties attributed to the increase of thecrosslinking density and molecule weight of polymer because there are more crosslink vacant para positions in the phenol part. Meanwhile, the introduction of fluorene ring to the chainbackbone improves the rigidity of polymer chain, which restrict the inner circumrotate andthermal movement of polymer chain, resulting in the improvement of thermal properties ofpolybenzoxazines. For different blocking the ortho, para, and meta positions in the aromaticrings of the monomer, the Tgvalue of polybenzoxazine of the free ortho and para sites of thephenolic ring such as phenol is the highest in all polymers. When either the ortho or para siteon the phenol is blocked, the glass transition temperatures of polymer drop dramatically,especially for the ortho-substituted compounds. The meta-substituted compounds, whicheither of the two primary reaction sites is not blocked, have a similar electron density withinthe benzene ring as the ortho and para-substituted compounds. Thus, the m-cresol basedpolybenzoxazine is below the unsubstituted compounds, but is well above those of the orthoand para-substituted benzoxazines. When the free ortho and para sites in phenolic ring arereplaced, the oxazine ring opening does not happen until high temperatures formingN,O-acetal-type structure of low molecule weight.Although the introduction of bulky fluorene into the benzoxazine chains improves thethermal properties of polymer, high rigidity of fluorene skeleton give rise to the increase ofbrittleness of polymers. Consequently, its commercial use is limited. The toughness propertiesof flourene-based benzoxazine resins are modified by the introduction of the ether bond tofluorene-based diamine skeleton. The thermal properties of cured polymers were evaluatedwith DSC and TGA. The result indicates decrease of the initial thermal decompositiontemperature, char yields, and Tgvalues of the polymers is unconspicuous, however, themechanical property of polymers is greatly improved.