Synthesis,Polymerization,and Performance Of Multifunctional Fluorene Based Benzoxazines

Polybenzoxazines,as a class of thermosetting phenolic resins formed by the cationic ring-opening of the corresponding benzoxazine monomers without any added initiator,provide some outstanding characteristics such as excellent thermal properties and flame retardance,no generation of byproducts,near zero shrinkage upon polymerization,low dielectric constant,and low moisture absorption.In particular,it is recognized that polybenzoxazines have extraordinary molecular design flexibility compared with other polymers.This allows designing of polybenzoxazine structures with a wide range of properties.Fluorene molecule contains two benzene rings linked with a five-membered ring and provides high overlaps of ?-orbitals.Polymers containing fluorenyl structure in the backbone have many excellent improved properties,such as good heat-resistance and flame-retardant,high char yield,high limited oxygen index,photo stability and chemical stability.Therefore,the development of these polymers,e.g.polyimide,epoxy resin,polycarbonate and polybenzoxazine etc.,has attracted extensive research interests during the past few years.In this paper,9,9-bis(4-aminophenyl)-2,7-dihydroxy-fluorene(BADHF)was synthesized via direct condensation of 2,7-dihydroxy-9-fluorenone and aniline using methylsulfonic acid as catalyzer.And then,a series of asymmetric N-wholly aromatic multi-functional fluorene-based benzoxazine monomers(N-WAMFB)were synthesized from the reaction of BADHF,aniline,paraformaldehyde,and unsubstituted/substituted phenol including phenol,p-(tert-octyl)phenol,nonylphenol,and cardanol by two-stage Mannich condensation reaction.Asymmetric N-semi-aromatic multi-functional fluorene-based benzoxazine monomers(N-SAMFB)were obtained from the reaction of BADHF,salicylaldehyde,paraformaldehyde,and primary amines including ethylamine,n-butyl amine,n-hexyl amine,n-octyl amine,cyclohexylamine,ethanolamine,2-(2-aminoethoxy)ethanol,allylamine,and 2-furfurylamine using the combination of two-step synthesis and classical one-step Mannich condensation reaction strategies.The structures of intermediates and benzoxazine monomers were identified via Fourier transform infrared(FT-IR)and hydrogen nuclear magnetic resonance(1H NMR).The polymerization behavior,curing kinetics,thermomechanical properties,and thermal stability of the obtained novel benzoxazines and their blending resins,including traditional phenol-aniline-based monofunctional benzoxazine monomer(P-a)or bisphenol-A-aniline-based benzoxazine monomer(BA-a),were measured by using rheometer,differential scanning calorimetry(DSC),FT-IR,dynamic themomechanical analyzer(DMA),and thermogravimetric analysis(TGA).Besides,the fracture surfaces of copolymer samples were observed by scanning electron microscope(SEM).The curing reaction of asymmetric multi-functional fluorene-based benzoxazines follows the thermal ring-opening polymerization of 1,3-benzoxazines.The chains of polymers are connected by Mannich bridge and inter-and intra-molecular hydrogen bonding.N-SAMFBs containing hydroxyl groups derived from ethanolamine and 2-(2-aminoethoxy)ethanol have short curing induction time,relatively low curing temperature,and increasing interaction of intermolecular and intramolecular hydrogen bonds in the network structure of polybenzoxazines due to the presence of-OH groups.The results by the dynamic DSC show that the curing behaviors of asymmetric multi-functional fluorene-based benzoxazines display the characteristic of auto-catalyzed reaction.The apparent activation energy values(Ea)of multi-functional fluorene-based benzoxazines calculated by Kissinger and Ozawa methods are 108.1?145.4 kJ/mol and 110.8?146.5 kJ/mol,respectively.For N-WAMFB,the Ea values remarkably decrease with the increasing alkyl chains on the substituted phenol.For N-SAMFB,however,there are small changes of the Ea values as the increasing segment length of aliphatic amines.N-SAMFBs containing hydroxyl groups display high Ea values due to abundant hydrogen bonds in the polymer network.Furthermore,the Ea values of N-SAMFB containing polymerizable groups,such as allyl and furan groups,are lower than that of other benzoxazines because of their high reactivity.The variation trend of activation energy versus degree of curing was estimated by Starink method.Except for N-SAMFBs containing hydroxyl groups,the activation energy values of multi-functional fluorene-based benzoxazines gradually increase with the degree of conversion from the start of curing intermediate stage.For bifunctional fluorene-based polybenzoxazines,pendent bulky fluorene unit protrudes vertically from the main chain of polymer.In this study,fluorene units are present in the chain backbone of polybenzoxazines,which restrain the internal rotations and thermal motion of polymer segments.Besides,multi-functional fluorene-based benzoxazines possess more polymerizable groups,resulting in the improvement of crosslinking density of polybenzoxazine.Therefore,the novel polybenzoxazines display high glass transition temperature(Tg)and high thermal decomposition temperature.For N-SAMFB,the Tgs and thermal stabilities of polybenzoxazines decrease with the increasing aliphatic chain length in benzoxazine structure,which are still higher than those of the corresponding bifunctional fluorene-based polybenzoxazines.In the meantime,these benzoxazine monomers with low melting point show good processibility.And the toughness of their polybenzoxazines greatly improves due to the introduction of flexible alkyl side groups in the backbone.For cured N-SAMFBs containing hydroxyl groups,the interaction of hydrogen bonding are enhanced in the network structure of polymers compared to the other polybenzoxazines,resulting in the higher Tg values ascribed to the tighter packing of plymer chains.However,the hydrogen bonding would weaken significantly above Tg,and the thermal stability of polymer decreases.The introduction of polymerized groups into benzoxazine chains can improve the crosslinking density of polymer.As a result,Tgs and thermal stabilities of polymer increase significantly.Finally,some multi-functional fluorene-based benzoxazines were used as a modifier to improve the performance of conventional phenol-aniline-based benzoxazine(P-a)and bisphenol A-aniline-based benzoxazine(BA-a)by blending with some multi-functional fluorene-based benzoxazines.The results indicate that the multi-functional fluorene-based benzoxazines can copolymerize with P-a or BA-a.By incorporation of fluorene ring and more oxazine rings into polymer backbone,the crosslinking density and chain rigidity of copolymers improve,leading to the enhancement of Tg and thermal stabilities.Among them,the Tg value of copolymer of BA-a and furfurylamine-based N-SAMFB of 30%is 55 ?higher than that of poly(BA-a).And the values of 5%and 10%weight loss temperatures and the char yield at 800 ? are increased by 30 ?,46 ?,and 26.1%,respectively,even higher than that of fluorene bisphenol-aniline-based polybenzoxazine.Besides,the storage modulus of copolymer observably decreases due to the introduction of flexible alkyl chains in the backbone,indicating that the brittleness of copolybenzoxazines are greatly modified.