Synthesis And Properties Of Quinoxaline-based Benzoxazines

Polybenzoxazines,one of thermosetting phenolic resins,which are formed by the thermal polymerization of their corresponding benzoxazine monomers without any catalysts,have become a hotspot of high performance polymers due to their near-zero volumetric change upon curing,excellent thermal stability,high char yield,good mechanical and dielectric properties,and low flammability.Conventionally,benzoxazine monomers are prepared from phenols,primary amines and formaldehyde.By varying the starting phenols and amines,a wide range of molecular design flexibility can be achieved.The phenol-substituted quinoxaline rings own large molar volume,high band energy,and weak polarity.The poly?phenylquinoxaline?s,which are derived from the phenol-substituted quinoxaline rings,have many desirable properties such as high glass transition temperatures,excellent processability,low dielectric constants,high chemical resistance,and good mechanical properties.In this paper,four kinds of quinoxaline-based benzoxazine monomers were synthesized via molecular design at first.A series of diamine-based benzoxazine monomers containing quinoxaline units were synthesized from the reaction of 2,3-bis-m-aminophenylquinoxaline,phenols and paraformaldehyde.The bisphenol-based benzoxazine monomers possessing quinoxaline groups were prepared from the reaction of 2,3-bis-p-hydroxylphenyl-6-aminoquinoxaline,primary amines and paraformaldehyde.Triamine-based benzoxazine monomers containing quinoxaline groups were derived from the reaction of 2,3-bis-m-aminophenyl-6-aminoquinoxaline,salicylaldehyde compounds and paraformaldehyde using a“one-pot”method.Monoamine-bisphenol-based benzoxazine monomers possessing quinoxaline units were prepared from the reaction of 2,3-bis-p-hydroxylphenyl-6-aminoquinoxaline,salicylaldehyde,primary amines and paraformaldehyde using the combination of“one-pot”method and classical Mannich condensation reactions.The structures of quinoxaline-based benzoxazine monomers and their intermediates were confirmed by fourier transform infrared?FTIR?,hydrogen and carbon nuclear magnetic resonance(¹H NMR and ¹³C NMR).The rheological properties and polymerization behavior of the produced benzoxazines were studied by rheometer,differential scanning calorimetry?DSC?and FTIR.After curing,the thermal and thermomechanical properties of cured quinoxaline based polybenzoxazines were measured by using DSC,dynamic mechanical analyzer?DMA?,and thermogravimetric analysis?TGA?.The results showed that the diamine-based benzoxazines containing quinoxaline groups possessed good fluidity at processing stage.Among the obtained benzoxazine monomers,the4-phenoxyphenol-diamine-based benzoxazine containing quinoxaline unit exhibited the broadest processing window,which is more than 100 ^oC,leading to an excellent processing property.The DSC curves of diamine-based benzoxazines were the typical heating exothermal curves of 1,3-benzoxazines,the peaking temperature was 254-267 ^oC.The storage modulus?E'?and glass transition temperature?T_g?of phenol-diamine-based polybenzoxazine were obviously higher than another diamine-based benzoxazines containing quinoxaline groups,since its structure has more crosslinking sites and none of soften groups such as methyl and ether groups.In addition,the thermal stability of diamine-based polybenzoxazines were better than that of bisphenol A-aniline-based benzoxazine due to the introduction of 2,3-quinoxalinediyl in their structures.For bisphenol-based benzoxazines owning quinoxaline groups,the existence of soften alphatic chains resulted in a broad processing window and low viscosity of aliphatic amine-bisphenol-basedbenzoxazines.Expectfor2-furfurylamine-bisphenol-based benzoxazine possessing quinoxaline group,the curing temperature of bisphenol-based benzoxazines and E',T_g,and weight loss 5%temperature?T₅?of their polymers are lower than the diamine-based polybenzoxazines,and decreased when the alkyl chains were incorporated and by increasing the alkyl chains length.Besides,the existence of polymerizable groups lead to a higher crosslink density,and thus higher T_g and char yield for2-furfurylamine-bisphenol-based polybenzoxazine.For triamine-based benzoxazines containing quinoxaline units,the processing window of salicylaldehyde-triamine-based benzoxazine reached 60 ^oC,indicating a good processing property.Triamine-based benzoxazines possess more oxazine rings,resulting in the improvement of crosslinking density of polybenzoxazine and tighter packing of polymer structure.Thus the triamine-based polybenzoxazines exhibited good thermal and thermomechanical properties,and were higher than bisfunctional quinoxaline-based benzoxazines.Among them,salicylaldehyde-triamine-based polybenzoxazine displayed the best thermal properties.The monoamine-bisphenol-based benzoxazines possessing quinoxaline groups affirmed excellet processing properties with the lowest viscosities were less than 1.6 Pa·s and processing windows were more than 120 ^oC.Their curing temperature are higher than the corresponding bisphenol benzoxazines.In addition,the heat resistance and thermal stability of cured polymers were also higher than corresponding bisfunctional quinoxaline-based polybenzoxazines.The E',T_g and char yield of 2-furfurylamine-salicylaldehyde-monoamine-bisphenol-based polybenzoxazine attained to 3349 MPa,423 ^oC and 69.2%,respectively.In this paper,a series of diffrent blends,i.e.,quinoxaline based benzoxazine/mono-functional benzoxazine?P-a?,quinoxaline based benzoxazine/fluorene-basedbenzoxazineoligomer?BAFO?,andquinoxalinebased benzoxazine/BAFO/P-a with multiple weight ratios were also performed.The curing behavior and properties were tested by DSC,DMA,TGA,uniseval testing machine,and SEM.The heat resistance and thermal stability of quinoxaline-based benzoxazine/P-a copolymers were improved due to the incorporation of quinoxalinediyl.When the weight percent was 30%,the T_gof2-furfurylamine-salicylaldehyde-monoamine-bisphenol-basedbenzoxazine/P-a copolymer was 77 ^oC higher than that of P-a resin.Then the structures and properties of BAFO with diferent mole ratio of phenols were studied.When the mole ratio of bisphenol-A and bisphenol-fluorene was 1:2,the ologimer showed excellent thermal and mechanical properties and was choosed as the modifier of bisfunctional quionxaline-based benzoxazines containing soften groups.The introduction of heat-resistant fluorenyl and increase of crosslinking densities improved the thermal and mechanical properties of quinoxaline-based benzoxazine copolymers.Finally,both quinoxaline-based benzoxazines and BAFO were utilized for the modification of P-a.The heat resistance,thermal stability,mechanical properties,and toughness of obtained copolymers were superior to that of poly?P-a?..