| Molecular composites can achieve excellent molecular strengthening and toughing effects by uniformly dispersing the rigid-rod macromolecules in the matrix at the molecular level,which provides a method to obtain composites with high performance,low density and good processability.However,although the concept of molecular composites has been put forward for 40 years,it is still difficult for rigid-rod macromolecules to be uniformly dispersed in flexible polymer matrix,which can easily lead to phase separation and poor reinforcement effect.For example,poly（p-phenylene terephthalamide）（PPTA）is a typical polymer with rigid-rod main chain,which has been widely used in the areas of high performance fiber and the fields of molecular composites because of its outstanding heat resistance and mechanical properties.However,PPTA can only to be processed when being dissolved in concentrated sulfuric acid,so it is difficult to disperse PPTA in the matrix at the molecular level.Therefore,improving the solubility of PPTA would help to further apply it to the preparation of molecular composites.In this study,the solubility of PPTA in organic solvents was significantly improved by introducing amino side groups into the main chain of PPTA.Moreover,the rigid-rod aminated PPTA was used to improve the strength and toughness of epoxy resin and their carbon fiber composites,and the microstructure,strengthening effect and strengthening mechanism were studied.First,paraphthaloyl chloride and 2-nitro-1,4-phenylenediamine were used as monomers to synthesize nitro-substituted poly（p-nitrophenylene terephthalamide）（NO2-PPTA）by low-temperature solution polycondensation and its solubility was studied.The mechanical properties of NO2-PPTA continuous fiber,NO2-PPTA/PMIA composite membrane and NO2-PPTA/PEI composite fiber membrane were further investigated.NO2-PPTA can be dissolved in DMAC/Li Cl for wet-spinning to obtain a golden continuous fiber with a maximum tensile strength of 465.5±14.6 MPa.The addition of 10%NO2-PPTA can increase the tensile strength of NO2-PPTA/PMIA composite membrane by 16%.Moreover,the addition of 10%NO2-PPTA can also increase the tensile strength of NO2-PPTA/PEI composite fiber membrane by 80%.Second,amino-substituted poly（p-animophenylene terephthalamide）（NH2-PPTA）was prepared by hydrogen-reduction of NO2-PPTA with a conversion of 91%.NH2-PPTA can react with epoxy and it was added as a reinforcing agent into triglycidyl-p-aminophenol（TGPAP）epoxy resin and epoxy/carbon fiber（EP/CF）composites.Chemical structure of NH2-PPTA and its dispersion in the epoxy matrix were characterized,the strengthening and toughening mechanisms of NH2-PPTA modified epoxy were investigated.At low contents,NH2-PPTA disperses at molecular level in the matrix.With the increase of NH2-PPTA content,the nanorod or nanofiber-like aggregates appeared.Stress can be effectively transfered from the matrix to the NH2-PPTA main chain with high modulus and high strength through the chemical bond between NH2-PPTA and epoxy resin,and a large amount of energy can be absorbed by the debonding,pull-out and deformation of NH2-PPTA.The addition of NH2-PPTA can significantly improve the mechanical properties of NH2-PPTA/EP,and does not reduce the heat resistance of the EP.The tensile strength,Young’s modulus,elongation at break and tensile toughness were increased by 74%,34%,30%and 131%,respectively;the flexural strength,flexural modulus and flexural toughness were increased by 41%,9%and 143%,respectively;the notch impact strength was also increased by 14%.Moreover,NH2-PPTA can also improve the mechanical properties of NH2-PPTA/EP/CF composites.Third,nitroterephthalic acid and 2-nitro-1,4-phenylenediamine were used as monomers to synthesize fully nitrated poly（p-nitrophenylene nitroterephthalamide）（（NO2）2-PPTA）by direct polycondensation in triphenylphosphine,hexachloroethane and pyridine solution.Different from those of NO2-PPTA,all the benzene rings in the main chain of（NO2）2-PPTA contain nitro groups,so it has better solubility in organic solvents.Fully amino-substituted poly（p-aminophenylene aminoterephthalamide）（NH2）2-PPTA was prepared by hydrogen-reduction of（NO2）2-PPTA.（NH2）2-PPTA was used as the strengthening and toughening agent of EP and EP/CF composites.The dispersion of（NH2）2-PPTA in epoxy and the strengthening and toughening effect and mechanism were explored.Compared with NH2-PPTA,（NH2）2-PPTA has better compatibility and stronger interfacial force with epoxy matrix,which can transfer the stress more efficiently and absorb more energy,so as to have a more remarkable improvement effect on the mechanical properties of epoxy resin.The addition of only0.5 wt%（NH2）2-PPTA significantly improves the tensile strength,Young’s modulus and tensile toughness of epoxy by 63%,44%and 72%,respectively,the flexural strength,flexural modulus and flexural toughness of epoxy by 58%,68%,36%,respectively,and the notch impact strength of epoxy by 41%.Among them,the tensile strength and flexural strength are up to 140.6±4.1 MPa and 237.9±12.3 MPa,while the tensile strength and flexural strength of epoxy resin nanocomposites modified by graphene,carbon nanotubes and carbon nanofibres reported so far are mostly no more than 100MPa and 200 MPa.Meanwhile,the addition of（NH2）2-PPTA significantly enhanced the interfacial strength and interlaminar toughness between CFs and epoxy matrix in（NH2）2-PPTA/EP/CF composites,which effectively prevented the abhesion of CFs and the matrix and improved the mechanical properties of the composites more significantly,the flexural strength,flexural modulus,flexural toughness and the interlaminar shear strength（ILSS）were increased by 22%,35%,13%and 25%,respectively.Finally,（NH2）2-PPTA was further used to modify heat resistant epoxy resin tetrachylglyceryl diaminodiphenyl methane（TGDDM）and EP/CF composites using4,4’-diaminodiphenyl sulfone（DDS）as the curing agent.Possibly because of the difficult dispersion of（NH2）2-PPTA in matrix due to the relatively high viscosity of TGDDM/DDS,the improvement of the mechanical properties of TGDDM system is slightly less than that of TGPAP system.However,stronge enhancement effect can be achieved with less（NH2）2-PPTA addition amount at 0.5 wt%,the tensile strength,Young’s modulus and tensile toughness of epoxy are improved by 28%,44%and 26%,respectively;the flexural strength,flexural modulus and flexural toughness of epoxy are improved by 42%,37%,56%,respectively;the notch impact strength of epoxy is improved by 31%.Meanwhile,the flexural strength,flexural modulus,flexural toughness and the ILSS of（NH2）2-PPTA/TGDDM/DDS/CF were increased by 17%,18%,12%and 16%,respectively.Therefore,（NH2）2-PPTA can be used to strengthen and toughen more epoxy resins,polymer matrixes and CF reinforced composites in the future. |
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