| Due to excellent mechanical properties,heat resistance and chemical resistance,para-aramid has been widely applied in military industry,aerospace and other fields.Kevlar,developed by DuPont in the 1970s,is the world’s first commercially available para-aramid and its specific strength is 5-6 times that of steel wire.In order to further improve the mechanical properties of para-aramid,the former Soviet Union develops heterocyclic aramid containing benzimidazole moiety,which has superior mechanical properties than Kevlar.However,its axial compressive strength is relatively low,like that of other high-performance organic fibers.The common methods to improve the compressive strength of aramid include introducing chemical crosslinking structure,blending inorganic nano-materials and coating inorganic materials.Although these methods can improve compressive strength of aramid,tensile strength always declines obviously after modification.Therefore,how to improve the compressive strength of aramid while maintaining its tensile strength is still a challenge.In addition to compressive strength,how to develop new polymerization methods of heterocyclic aramid is also important.The introduction of diamine monomers with asymmetric structure destroys the chains regularity of heterocyclic aramid and reduces the chain rigidity simultaneously,which makes it soluble in a mixture of N,Ndimethyl acetamide(DMAc)/lithium chloride(LiCl),and can be directly processed through wet-spinning.However,the addition of co-solvent LiCl is relatively high,and LiCl is expensive,which leads to high production cost.In addition,due to high reactivity of amino and acyl chloride in the polymerization process,low-temperature polymerization is required in the initial stage.Molecular weight distribution of the product is relatively wide,and the chain sequence structure is difficult to be precisely controlled.Therefore,it is of great significance and value to develop a low-cost solvent system suitable for polymerization of heterocyclic aramid and a new method which can control molecular weight distribution and chain sequence structure.Based on problems mentioned above,heterocyclic aramid fiber with benzimidazole structure was selected for the research.In order to enhance compressive strength of heterocyclic aramid,we proposed two post-treatment methods to construct crosslinking structure in heterocyclic aramid.The first was to utilize trace amount of oxygen to catalyze cross-linking between benzimidazole units and the second was to bring external crosslinker into fiber during swelling process.The excellent tensile strength of the fiber was maintained while the compressive strength was improved remarkably.Further,inspired by space grid structure in architecture science,we proposed constructing dendritic support structure in heterocyclic aramid to improve its compressive strength.In order to develop the new polymerization method of heterocycle aramid,we proposed utilizing in-situ ionization of HCl,the by-product of aramid polymerization,to produce Cl-to destroy the interaction between macromolecular chains.The solubilization of heterocyclic aramid was realized without addition of LiCl,and the low-cost solvent system of heterocyclic aramid fiber was developed.Secondly,the condensation reaction of amino and acyl chloride was regulated through the coordination reaction between amino and heavy metal ions,which realized the control of its chain sequence structure and molecular weight distribution.The results of this study can provide a theoretical basis for the polymerization of heterocyclic aramid and preparation of fibers with high compressive strength.The main research results of this paper are as follows:(1)Preparation of heterocyclic aramid with high compressive strengthHow to construct chemical crosslinking structure under mild conditions is the key to improve the compressive strength and maintain tensile strength of aramid.In this study,heterocyclic aramid(PABI)containing benzimidazole moiety was prepared by solution polymerization,followed by wet spinning and high-temperature annealing.Due to conjugation effect of benzimidazole units,the benzene ring in the benzimidazole moiety of PABI fiber has higher electron density,which leads to the dehydrogenation coupling reaction above 330℃ and forms cross-linked structure.At the same time,it is found that oxygen has a significant catalytic effect on the dehydrogenation coupled cross-linking reaction,and the introduction of trace oxygen can effectively improve the cross-linking density of the fiber.When oxygen concentration is 50ppm during heat-treatment,the compressive strength of the fiber increases by 27%,and the tensile strength maintains nearly 94%.However,with the increase of oxygen concentration,the tensile strength of the fiber gradually decreases,due to degradation caused by oxygen at high temperature.Therefore,the tensile strength and compressive strength of the fiber can be effectively balanced by adjusting oxygen concentration.In addition,the interaction between skin and core layers of the fibers is further improved by cross-linking.So,no detach of the skin layer is found after debonding of the interface and the interfacial shear strength(IFSS)of the fiber increases by nearly 27%.Although trace amount of oxygen can significantly catalyze the dehydrogenation coupling reaction of benzimidazole moiety in heterocyclic aramid and construct crosslinking structure between macromolecular chains.However,the introduction of oxygen will inevitably lead to breakage of polymer chains at high temperature,which will lead to the decrease of tensile strength.Therefore,we proposed to use benzimidazole moiety of heterocycle aramid as the grafting site to avoid the damage of chemical structure.We selected 4-bromomethylbenzoic acid(BBA)as the crosslinking agent and dissolved it in DMAc,followed by swelling heterocyclic aramid to bring BBA into the fiber and heat-treating at 320℃.During heat-treatment,the bromomethyl of BBA will react with the NH of benzimidazole units in heterocyclic aramid above 80℃,and then BBA is grafted onto polymer chains.When the heat treatment temperature is above 300℃,the carboxylic groups of BBA grafted on the polymer chains undergo decarboxylation coupling reaction,and then the cross-linking structure forms.Crosslinking density of the fiber can be adjusted by controlling concentration of crosslinker.The compressive strength of the fiber modified by 0.3wt%BBA solution can be increased by nearly 90%and the tensile strength can maintain by more than 97%.In addition,some benzimidazole moieties on the surface of the modified fiber did not graft with BBA.Therefore,the residual benzimidazole units on fiber surface was utilized for second modification.At lower temperature(<300℃),BBA was grafted with benzimidazole units on fiber surface while maintaining its carboxyl groups,thus the polarity of the fiber surface was improved.After modification,the water contact angle of the fiber decreases from 107° to 100°,and IFSS increases by nearly 27%.Therefore,the axial compressive strength and interfacial shear strength can be improved synergistically by the post-treatment modification.However,this method is not applicable to other high-performance organic fibers without benzimidazole units.To further expand the application of the method,we synthesized a cross-linking agent(APA)containing alkynyl group,which was introduced into the fiber by swelling.The alkyne group can cross-link and form semi-interpenetrating structure with the macromolecular chain of the fiber above 230℃,which enhances its transverse interaction.The compressive strength of Kevlar49,PBO-HM and heterocyclic aramid fibers modified by this method is increased by 70%,36%and 72%respectively.Because this method does not depend on the chemical structure of the fiber itself,it shows obvious enhancement effect on the compressive strength of various high-performance organic fibers.In order to further improve compressive strength of aramid,we constructed dendritic support structure in heterocyclic aramid.Nano SiO2 was selected as the supporting center,and hyperbranched aromatic polyamide was grafted on its surface.Firstly,nano-SiO2 was modified by KH550,and the amino group was introduced on the surface.The hyperbranched polyamide was grafted onto SiO2 by the reaction of 3,5diaminobenzoic acid with amino groups on surface of SiO2.Then,the SiO2 derivative was blended with heterocyclic aramid solution,followed by wet-spinning and heattreatment to prepared aramid-SiO2 composite fiber.Because the grafted hyperbranched polyamide has similar chemical structure with heterocyclic aramid,it has good compatibility with fiber matrix.In addition,the grafted hyperbranched polyamide can form hydrogen bond interaction and π-π interaction with heterocyclic aramid,which makes SiO2 derivates show good dispersibility in matrix.More importantly,the chains of hyperbranched polyamides tend to grow in multiple directions due to the steric hindrance effect according to the results of molecular simulation.Compared with linear aromatic polyamides,hyperbranched polyamides exhibit a unique threedimensional structure,which makes it have unique properties in stress transfer and dispersion.When content of SiO2 derivate modified by hyperbranched polyamide in matrix is 0.5wt%,the compressive strength and tensile strength of heterocyclic aramid can be increased by 114%and 13%respectively.But adding the same content of SiO2 derivate modified by linear polyamide,compressive strength of the fiber increases by only 53%.Therefore,it is proposed that stress can be transferred from polymer chains to supporting center(SiO2)along multiple directions due to three-dimensional structure of the hyperbranched polyamide,like dendritic support structure.The structure is more beneficial for stress dispersion and transfer,which significantly improves the compressive strength of the fiber.In addition,it is found that there are lots of amino groups on surface of SiO2 derivate modified by hyperbranched polyamide according to the results of X-ray photoelectron spectroscopy(XPS).Therefore,C=C was introduced on its surface by reaction of amino group with acryl chloride.Then,it was blended with the heterocyclic aramid oligomer end-capped with C=C,and the composite film was prepared by poor solvent induced phase transfer method.After heat treatment at 360℃,the C=C on the surface of SiO2 derivatives reacted with the C=C of the heterocyclic aramid oligomer.Then,a three-dimensional network structure is constructed in the heterocyclic aramid film by utilizing the threedimensional structure of hyperbranched polyamide,which can extend path tortuosity in the process of dendritic breakdown.Breakdown strength of the film can be increased from 445kV/mm to 624kV/mm,and a novel aramid film with high breakdown strength is developed.(2)Exploration of polymerization method of heterocyclic aramidDue to rigid chain structure and intermolecular hydrogen bond interaction,heterocyclic aramid exhibits poor solubility in common polar organic solvents such as DMAc.In order to make it dissolve,it is often necessary to add LiCl in the solvent.Cl-produced by LiCl is a strong hydrogen bond acceptor,which can interact with amide bonds of aramid and form hydrogen bonds,thus breaking inter-molecular hydrogen bonds and making it dissolved.HCl is a by-product of the polymerization process of aramid,and can also be used as a potential donor of Cl-.However,HCl mainly exists in the form of molecules in DMAc,and can’t ionize a large amount of free Cl-.Therefore,if the by-product HC1 can be in situ converted into Cl-,it is possible to replace LiCl,and then realize the solubility of heterocyclic aramid.According to the proton theory of acid and base proposed by Br?nsted and Lowry,conjugate bases of weak acids,such as acetate ion,can react with strong acids,such as HCl,to form acetic acid and Cl-.Therefore,sodium acetate was selected as the co-solvent,and the byproduct HCl was in situ converted into Cl-by acetic acid ions.In the presence of sodium acetate,heterocyclic aramid complexed with HCl could be dissolved in DMAc and showed good solubility.In addition,potassium acetate,zinc acetate and other acetate salts can also show good solubility effect on heterocyclic aramid.Then,the mechanism of solubility was further studied by ultraviolet spectroscopy(UV-Vis),atomic absorption spectroscopy(AAS)and conductivity test of solution.It is found that in DMAc,strong acids such as HCl can react with the conjugate base of weak acids such as acetate ion to form acetic acid which is difficult to ionize and strong acid conjugate base Cl-.The Cl-can interact with the amide bond to form hydrogen bond and thus dissolve the heterocyclic aramid.Therefore,HCl as a by-product of polymerization is used to dissolve heterocyclic aramid without adding Cl-donor.However,low solubility of acetate in DMAc limits its application in the polymerization.Based on similar mechanism,imidazole with excellent solubility in DMAc was used as the co-solvent to solve this problem.Due to its stronger proton affinity,imidazole can rapidly complex with HCl and form imidazole hydrochloride,which is a strong electrolyte and can ionize large amount of Cl-.The ionized free Clalso shows significant solubility effect on heterocyclic aramid.In the polymerization process of heterocyclic aramid,the reactivity of amino group and acyl chloride is high and the reaction rate is comparatively fast.Therefore,it is difficult to control the reaction kinetics and the chain sequence structure of the products.The condensation reaction of amino and acyl chloride is a nucleophilic substitution reaction,which is realized by the nucleophilic attack of p-electrons of amino.So how to control the nucleophilicity of amino group is the key to control the polymerization.In this research,the nucleophilicity of the amino was inhibited by the coordination of Cu2+ with the p-electron of the amino,and then the reaction kinetics was regulated.The coordination ability of Cu2+ with different diamine monomers is significantly different.For example,coordination ability of p-phenylenediamine(PDA)and 3,4-diaminodiphenyl ether(3,4-ODA)with Cu2+ is strong,and coordination reaction equilibrium constant is high.Therefore,Cu2+ has a more significant inhibition effect on its reactivity.As for diamine monomers with-CF3 side group,the coordination ability of amino is reduced due to the strong electron-absorbing effect of-CF3.Therefore,Cu2+ has little effect on its reactivity.The differences in the coordination ability between different diamine monomers and Cu2+ can significantly’enlarge’ their reactivity differences with acyl chloride.Then,the diamine monomer with weak coordination ability can react with acyl chloride to form corresponding chain segments in the early stage of reaction.Because the coordination reaction is reversible,the diamine monomer with stronger coordination ability begins to undergo the un-coordination reaction with Cu2+,releasing the amino p-electron,restoring its reactivity with acyl chloride and forming the corresponding chain segment when the monomer with weaker coordination ability is exhausted.Thus,the block polymer was prepared by the ’one-pot polycondensation strategy’.Results of 1H-NMR spectra show that the copolymer prepared by this method has the chemical structure characteristics of block polymer.In addition,Cu2+ can reduce the reaction rate of amino and acyl chloride,avoiding heat accumulation caused by rapid reaction,and narrowing the molecular weight distribution of products. |
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