Preparation,Structure And Mechanical Properties Of Organic/Organic Hybrid Heterocyclic Aramid Fiber

Para-aramid fiber possesses high strength,high modulus,low density,outstanding heat resistance and thermal stability.It has been widely applied in many fields,such as national defense,military industry,and national production and life.Generally,copolymerization is an effective method to improve the properties of aramid fiber.Heterocyclic aramid fiber is a copolymer synthesized by introducing a monomer containing benzimidazole structure into PPTA chains.It has superior tensile strength,compressive strength and interfacial shear strength compared to PPTA fiber.However,the performance of heterocyclic aramid fiber still needs to be further improved to meet the higher requirements in application,because the application field is becoming more extensive.In addition to the compressive strength and interfacial property,the tensile strength is the most basic and most important property of heterocyclic aramid fiber.Therefore,it is of great significance to research new preparation methods and principles to further improve the mechanical properties of heterocyclic aramid fiber.Herein,we prepared heterocyclic aramid fiber with higher mechanical properties based on the idea of organic/organic blending and studied the relationship between structure and performance of heterocyclic aramid fiber in detail.Firstly,through computer simulation,it is found that there exists quite a lot of free hydrogen bonding(H-bonding)sites between rigid-chain macromolecules due to high energy barrier of conformation adjustment and difficulty of chain movement studied by computer simulation.The oligomer can bridge the macromolecular chains through forming new H-bonds by virtue of its strong movement ability,which could enhance the strength of intermolecular H-bonds effectively.In addition,it is demonstrated that ultrahigh molecular weight heterocyclic macromolecule could help build a special“mainstay-body structure”in heterocyclic aramid fiber,which can improve tensile stress resistance of the fiber.Based on the results by computer simulation,we synthesized heterocyclic aramid oligomer(OPBIA)and ultra-high molecular weight heterocyclic aramid(HMPBIA),and then prepared PBIA/OPBIA and PBIA/HMPBIA hybrid fibers,respectively.The synergistic enhancement in tensile strength and compressive strength of heterocyclic aramid fiber was realized by filling the free H-bonding sites through oligomer.And the synergistic improvement of tensile strength and toughness of heterocyclic aramid fiber was achieved through constructing a special structure in the fiber.Then,PBIA/HMPBIA/OPBIA ternary organic hybrid fiber was prepared.Based on the rheological properties and spinnability of the blending solution,an optimized spinning process was established and heterocyclic aramid fiber with better mechanical properties was obtained.Finally,the solubility of heterocyclic aramid was regulated by changing the composition in its copolymerization,and then the heterocyclic aramid nanofiber was obtained by one-step preparation with high efficiency.Then,heterocyclic aramid spinning solution was prepared by in-situ copolymerization.Enhancement in tensile strength and modulus of heterocyclic aramid fiber was achieved by introducing HANF.The specific research contents and results are as follows:Chapter 2:Computer Simulation Research on the Structures and Properties of Organic/Organic Hybrid Heterocyclic Aramid FiberFirstly,the amorphous cell(AC)models of flexible chain nylon 66(PA66),semi-rigid chain nylon 6T(PA6T)and rigid chain PPTA with the same degree of polymerization and number of chains were constructed by Materials Studio(MS)software,respectively.The models were optimized under COMPASS force field.Then,inter-chain H-bonds were calculated by radial distribution function(RDF)and“H-bond stats”scripting.The results indicate that both the total number of H-bonds and the proportion of strong H-bonds decrease with the increase of chain-rigidity.Research by mean square displacement(MSD)and Density Functional Theory(DFT)calculation demonstrate that the movement ability of molecular chains reduces as the chain-rigidity increases,and the energy barrier of corresponding conformation adjustment is also increased,which are the fundamental reasons of forming free H-bonding sites between rigid chains.The introduction of diamine containing benzimidazole structure into PPTA chains provides quite amount of potential H-bonding sites,C=N and N-H,and they enhance H-bonding strength between macromolecular chains significantly.But,the proportion of strong H-bonds in the model drops from 49.2%to 45.9%.DFT calculation shows that the conformation rotation energy barrier of benzimidazole ring is rather high,which limits the utilization of H-bonding sites in benzimidazole structure.After a certain amount of OPBIA was added into PBIA system,the total number of H-bonds and the proportion of strong H-bonds in PBIA system are both improved notably.MSD and WAXD results confirm that oligomers have the characteristics of strong mobility,which could fill the position that chains cannot form H-bonds due to conformation adjustment.Thus,oligomers play a role in bridging to macromolecular chains.PBIA/HMPBIA hybrid AC model was also built and studied by MD simulation.The PBIA and PBIA/HMPBIA models were imposed by tensile stress in a single direction to simulation the heat treatment of fiber.The geometric parameters of the models were recorded.The results show that the maximum tensile stress that PBIA/HMPBIA can withstand at 633K is 0.24GPa,which is higher than 0.20GPa of PBIA system.Subsequently,the orientation of molecular chains along the tensile direction was analyzed through the torsion distribution of amide group.Although the degree of orientation of chains in PBIA/HMPBIA is lower than that in PBIA model at the same tensile stress of 0.20GPa,it is possible to apply a larger tensile stress to PBIA/HMPBIA model to make up for the deficiency in degree of orientation.When the tensile stress imposed on PBIA/HMPBIA system increases to 0.24GPa,HMPBIA in the system presents a highly stretched chain conformation,which runs through the whole model.It helps to form a special“mainstay body structure”in the model,which inhibits the slippage between PBIA chains,thus enhancing the tensile property of PBIA/HMPBIA.Chapter 3:Structure and Mechanical Properties of Oligomer Reinforced Heterocyclic Aramid FiberBased on the results of computer simulation,the structure and properties of PBIA/OPBIA hybrid fiber were studied.Firstly,heterocyclic aramid oligomers with polymerization degree of 5 and 100 were synthesized,and then a certain amount of OPBIA was added into the spinning solution of PBIA.Hybrid PBIA/OPBIA fiber was obtained by wet-spinning and heat treatment.The FTIR spectra of PBIA/OPBIA fibers show that H-bonds between macromolecules are enhanced by OPBIA effectively.Then,the evolution of H-bonds in PBIA,PBIA/OPBIA5 and PBIA/OPBIA100 was monitored by in situ variable-temperature FTIR.Through curve fitting and the calculation of Lambert Beer law combined with van der Hoff equation,enthalpy change of destruction of H-bonds was obtained.The value of PBIA isΔH=-13.14k J/mol.The value of PBIA/OPBIA5 isΔH=-19.37k J/mol,whose absolutely value is notably higher than that of PBIA.WhileΔH of PBIA/OPBIA100 is-14.58k J/mol,not much different from PBIA.It is clearly confirmed that oligomers enhance the inter-chain H-bonding strength notably.Tensile strength and compressive strength of PBIA/OPBIA5 fiber are increased by 23.1%and 24.9%compared with PBIA fiber,respectively.In addition,DMA analysis shows that there is a plasticizing effect of OPBIA on PBIA fiber at high temperature,which is beneficial to increase the stretchability of PBIA fiber during heat treatment,so as to further improve the mechanical properties.Chapter 4:Study on the Structure and Mechanical Properties of HMPBIA Enhanced Heterocyclic Aramid FiberOn the basis of results by computer simulation,the structure and properties of PBIA/HMPBIA fiber were studied.HMPBIA was synthesized by controlling the polymerization conditions,and then it was added to PBIA spinning solution.The rheological behaviors of hybrid solutions with different content of HMPBIA were tested by dynamic frequency scanning mode.The relaxation time spectrum of PBIA solution at 30~oC was derived by curve fitting of loss modulus G”(ω).The relaxation timeλ=11.62s,which represents the motion of whole molecular chain,is selected for analysis.It is proved that the relaxation time of PBIA is determined by the high molecular weight component.Because the longer relaxation time of HMPBIA is conducive to maintaining the high orientation state of macromolecular chains that are oriented by strong shear during spinning.As a result,the degree of orientation of PBIA/HMPBIA(100/15)is increased to 0.78 from 0.75 of PBIA.However,the further improvement in degree of orientation of as-spun fiber during heat treatment is inhibited to a certain extent because of increased relaxation time.The results of computer simulation shows that HMPBIA constructs a“mainstay-body”structure in PBIA,which helps to improve the ability of the system to withstand external tension.So,we optimized the heat treatment process of PBIA/HMPBIA fiber.The tensile strength and elongation at break of PBIA/HMPBIA fiber are increased by 17.7%and 16.7%,respectively,by heat drawing with higher draw ratios.So,the synergistic improvement of tensile strength and toughness is realized.Finally,the length of micro-fiber in PBIA/HMPBIA fiber was calculated by SAXS,and the fracture section of PBIA/HMPBIA fiber was observed by SEM,both of which verify the rationality of the structural model of“mainstay-body”in the fiber.Chapter 5:Preparation of Ternary Organic/Organic Hybrid Heterocyclic Aramid Fiber and Optimization of Wet-Spinning ProcessIn this chapter,PBIA/HMPBIA/OPBIA ternary hybrid fiber was prepared to further explore the relationship between structure and properties when oligomer and ultra-high molecular weight component are added to the heterocyclic aramid fiber meanwhile.The relationship between rheological properties and spinnability of spinning solution was established.The oligomer reduces the extensional viscosity of the spinning solution,which reduces the stability of the plasticizing drawing process of the fiber in coagulation bath during wet spinning.But the ultra-high molecular weight component prolongs the relaxation time,and it improves the extensional viscosity and enhances the tensile properties of fiber in coagulation bath.The proportion of PBIA/HMPBIA/OPBIA was optimized,and then the spinning speed was increased by 4%which endows hybrid fiber high tensile strength.In addition,the plasticizing effect of oligomer and the improvement in strethability by ultra-high molecular weight component play a synergistic effect during heat treatment,which helps to further improve the degree of orientation of the fiber,so as to achieve a higher tensile strength.Finally,the evolution of chain conformation during heat treatment was analyzed by MD simulation.Results show that as higher tension was applied for the fiber which possesses superior ability to bear tensile stress brought by HMPBIA,the plasticizing effect of oligomer promotes the movement of macromolecular chains.The synergistic effect of these two roles endows the fiber higher degree of orientation after heat treatment.Chapter 6:Preparation of Organic Heterocyclic Aramid Nanofiber by One-Step Method and Study on Structure and Mechanical Properties of Heterocyclic Aramid Fiber Enhanced By the NanofiberNano composite reinforcement is also an effective method to improve the mechanical properties of fibers.By adjusting the solubility of heterocyclic aramid,we realized one-step preparation of heterocyclic aramid nanofiber(HANF).It is found that the solubility of heterocyclic aramid is the key factor for the formation of HANF.The crystallinity of HANF is about 43%.The average diameter is 13.6nm,and their length is in the micron scale.HANF can disperse stably in DMAc/Li Cl for more than3 days at room temperature without dispersant.However,it is easy for HANF to aggregate after in-situ polymerization of PBIA.Then,heterocyclic aramid oligomers were grafted onto the surface of HANF by reactive amino groups to avoid aggregation.The dispersion stability of HANF in DMAc/Li Cl and PBIA solution was effectively improved with destroying the morphology of nanofibers.The rheological behavior of PBIA/HANF-g-OPABI was tested,and it is found that HANF-g-OPABI decreases the non-Newtonian index of PBIA solution and improves its shear thinning.Meanwhile,the heterocyclic aramid nanofiber can maintain its highly orientated conformation when sheared during wet-spinning,thus improving the orientation degree of as-spun PBIA fiber.In addition,the self-orientation of PBIA chains are improved by the nanofiber,so the orientation of hybrid fiber is higher than that of PBIA fiber after heat treatment.Compared with PBIA fiber,the tensile strength and modulus of hybrid fiber increased by 12.2%and 11.3%,respectively.Moreover,the compressive strength of hybrid fiber increased by 44.1%because of the oligomer grafted on the surface of HANF that enhances the transverse interactions between PBIA molecular chains.