Preparation,Structure And Properties Of Polyimide Fibers Containing Hydroxyl Groups

Polyimide（PI）fiber is a new type of high-performance organic fiber with rich chemical structure of molecular chain.Because the molecular chains are mainly composed of benzene rings,cyclic imide,aromatic heterocycles and other characteristic groups bonded to each other,they have outstanding mechanical,thermal,flame retardant,aging resistance,corrosion resistance and low dielectric properties,etc.Compared with the mature application of UHMWPE fiber and aromatic fiber,PI fibers have the advantages of high glass transition temperature,weather resistance and dimensional stability,so it exhibits broad development potential in fiber-reinforced resin matrix composites under extreme environmental fields such as ocean,aerospace,transportation industry,wind power energies,and other fields.However,the traditional PI fiber lacks active groups in the molecular chain and shows smooth“skin”,which results in poor interfacial adhesion of the composites prepared after impregnation with resin matrix,and the fiber is difficult to play their role of strengthening and toughening.Therefore,from the perspective of designability to PI fiber molecular chain chemical structure,this thesis improved the surface activity of fiber itself by introducing hydroxyl groups,and systematically studied the relationship between molecular chain structure-condensed state-properties of PI fiber.To prepare surface-active PI fiber,3,3’,4,4’-biphenyl tetracarboxylic diandhydride（BPDA）/2-（4-aminophenyl）-5-amino benzimidazole（BIA）/p-phenylene diamine（p-PDA）were selected as the basic system in this experiment.The reactive monomer 4,4’-diamino-（1,1’-biphenyl）-3,3’-diol（HAB）was introduced into the molecular chain of the matrix molecular chain by low-temperature polycondensation reaction to improve the chemical activity of the fiber.Two-step wet spinning technology prepared five kinds of copolymer PI fiber of BPDA/p-PDA/HAB/BIA（BPHB）system with different surface activities.Research shows that:（1）With the increase of HAB molar ratio,the intensity of the characteristic peak of—OH group at 1191 cm^-1 in ATR-FTIR spectrum also enhanced correspondingly,accompanied by the red shift phenomenon of 1704 cm^-1 peak,i.e.the molecular chain gradually constructed strong intermolecular forces;when the p-PDA/HAB/BIA molar ratio was 4/3/3,PI fiber showed a maximum wavenumber shift of 15 cm^-1.2D SAXS and SEM data indicated that the narrow microvoids radius distribution was conducive to the formation of dense and uniform internal structures;2D WAXD clarified that the introduction of HAB destroyed the orientation of polymer chain along the fiber axis.For the fibers with 30 mol%HAB,the distribution of microvoids radius is the narrowest and the average radius is the smallest of 8.486 nm,and the（002）crystal plane has an axial orientation degree of 74.04%.Structure determines the properties,i.e.strong intermolecular H-bonding,narrow microvoids radius and high polyimide chain orientation are the foundations for the high mechanical properties of the fiber.Consequently,the PI fiber with a diamine molar ratio of 4/3/3 displayed the best mechanical properties,with tensile strength and modulus of 2.62 GPa and 100.15 GPa,respectively.（2）Taking the p-PDA/HAB/BIA molar ratio as 2/5/3 as the research object,the effects of imidization temperature and drawing ratio on the final properties of PI fibers containing hydroxyl groups were investigated.Accoding to data analysis,under the same drafting ratio condition,the appropriate imidization temperature not only made the fiber obtain a higher imidization degree,but also contributed to the orderly arrangement of the polymer chains along the drawing direction;when the temperature is too high,the molecular chain disorienting ability was enhanced leading to the enlargement of the inner microvoids.Under the same imidization temperature,the tensile strength and modulus of the fibers increased from the initial values of 1.10 GPa and 54.25 GPa to the best values of 2.11 GPa and 119.05 GPa after 1.6 times hot drawing.The effect of drafting ratio on the hydrogen bond interaction between chains,the degree of orientation and the internal micropore radius is the key factor for improving the mechanical properties of the fiber.Moreover,DMA and TGA analyses indicated that the thermal performances of prepared fiber were also related to the aggregation structure.（3）Unidirectional PI/EP composites:when the HAB content of mixed diamine gradually increased from 0 mol%to 70 mol%,XPS surface analysis showed that the S_O-C/S_O1S value of the fiber raised significantly from 18.51%to33.84%;the static contact angle of monofilament-epoxy microsphere also decreased from 44.01°to 31.56°.The interlaminar shear strength of composites and interfacial shear strength of monofilament-epoxy microsphere increased from 28.51 MPa and 40.78 MPa to 41.41 MPa and 56.47 MPa,respectively.SEM interface failure morphology analysis showed that the surface active—OH groups of the fiber established a stable interaction with the matrix at the interface phase through H-bonding and ring-opening reaction,thus advancing the interfacial adhesion between PI fibers and epoxy matrix.