Preparation,structure And Properties Of High Strength And Low Dielectric Polyimide Fibers

Due to the excellent dielectric properties,especially low dielectric constant ? and dielectric loss tan?,polyimide fiber has been used in the fabrication of wave transmissibility materials,but the dielectric constant of polyimide is usually in the range of 3.4?3.5,which is still relatively high.Therefore,reducing the dielectric constant while avoiding the reduction of other properties is one of the development directions of polyimide fibers.Although polyimide with lower dielectric constant can be obtained by introducing pore structure in the material,the mechanical properties are often decreased with the existence of the pore structure.The introduction of fluorine-containing groups has a great advantage in maintaining the excellent mechanical properties of polyimide.Therefore,we use this method to prepare low dielectric polyimide fibers with excellent mechanical properties.In order to analyze the influence of the introduction of fluorine-containing diamines on the properties of polyimide,we constructed four different fluorine-containing monomers and corresponding structural units as well as molecular chain model and condensed state model.Simulation result shows that due to the strong electron absorption of fluorine groups,the charge transfer in the molecular chain is inhibited,which is conducive to the reduction of the dipole moment.The strong electronegativity of fluorine atoms can reduce the fluidity of ? electrons on the benzene ring,making it difficult to polarization in electric field,thus reducing the molecular polarizability.Trifluoromethyl can occupy large volume,which can effectively reduce the dipole density and average polarizability in unit volume,thus greatly beneficial to the reduction of the dielectric constant.Due to the volume effect,trifluoromethyl attached to the benzene ring weakens the ability of the molecular chain to change conformation and maintains the highly rigid structure of the molecular chain.The results show that the introduction of trifluoromethyl can improve the system FFV more effectively.Since increasing the volume of substituents can more effectively reduce the dipole density and average polarizability per unit volume than increasing the number of fluorine-containing groups,leading to remarkably decreased dielectric constant,we chose TFMB monomer for further study.In the experiment,TFMB was introduced into the BPDA/PDA backbones and then spinning into fiber.It was found that the fiber prepared with the when the PDA:TFMB molar ratio of 7:3 exhibited the best mechanical properties with a strength of 1.40GPa and modulus of 83.30GPa.However,the further introduction of TFMB would significantly reduce the mechanical properties.At the same time,with the increase of TFMB content,the dielectric constant and dielectric loss of the fiber decreased significantly.The dielectric constant of the fiber decreased to 2.48 at the lowest.In order to improve the mechanical properties of the fibers,we introduced BIA into the system to improve the mechanical properties by establishing intermolecular hydrogen bonds.FT-IR showed that hydrogen bonds had been established between molecular chains after the introduction of BIA,and the strength and modulus of the system increased significantly to 2.63GPa and 96.79GPa.Intermolecular hydrogen bonding associations plays an important role in the mechanical properties of fibers.However,with the increase of TFMB content,the hydrogen bonding in the fiber gradually weakened,and the fiber with more TFMB content exhibits weakened mechanical properties while the dielectric constant of the fiber reduced from 2.71 to 2.43,and the dielectric loss tan? also decreased.The 2D WAXD shows that the orientation of molecular chains along the axial direction increases with the increase of TFMB content.At the same time,the glass transition temperature and thermal decomposition temperature of the fiber also decreased with the increase of TFMB content.The properties of fibers are not only affected by the chemical structure of the molecular chain itself,but also by the structural parameters such as the stack structure of the molecular chain,the degree of crystallinity and orientation.The effects of different process conditions on the properties and structure of the fibers are also investigated.The experimental results show that the imidization temperature,draft ratio and heat treatment time applied in the spinning process are interrelated,which have a significant effect on the structure and properties of the fiber.When the fibers are imidizied at higher temperatures,molecular chains have higher mobility and easier to form complete chemical structures,and highly oriented structures can be formed at higher draft ratios.However,increasing the draft ratio will change the velocity distribution on the fiber,which will inevitably affect the heat and mass transfer in the imidization process.We also studied the effect of coagulation bath conditions on the fiber structure and properties,and found that the change of coagulation bath concentration had a great effect on the microstructure of the fiber.The lower the temperature of coagulation bath,the more regular the fiber appearance,the less the surface and internal defects,leading to improved mechanical properties of the fiber.It is possible to increase the surface defect of the fiber and the pore size of the fiber by increasing the jet drawing ratio.Washing and drying are the intermediate process of the transformation from polyamide fiber to polyimide fiber,during which a series of physical and chemical structure changes take place.In general,in order to maintain high mechanical properties,it is not suitable to use longer water bath.At the same time,the increase of drying temperature can reduce the internal defects to a certain extent,but the defects on the fiber surface have not been significantly reduced.