Preparation Of High Strength Polyimide Microfibers By Electrospinning Of Water-soluble Poly (Amic Acid) Salt

With the rapid development of aerospace,environmental protection,microelectronics,the demand for polymer materials with higher strength,higher modulus,higher heat deformation temperature and lower density is becoming growing.Polyimide is a class of polymer materials which contains imide ring in the molecular chain.The aromatic ring and imide ring on the main chain endows polyimide material excellent performances.The polyimide nanofibers prepared by electrospinning have larger surface,higher strength and outstanding heat resistance.The electrospun nanofiber membranes are porous material,which is widely applied in lithium ion battery as separators,in high temperature filtration for air purification,in aerospace as composites and etc.Although the polyimide nanofibers prepared by electrospinning in the previous study had a high strength,the load force of the polyimide nanofibers at breaking was small due to the thinner diameter of the fibers.And at the same time,the concentration of the precursor PAA solution used for electrospinning is usually less than 20%,and a large amount of toxic and non-environmentally friendly solvents,such as DMAC,DMF,NMP and etc.,be used during the electrospinning processing.In order to solve the above problems,the polyamic acid ammonium salt(PAAS)nanoparticles,which are soluble in water,were prepared in this work.And the polyimide microfibers with higher strength and higher breaking load were prepared by electrospunning PAAS/water-ethanol(PAAS/H2O-C2H5OH)solution.The specific content of this thesis is mainly divided into the following three parts:1.Preparation and characterization of polyamic acid ammonium salt(PAAS):PAAS powders were prepared by a polycondensation at low temperature and a two-step solvent exchanging method.The results show that the intrinsic viscosity of PAA prepared by the low temperature polycondensation is 4.49 dl/g,and the intrinsic viscosity of PAAS prepared by the two-step solvent exchange method is 6.36 dl/g.At the first step of the solvent exchange process,a diluted solution has a great influence on the preparation of PAAS powder: a solution containing 5?6 wt % PAA was used for the preparation of PAAS powder with a nano-scaled particle size and a feature easy to filter.At the same time,the solubility of PAAS in water also depends on thechemical main chain structure of the PAAS own.A higher density of the ammonium salt groups in the molecular chain will facilitate the dissolution of the PAAS in water.2.Preparation and characterization of polyimide microfibers: The water-soluble PMDA/ODA-PAAS nanoparticles were used to prepare the PAAS micro-fiber membrane by "green" electrospinning,and the polyimide micro-fiber(PI-MF)membranes were obtained by the thermal imidization of PAAS micro-fiber membranes.SEM test results showed that the PI-MF fibers have a continuous length and a smooth surface.The diameter of fiber distributed from 1.2 ?m to 3.2 ?m,with an average of 1.91 ? m.The best mechanical performance of the PI-MF fiber membrane was obtained when the imidization temperature was at 430 ? C,and the corresponding tensile strength,modulus and elongation at break were 229 MPa,2GPa and 16.1%,respectively.TGA results revealed that the PI-MF fiber membrane had excellent heat resistance,the thermal decomposition temperature at 5% weight loss was 481~519 ? C,and the carbon residue percentage at 800 ? C was 53%.The dynamic mechanics analysis(DMA)showed that the glass transition temperature of the PI-MF fiber membrane is 382?C,and the storage modulus has a good retention proportion even at high temperatures.3.Preparation and characterization of polyimide single microfiber with high strength and high break load force: the PI-MF single fibers were prepared.The mechanical tensile tests showed that the PI-MF single fiber with a diameter of 1.94?m has a tensile strength of 1036 MPa,modulus of 5.2 GPa and break load force of3061 ?N,respectively;while the fiber with a diameter of 3.06 ?m has 989 MPa,8.3GPa and 7270 ?N,respectively.The results show that there is no significant difference in the tensile strength and modulus of the PI-MF single fibers with different diameters.However,the break load force of the PI-MF single fiber significantly increased from 3061 ?N to 7270 ?N when the diameter increased from 1.94 ?m to3.06 ?m.Polarization Raman Spectroscopy tests showed that the degree of orientation of PAAS-MFs was 70%,and the orientation of PI-MFs imidized at 430?C was 91%.XRD results indicated that PI-MFs are partially crystalline.Both the presence of a large amount of charge in the electrospinning solution and the rapid evaporation of the solvent are beneficial to the crystallization of the electrospun fibers.