Synthesis And Properties Of Phenylacetylene-terminated Polyimide Resin With Low Dielectric Constant

Wave-transparent materials are widely used in radar and other fields due to their low dielectric constant,low dielectric loss and good weather-resistance.Compared with inorganic materials,organic materials have a broad application space because of strong molecular designability,excellent processability,high-strength lightweight.With the increasing improvement of aircraft speed and signal accuracy,higher requirements have been put forward for the heat resistance and dielectric properties of the organic resin matrix.Therefore,it is of great significance to develop a wave-transparent resin with high heat resistance,low dielectric constant and high dielectric stability for radar and other fields.Phenylacetylene-terminated polyimide resin(PETI)has high temperature resistance,good radiation resistance,excellent physical properties and chemical resistance due to its unique chemical structure.At the same time,the active terminal group of phenylacetylene endows polyimide resins with a wider processing window and expands the application fields of PETI.However,the dielectric constant of PETI is generally 3.5?3.0,which can not meet the requirements of the new generation of wave-transparent materials.Therefore,it is urgent to develop polyimide resin with high heat resistance,low dielectric constant and loss,and excellent processing properties.In this paper,a new type of PETI was synthesized based on molecular structure design,a series of low dielectric constant PETI with excellent comprehensive properties were prepared by introducing thermal rearrangement structure,small molecular active diluent and nano hybrid particles into the resin matrix,and their structures and performance have been systematically studied.The main contents are as follows:1)A new type of phenylacetylene-terminated polyimide resin(PI-n)was designed and synthesized to solve its manufacturing difficulties,and its processability and curing kinetics were systematically studied.Flexible ether ketone structure and asymmetric structure were introduced into PI-n by structural design,which made the prepared oligoimides have higher solubility(the solubility of oligoimides in NMP and DMAC were more than 30wt%)and lower melt viscosity(<28 Pa.s).The optimum curing conditions were obtained by simulating the curing kinetics of PI-1,and the further study of curing mechanism and kinetic equation of PI-1 provided theoretical support for the subsequent modification and processing.The relationship between processability,thermal stability,mechanical properties and polymerization degree of PI-n was analyzed.The results indicated that the glass transition temperature(T_g)of cured resins decreased with the increase of the degree of polymerization(154?210?),and the cured resin showed excellent mechanical properties and thermal decomposition properties.2)In order to further improve the heat resistance and dielectric properties of the matrix resin,ply(imide-co-hydroxyimide)copolymer resins(PI-co-HPI)with low dielectric constant and excellent mechanical properties were prepared by introducing ortho-hydroxydiamine unit with thermal rearrangement structure into the backbone of resin.The formation process of benzoxazole structure with high rigidity and low polarity through thermal rearrangement reaction of ortho-hydroxyimide at high temperature was studied in detail,and the effects of thermal rearrangement reaction on the molecular stacking state,thermal stability,mechanical properties,dielectric properties and bonding properties of resin with reinforced fiber were investigated.The results showed that the presence of polar hydroxyl groups in the oligomer increased its solubility(up to 50wt%in NMP).the thermal rearrangement reaction and the introduction of low polar trifluoromethyl in the copolymerization unit made the dielectric constant of the cured copolymer resins PI-co-PBO decreased significantly(the dielectric constant of pure PI resin is 3.55),which is2.56?3.4.At the same time,the 5wt%weight loss temperature(T_d5)of the cured resin increased from 455? of pure PI resin to 491? of PI-co-PBO-60.3)In order to further reduce the melt viscosity of the oligomer and improve the glass transition temperature of the cured resin,a reactive small molecular diluent containing cardo structure was designed and synthesized,and a series of blend resin matrix PI/Cardo were prepared by blending.The large volume of Cardo increased the free volume of blend matrix,the small molecular rigid structure reduced the melt viscosity of oligomer,the active end group of Cardo-HPI increased the cross-linking density,and further improved the thermal stability of cured resin.The results showed that the introduction of Cardo-HPI can effectively reduce the melt viscosity of the matrix resin and improve their processability.The T_g of the cured PI/Cardo increased from 264? of the pure PI matrix to 403? of PI/Cardo-40,and its heat resistance was greatly improved.The dielectric constant of cured PI/Cardo decreased from 3.4 to 2.5 due to the large volume of Cardo group,the loose molecular stacking caused by thermal rearrangement reaction and the low polarity benzoxazole structure.More importantly,due to the increase of crosslinking density,the dielectric constant and dielectric loss of the cured PI/Cardo film showed excellent stability in the range of-150?250?.Therefore,the PI/Cardo resin matrix with good processability,high thermal stability and excellent dielectric properties has great application prospects in the field of high temperature wave-transparent materials.4)In order to prepare polyimide resins with low dielectric constant and excellent adhesion properties,a series of composite resins PI/HBPSi were prepared by embedding the multi-amine hyperbranched polysiloxane(NH₂-HBPSi)into the main chain of polyimide resin through in-situ polymerization.The effects of NH₂-HBPSi on processability,molecular stacking,thermal stability,dielectric properties and bonding properties of resin were investigated.The results showed that the large volume branched structure of NH₂-HBPSi increased the free volume of matrix,and the"dielectric confinement effect"of nanoparticles reduced the dielectric constant of cured resin.As the content of NH₂-HBPSi increased from 0 to 30wt%,the dielectric constant of the cured composite films decreased from 3.29 to 2.19 without compromising its processability and thermal stability.The interfacial shear strength(IFSS)between PI/HBPSi-10and Kvelar fiber reached the highest value of 35.8 MPa(IFSS of pure PI resin=27.34 MPa),and the bond strength with metal alloys was also 50%higher than that of pure PI resin.Therefore,the ultra-low dielectric constant and high bonding properties resin matrix provides a way for the production of the next generation of wave-transparent composites.