Preparation And Study Of High In-plane Orientation Polyimide Film And Its Graphite Film

With the gradual development of high-power devices towards lightweight and high density,flexible graphite materials with high heat dissipation play a crucial role.Graphite films with excellent properties can be prepared by using polyimide(PI)films with high in-plane orientation as precursor films.Through the selection of different monomers,the PI molecular framework can be adjusted to achieve the purpose of preparing PI films and graphite films with high in-plane orientation.In this paper,high in-plane orientation PI films were prepared by two-step thermal imidization method.PMDAm-BPDAn-PDA and PMDA-ODAm-PDAn ternary PI films were prepared by selecting rigid monomers PMDA,BPDA,PDA and flexible 4,4'-diaminodiphenyl ether(ODA).The PI films were systematically characterized and graphitized.Then quaternary PI films of PMDA5-BPDA5-PDA and PMDA-ODA4-PDA6 systems were prepared by adding the fourth monomer:2-(4-aminophenyl)-5-aminobenzimidazole(BIA),3,3'-diaminobenzidine(DAB)and 2-(4-aminophenyl)-5-aminobenzoxazole(DAPBO),The effect of molecular chain structure on the in-plane orientation of PI films was further studied.At the same time,all PI films were graphitized to obtain graphite films.The microstructure and the conductivity of the graphite film were studied.finally,the quaternary film On this basis,Nanodiamonds(NDs)and nanosilica(Si O₂)were further added to construct the interlayer thermal conductivity network to explore its mechanism of improving the interlayer thermal conductivity of graphite films.The study concluded that:(1)Two kinds of PI films,PMDAm-BPDAn-PDA and PMDA-ODAm-PDAn,were prepared.With the increase of rigid monomer,the molecular chain rigidity of the films increased,and the birefringence increased in the range of 0.17?0.22 and 0.08?0.163,respectively.When the PMDA content of the former was 50 mol%,the graphite film was successfully prepared.At this content,the birefringence of the PI film reached 0.21591,the corresponding graphite film layer spacing was 0.336 nm,and the conductivity reached1.26×10⁴ S/m.When the content of PDA is 60 mol%,the double refractive index of PI film reaches 0.15158,while the graphite film spacing is 0.337 nm,and the conductivity reaches1.21×10⁴ S/m.(2)The fourth monomers DAB,BIA and DAPBO were introduced in PMDA5-BPDA5-PDA and PMDA-ODA4-PDA6.Among them,DAB was introduced into the two systems,and a part of the cross-linking structure was formed in the molecular chain to make the birefringence of PI films reach the maximum values of 0.223 and 0.179,respectively.Among them,the graphite films were successfully prepared by introducing DAB,BIA and DAPBO into the PMDA5-BPDA-PDA system,with the interlayer spacings of 0.338 nm,0.338 nm and 0.34 nm,and the electrical conductivities of 1.5×10⁴ S/m,1.35×10⁴ S/m and 1.28×10⁴ S/m,respectively.(3)Inorganic nanofillers(NDs/nano Si O₂)were introduced into PMDA5-BPDA5-PDA-DAB PI films to prepare graphite films with high thermal conductivity between layers.The functional groups on NDs surface reacted with PI molecules to make a small part of PI“grafted”on NDs surface.After graphitization,the PI structure grafted on the NDs surface was transformed into the graphite structure to bridge the upper and lower layers,and the thermal conductivity network was constructed between the layers of the graphite film.When the NDs content was 0.5 wt%,the thermal conductivity between the layers of the graphite film reached 20.141 W/m·K,which was 115.6%higher than that of the graphite film without fillers(9.342 W/m·K).