Study On Structural Evolution And Thermal Conductivity Of Flexible Graphite Films Prepared By C-Doped Polyimide

Polyimide(PI)is one of the most promising precursors because of its simple process of preparation,low cost,high carbon yield,easy control of carbonization and graphitization process.In the heat treatment process of PI films,N,H,O elements escape in the form of gas,resulting in porosity,high shrinkage,low conductivity and so on,which limits its application in the field of heat conduction.The researchers believe that adding inorganic filler to PI films can improve the mechanical and thermal properties of composite films.Meanwhile,the thermal conductivity of carbon films and graphite films prepared by PI films are improved.Carbon nanoparticles are regarded as the best filler to improve the performance of carbon films and graphite films due to their special carbon structure and excellent conductivity of thermal conductivity.However,carbon nanoparticles with complete structure have strong hydrophobicity and are prone to agglomerate in the process of polymerization.For polymer-based nanocomposites,uniform dispersion of nanoparticles in the matrix is essential important.By modifying the nanoparticles,the compatibility between matrix and filler can be improved and the interface interaction can be increased,achieving uniform dispersion of the filler in the matrix.In this paper,the effects of graphene(GN)and carbon nanotubes(CNTs)on the performance of PI films were investigated.GO with oxygen-containing functional groups on the surface was prepared by modified Hummers method.The GO before and after modification was characterized by FTIR,XRD,XPS and TEM.The results showed that GO was successfully synthesized.Reduced graphene oxide(rGO),carboxylic graphene(GO-COOH)and aminated graphene(NH2-GO)were prepared by chemical modification of GO.Single-walled carbon nanotubes(SWCNT)have large aspect ratio and are prone to agglomeration and entanglement.To solve this problem,carboxylic carbon nanotubes(SWCNT-COOH)and aminated carbon nanotubes(SWCNT-NH2)were prepared by chemical modification of SWCNT.The success of the modification of GO and SWCNT is demonstrated by the FTIR and XPS characterization of GO and SWCNT derivatives.Diaminodiphenyl ether(ODA)and Pyromellitic dianhydride(PMDA)were selected as the monomers to prepare PI films.Graphene oxide/polyimide(GO/PI),reduced graphene oxide/polyimide(rGO/PI),carboxylic graphene/polyimide(GO-COOH/PI),aminated graphene/polyimide(NH2-GO/PI),carboxylic carbon nanotubes/polyimide(SWCNT-COOH/PI)and aminated carbon nanotubes/polyimide(SWCNT-NH2/PI)with 0.5wt%,lwt%,2wt%and 3wt%contents of graphene and carbon nanotubes were synthesized by in-situ polymerization.The structure and properties of the composite films were characterized and analyzed by XPS,XRD,SEM and TG.When the content of graphene and carbon nanotubes is 3wt%,the obvious agglomeration phenomenon of nanoparticles appears in the films,which is not conducive to the improvement of the performance with composite films.Therefore,the 3wt%content of garphene and carbon nanotubes in the films is not considered in the discussion of the results in this paper.The reaction mechanism of the composite films with different carbon nanoparticles was analyzed.The carboxyl and amino groups on the surface of the modified graphene and carbon nanotubes could be connected to the PI molecular chain through covalent bonds to generate forces.The structure of the composite films was a network structure and the interfacial force between the nanoparticles and the PI molecular chain could be generated.The interfacial force becomes stronger with the improvement of dispersion.It was found that in the modified graphene,such as GO,GO-COOH and NH2-GO can significantly improve the tensile strength,thermal conductivity and thermal stability of the composite films.The tensile strength of PI films is 83MPa and 2wt%GO-COOH/PI composite films is 143MPa,which shows 73%improvement compared to PI films.The thermal decomposition temperature of PI films is 503?.The thermal decomposition temperature rises to 529? after adding 2wt%GO-COOH and NH2-GO into PI films.The surface oxygen-containing functional groups decrease after GO reduction,the graphite layer spacing decreases and the thermal conductivity rises.Therefore,the thermal conductivity of the composite films prepared as nanoparticles is improved.After modification,the dispersion of rGO decreases and the agglomeration is serious which cause the decreased interface interaction.Therefore,the tensile properties of rGO/PI composite film increase first and then decrease.The tensile strength of 2wt%rGO/PI is 78MPa.The increase of SWCNT with large aspect ratio can provide more channels for thermal conduction,leading to the increase of thermal conductivity of polyimide composite films.Some carbon nanotubes still exist in the form of agglomeration in the composite films with the increasing of carbon nanotubes,which cause the reduced tensile strength and thermal stability.The induction and toughening effects of modified graphene and carbon nanotubes with different content in the process of carbonization and graphitization of polyimide were studied.The carbonization and graphitization products of PI films and composite films were characterized by XRD and Raman Spectroscopy.The results showed that carbon nanoparticles could improve the degree of order,graphitization degree and grain size of the graphitization films which play a good role in inducing graphitization.TEM was used to characterize the microscopic morphology of carbonization and graphitization products.The results showed that the structure of carbon films treated with 1500? was turbostratic carbon.When the temperature rose to 2500?,the disordered structure of the films changed into an orderly graphitization structure.The graphite films prepared by composite films exhibited smaller graphite layer spacing compared to PI films which were treated with the same temperature.The graphite layer spacing of PI films heat treated with 2500? is 0.358nm and the graphite layer spacing of 2wt%GO-COOH/PI composite films is 0.341nm which is closer to the ideal graphene layer spacing(0.334nm).The AFM Characterization of composite films heat treated with 2500? shows that the graphite films prepared by PI films is rough and the surface of graphite films has defects such as holes and grooves.After adding carbon nanoparticles,the surface defects are obviously decrease and the roughness decreases in different level.It is shown that carbon nanoparticles can not only induce graphitization in the process of carbonization and graphitization,but also reduce defects of composite films in the process of graphitization.The defects of the films will lead to decrease of toughness of the camon films and the graphite films prepared by heat treatment is easy to produce cracks and larger contractility etc.,causing complete films changing into irregular fragments.After adding carbon nanoparticles to PI films,the integrity of carbon films and graphite films is improved.When the temperature of heat treatment is not exceeding 2200?,the surface of the films shows obvious folds.When the temperature rises to 2500?,the folds on surface of the films disappear and the surface becomes smooth and flat.The thermal conductivity of carbon films and graphite films was measured by heat flow method and laser flash method.The thermal conductivity is related to the form of carbon in the films after heat treatment.When the temperature was not exceeding 1800?,the thermal conductivity of the films increased slowly.The structure of the films is disordered and the graphitization degree is low,which is not conducive to the thermal conductivity.As the temperature continues to rise,the degree of graphitization and the degree of order increases,which is in favour of the heat conduction.Modified carbon nanoparticles can significantly improve the thermal conductivity of the films.The carbon nanoparticles in the composite films can form a network structure with PI.After carbonization and graphitization,the graphite structure of the films is a network six-element ring structure,providing a channel for the thermal conductivity,thus improving the thermal conductivity of the graphite films.It is found that the graphite films prepared by GO-COOH/PI outperform the best thermal conductivity(760.386W/mK)which gives 128%improvement compared with that of PI films(332.53W/mK).Compared with other carbon nanoparticles,GO-COOH exhibits the best graphite induction effect on PI films.