Electron Irradiation Damage Behavior Of Polyimide Film Under Multi-physical Field Coupling Condition

Polyimide films are widely used in space.The research is base on the degradation behavior of polyimide films after electron irradiation,and the combined effect of electron beam,heavy tensile stress and constant high temperature were carried out through the self-designed experimental device.The main purposes of the research are to reveal the mechanism of surface damage and microstructure evolution after coupling condition,and to analyze the relationship between properties and microstructure of polyimide,such as mechanical properties and stress relaxation behavior.The effect of nano-TiO₂ layer on the irradiation behavior of polyimide film in TiO₂/polyimide is investigated to predict the lifetime of the polyimide film in a complex environment.The surface of the polyimide film is demaged after irradiation and coupling condition in some degrees.After irradiation with electron beams,irregular defects form on the surface of the polyimide film.After irradiation-load condition,the microcracks and air gaps are found on the surface of the polyimide film.After the coupling condition of irradiation,tensile stress and high temperature,the surface of the film exhibits the morphology of high-densiy granular protrusions with a diameter of 50-100 nm.At the same time,the surface hydrophobicity,hardness and elastic modulus of the polyimide film decrease after irradiation and irradiation-load.After coupling condition,the surface hydrophobicity enhances,and the surface contact angle increases from 80°to 96°.The hardness and elastic modulus increase by 7.61%and 2.41%,respectively.Electron beam irradiation can cause damage to the mechanical properties of polyimide film.After irradiation,the tensile strength and elongation at break are 7.30%and 21.59%lower than that of the pristine polyimide,respectively.The tensile strength and elongation at break decreased by 9.90%and 34.08%after irradiation-load condition,respectively.After coupling condition,the elongation was 5.48%and31.10%lower than the original sample,respectively.The creep and recovery behavior of polyimide film at 150°C after different irradiation conditions are studied.The results indicate that after irradiation and stress coupling condition,the deformation ratio of the samples decrease.Meanwhile,the ratio of elastic deformation and viscous flow deformation increase.The ratios of viscous flow deformation increase from 15.05%to 21.64%and 17.59%,respectively.After coupling condition,the ratio of viscous flow deformation decreases to 4.82%.At the same time,the bulk viscosity of polyimide increases significantly,which is higher than that of the only-irradiated and irradiation-load coupling treated ones.With the increase of irradiation dose,the results indicate that the process of evolution increases first and then decreases.In addition,after coupling condition,the glass transition point of the material increases from 405.4°C to 412.5°C and then decreases to 404.5°C.The results indicate that the material maintains high performance of thermal stability after different irradiation conditions.These evolution performances provide strong data support for the reliability and longevity prediction of polyimide films in the complicated applications.Irradiation reduces the order of the molecular chain structure of the polyimide.And the coupling condition lead to the changes of the aggregate structure of the molecular chain in the polyimide,and the transition from the semi-crystalline state to the amorphous state.During irradiation,the chemical bonds of the main chain and the side groups of the polyimide are broken,and the degree of reaction gradually increases with the increase of the irradiation dose.The coupled stress field will aggravate the degradation reaction of C=O and NH₂ group.Under the multi-physical field coupling condition,the high temperature causes the cross-linking reaction,at which time the film exhibits the combined chemical reaction of ring opening and surface oxidation,and aliphatic C-C bonds and a large number of C?N bonds are newly formed in the film,and the oxygen-containing groups in the surface layer are newly formed which are CH₂-O bond.Based on the above research,this research initially explores the preparation of high-stability nano-TiO₂ layer on the surface of polyimide,and analyzes the influence of the deposited layer on the electron irradiation behavior of polyimide film.After the high-energy electron irradiation of TiO₂/polyimide,the presence of the deposited layer protects the polyimide substrate,which significantly reducing the surface defects caused by the irradiation and the damage to the mechanical properties.At the same time,the damage of Ti-O-Ti and Ti-O-N bonds and the formation of N-O bonds occur in the deposited TiO₂ after irradiation,and the degradation of the carbonyl group in the matrix is reduced.It can be concluded that the decrease of radiation protection effect is caused by the damage of TiO₂ layer after coupling condition.