Microstructure evolution of polyimide films induced by electron beam irradiation-load coupling treatment
Introduction
Due to its remarkable thermal stability and mechanical properties, polyimide films have been widely used as structural materials in a number of fields with extreme conditions, such as in space missions and nuclear installations, e.g. in thermal blankets, shielding, and reflective materials [1,2]. The high-energy charged particles in space, such as electrons, protons, and heavy ions, might affect the microstructure of polyimide significantly, and the corresponding changes in the polyimide have to be analyzed [[3], [4], [5], [6]]. When exposed to energized particles, the irradiation effect on the polyimide could be classified into ionization and displacement effect. The charged particles, such as electrons, protons, and heavy ions, might lead to an ionization effect on the polyimide. Meanwhile, the displacement damages in polyimide would be caused by protons and heavy ions. Generally, the ionization and displacement effect induced by energized particles could lead to scission and crosslinking of molecular chains, and thus change the microstructure of the polyimide. These complex chemical reactions may lead to the breakage of original chemical bonds in polyimide and the formation of new chemical bonds as well as volatile species [7,8]. Further, the free radicals induced by irradiation would recombine and react with oxygen in the polyimide [9,10].
Previous studies confirmed that the tensile strength and elongation dropped over 50% after 3 MeV proton irradiation [11]. Further, it also decreased the glass transition temperature due to the scission of molecular chains in the polyimide [12]. It was reported that the energized electrons could decrease the surface roughness and deteriorate the mechanical properties and thermal stability due to the breakage of chemical bonds in the polyimide [[13], [14], [15]]. Further, some researchers performed experiments with electron and proton irradiation in air to study the microstructure evolution and degradation behavior of polyimide films, which also provided useful information for the application of polyimide films in space [12,15,16]. The heavy ions also led to severe polyimide degradation and new chemical bonds were detected after irradiation [17]. Consequently, the surface quality, and mechanical, optical, and dielectric properties of the polyimide tended to degrade after irradiation with charged particles and thus influence the durability of the components [3,[18], [19], [20], [21], [22], [23]].
Generally, when analyzing the polyimide degradation behavior, studies have mainly focused on the factors related to irradiation such as particle types, irradiated doses, and accelerating voltage. However, the components in extreme environments based on the polyimide matrix (e.g., solar panels and sail) not only get irradiated by the energized particles but also experience tensile stress, which might lead to creep and plastic deformation in the polyimide matrix. Further, the molecular chains in polyimide films tend to be stretched under tensile stress. In this case, the degradation behavior of irradiation-only polyimide films might be different from that of films treated with the combination of external tensile stress and high-energy irradiation. According to previous reports, the tensile stress level of 1.4 MPa was based on the nominal stress of the base films of the ADEOS-I solar panels [24]. However, in order to estimate and investigate the performance and microstructural evolution in space with extreme environments, most researchers used higher tensile stress levels and high loading conditions. Results have shown that low-energy (200 keV) electron beams and tensile stress (<7 MPa) coupling treatment had little impact on the mechanical properties [24,25]. However, it was suggested that the tensile stress led to increasing roughness in polyimide and initiated rapid degradation, especially when a high load (≥40 MPa) was applied [23,26]. Therefore, the stress state is an important factor to be considered in order to maintain the reliability in complex environments. However, few studies have investigated the irradiation evolution and mechanical performance after combination treatment with high-energy electron beam irradiation and high tensile stress.
It can be concluded from the discussed previous work that the tensile stress could significantly degrade polyimide during irradiation in specific conditions; however, few previous studies have considered the effect of tensile stress during electron beam irradiation. In this study, the effect of coupling treatment is investigated by applying a combination of high-energy electron beam irradiation (1.2 MeV) and high tensile stress (50 MPa). The aim of this work is to shed light on the effect of tensile stress as an external factor on the microstructure and mechanical properties after irradiation-load coupling treatment.
Section snippets
Experimental
Polyimide films (50 μm thick) composed of pyromellitic dianhydride (PDMA) and 4,4′-oxydianiline (ODA) structure were supplied by China Academy of Space Technology. The corresponding chemical unit of PMDA-ODA is shown in Fig. 1. The polyimide film was anisotropic, which was manufactured by the tape casting process and then stored in rolls. The samples were cut from the center with dog-bone shapes for the following irradiation and load coupling experiment. The longitudinal direction of the
Results and discussion
X-ray diffraction patterns of the pristine polyimide films are presented in Fig. 3, in which the films show semi-crystalline characteristics. There are three Bragg's peaks at 5.8°, 18.2°, and 25.3°, which could be indexed as (002), (110), and (210) of the polyimide, respectively [29,30]. According to previous works, the polyimide derived from PMDA and ODA would tend to form orthorhombic crystals with the space group of Pda2 [31]. The results indicate that the polymer chains in the pristine
Conclusions
The degradation process and mechanical behavior of polyimide films treated with a combination of high-energy electron beam irradiation (1.2 MeV) and high tensile stress (50 MPa) have been investigated. When irradiated with the electron beam, the number of defects and surface roughness increased concomitantly with increasing electron fluence. Further, the decrease in the crystallinity and breakage of chemical bonds occur in the polyimide film. Modifications in the structural properties of
Acknowledgement
We would like to acknowledge Si-Jing Chen for testing the mechanical properties of our samples.
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