High dimensional stability polyimide membrane material for space optical imaging system

Overview: Polyimides are a kind of high performance polymer with excellent chemical resistance, thermal stability, and mechanical properties. It is a good candidate material for building space large aperture lightweight optical imaging system. For example, to provide geosynchronous-orbit Earth observation capabilities the Membrane Optical Imager Real-time Exploitation(MOIRE) program, sponsored by the Defense Advanced Research Projects Agency (DARPA) seeks to further the technology development related to ultra light diffractive polyimide membrane-based telescopes. But there are strict requirements for the mirror material because of the harsh space application environment and the high optical imaging quality requirements. Under the space thermal alternation application environment, the temperature change can lead to deformation of the microstructure on the membrane, and then cause image distortion. A membrane optic is an optical system component with a large size, small thickness, and optical precision, so the membrane material and the manufacturing process are nontrivial. The dimensional stability and the optical homogeneity are two essential factors for large aperture optical mirror material. However, it's difficult for traditional aromatic PIs to simultaneously meet the challenging requirements of high dimensional stability, optical transmission, good thermal stability, and mechanical properties. Also it's hard to get large aperture PI films with good optical homogeneity based on the current membrane preparation process. In this research, based on the molecular structure design, rigid molecular chain and hydrogen chain have been introduced to polyimide to improve the dimensional stability and guarantee the excellent mechanical, optical, and thermal properties of the polyimide membrane. At the same time, by optimizing the membrane forming process, the wave-front error of the PI film can meet the requirements of optical use in diffractive imaging system. Compared with commercial Kapton polyimide membrane, the CTE of the new designed PI is ultra-low which is only -1.71×10^-6/℃ in the temperature range of -150 ℃~100 ℃. Also, the tensile strength of the new designed PI is 200.6 MPa. The glass-transition temperature of it is 306.81 ℃. And the average transmittance of designed PI at 500 nm~800 nm is 82.9%. By optimize the membrane fabrication process parameters, the PV and RMS of Φ300 mm membrane can reach to 0.587λ/0.059λ(λ=632 nm). Also, the designed PI shows a good optical stability of the space environment. The obtained optical grade polyimide with high dimensional stability and good optical homogeneity. The excellent comprehensive properties is a good candidate for diffractive lightweight optical application. Also, it will be a good candidate for the optical system of many other fields like high power lasers and solar cells.