UV–VIS-NIR optical properties of micrometric-thick polyimide membranes for lightweight devices in space

The optical properties of micrometric-thick polyimide membranes were analyzed between 200 nm and 2500 nm to assess their potential use as substrates for the fabrication of wide optical devices for space applications. These membranes were prepared from fully aromatic polyimides containing trifluoromethyl functional groups which have already been used in space and have shown excellent resistance to the space environment. Various membranes have been synthesized using two different solvents and thermal treatments. Spectrophotometric and ellipsometric measurements were performed to estimate the spectral refractive index (n) and extinction coefficient (k) of various membranes synthesized using a standard solvent, such as N,N-dimethylacetamide (DMAc) and an alternative bio-based solvent, that is dimethyl isosorbide (DMI). The investigation has been extended to consider the morphological properties of the surface and the consequences they have on the diffusion of incident light. This work demonstrates that it is possible to use a green and non-hazardous solvent for the synthesis of thin polyimide membranes, even in the perspective of a large-scale manufacturing process that is greener and more sustainable. Using similar synthesis methods, the surface topography of the polyimide obtained with DMI is found to be smoother. Results suggest that membranes obtained by this bio-based solvent can also be used for optical devices where the scattered component of reflected and transmitted radiation needs to be minimized.