Abstract
Optical polyimide (PI) membranes have been increasingly attractive in optoelectronic substrate and optical element material applications. Controlled stress distribution is very important to optical PI membrane-based optics. However, nondestructive absolute stress measurement inside optical PI membranes remains challenging. In this letter, we adopted the stress birefringence method to experimentally investigate the correlation between stress and retardation in uniaxially, biaxially, and circularly stretched PI membranes. The calculated value of the photoelastic coefficient was found to be around 400 nm/Mpa·cm. A theoretical model was established where the retardation angle is the negative arctan of the principal stress ratio in the biaxially stretched membrane. We also found that the average retardation angle is an important parameter for evaluating the uniformity of stretching force in the circularly stretched membrane. This work provides a better understanding of the stress birefringence measurement of membrane materials.
Highlights
Polyimide (PI) membranes have been increasingly attractive in optoelectronic substrate and optical element material applications due to their light weight, good mechanical and chemical stability, and optical transparency [1,2,3,4]
We can adjust the tightness of each screw connection depending on the average retardation angle. This experiment helps establish a better understanding of the stress birefringence measurement of the polymer membrane in the uniaxially, biaxially, and circularly stretched state, and provides a possible method to evaluate the uniformity of the stretching force in a polymer membrane without contact
We adopted a stress birefringence method to investigate the correlation between stress and retardation in a uniaxially stretched PI membrane, and experimentally obtained a photoelastic coefficient around 400 nm/Mpa·cm
Summary
Polyimide (PI) membranes have been increasingly attractive in optoelectronic substrate and optical element material applications due to their light weight, good mechanical and chemical stability, and optical transparency [1,2,3,4]. The optical PI membrane is a promising isotropic material for large-aperture-membrane diffractive lenses applied in next-generation space telescopes [11]. Birefringence, or the double refraction of light, is a natural phenomenon of many crystals and anisotropic materials such as calcite and mica. Isotropic materials such as fused silica and optical polyimide have a uniform index of refraction in all directions and do not exhibit native birefringence. When mechanical stress is applied to isotropic materials, residual birefringence is generated. This birefringence is linearly proportional to the stress inside the material. If the material thickness and photoelastic coefficient of the material are known, the birefringence can be used to Coatings 2020, 10, 56; doi:10.3390/coatings10010056 www.mdpi.com/journal/coatings
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