Unmodified types of plastics that are regarded as having unacceptable resistance to UV are POM (Acetal), PC, ABS and PA6/6. Other plastics such as PET, PP, HDPE, PA12, PA11, PA6, PES, PPO, PBT and PPO are regarded as fair. Note that a PC/ABS alloy is also graded as fair. Good resistance to ultraviolet rays can be achieved from polymers extruded by Zeus such as PTFE, PVDF, FEP, and PEEKTM. The only plastics found with excellent resistance are the imides, Polyimide (PI) as used in the Hubble Space Telescope and Polyetherimide (PEI).

http://www.coleparmer.com/TechLibraryArticle/834

Dear Mushtaq Ahmad,

Thanks so much for your answer.

If it is transmittance, Musthaq Ahmed give an elaborated answer but if it is resistance to UV, you should think seriously about Michale Pach hint. Please make it clear for appropriate answer.

@Peter Kapusta: Thanks so much for your answer. PMMA is a good choice. But PMMA cannot be flexible or stretchable. Now I am trying with PDMS, which has a transmittance of 70% for 254 nm UV light.  

PMMA is really good but not as stable as polyimides. DuPont offers now new type of colorless PI films used as a flexible substrate and protecting coating for organic solar cells. They do not disclose the structure. But I guess it must be UV-transparent fluorinated polyimide.

@Dmitri Likhatchev: Thanks for your answer. I think colorless PI is a good choice. It can stand much high temperature than other plastic.

hello mr Zhang Yonghui ! " Based on our evaluation, UV-C light which is used for germicidal processes will not penetrate standard acrylic plastic sheets, so they will not work for your proposed application" (Would acrylic plastic about 1 centimeter (cm) thick prevent ultraviolet-C (UV-C) radiation from passing through?)

"The ability of UV-C light to penetrate materials will depend on the chemical composition of the materials. Most acrylic plastics will allow light of wavelength greater than 375 nm to pass through the material, but they will not allow UV-C wavelengths (100–290 nm) to pass through. Even very thin acrylic sheets of less than 5 millimeters (mm) do not let UV-C light penetrate. Therefore, you will not be able to use standard acrylic plastic for your application.

Because the ability of UV light to penetrate acrylic plastic is dependent on the chemical formulation, you may be able to find plastic sheets that will allow the UV-C to penetrate."

you could try visit the site Mr Mushtaq Ahmad suggested for more information

special UVC transparent plastics can be found in medical applications

https://topas.com/tech-center/performance-data/uv-transmission

These polymers do not transmit appreciable amounts of 270nm UVC. In fact they block UVC transmission. Your lens will need to be fused silica or quartz or perhaps sapphire. One inventor in 2001 claims to have developed a plastic that will transmit UVC, but ithe idea does not seem to have entered the manufacturing phase.

ETFE film, like F-Clean Clear, has high (>80%) transmission to UVC light.

Cheap material, compared to quarz.

https://www.agcchem.com/wp-admin/admin-ajax.php?juwpfisadmin=false&action=wpfd&task=file.download&wpfd_category_id=38&wpfd_file_id=2105&preview=1&embedded=true