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fluorinated ethylene propylene (FEP) is close relative of poly(tetra-fluoroethylene) (PTFE), both only contain fluorine and carbon atoms,both have very similiar properties yet FEP is relatively transparent while PTFE is opaque, why?

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Otherwise clear plastics lose their transparency from crystallization of the polymer. The crystalline phases have different refractive indices and when a sufficient number form the light becomes so scattered that the material is no longer transparent. Note this is an extensive property and if a material is thick enough it will become opaque, while a thin film of the same material could be translucent.

FEP lacks the regularity in polymer structure that PTFE has, and thus does not crystallize as readily. Since the crystallite density is lower, the polymer is more amorphous and the medium does not scatter as much and thus appears more transparent.

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The transparency of a plastic does not depend on its nature but on how it is made.

To get a transparent plastic it must be cooled very fast when it is in molten state. If the cooling is too slow, the polymer begins to order it self in more or less crystalline domains.

When it is not isotropic anymore, it is also not transparent.

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    $\begingroup$ I cant imagine what non-isotropic plastic is.What is the difference between normal isotropic and non-isotropic plastic? $\endgroup$ – wav scientist Aug 27 '18 at 18:55

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