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Chapter 21: Problem 22

Briefly explain why amorphous polymers are transparent, while predominantly crystalline polymers appear opaque or, at best, translucent.

Short Answer

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Question: Explain why amorphous polymers tend to be transparent, while crystalline polymers usually appear opaque or translucent. Answer: Amorphous polymers are transparent because their random molecular chain arrangement does not provide specific planes for light reflection, allowing light to pass through without scattering. In contrast, crystalline polymers have an ordered arrangement of molecular chains that form crystalline regions, causing light scattering due to a change in refractive index at the interfaces between the crystalline and amorphous regions, resulting in opacity or translucency.

Step by step solution

01

Discuss amorphous polymers

Amorphous polymers are characterized by a random arrangement of their molecular chains. These molecular chains do not exhibit an ordered, crystalline structure. Instead, the chains are tangled and irregular. A common example of an amorphous polymer is polystyrene.
02

Discuss crystalline polymers

Crystalline polymers, on the other hand, have a more ordered and regular arrangement of their molecular chains. This structure creates a repeating pattern, which forms a crystalline structure. Some examples of predominantly crystalline polymers include polyethylene and polypropylene.
03

Explain light transmission through amorphous polymers

In amorphous polymers, because of the random arrangement of the chains, there is no specific plane for light reflection, unlike crystalline polymers. As the light passes through the material, it continues to travel without scattering or reflection. This allows most of the light to pass through, and as a result, the amorphous polymer appears transparent.
04

Explain light transmission and scattering through crystalline polymers

Crystalline polymers have an ordered arrangement of molecular chains that form crystalline regions. When light enters these regions, it encounters changes in refractive index and strong interfaces between crystalline and amorphous regions. These interfaces cause light to scatter in different directions, and some light may also be absorbed by the material. As a result, only a small portion of the light passes through, making the crystalline polymer appear either opaque or translucent.
05

Conclusion

The transparency of amorphous polymers and the opacity or translucency of predominantly crystalline polymers can be chalked up to their molecular structures. The random arrangement of chains in amorphous polymers allows light to pass through without scattering, while the ordered structure in crystalline polymers causes light scattering due to a change in refractive index at the interfaces between the crystalline and amorphous regions.

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Most popular questions from this chapter

The index of refraction of corundum \(\left(\mathrm{Al}_{2} \mathrm{O}_{3}\right)\) is anisotropic. Suppose that visible light is passing from one grain to another of different crystallographic orientation and at normal incidence to the grain boundary. Calculate the reflectivity at the boundary if the indices of refraction for the two grains are \(1.757\) and \(1.779\) in the direction of light propagation.

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