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Traditionally, solids are classified as being either crystalline (well-ordered, periodic lattice structure at large spatial scales) or amorphous (disordered structure).

A well-ordered polymer is a crystalline polymer, whereas one which is disordered is an amorphous polymer.

My questions: is a crystalline polymer categorically different from a crystal? Is it simply a sub-type of the class of materials we call crystals? Is an amorphous polymer simply an amorphous solid with a bit more order (containing strings of molecules chained together)?

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    $\begingroup$ Crystalline doesn’t necessarily imply periodic anymore since the 1980’s and the discovery of quasicrystals. $\endgroup$
    – penovik
    Commented May 28, 2019 at 18:54

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In my material science course, we divided solids under three categories: Crystalline solids, amorphous materials, and polymers. So... I guess your question can have different answers from different people. Plus this distinction is not that certain: For example, we can have grained materials. One can probably claim a well-structured polymer is a crystal, yet we still know that that structure will not always behave like a normal (metallic-ceramic) crystal. The existence of more and more exotic properties of polymers, compared to its crystalline and amorphous counterparts, such as high elasticity, are some good reasons to make them a separate category. I advise you to find a source that investigate all three and compare their properties.

(This was initially a comment, so sorry for the formatting).

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Polymers form molecular solids. These can be crystalline or disordered. See https://en.m.wikipedia.org/wiki/Molecular_solid. Other examples are solid CO2, H2.

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    $\begingroup$ But the structural arrangement and mechanical properties are very different from molecular solids of small identical molecules. Polymers are long and get entangled like spaghetti after cooking. And the molecules do not usually have the same length. $\endgroup$
    – user137289
    Commented May 28, 2019 at 8:57

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