It's been known since the 1930s that polymers (plastics etc) are not entirely random amorphous molecular chains as was once thought. In ordinary plastics for example, between 10 and 80% of the structure is 'crystalline' - or, in other words, atomically arranged in regular repeating patterns.

As might be expected, the 'crystalline' zones can have very different properties to the 'amorphous' zones.

It's known that many different factors - e.g. temperature, physical stresses, and the presence of 'nucleating seeds' (from impurities etc.) - can radically affect the extent and type(s) of crystal formation.

But an overall theory explaining all forms of polymer crystal formation has not yet been found.

In the last 100 years, the study of polymer crystallization theory has achieved many breakthroughs, and a large number of models and theories have been proposed. However, each of them can only partially explain the crystallization behavior of polymers. A uniform understanding of polymer crystallization remains a huge challenge."

Source :Polymer, Volume 309, 12,, 127464 ^{[ paywalled ]}

A full explantion of all aspects of polymer crystallization might allow the creation of pupose-made large-crystal polymers with very different and useful properties.

Further technical reading : A Review on Polymer Crystallization Theories, Crystals, Volume 7, Issue 1.