More than 50 years ago, a research group in Japan began characterizing polymers in the presence of water under different conditions. They discovered that two different polymers, polyoxocyclobutane (POCB) and polyethyleneimine (PEI), can bind to water and form crystalline hydrate structures upon cooling. The unit cells of these hydrates were altogether different from those of the pure polymers, and this was not a case of a homopolymer crystal absorbing water into interstitial spaces. Instead, the water and polymer were synergistically combining into entirely new ordered structures.
Co-crystallization of two distinct species is relatively common among metal alloys, and numerous intermetallic compounds are known. Co-crystallization is less common among organic compounds. In fact, water can itself crystallize with small molecules such as methane, carbon dioxide, and cyclopentane to form clathrate hydrates. Even a relatively large molecule like t-butyl ammonium bromide (TBAB) can form a hydrate. However, polymer co-crystallization with any kind of small molecule is unusual, whereas co-crystallization of a polymer with water is exceptionally rare.
Since the original research from the 1970s,...
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