Highly crystalline, thermally stable, and mechanically flexible polymers are desirable. Unfortunately, most polymers are not very crystalline (if at all), and an increase in polymer crystallinity usually results in increased brittleness. In addition, crystallizing polymers on an industrial scale can be very difficult.
Dejan-Krešimir Bučar, University College London, UK, Dritan Hasa, University of Trieste, Italy, and colleagues have used a crystal engineering approach to enhance the crystallinity and thermal stability of polyethylene glycol (PEG) while maintaining its mechanical flexibility. This was achieved through cocrystallization, in particular, the incorporation of small molecules into the solid-state structure of the polymer. The team prepared one type of cocrystal composed of caffeine, anthranilic acid, and poly(ethylene glycol) dimethyl ether (PEG-DME) and a second one comprised of caffeine, 6-fluoro-anthranilic acid, and PEG-DME.
The researchers found that cocrystallization of PEG-DME with caffeine and anthranilic acid significantly boosts its thermal stability from 36 °C to 98 °C without compromising the mechanical flexibility. The replacement of the anthranilic acid cocrystal component with 6-fluoro-anthranilic acid further boosted the melting point of the polymer to 128 °C. The formation of solid solutions allowed the team to fine-tune the polymer formulation melting point over a temperature range from 98 °C to 128 °C.
The ability to thermally stabilize polymers without compromising their mechanical flexibility could have implications, e.g., for the development of pharmaceuticals or polymers for electronics. According to the researchers, the use of polymers as cocrystal formers could enhance, for example, the “tabletability” of otherwise brittle pharmaceutical solids, and the use of small molecules as cocrystal formers could enhance the crystallinity and thermal stability of organic semiconducting polymers.
- Modulating thermal properties of polymers through crystal engineering,
Luzia S. Germann, Elvio Carlino, Antonietta Taurino, Oxana V. Magdysyuk, Dario Voinovich, Robert E. Dinnebier, Dejan-Krešimir Bučar, Dritan Hasa,
Angew. Chem. Int. Ed. 2023.