Now imagine that you wake up one day and your flexible smartphone is suddenly inflexible, and the colours on the screen have distorted. The FMLs that make up the devices have solidified without warning. This is a real potential problem with FMLs, and one that Takashi Nakanishi and his team at the National Institute for Materials Science in Japan are working to pre-empt.
The challenge with FMLs is they are prone to a phenomenon called 'supercooling'. "When cooling down water to below 0 °C, it generally freezes into ice," explains Takashi. "However, if its cooling rate is very slow, even below 0 °C, i.e. at -10 °C, water can remain in its liquid phase. Water in such a state is called 'supercooled water'." Because the water is below its natural freezing point, it only takes the smallest change, such as a vibration, or an impurity being added, to cause the water to suddenly freeze.
FMLs can also experience a similar effect, but unlike water they might supercool over a period of months, and we often don’t know what the true freezing point is. This means an FML could be in a supercooled state without anyone realising. "Worse", says Takashi, "the optoelectronic properties of FMLs can change upon freezing – from liquid to solid. This can be quite frustrating when fabricating an FML into electronics, because the FML may solidify suddenly and the device performance may change and show unexpected performance."
Takashi and his team have devised a method of quickly identifying which FMLs are susceptible to supercooling. They have found that small changes in the chemical structure of the molecules can affect their properties – including whether or not they supercool. Based on this they have designed ways of modifying the chemical structure of various molecules in order to prevent supercooling.
"We believe this study will greatly smooth the application of FMLs in various electronics", says Takashi. "In the near future, a huge number of FMLs-based flexible and stretchable electronics may be developed and become a part of our daily life."
This article is free to read in our open access, flagship journal Chemical Science: Fengniu Lu et al., Chem. Sci., 2018, Accepted Manuscript. DOI: 10.1039/C8SC02723D. You can access all of our ChemSci Picks in this article collection. Read more like this