June's Paper of the Month

Polarised optical photomicrographs of liquid crystals show the change in texture caused by slow cooling.

Polarised optical photomicrographs of liquid crystals show the change in texture caused by slow cooling.

Polymorphism is the existence of more than one form. In the case of liquid crystals, this is when a material can exist in two or more crystal structures. As the structures vary, this in turn affects their function and properties. Finding liquid crystal polymorphs would be advantageous for many different fields including engineering, pharmaceuticals and sensors. 

Real polymorphisms are difficult to find in rod shaped liquid crystals. Previous studies have shown that bent-core liquid crystals, although their phases can vary depending on cooling rate, havesmectic structures and x-ray diffraction patterns that are almost identical. 

A collaborative research effort from the Kent State University and Lawrence Berkeley National Laboratory found a polymorphic bent core liquid crystal that has structurally and morphologically independent liquid crystal phases that are cooling rate dependent. As their structures differ, so does the structural colour, paving way for a range of potential applications. 

The group conducted several different experiments to identify the liquid crystal polymorphs. They used Polarised Light Optical Microscopy to visualise the cooling rate dependant formation. To do so, the team used the Linkam LTS420E to conduct their temperature-controlled experiments, both heating and cooling the samples. 

They found that upon slow cooling oblique columnar phase forms and on rapid cooling, helical microfilament phase forms were produced. This change in structure was also accompanied by a unique colour change. 

This novel finding highlights the ability to control liquid crystal structure through temperature control. The change in colour facilitated by the structural transformation, could be used in future applications of thermal sensors and security tags.  

By Tabassum Mujtaba

Hegmann et al., An unusual type of polymorphism in a liquid crystal. (2018) Nature Communications volume 9, Article number: 714