Researchers in Germany say they can explain the apparently contradictory results seen when liquid crystals are mechanically stretched.

Smectic liquid crystal elastomers (SLCEs) are rubber-like liquid crystals in which molecules, known as mesogens, are grouped in layers that are coupled with an elastic polymer backbone. When films of SLCEs are stretched in one direction, they shrink along another direction to conserve volume.

 

      

 

Ralf Stannarius at Otto-von-Guericke-Universität Magdeburg and colleagues claim that SLCEs fall into two distinct types that have subtly different microstructures due to the different ways they were made.  The researchers say these small differences in structure might explain why the liquid crystals behave differently under mechanical stress.

'Smectic elastomers show a number of unexpected, and so far not satisfactorily understood, mechanical properties,' said Stannarius. While other scientists found that their films exclusively contract within the layer, Stannarius said he has observed films shrinking perpendicular to the layers. 

Stannarius said the most likely explanation for the films shrinking perpendicular to the layers is that mesogens in adjacent layers interpenetrate. Usually this would require a lot of energy, but it is the only model that explains all the observations, said Stannarius. 

He claims the method of synthesis can have a strong influence on the layer structure as the sample is stretched. The result is that SLCEs prepared by different methods behave differently.

'The suggested mechanism is an interesting speculation,' said Mark Warner, professor of physics at the University of Cambridge. 'One would need to demonstrate that the crosslink distribution and homogeneity is different in the two cases.'

Stannarius's goal is 'ordered materials that allow the controlled manipulation of sample shapes and dimensions by external fields in technical devices, thus serving as smart actuators or sensors.' He says this requires a better understanding of the interactions within the SLCE network. 

Caroline Moore