New insights into the nature of semi-soft elasticity and “mechanical-Fréedericksz transitions” in liquid crystal elastomers

Devesh Mistry, Philip B. Morgan, John H. Clamp, Helen F. Gleeson
  • Soft Matter, January 2018, Royal Society of Chemistry
  • DOI: 10.1039/c7sm02107k

The mechanical richness of Liquid Crystal Elastomers continues to grow and surprise us!

What is it about?

Liquid Crystal Elastomers (LCEs) couple the anisotropy and ordering of liquid crystals with the elasticity and form retention of crosslinked polymer systems. The result is a highly responsive and mechanical anisotropic soft material with bio-reminiscent properties such as actuation and mechanical anisotropy and non-linearity. Although numerous studies have been performed on the mechanical properties of LCEs to explore phenomena called "semi-soft elasticity" (SSE, >100% extensions costing little to no energy) and "mechanical-Fréedericksz transitions" (MFTs, sharp, critical rotation behaviour of the liquid crystal director) the reasons why one LCE display either SSE or MFTs remains unknown. Here we report a new acrylate-based LCE and explore its mechanical properties using bespoke testing equipment which allows us to study in unprecedented detail the orientation and behaviour of the liquid crystal component of the LCE during tensile load testing. Critically we discover the LCE displays hallmarks of both SSE and MFTs, possibly suggesting a new deformation mode for LCEs and further that the processes of SSE and MFTs may be more closely linked than previously thought. Perhaps most interestingly, we observe that our LCE passes through a state of negative liquid crystal order parameter - a counterintuitive state of liquid crystal ordering never previously reported for a LCE. The work shows that the richness of the mechanical behaviour of LCEs continues to grow and that there is still much to be done to fully understand their physics. Such an understanding is particularly important as the community moves closer to the realisation of LCE-based commercial devices.

Why is it important?

Monitoring both the tensile- and director rotation-mechanical aspects of a LCE's deformation behaviour is critical to understanding the unique anisotropy and nonlinearities observed. However, bizarrely both aspects of a LCE-s deformation behaviour are rarely reported together. If we had only performed tensile- or director-rotation-mechanical testing we would have incorrectly concluded that our LCE conforms to SSE-like or MFT-like mechanical behaviour and we would have missed the conflicting simultaneous observation of hallmarks of both behaviours. Our results therefore offer an insight into potentially what links these two deformation modes allowing a greater understanding of LCEs to be developed. Moreover, by using crossed polarisers to observe the behaviour of the liquid crystal component during deformation of the LCE we can clearly observe a mechanically induced decrease in the nematic order parameter which we deduce must take a negative value at a critical point in the deformation behaviour. If we had instead used X-ray diffraction or IR dichroism we would not have easily been able to discern the difference between a reduction in the liquid crystal order parameter or the formation of a polydomain (scattering) state as has previously been observed.


Devesh Mistry (Author)
University of Leeds

LCEs display remarkable mechanical behaviours which appear to be close to translation into real-world devices in the near future. However, in our work we show that there still remain open questions surrounding the fundamentals of LCE deformation behaviour. Alongside research into devices, future research should also be performed to answer the questions posed by this paper. Doing so could lead to a greater understanding of how LCEs are best used in mechanical-based devices and could open new opportunities for the applications of LCEs.

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