CONCLUSION AND FUTURE OUTLOOK
6.2 Future outlook
such as shape memory alloys, which are commercially available and accepted in industry [11, 51]. The finite element model using our constitutive relation from Chapter 4 is a significant step towards a tool that can study nematic elastomers as an engineering material.
Experimental characterization of rate dependence and temperature depen- dence in polydomain and monodomain nematic elastomers
Finally, Chapter 5 described the synthesis of nematic elastomer samples that were tested in our experimental tensile test apparatus, which featured a temperature- controlled chamber. We presented the results for the uniaxial stretch of polydo- main and monodomain nematic elastomers tested at their isotropic and nematic temperatures. The thermo-mechanical coupling inherent in nematic elastomers is responsible for the characteristic stress-strain curves presented in this chapter, for example in the polydomain-to-monodomain transition and in the monodomain- pull-perpendicular experiments that yielded the stripe domain patterns. We could clearly identify the distinct regimes in the stress-strain plots where the liquid crystal mesogen reorientation dominates vs. where the elasticity of the polymer backbone dominates.
The experimental setup also featured polarized light microscopy capabilities. Be- cause of nematic elastomers’ optical properties derived from the underlying liquid crystals, we were able to gain valuable information from viewing samples under cross-polarizers. We observed the formation of fine-scale microstructure, exhibited by stripe domains in monodomain samples pulled perpendicular to their nematic alignment. Finally, we also studied viscoelastic effects by studying the hysteresis at various strain rates and conducting a cyclic loading test.
for designing structures with multiple stable solutions for energy-efficiency reasons, so that the only energy expended is moving between the various stable states. The snap-through instability of Chapter 2 and the kinking instability of Chapter 4 are both interesting phenomena that result from finite elasticity at large deformation, and it would be interesting to continue studying other such instabilities in nematic elastomers.
Computational characterization
To build upon the computational work of Chapter 4, one could perform simulations of the universal deformations inABAQUSusing the constitutive relation for non-ideal polydomains, and investigating the effects of viscoelasticity, for instance plotting the hysteresis between loading and unloading. We also saw that interesting instabilities can arise, for instance in the torsion of a cylinder, opening the door to further investigation regarding the onset of such instabilities in nematic elastomers, the dependence of the instability upon material parameters and geometry, and more.
With theUMATbuilt, further finite element simulations answering such questions are straightforward to run and analyze.
Experimental characterization
The natural next step in experimentally characterizing these materials is to expand upon the experimental results of Chapter 5 to build a complete set of material parameters to match an Ogden model for nematic elastomers for the ideal Bladon- Terentjev-Warner model with the non-ideality in Chapter 2, the relaxed generalized Mooney-Rivlin model of Chapter 3, and the constitutive relation for non-ideal polydomains for Chapter 4. Then, one can perform experiments on the expansion of a monodomain nematic elastomer balloon and quantitatively compare the expansion and twist parameters from Section 2.3, as well as manufacturing and testing a pump made from this monodomain balloon to construct pressure-volume curves at different anisotropy parameters, as described in Section 2.4. The physical size of such a pump could be on the order of centimeters, such as the balloon found in [34].
Furthermore, one can perform experiments within various classes of universal defor- mations, e.g. the bending of a polydomain block, inflation of polydomain balloons (spherical and cylindrical), and cavitation of a polydomain disk, and match the ex- perimental results with the theoretical results of Chapter 3. Other experiments that were traditionally performed on thin films, such as the bulge and blister tests, would also be useful avenues of exploration to characterize the material.
Nematic elastomers and other active materials
Within the field of active materials, there are exciting paths forward leading to- wards the multifunctional, the adaptable, and the autonomous. We can think of the integration of active materials with origami/kirigami for shape-morphing applica- tions, as well as designing adaptable features such as roughness and stickiness for bio-inspired soft robotics applications.
Thermotropic nematic elastomers are quick to heat, but the cooling time can be slow in ambient air based on the temperature differential and the geometry of the sample.
Actuating within a bath increases response times but can be limiting depending on the application. Phototropic nematic elastomers, for instance, have better response times, although there are other issues associated with penetration depth of the light.
The combination of such nematic elastomers responding to multiple stimuli, or the combination of various active materials responding to multiple stimuli, can create multifunctional structures in which the order and extent of the responses can be controlled and tuned for the desired actuation. Additionally, composites of nematic elastomers can be designed for one or more desired properties, e.g. fiber-reinforced elastomers for augmented mechanical behavior, stretchable wiring for augmented electrical capabilities. Composites of active materials can be optimized for various loading configurations using topology optimization.
As mentioned previously, we observe three distinct length scales in nematic elas- tomers: the nematic mesogens (order of nanometers), domains of nematic alignment (order of microns), and the macroscale (on the order of centimeters). However, if a desired application is of a different macroscopic length scale than this, perhaps designing artificial nematic elastomers, featuring a fundamental phase transition occurring at the smallest length scale fully coupled with shape change at the macro- scale, might be a fruitful area of exploration.