Mechanotropic Elastomers

Abstract

Liquid crystal elastomers (LCEs) are anisotropic polymeric materials. When subjected to an applied stress, liquid crystalline (LC) mesogens within the elastomeric polymer network (re)orient to the loading direction. The (re)orientation during deformation results in nonlinear stress‐strain dependence (referred to as soft elasticity). Here, we uniquely explore mechanotropic phase transitions in elastomers with appreciable mesogenic content and compare these responses to LCEs in the polydomain orientation. The isotropic (amorphous) elastomers undergo significant directional orientation upon loading, evident in strong birefringence and x‐ray diffraction. Functionally, the mechanotropic displacement of the elastomers to load is also nonlinear. However, unlike the analogous polydomain LCE compositions examined here, the isotropic elastomers rapidly recover after deformation. The mechanotropic orientation of the mesogens in these materials increase the toughness of these thiol‐ene photopolymers by nearly 1300 % relative to a chemically similar elastomer prepared from wholly isotropic precursors.

Isotropic Liquid Crystal Elastomers as Exceptional Photoelastic Strain Sensors

“…While it is well-known that nematic LCEs will undergo a softening of the nematic-to-isotropic transition when under strain (so-called paranematic transitions), the observation of strain-induced nematic order parameters in completely isotropic elastomers has only recently been observed. − Here, LCEs 1, 2, and 3 all demonstrate linear increases in their nematic order parameter with strain–in agreement with the prediction (eq ) made using Warner and Terentjev theory. LCEs 1 and 3 have larger couplings, a , between the polymer chain and mesogenic groups (0.51 ± 0.02 and 0.66 ± 0.02, respectively), than LCE 2 (0.37 ± 0.02)a fact rationalized by the fact that LCEs 1 and 3 have mesogenic cross-linking groups which are main-chain in nature and so align strongly with the applied stress axis.…”

“…Our LCEs would therefore be simple to model and work with in photoelastic experiments. By comparison, conventional LCEs are well-known for their highly complex nonlinear and anisotropic mechanical behaviors. ,,,, …”

“…Recently, there has been a growing interest in isotropic elastomeric networks containing liquid crystal moieties. − Donovan et al. described strain-induced nematic phases in thiol-ene LCEs which demonstrated a nematic order parameter, ⟨ P 2⟩, of ∼0.3 at the material’s maximum strain of 2.25, a value most likely indicative of a biphasic state containing paranematic and nematic phases .…”

“…described strain-induced nematic phases in thiol-ene LCEs which demonstrated a nematic order parameter, ⟨ P 2⟩, of ∼0.3 at the material’s maximum strain of 2.25, a value most likely indicative of a biphasic state containing paranematic and nematic phases . Their study found that their isotropic LCEs had a significantly greater toughness, less creep, and faster recovery times compared to polydomain LCEs of similar chemistryalthough comparisons with the performance of conventional isotropic elastomers were not made …”

Exchangeable Liquid Crystalline Elastomers and Their Applications

“…These properties make LCEs highly attractive in biomedical engineering, 11 , 12 robotics, 11 , 13 smart textiles, 14 , 15 damping, 16 , 17 adhesive systems, 11 , 18 surface modifications, 14 , 19 and many other fields of modern engineering. 20 − 23 …”