Microscopy of Diffuse Nematic–Isotropic Transition in Main-Chain Nematic Liquid-Crystal Elastomers

Abstract

Nematic−isotropic phase transition in liquid-crystal elastomers (LCEs), which drastically alters the macroscopic properties of the material, is attractive for both fundamental physics and applications. Herein, we introduce new experimental methods based on polarized and fluorescence optical microscopy to characterize the phase transition of polydomain LCEs, monitoring the order parameter and the nematic domain structure. It is demonstrated that polydomain main-chain nematic LCEs exhibit an ordering transition very different from the classical first-order one, having a diffuse continuous nature. Unexpectedly, the characteristic correlation length of the fading polydomain structure remains unchanged across the transition.

Impact of Crystallites in Nematic Elastomers on Dynamic Mechanical Properties and Adhesion

“…Figure 6c shows a polarized optical microscopy (POM) image of a very thin film of LC6-Az4 after partial UV irradiation. Note that the polydomain texture, a typical characteristic of nematic LCEs 21,25,60 disappears at the cisrich part, indicating the destruction of the nematic order by the cis state. 23 Moreover, since the cis-rich part probably contains crystallites, the black appearance, i.e., no birefringence at the scale of optical resolution limit of ∼0.5 μm, confirms that the crystallite length scale is much smaller.…”

Photo‐Rewritable Glaring Patterns Composed of Stripe Domains in Nematic Elastomers

“…[ 27 ] Because the POM intensity depends on the degree of birefringence, the nematic order Q or the change can be qualitatively estimated (Figure 2b, red circles). It is shown that Q starts increasing soon after the termination of UV irradiation and recovers the clearly visible polydomain [ 24,25 ] pattern in 10 min (Figure 2b, right inset). The cis%‐dependent isotropic‐to‐nematic phase transition occurs ≈6 min after UV irradiation, at which the cis% is ≈97%.…”

“…This self-organized structure can be used as an anisotropic optical diffuser with low preparation costs. In our previous study, [13] we first stretched a polydomain LCE film, composed of randomly oriented nematic microdomains, [24,25] to obtain the monodomain state. The applied strain is then released to prepare the arrested state, which retains a specific residual strain without externally applied stress and preserves the nearly aligned director state.…”

Effects of photo-isomerizable side groups on the phase and mechanical properties of main-chain nematic elastomers

“…The second heating cycle scans, after full recovery of the trans states induced by the high temperature, indicate the phase behavior of the LCEs in the trans state. Because the endothermic nematic-to-isotropic transition bands of some systems could not be clearly recognized in their DSC curves, the birefringent polydomain textures 1,50,[53][54][55][56] in the POM images and the SS curves at 24 °C were also investigated to identify the phase at room temperature. The estimated phase behaviors are presented in Table 1.…”

“…Although the analytical expression is non-trivial for the other temperature ranges, Q should monotonically increase with decreasing T as the thermal fluctuation of the main chain subsides, which is also associated with the change in birefringence. 1,56,58 Thus, T NI and ΔT seem to be good parameters to evaluate Q at a specific temperature in the nematic phase; a higher T NI or ΔT could imply a higher Q at the temperature of interest. Another experimentally available parameter for estimating Q is the soft elastic strain range, e s , in the SS curves (Fig.…”