Real-Time Alignment and Reorientation of Polymer Chains in Liquid Crystal Elastomers

Abstract

Liquid crystal elastomers (LCEs) exhibit soft elasticity due to the alignment and reorientation of mesogens upon mechanical loading, which provides additional mechanisms to absorb and dissipate energy. This enhanced response makes LCEs potentially transformative materials for biomedical devices, tissue replacements, and protective equipment. However, there is a critical knowledge gap in understanding the highly rate-dependent dissipative behaviors of LCEs due to the lack of real-time characterization techniques that probe the microscale network structure and link it to the mechanical deformation of LCEs. In this work, we employ in situ optical measurements to evaluate the alignment and reorientation degree of mesogens in LCEs. The data are correlated to the quantitative physical analysis using polarized Fourier-transform infrared spectroscopy. The time scale of mesogen alignment is determined at different strain levels and loading rates. The mesogen reorientation kinetics is characterized to establish its relationship with the macroscale tensile strain, and compared to theoretical predictions. Overall, this work provides the first detailed study on the time-dependent evolution of mesogen alignment and reorientation in deformed LCEs. It also provides an effective and more accessible approach for other researchers to investigate the structural-property relationships of different types of polymers.

Rate-dependent stress-order coupling in main-chain liquid crystal elastomers

“…Fig. 11a and b illustrate the behavior of the director and shear strain based on eqn (19) and (20) for a = 0.1. The director and shear strain start with y = 901 and l 21 = 0 when l 22 = 1.…”

“…1 Moreover, external stimuli, such as light irradiation, [5][6][7] magnetic fields, [8][9][10] and electrical fields [11][12][13][14] can trigger the phase transition or reorient the director of LCEs, which can lead to spontaneous strain approaching 400% 15,16 or stress if constrained. On the other hand, mechanical deformation not parallel to the director can reorient the director to the stretching direction, 1,[17][18][19][20][21][22] inducing a stress plateau in the stress-strain relation. 1,23 The unique mechanical behaviors endow LCEs with many potential applications, including soft robots, 5,[24][25][26] thermomechanical actuators, 27,28 artificial muscles, 15,29 and so on.…”

“…Conversely, polarized optical microscopy is an alternative method to capture mesogen reorientation under fast loading. 19,20,46,49 Recently, Luo et al 20 used crossed-polarized optical measurements to evaluate the director rotation in monodomain LCEs at different loading rates. Mistry et al 19 used polarized optical microscopy to measure the director distribution in LCEs subjected to step stretching almost perpendicular to the initial director.…”

“…Azoug et al 31 and Martin Linares et al 35 studied the main-chain polydomain LCEs under uniaxial tension, reporting rate-dependent anisotropic stress responses. Moreover, Luo et al 20 evaluated the director alignment of main-chain monodomain LCEs by optical measurements, finding the director almost reaches equilibrium rotation at the loading rate of 450% per min. However, it is not clear what the relaxation time scales of the network extension and director rotation are for main-chain LCEs, and how they influence the macroscopic stress–strain behavior and microscopic director rotation.…”

Recent advances in molecular programming of liquid crystal elastomers with additive manufacturing for 4D printing

“…Since LCE's stimuliresponsive deformation is driven by re-orientation of their own molecular structure, they require neither external force nor aqueous environment for self-deformation. In addition, LCEs also possess other unique properties including high work density, [10][11][12][13] anisotropic mechanical/thermal/optical properties, [14][15][16][17] and high viscoelastic dissipation. [18][19][20][21] The extraordinary properties of LCEs, their chemistry and responsiveness to different physical cues, and how they are utilized to enable 4D printing have been extensively discussed in recent reviews.…”