Theoretical research in soft matter physics has progressed

The coupling between the macroscopic shape change of the liquid crystal elastomer and the direction of the microscopic liquid crystal unit inside the liquid crystal elastomer makes it possible to perform mechanical work by adjusting the orientation order of the liquid crystal unit inside the liquid crystal elastomer. Single-domain liquid crystal elastomers with well-arranged liquid crystal molecules have excellent mechanical properties, such as reversible large strain deformation, high strength and excellent toughness. However, due to the difficulty of random quenching effect to achieve a uniform arrangement of liquid crystal cells and the non-recyclability of the material (similar to other common thermoset materials), liquid crystal elastomers have not been widely used in industry until now. In order to overcome the above difficulties, the bond exchange liquid crystal elastomer was successfully prepared in 2014; Unlike the high cross-linking energy barrier in liquid crystal elastomers, which makes it difficult to recombine cross-linked bonds, the bond exchange reaction rate in bond exchange liquid crystal elastomers can be adjusted by adding catalysts and changing temperature. Although remarkable experimental progress has been made in the preparation of different bond-swapped liquid crystal elastomers, there is still a lack of theoretical framework for understanding the physical properties of such liquid crystal elastomers, and the main difficulties in constructing this theory come from the coupling of various physical factors such as dynamic evolution nematic order, entropy elasticity of polymer materials and energy dissipation caused by bond exchange reactions.

Recently, Meng Fanlong’s research group of the Institute of Theoretical Physics, Chinese Academy of Sciences, constructed a continuum theoretical model of bond-switched liquid crystal elastomers by considering the microscale bond exchange reaction, and revealed the universal rheological characteristics that bond-switched liquid crystal elastomers may present in different application scenarios.

Figure 1: Schematic diagram of a bond-swapping liquid crystal elastomer whose cross-linked bonds can be exchanged through certain chemical exchange reactions.

Stress hesitation: By applying a fixed amplitude of deformation, the bond-swapping liquid crystal elastomer creates nematic order while applying the deformation. After that, the stress of the bond-swapping liquid crystal elastomer decays exponentially with time, and the sequence parameters accompanying the column order also decay exponentially with time (Figure 2).

Figure 2: Stress hesitation of a bond-swapped liquid crystal elastomer.

Stress-strain relationship at fixed strain rate: By applying strain at a fixed strain rate, the stress-strain curve of the bond exchange liquid crystal elastomer will exhibit non-monotonic characteristics of first increasing and then decreasing, in which the highest point of the curve is defined as the yield point. Due to the coupling of deformation to its nematic order, the stress-strain response of bond-swapped liquid crystal elastomers is “softer” than that of vitrimers without directed nematic order. Increasing the fixed strain rate, the stress-strain curve of the bond-swapped liquid crystal elastomer will move upward and approach the stress-strain curve of the cross-bonded permanent liquid crystal elastomer (Figure 3).

Figure 3: Stress-strain relationship of bond-swapped liquid crystal elastomer at a fixed strain rate (the solid line is the bond-swapping liquid crystal elastomer, and the dashed line is the vitrified-like elastomer).

Creep yield: By applying a fixed amplitude of stress, a bond-swapping liquid crystal elastomer generates an instantaneous strain. Due to the energy dissipation caused by the bond exchange reaction, its strain increases over time and takes on the “flowing” properties like a liquid. By defining the effective viscosity, the authors found that its effective viscosity decreases with increasing fixed stress. At the same time, by analyzing the temporal evolution of strain, the authors found that the bond-switched liquid crystal elastomer has strain thinning behavior (Figure 4).

Figure 4: Creep yield of key-swapped liquid crystal elastomer (solid line is key-swapped liquid crystal elastomer, dashed line is vitrified elastomer).

The research results were recently published in the journal Physical Review Letters, with Zhao Jiameng, a 2022 doctoral student from the Institute of Theoretical Physics, as the first author of the paper, and researcher Meng Fanlong as the corresponding author. (Source: Institute of Theoretical Physics, Chinese Academy of Sciences)

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