Programming actuation onset of a liquid crystalline elastomer via isomerization of network topology

Abstract

Tuning actuation temperatures of liquid crystalline elastomers (LCEs) achieves control of their actuation onsets, which is generally accomplished in the synthesis step and cannot be altered afterward. Multiple actuation onsets in one LCE can be encoded if the post-synthesis regulation of actuation temperature can be spatiotemporally achieved. This would allow realizing a logical time-evolution of actuation, desired for future soft robots. Nevertheless, this task is challenging given the additional need to ensure mesogen alignment required for actuation. We achieved this goal with a topology isomerizable network (TIN) of LCE containing aromatic and aliphatic esters in the mesogenic and amorphous phases, respectively. These two ester bonds can be distinctly activated for transesterification. The homolytic bond exchange between aliphatic esters allows mechanically induced mesogen alignment without affecting the mesogenic phase. Most importantly, the heterolytic exchange between aromatic and aliphatic esters changes the actuation temperature under different conditions. Spatial control of the two mechanisms via a photo-latent catalyst unleashes the freedom in regulating actuation temperature distribution, yielding unusual controllability in actuation geometries and logical sequence. Our principle is generally applicable to common LCEs containing both aromatic and aliphatic esters.

Fig. 1. Synthesis and programming of the topology-transformable liquid crystalline elastomer.

a Synthesis route. b Two sorts of esters within the network. c Homolytic and heterolytic bond exchange reactions. d Network reconfiguration and isomerization during alignment programming. e Spatial programming of the alignment and T_NI. f Sequential actuation upon progressive heating/cooling.

Fig. 2. Actuation programming of LCEs (catalyst: 1% neutralized TBD, pre-stretched strain: 50%).

a Stress relaxation under different T_ps. b Isotropic-to-anisotropic transformation of the 2D-WAXD pattern before and after alignment programming. c LCE linear actuation (T_p: 120 °C, t_p: 10 min). d Correlation between actuation strain and t_p at different T_ps. Error bars represent standard deviation, n = 5. e T_NIs of the LCEs programmed at different T_ps (t_p: 10 min). f Relationship between actuation strain and temperature upon cooling (t_p: 10 min). g DMA cyclic actuation curves (T_p: 120 °C, t_p: 10 min).

Fig. 3. Actuation programming of the LCEs (catalyst: PBG, pre-stretched strain: 50%, T_p: 120 °C).

a Dependence of the actuation strain on t_p (catalyzed via unexposed PBG). Error bars represent standard deviation, n = 5. b Relationship between the actuation strain and the irradiation time (t_p: 10 min). Error bars represent standard deviation n = 5. c Correlation between the actuation strain and the temperature of the samples irradiated for different times (t_p: 10 min). d Relationship between T_act and the irradiation time (t_p: 10 min). e Residual actuation strain of a monodomain LCE irradiated at 120 °C for different times (without stress). Error bars represent standard deviation, n = 5. f Visual demonstration of actuation erasing (irradiation time: 120 s). g 3D active LCE structures fabricated via spatially erasing actuation strain. Inset: the left one is the employed photomask, where the yellow areas are the exposed regions; the right one is the schematic illustration of the LCE sample, where the arrows indicate the alignment direction and the solid lines represent the cutting lines of the kirigami patterns. Scale bars: 1 cm.

Fig. 4. Complex actuation manners of LCEs.

a Active LCE strip with two distinct T_NI patterned via UV exposure exhibits a sequential actuation. b Reversible multi-shape actuation. The white areas are exposed to UV light for 2 min. Square size: 1 mm × 1 mm. c, d Self-locked and self-correctable actuation behavior of the four-leaf clovers with single and quadri T_NIs. The number in each petal indicates the irradiation time (s). Scale bar: 1 cm.