As a class of stimuli-responsive materials, liquid crystal elastomers (LCEs) have attracted increasing attention due to their ability to translate microscale orientational changes into macroscale deformation. Considerable efforts have been devoted to controlling the molecular alignment of LCEs to enable their functionality across various applications. Among them, intrinsic auxetic liquid crystal elastomers (IALCEs) represent an emerging system, where the regulation of auxetic responses through alignment control remains unexplored. Moreover, there is significant potential for designing practical applications based on their tuneable auxetic responses. This thesis aims to achieve orientation control of IALCEs via electric fields, by introducing the Fréedericksz transition alignment, thereby enabling tuneable auxetic responses in IALCEs polymerised under electric fields. Furthermore, by patterning distinct auxetic responses, this work demonstrates a pathway toward tuneable and functional implementations of IALCEs, with particular emphasis on applications such as information encryption.
Due to the elastic and dielectric properties of liquid crystals, the director can deform under an external electric field. In this thesis, we conduct electro-optical experiments to characterise the threshold voltage of the IALCE precursor, which was determined to be 0.9 ± 0.1 Vrms. Together with the measured elastic constants (K11 = 4.6 pN and K33 = 4.3 pN), we construct a detailed schematic illustrating the relationship between applied voltages and the director profile under Fréedericksz transition alignment in the IALCE precursor. Through applying electric fields during polymerisations, we represent the first successful attempt to obtain high-quality homeotropic alignment in 100 μm thick films of IALCEs. We also fabricate planar-aligned samples for comparison. Both types of LCE sample show an auxetic response with threshold strains in excellent agreement: 0.56 ± 0.05 for the homeotropic alignment sample and 0.58 ± 0.05 for the planar alignment sample. Furthermore, we demonstrate that the system became biaxial even at very low strains with the high-quality homeotropic sample. To explore the tunability of auxetic responses, we introduce Fréedericksz transition alignments by polymerising the IALCE precursor under varying electric field strengths. By adjusting the applied voltages, we fabricate IALCEs with distinct director profiles, exhibiting the maximum angle induced in the Fréedericksz transition alignment from 40° to 88°, and realise a wide tuneable range of threshold strain of auxetic responses from 0.58 ± 0.05 to 0.91 ± 0.05. Thermal deformation and birefringence measurements confirm the successful introduction of the Fréedericksz transition alignment, showing that IALCEs with the Fréedericksz transition alignment exhibit minimised actuations due to modified order parameters. DSC and stress-strain curve measurements confirm consistent glass transition temperatures (15.0 ± 1.0 °C) and hyperelastic properties across all samples, indicating that the tunability of the auxetic response originates solely from variations in the director alignment.
To explore the potential of tunable devices based on IALCEs, we fabricate two demonstrations of multilevel, multidimensional information storage and encryption by integrating electric field-assisted polymerisation with photomask patterning. These devices respectively showcase 2D optical and 3D tactile information encoding. For the 2D optical information, binary codes (000 100 110), (011 001 000), and (100 010 001) are encrypted into a single IALCE film via masked polymerisation under electric fields. The encoded patterns are sequentially decrypted by stretching the film to specific strain levels of approximately 0.60, 0.70, and 0.80, respectively. In the case of 3D tactile information, we encode the haptic letters "K", "O", and "R" using a set of four IALCE films. Upon sequential stretching to strains of approximately 0.75, 0.85, and 1.00, the corresponding haptic letters appear in order. Owing to the intrinsic auxetic response of the material, a fourth haptic letter, "W", emerges on the reverse side of the IALCE set, completing the word "WORK" during the final stage of decryption. The decryption processes for the 2D optical and 3D tactile information remain mutually independent and highly secure, as successful decryption relies not only on the strain magnitude but also on the strain direction.
In summary, this work demonstrates that polymerisation under electric fields enables precise control over the director profile in IALCE precursors, thereby tuning the auxetic responses of polymerised IALCEs. The successful fabrication of high-quality homeotropic alignment confirms that the emergence of biaxiality is an intrinsic feature of the auxetic response in nematic LCEs. What’s more, by patterning distinct auxetic responses, we realise multilevel, multidimensional information storage and encryption, highlighting the potential of IALCEs in tuneable information devices. This study offers a new strategy for designing and implementing tuneable devices based on programmable auxetic responses in IALCEs.
