Dynamic self-stiffening in liquid crystal elastomers

Abstract

Biological tissues have the remarkable ability to remodel and repair in response to disease, injury and mechanical stresses. Synthetic materials lack the complexity of biological tissues, and man-made materials that respond to external stresses through a permanent increase in stiffness are uncommon. Here we report that polydomain nematic liquid crystal elastomers increase in stiffness by up to 90% when subjected to a low-amplitude (5%), repetitive (dynamic) compression. Elastomer stiffening is influenced by liquid crystal content, the presence of a nematic liquid crystal phase and the use of a dynamic as opposed to static deformation. Through rheological and X-ray diffraction measurements, stiffening can be attributed to a mobile nematic director, which rotates in response to dynamic compression. Stiffening under dynamic compression has not been previously observed in liquid crystal elastomers and may be useful for the development of self-healing materials or for the development of biocompatible, adaptive materials for tissue replacement.

Figure 1. Synthesis and dynamic strain stiffening of polydomain LCEs

a, Schematic for the synthesis of a polydomain LCE. A representative LCE sample is shown on the right-hand side, and dynamic mechanical testing was carried out on LCEs with dimensions of 1.5 mm x 1.5 mm x 1 mm. b, Change in stiffness (%) versus time for a LCE (LCE90) under repetitive (dynamic) compression. The LCE is dynamically compressed between two flat plates at 5 Hz, 45 °C, a pre-load of 0.01N, and a 5 % strain amplitude using a DMA Q800. Schematics in the bottom-left and top-right show the alignment of nematic domains in LCEs subjected to repetitive compression. The inset in the bottom right shows schematic of experimental protocol employed for the dynamic compression of polydomain LCEs. The data shown are plotted on linear-log axes, and data on linear-linear axes is provided in the Supplementary Figure S5. A plot of the time-dependent strain applied to the sample during measurement is shown in Supplementary Figure S6.

Figure 2. Dynamic strain stiffening in LCEs with varying mesogen content

Change in stiffness (%) versus time for a series of LCEs varying in mesogen content and PDMS under dynamic compressive strain. The data shown are plotted on linear-log axes, and data on linear-linear axes is provided in the Supplementary Figure S4. The extent of stiffening is correlated with LCE mesogen content. Measurements were carried out at 5 Hz, 45 °C and at 5% strain amplitude. The top inset compares dynamic strain stiffening in LCE90 under dynamic load, static load and dynamic load above T_NI (80 °C) and demonstrates that stiffening is only observed in the nematic phase under dynamic compression. For static load tests, samples were subjected to a compressive strain of 6%, both greater than the corresponding stress and strain values applied during dynamic loading.

Figure 3. Microstructure analysis of LCEs after dynamic compression

a, Schematic of geometry during dynamic compression experiment and 2DWAXD pattern of unstressed, polydomain LCE90. All LCEs exhibit similar 2DWAXD patterns before dynamic compression. b, Polarizing optical microscopy images of dynamically stressed LCE90 along three different faces. Reduced light transmission through the x-z and y-z faces when crossed polarizers are oriented parallel to the compression direction (z-axis) indicates reorientation of the nematic director perpendicular to the z-axis. All scale bars represent 0.1 mm. c, 2D WAXD patterns of LCEs with varying mesogenic content subjected to compressive dynamic load (5 Hz, 5 % strain) for at least 16 hours. The patterns are shown for three independent LCE faces (x-z plane, x-y plane, and y-z plane), and the arrow on the bottom right hand side indicates the direction of compression. The anisotropic scattering pattern observed for the x-z and y-z planes indicates that the LCE nematic director rotates away from the z-axis (compressive direction) to lie primarily in the x-y plane, but the nematic director remains disordered in the x-y plane. d, Macroscopic alignment parameter S for dynamically stressed LCEs extracted from model fit of 2DWAXD pattern. The order parameter S is defined with respect to the x-y plane. The order parameters for the x-z and y-z planes is an average of the order parameters measured in each plane.

Figure 4. Reorientation of LC polymer chains under dynamic compression

a, Finite element modeling (FEM) simulation showing stress lines as a result of compression. FEM simulations were performed out using COMSOL Multiphysics 4.2 simulation package. b, Schematic depiction of a prolate LC polymer chain. Note that the mesogenic side-groups are oriented preferentially parallel to the polymer backbone. c, Reorientation of prolate LC polymer chains under compressive strain. d, Stress-Strain relationship of (i) polydomain LCE under uniaxial tension (data reproduced with permission from Clarke et al.) and (ii) polydomain LCE90 before and after dynamic compression. For LCE90 stress-strain profiles are recorded along the compression axis.