Post-Wrinkling Behaviors in Layered Elastic Polymers

Surface instabilities include a variety of different modes such as wrinkles, folds, and creases. Such surface instabilities have been used in numerous contexts including changing the wetting, optical, and mechanical properties of different material surfaces for applications in flexible electronics, tissue engineering and biosensors. Interesting surface morphologies can arise due to differences in pre-strain, stiffness, and thickness between the film and substrate; however, the effects of these differences are not well understood. We have developed an experimental system, complemented by finite element simulations, to systematically vary the pre-stretch of the substrate, modulus contrast and thickness contrast to study the transitions of surface instabilities in compressed bi-layers as strain is increased. We find that for systems with large modulus contrast, but varying degrees of substrate pre-stretch, the pre-stretched substrates not only substantially shift the critical strain for post-wrinkling bifurcations, but also qualitatively change the post-wrinkling modes observed. Understanding that pre-stretching the substrate affects post-wrinkling behaviors, the remaining experiments were conducted with no substrate pre-stretch. Examining the effect of modulus contrast, we report that below a film to substrate elastic modulus ratio of approximately 2, the flat state transitions directly to the creased state. For slightly larger contrasts, wrinkling occurs first but there are two distinct types of post-wrinkling behaviors: (1) wrinkles that transition into creases without period-doubling; and (2) wrinkles to creases preceded by period doubling. Finally, considering the effect of thickness contrast between the film and substrate, we discover that the critical strain for post-wrinkling behavior increases as thickness contrast decreases leading to wrinkles with aspect ratios (defined as amplitude normalized by wavelength) as high as 0.6. Further increases in aspect ratio can be achieved by increasing the modulus contrast, leading first to the formation of localized ridges, and then uniform, high aspect ratio wrinkles. Understanding surface instabilities in such non-ideal bi-layer systems provides important insight into the behavior of natural and synthetic systems with varying substrate pre-strain and modulus and thickness contrast

Synthesis of Elastomeric Liquid Crystalline Polymer Networks via Chain Transfer

Materials capable of complex shape changes have broad reaching applications spanning biomimetic devices, componentless actuators, artificial muscles, and haptic displays. Liquid crystal elastomers (LCE) are a class of shape programmable materials which display anisotropic mechanical deformations in response external stimuli. This work details a synthetic strategy to quickly and efficiently prepare LCEs through the usage of chain transfer agents (CTA). The polyacrylate materials described herein exhibit large, reversible shape changes with strains greater 475%, rivalling properties observed in polysiloxane-based networks. The approach reported here is distinguished in that the materials chemistry is readily amenable to surface alignment techniques. The facile nature of the materials chemistry and the compatibility of these materials with directed self-assembly methods could further enable paradigm shifting end uses as designer substrates for flexible electronics or as actuating surfaces

Electrical Control of Shape in Voxelated Liquid Crystalline Polymer Nanocomposites

Liquid crystal elastomers (LCEs) exhibit anisotropic mechanical, thermal, and optical properties. The director orientation within an LCE can be spatially localized into voxels [three-dimensional (3-D) volume elements] via photoalignment surfaces. Here, we prepare nanocomposites in which both the orientation of the LCE and single-walled carbon nanotube (SWNT) are locally and arbitrarily oriented in discrete voxels. The addition of SWNTs increases the stiffness of the LCE in the orientation direction, yielding a material with a 5:1 directional modulus contrast. The inclusion of SWNT modifies the thermomechanical response and, most notably, is shown to enable distinctive electromechanical deformation of the nanocomposite. Specifically, the incorporation of SWNTs sensitizes the LCE to a dc field, enabling uniaxial electrostriction along the orientation direction. We demonstrate that localized orientation of the LCE and SWNT allows complex 3-D shape transformations to be electrically triggered. Initial experiments indicate that the SWNT–polymer interfaces play a crucial role in enabling the electrostriction reported herein