Extreme Dissipation Behavior of Main-Chain Liquid-Crystal Elastomers and Structures
Abstract
Liquid crystal elastomers (LCEs) are a class of soft stimuli-responsive materials composed of stiff polymer segments, composed of 2-3 ring molecules, bound in an elastomeric network of flexible polymer chains. The mesogens can be incorporated in the elastomeric network either directly in the polymer backbone (main-chain) or as side groups (side-chains), and can develop order/disorder in response to external stimuli, such as temperature and mechanical deformation. The mesogens can develop either orientational order (nematic) or positional and orientational ordering (smectic). This allows LCEs to undergo reversible phase transitions between the polydomain, monodomain, and isotropic states. The motion of the mesogens relative to the polymer network also leads to unusual behavior, including large reversible actuation in response to temperature or light, soft-elasticity, and enhanced dissipation in a load-unload cycle. The goal of this research is to investigate the effects of the mesogen and polymer chain structure on the enhanced dissipation behavior of a main-chain nematic LCE material. We focus our investigation on main-chain nematic materials because they have greater dissipation compared to side-chain or smectic materials. The research plan will utilize recently discovered LCE synthesis methods to design and process LCE materials with different mesogen order and chain alignment. To separate the influence of mesogen ordering and chain alignment on the dissipation behavior, we will develop an innovative experiment that uses near real-time wide- angle X-ray scattering (WAXS) with mechanical tests to measure the relaxation behavior of the mesogen structure and polymer chains. We will develop an experimentally validated thermoviscoelastic model to investigate the interactions between the viscoelastic behavior of the anisotropic network and the dynamics of mesogen ordering and rotation. This integrated experimental and modeling approach applied to LCEs with tailored structures will allow us to isolate the relaxation behavior of the mesogen and the polymer chain structures and analyze for their contributions to the overall dissipation behavior of LCEs. Finally, we will apply the model to design new LCE-based architected cellular structures with extreme dissipation behavior and expanded functionality compared to existing elastomer-based architected structures. The dissipative properties of LCEs have been understudied compared to the stimuli-responsive behavior. This research will answer fundamental questions regarding the processing-structure-properties relationships of LCEs such as: (1) how does the relaxation behavior of the mesogens and polymer chains combine to give the overall dissipation behavior of the LCE, and (2) what is the effect of the microscopic and macroscopic mesogen order and chain orientation on the dissipation behavior? A fundamental understanding of the structure-function relationship of the LCEs will enable the design of new elastomeric materials that can paradoxically exhibit the dissipative behavior of a viscous liquid. The superior dissipation behavior of LCEs will be harnessed to develop new cellular structures for energy absorption and control of wave propagation with vastly improved performance compared to current elastomer-based structures. The ability to locally tune and spatially pattern the chemical structure, mesogen order, network structure (e.g. crosslink density and chain orientation), and cellular architecture provides a large multi-scale design space for optimization of the dissipative properties.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Oct 16, 2018
- Source ID
- W911NF1710165
Entities
People
- Thao D. Nguyen
Organizations
- Army Contracting Command
- Johns Hopkins University
- United States Army