Liquid crystalline elastomers (LCE) are rubber-like polymer networks that usually contain rigid structural units (mesogens), which exhibit spontaneous orientational and/or positional anisotropy, producing strong deviations from ordinary rubber elasticity. LCE exhibit remarkable physical phenomena, such as spontaneous shape changes triggered by electric fields or heating, and anomalously strong mechanical damping (tan δ > 0.5) over a wide range of frequencies. As their exceptional dynamic mechanical response distinguishes LCE from all other elastomers, recent efforts have focused on understanding and controlling the molecular factors governing their mechanical behavior.

Smectic LCE, which are among the most frequently studied, possess lamellar mesophases characterized by robust relaxation dynamics that are strongly perturbed by the constraints of layering. Recent work in our group revealed that main-chain smectic polysiloxane elastomers exhibit a high mechanical loss factor (tan δ = G”(ω)/G'(ω)) over a broad range of temperatures and frequencies, suggesting use as tunable vibration-damping materials. We are investigating the fundamental basis for high mechanical loss in smectic elastomers, which arises from a complex array of relaxation processes occurring at different length scales.

Simple variations in chemical structure of the mesogen, combined with deliberate introduction of high fractions of pendant chains, produce damping elastomers that perform over an extremely broad temperature/frequency range. We recently characterized how relaxation of pendant (dangling) chains (which are present in all polymer networks) influences the mechanical relaxation spectra of smectic LCE. By comparing "optimal" networks with minimal concentrations of dangling chains to "imperfect" networks with higher concentrations thereof, we observed the enhancement of tan δ only in the isotropic state above the clearing temperature. Current work seeks to identify the physical origin of a large loss peak at the LC-isotropic phase boundary, which also appears in rheological studies of chemically similar smectic linear polymers. By simple modifications to the chemical microstructure of the mesogens, we are able to manipulate the temperature/frequency range at which the loss peak is observed, presenting the possibility of "tunable" vibration damping elastomers.