SGER: Stress Relaxation Mechanisms in End-Linked Main-Chain Smectic Elastomers

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TECHNICAL SUMMARY:Liquid crystalline elastomers (LCE) are rubber-like polymer networks that exhibit strong deviations from ordinary rubber elasticity due to the presence of mesomorphic ordering. "Main-chain" smectic LCE, which form lamellar mesophases due to mesogens embedded in the backbones of the network chains, exhibit complex dynamic mechanical behavior with broad relaxation time spectra. Recent studies of main-chain smectic elastomers prepared by non-linear polycondensation made some progress in understanding their dynamic mechanical response, but further progress depends on achieving better control over the molar mass distribution of elastic chains between crosslinks (Mc). This SGER involves a challenging synthesis of architecturally well-defined smectic LCE by end-linking, which allows control of Mc. "Model" end-linked networks having minimal amounts of defects and "imperfect" networks having deliberately introduced pendant or free chains will be synthesized. Mechanical properties of end-linked MCLCE will be characterized by small-strain oscillatory deformation and tensile stress relaxation. Domain size and the volume fraction of smectic vs. amorphous material will be characterized by X-ray diffraction. Comparing model and imperfect elastomers will reveal how segment-level or domain-level relaxation processes are tied to macroscopic dynamic mechanical response. This study will be the first to use end-linked networks to sort out the complex array of relaxation processes in smectic LCE, offering the potential to catalyze rapid and innovative advances in understanding the physics of liquid crystalline polymer networks.NON-TECHNICAL SUMMARY:Main-chain liquid crystalline elastomers (MCLCE) are materials that combine the flexibility and toughness of a rubber-like polymer with the molecular ordering of liquid crystals. MCLCE have unique mechanical properties that potentially make them useful as vibration damping or impact-absorbing rubber coatings, or as soft actuators with properties similar to muscle tissue. To better understand the connections between molecular structure and macroscopic properties, "model" MCLCE with controlled chemical structures and minimal amounts of defects must be studied. This SGER grant supports preparation of model MCLCE by a technique called end-linking, which will be followed by experimental research aimed at characterizing their physical properties. The project will advance basic understanding of smectic MCLCE, which have layered ordering at the nanometer scale. The research supports valuable educational activities at Penn State University including graduate and undergraduate research, supporting the work of students who actively participate in outreach programs that introduce women and high-school students to polymers and materials science.