Hairpin Rubber Elasticity: Molecular Basis for Cold Drawing in Smectic Elastomers

Texas Tech University, Lubbock TX

Investigators

Abstract

TECHNICAL SUMMARY: Cold-drawing is a well-known phenomenon in both glassy amorphous and semicrystalline polymers, which is seldom observed in rubber-like networks above the glass transition temperature. Cold-drawing and neck formation have recently been reported in smectic main-chain liquid crystalline (LC) elastomers under certain conditions of temperature and elongation rate, however. Cold-drawing in LC elastomers, glassy amorphous polymers, and semicrystalline polymers clearly cannot be attributed to common morphological features, but it may have common origins in conformational transitions at the chain level. This investigation is aimed at elucidating the underlying molecular basis for cold-drawing in polydomain smectic LC elastomers. The necking instability is proposed to arise from strong energetic contributions to the elastic free-energy upon elongation, in contrast to the classical picture of entropy-driven rubber elasticity. Elastomers containing larger, more stable domains are hypothesized to exhibit a larger yield stress and to be more prone to mechanical instability due to an increased energetic penalty for disrupting smectic ordering during elongation. Macroscopic mechanical response of polydomain smectic LCE will be linked with conformational instability at the chain level by X-ray measurements of domain size and small-angle neutron scattering (SANS) measurements of chain dimensions. A pressing fundamental issue underlying all of the work is the concept of hairpins, chain folds by which the backbone reverses direction upon itself. The number of hairpins per elastic chain affects the domain size along the layer normal, which will be probed by X-ray lineshape analysis. Using SANS, the number of hairpins per chain will be measured as a function of chain length, temperature, and deformation history. The combined results of X-ray diffraction and SANS will provide potentially transformative insights regarding the role of hairpinned chain statistics in promoting cold-drawing and necking in elastomers, and possibly more broadly in other polymers. NON-TECHNICAL SUMMARY: Smectic liquid crystalline elastomers (LCE) are rubber-like materials that possess the flexibility and toughness of a rubber-like polymer, but have layered molecular ordering at the nanometer scale. Smectic LCE 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. One of their unusual features is cold-drawing, a process by which the material yields and elongates drastically when placed under tension, while forming a contraction or "neck." Most rubber-like polymers do not undergo cold-drawing or necking. To better understand how molecular structure in smectic LCE leads to cold-drawing, experimental methods of small-angle neutron scattering and X-ray diffraction will be applied to characterize structural changes at the molecular level due to mechanical deformation. The results of our study will broaden understanding of the mechanical behavior of rubber-like polymers, and possibly uncover broader insights regarding mechanical instability in polymers. This project supports valuable educational activities at Texas Tech University, including graduate and undergraduate education, ethics training for all researchers involved, and mentoring of postdoctoral researchers. The students and postdocs will actively participate in outreach programs that introduce honors students (including women and minorities) to polymers and Chemical Engineering fundamentals, fostering diversity among future researchers in scientific and engineering disciplines.