(447c) Stress-Strain Behavior of Smectic Main-Chain Elastomers | AIChE

(447c) Stress-Strain Behavior of Smectic Main-Chain Elastomers


Hedden, R. C. - Presenter, Texas Tech University
Lentz, D. M. - Presenter, Penn State University
Patil, H. P. - Presenter, Penn State University

Liquid crystalline elastomers (LCE) are rubber-like polymer networks that usually contain rigid, rod-like chemical units (mesogens). Because of spontaneous orientational or positional ordering of the mesogens, LCE exhibit remarkable deviations from ordinary rubber elasticity, leading to fascinating physical phenomena such as spontaneous shape changes triggered by electric fields or heating. 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 static and dynamic mechanical response at both low and high strains.

Smectic LCE are those that form a layered mesophase having inter-layer spacings of about 30 Å. Crosslinking a smectic polymer in the absence of an aligning field produces a polydomain elastomer containing numerous randomly oriented microdomains. Under uniaxial tension, polydomain smectic elastomers are well known to undergo a transition to a globally oriented monodomain state. The mechanism for this transition has been debated, as domain rotations and/or transient disordering of domains have each been suggested to occur in response to applied strain. New studies of smectic elastomers having mesogens embedded in the polymer backbone (main-chain elastomers) suggest a three-stage deformation process. At low strains, amorphous (non-smectic) material deforms readily, with minimal change to the size or orientation of smectic domains. At intermediate strains, disordering of smectic microdomains via unfolding of hairpin structures is the dominant mechanism for elongation. At very high strains, elastic chains approach the finite extensibility limit, and layer buckling becomes a significant mechanism for elongation. Because of crude chain-folding in the polydomain state, the stress-strain behavior of smectic elastomers bears similarities to the cold drawing of semicrystalline polymers. A yield stress is found in plots of the nominal stress vs. strain, the value of which increases with increasing domain size. The observed yield stress is shown to result from a "necking" instability, which we have reported in a smectic LCE for the first time.