(488m) Stress-Strain Behavior of Smectic Main-Chain Polydomain Elastomers

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.^1-3

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, any amorphous (non-smectic) regions deform elastically. In addition, local director rotations within smectic regions slightly favor the anomalous (perpendicular) alignment of chain axes with respect to the draw direction. 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 is observed under certain conditions of temperature and strain rate. Because of crude chain-folding in the polydomain state, the stress-strain behavior of smectic elastomers bears superficial similarities to the cold drawing of semicrystalline polymers, despite morphological differences.

A yield stress is observed during uniaxial elongation at a constant strain rate. The yield stress results from a necking instability that initiates with unfolding of hairpinned smectic microdomains. The polydomain-monodomain transition occurs in a discontinuous fashion, localized within the transition region between the necked and non-necked regions. The magnitude of the observed yield stress increases as temperature decreases below the smectic-isotropic clearing temperature. The yield stress also increases as strain rate increases. In addition, the transition region is narrower and the neck is better defined at lower temperatures and higher strain rates. The magnitude of the yield stress is governed by the product of the strain rate with the longest relaxation time of the polymer, which might be associated with local director rotations. Under conditions where director rotations are essentially complete prior to the onset of the P-M transition, little or no neck is observed. Under conditions where the time scale for director rotations is slow compared to the deformation time in the polydomain state, a strong necking transition results because uncoiling of hairpins initiates before director rotations are complete.

References

1. H. P. Patil, D.M. Lentz, and R. C. Hedden, "Necking Instability during Polydomain-Monodomain Transition in a Smectic Main-Chain Elastomer." Macromolecules 2009, In Press.

2. H. P. Patil, J. Liao, and R. C. Hedden, "Smectic Ordering in Main-Chain Siloxane Polymers and Elastomers Containing p Phenyleneterephthalate Mesogens." Macromolecules 2007, 40, 6206-6216.

3. H.P. Patil and R. C. Hedden. "Effects of Structural Imperfections on the Dynamic Mechanical Response of Main-Chain Smectic Elastomers." J. Polym. Sci. B: Polym. Phys. 2007, 3267-3276.