Mechanical Response of Polydomain Smectic Elastomers: Influence of Thermal History

            Smectic main-chain liquid crystalline elastomers (S-MCLCE) with polydomain morphology are rare examples of elastomers that can yield and undergo cold drawing under tension.^1,2  Yielding and formation of a "neck" are observed only under certain conditions of strain rate and temperature.  Inhomogeneous deformation is increasingly favored as the strain rate increases at constant temperature, or as the temperature decreases toward T_g.²  As cold drawing proceeds, significant creep occurs continuously within the neck.  Thermal imaging of the MCLCE during elongation corroborates this observation, yet illustrates that viscous heating is not a prerequisite for neck formation.²  Rather, inherent softening of the material during yielding due to morphological changes leads to an enhanced rate of deformation and contraction at the neck. 

            The physical basis for the observed mechanical instability is of considerable interest, given the rarity of necking phenomena in elastomers.  Recent studies revealed that thermal history of the material plays a significant role in promoting mechanical instability, which is enhanced in elastomers having large, stable microdomains.  Polydomain S-MCLCE cooled quickly from above the clearing temperature (T > T_si) to room temperature exhibit a low nominal yield stress, a lessened tendency to form a neck during elongation, and high extensibility.  S-MCLCE cooled slowly from T > T_si and annealed for an extended period of time at T slightly less than T_si exhibit increased yield stress, well-defined necking, and in some cases, lower elongation at break.  Analysis of the lineshape of the low-angle X-ray reflection (associated with smectic layering) reveals a significant growth in domain size with increased annealing time.  In addition, very long annealing times can induce formation of a second, higher-order smectic phase.  The increase in the yield stress during annealing is initially due to the growth of domain size, which increases the energetic penalty for yielding (via transient disordering of smectic microdomains).   After a long annealing time, the material can become brittle due to the formation of the higher-order phase. 

References

[1] H.P. Patil, D.M. Lentz, R.C. Hedden, Macromolecules 42, 3525-3531 (2009).

[2] D.M. Lentz, H.P. Chen, Z.Y. Yu, H.P. Patil, C.A. Crane, R.C. Hedden, J Polym Sci Pol Phys 49(8), 591-598 (2011).