(337g) 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).