(302f) Effects of Domain Size and Crosslink Density On Stress-Strain Behavior of Smectic Polydomain Networks

            Smectic
main-chain liquid crystalline elastomers (S-MCLCE) are soft, flexible networks
that have attracted attention due to their ability to yield and undergo cold
drawing under tension,^1,2 unlike most elastomeric materials.  Smectic networks prepared by crosslinking in
the absence of an external aligning field generally contain numerous randomly
oriented domains on a micrometer or sub-micrometer length scale (polydomain
morphology).  The mechanical response of
polydomain S-MCLCE differs significantly from that of isotropic elastomers
because deformation of smectic microdomains produces a significant internal
energy increase, resulting in higher stiffness and promoting mechanical
instability. 

            Two
observations regarding the mechanical behavior of S-MCLCE motivated the present
study.  First, the mechanical response of
S-MCLCE is quite sensitive to annealing slightly below the clearing temperature
(T_si), unlike isotropic rubbers. 
Second, whereas the modulus of an isotropic rubber increases as
crosslink density increases, S-MCLCE exhibit a more complex dependence of
modulus on crosslinker concentration. 
Under certain conditions, the Young's modulus can actually decrease as
crosslinker concentration increases. 
Thermal history and crosslink density each play a significant role in
promoting mechanical instability as well. 
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 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.  S-MCLCE having low
crosslink density are more prone to necking and cold drawing at temperatures
slightly above Tg.   The goal of this
work is to apply X-ray lineshape analysis to identify morphological factors
underlying these observations.

            The
observed effects of annealing and crosslink density on mechanical response can
be explained in terms of changes in the average domain size.  Analysis of the low-angle X-ray reflection
(associated with smectic layering) reveals a significant growth in domain
thickness with increased annealing time, much as observed in annealing of
semicrystalline polymers.  Very long
annealing times can in fact produce domains large enough to be comparable to
the beam diameter (~ 1 mm), such that X-ray patterns suggest the material
possesses monodomain order locally. 
Because there is a greater internal energy penalty for deforming larger,
more stable domains, the material stiffens and mechanical instability becomes
more pronounced.  The effects of
crosslinker concentration on the modulus can be understood in terms of a
competition between domain size effects and elastic chain concentration
effects.  As crosslink density increases
from zero to some small value, smaller domains are at first produced because
crosslinkers cannot fit into the smectic lattice, generating defects in the
layering.  Thus, the internal energy
penalty for deformation of the domains decreases, and the material softens
initially.  However, as the concentration
of crosslinker molecules increases further, the increase in the number of elastically
effective chains increases the entropic penalty for deformation, and the
material stiffens.     

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).