281990 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 (Tsi), 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 > Tsi) 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 Tsi 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.
 H.P. Patil, D.M. Lentz, R.C. Hedden, Macromolecules 42, 3525-3531 (2009).
 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).