Smectic main-chain liquid crystalline elastomers (S-MCLCE) are soft, flexible networks that have attracted attention due to their unusual ability to yield and undergo cold drawing under tension above the glass transition temperature. Smectic networks prepared by crosslinking in the absence of an external aligning field generally contain numerous randomly oriented domains on a micrometer or smaller length scale (polydomain morphology). During elongation, deformation of the smectic microdomains significantly increases the internal energy, in contrast to ordinary isotropic rubbers, resulting in higher Young's modulus. Internal energy effects also promote the observed mechanical instability. Two observations regarding the mechanical behavior of S-MCLCE motivated the present study. First, the mechanical response of S-MCLCE was noted to be quite sensitive to their thermal history, in contrast to ordinary isotropic rubbers. Second, whereas the modulus of an isotropic rubber increases as crosslink density increases, S-MCLCE were noted to exhibit a more complex dependence of modulus on crosslinker concentration. Under certain conditions, the Young's modulus can actually decrease as crosslinker concentration increases. The goal of this work is to apply X-ray lineshape analysis to identify morphological factors underlying these observations.
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