Mesoscale Ordering and Mechanical Response in Polydomain Smectic Networks

Smectic Main-Chain Liquid Crystalline Networks (S-MCLCN) are semi-flexible polymer networks with main-chain mesogenic units.   These materials behave like normal isotropic rubbers at temperature above their clearing temperatures (T_i), while showing strong deviations from ordinary rubber-like mechanical behavior below T_i due to nanometer-scale segmental layering.  Properties like shape-memory, soft actuation, and energy-dissipative characteristics have attracted much attention in recent years. The anomalous mechanical behavior of S-MCLCN is partially attributed to the large internal energy cost for deformation of smectic microdomains, which promotes mechanical instability during deformation.

            Two factors affecting mechnical response of S-MCLCN have been considered in this study. First, the effects of thermal history on the domain thickness were characterized to elucidate the reasons for stiffening of the material during aging or annealing. Second, the effects of crosslink density on domain size were examined to explain an unexpected trend in the Young's modulus at low strains.

X-ray lineshape analysis was applied to quantify domain size after different thermal treatments and identify morphological factors relating to mechanical behavior.  A significant growth in domain thickness with increased annealing time was observed, similar to the thickening of crystal lamellae in semicrystalline polymers during annealing.  Because there is a greater internal energy penalty for deforming larger, more stable domains, the material stiffens and mechanical instability becomes more pronounced as annealing continues. 

The effects of crosslink concentration on the modulus can be understood in terms of a competition between domain size effects and elastic chain concentration effects.  Surprisingly, under certain conditions, the Young's modulus can actually decrease as crosslinker concentration increases because the crosslinkers decrease the average thickness of smectic microdomains.   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.  However, as the concentration of crosslinker molecules increases further, the increase in the number of elastically effective chains outweighs the effects of domains size, and the material stiffens again.

In addition, a recent study of flexible or semiflexible guest molecules in S-MCLCN will be described in this work. Two kinds of guest molecules have been synthesized.  Low molar mass liquid crystals having nearly the same structure as the repeat unit of the S-MCLCN were used to decrease the glass transition temperature of a MCLCN without destroying smectic ordering.  These guest molecules act as plasticizers that can both lower the T_g of the material and hinder the formation of higher order mesophases, both of which are desirable changes.  A second class of guest molecules are modified chromophores with flexible oligosiloxane tails. The orientational response of the modified dichroic dyes during elongation of the host network reveals how host-guest coupling affects the mechanical response of plasticized S-MCLCN.