Thermal and mechanical properties of smectic main chain liquid crystalline networks

Date

2015-05

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Abstract

Smectic Main Chain Liquid Crystal Networks (MCLCNs) are semi-flexible networks with main-chain mesogenic units that behave like normal isotropic rubbers at temperatures above their isotropic temerperature (Tsi). However, MCLCNs show strong deviations from ordinary rubber-like mechanical behavior below Tsi due to the presence of nanometer-scale segmental layering. Applications like shape-memory materials, soft actuators, energy-dissipative coatings, and fundamental scientific styduies of soft-elastic materials having internal degrees of orientational freedom (similar to Cosserat solids), have attracted lots of attention. For semi-crystalline polymers, the relationship between mechanical behavior and thermal history or crystallinity has been well studied. However, no sytematic study on the relationship between mechanical behavior and thermal history for polydomain (unoriented) liquid crystalline networks has been conducted. The non-ideal mechanical behavior of smectic MCLCN is due to the significant internal energy increase during deformation of smectic microdomains, which results in higher stiffness and promotes mechanical instability. In this thesis, the molecular and mesoscale factors affecting the shear and tensile moduli of smectic MCLCNs are investigated with a model MCLCN system having controlling crosslink density and thermal history. As the results show, under certain conditions, the modulus can actually decrease as crosslinker concentration increases, surprisingly. Another problem for commercializing liquid crystalline networks is usually their high glass transition temperature (Tg) and Tsi, which limits their useful temperature range in applications. To explore the possibility of decreasing the Tsi or Tg of MCLCNs, two types of low molar mass guest molecules were designed and infused into smectic MCLCNs. The thermophysical properties and shape memory behavior of plasticized smectic MCLCNs were investigated for the first time. The first guest molecule was a flexible, oligosiloxane "A" molecule similar to the flexible blocks in the MCLCN elastic chains. The second guest molecule is a low molar mass liquid crystal, "ABA," containing a liquid crystal mesogen (B) similar to the mesogens in the backbone of the MCLCN elastic chains. The molecular-level organization of plasticizers in the MCLCNs was identified by WAXD, and the brittleness problem for MCLCNs with long aging or annealing time was solved by adding plasticizers. Also, the shape memory behavior was revealing that studied after infusing guest molecule into the MCLCNs. This is first time that a systematic study of thermal and mechanical properties of smectic MCLCNs with different micro-structures (domain size, d-spacing between layers, free volume) is presented, which provides an in-depth understanding of structure-property relationship of smectic MCLCNs.

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Keywords

Liquid crystalline elastomer, Smectic

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