Abstract
Shape memory polymers (SMP) are a category of smart materials that are able to return to their original shape from adeformed state when exposed to external stimuli. SMPs generally consist of cross-linked polymer networks, which determine the permanent shape of the material, and switching segments, which can be oriented and solidified to fix a
temporary shape.1 The shape recovery is driven by the entropic force of the switching domains which tend to gain entropy and return to a random conformation when undergoing phase transitions, such as glass transition, liquid crystalline (LC) transition, and melting transition.2,3 Liquid crystalline elastomers(LCE) represent a special class of SMPs that are defined by a reversible LC phase transition and a unique coupling between LC mesogens and polymer networks. They exhibit reversible shape change when exposed to external stimuli, such as heat,4 light,5−8 or magnetic field,9,10 which makes them excellent candidates for artificial muscles, sensors, lithography substrates, and shape memory materials.11−16 A number of LCEs with different LC phases and network structures have been synthesized and characterized, including nematic main-chain,17−19 smectic main-chain,20,21 nematic sidechain, 22,23 and smectic side-chain LCEs.24−26 These materials exhibit a wide variety of shape memory and actuating behaviors.
However, in spite of their promising properties and remarkable potential, practical applications of LCEs are limited because of the difficulties encountered when tailoring thermal transition temperatures and thermomechanical properties of the materials for end-use applications. Several methods have been proposed to prepare LCEs with tunable shape memory properties. A smectic main-chain LCE has been designed and synthesized through copolymerization of two benzoate-based vinyl monomers with different LC phase transition temperatures.27
It has been shown that both LC transition and thermomechanical properties of LCEs could be tailored by changing the ratio of the two monomers. On the other hand, a series of smectic side-chain LCEs with methylene spacers of different lengths have been synthesized. The effect of these microstructure changes on the shape memory properties of the LCEs was investigated.28 The smectic polymorphism was successfully modified by varying the length of the spacers and the resulting LCEs exhibited different shape memory behavior. In the present work, a different approach was used: a smectic main-chain LCE was produced by polymerizing a biphenylbased epoxy monomer with an aliphatic carboxylic acid curing agent. By adjusting the stoichiometric ratio between the monomer and the curing agent, it was possible to control liquid crystallinity, cross-linking density, and network rigidity of the LCEs, thereby providing an easy way to tailor the LC transition and thermomechanical properties of the material. The prepared LCEs exhibited significant differences in phase transition temperature, stress−strain, shape memory, and thermal degradation behavior as well as in dynamic mechanical, thermomechanical properties that were attributed to the
tailored microstructure.
