Side Chain Liquid Crystalline Thermoplastic Elastomers for Actuator and Electromechanical Applications
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Side Chain Liquid Crystalline Thermoplastic Elastomers for Actuator and Electromechanical Applications Eric Verploegen1, LaRuth C. McAfee2, Lu Tian2, Darren Verploegen2, and Paula, T Hammond2. 1
Massachusetts Institute of Technology, Department of Materials Science and Engineering, Cambridge, MA 2 Massachusetts Institute of Technology, Department of Chemical Engineering, Cambridge, MA INTRODUCTION Liquid crystal research has gained interest due to the usefulness of liquid crystals in many applications other than displays. Some of these applications include memory devices, sensors, and variable light valves. Currently, liquid crystals are in the form of small molecules or LC homopolymers due to the fast response time that these molecules or polymers can achieve. However, there are many advantages to using block copolymers in these applications, such as surface stabilization caused by the block copolymer morphology, and recent research has increased in the area of LC block copolymers1-6. In this group, LC block copolymer research has focused on diblock copolymers with one amorphous block and one side-chain LC block2,7. The main drawback of these block copolymers is that the block copolymer interface and high viscosity of the smectic phase increase their response time. This research seeks to examine the potential use of ferroelectric and nematic LC block copolymer elastomers as actuators. These devices can be used as artificial muscles, in microrobotics, in micromachinery, in MEMS, and in other applications that require gates or valves. Artificial muscles have previously been prepared using multilayer composites of conducting polymers and nonconducting materials that may or may not be polymers8,9. Similar functionality could also be accomplished by preparing an amorphous-LC block copolymer with cylindrical morphology. One group has studied the possibility of using elastomeric main-chain LC polymers as actuators10 due to the ability to mechanically orient ferroelectric materials using an electric field. The proposed polymers for this research offer unique processing, mechanical, and electrical advantages over the current technologies because they are both block copolymers and elastomers. The main issues that need to be examined when designing such a material are the response time after an electrical pulse has been applied, the amount of strain achievable, and, if used in biological applications, the biocompatibility of the materials. This project specifically studies the first two issues. In order to do this, block copolymers with side-chain liquid crystal mesogens have been synthesized and their properties are currently being studied. Initially, the polystyrene-b-polyvinylmethylsiloxane diblock copolymer backbone with sidechain LC mesogens was studied. Diblock studies are useful as model systems and have potential for electro-optic applications. However, studies on PS-b-PVMS-b-PS triblock copolymers allow us to make a true elastomer. The mesogens were chosen such that the nematic or smectic C* phase wil
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