Shape Memory Behavior of Side-Chain Liquid Crystalline Polymer Networks Triggered by Dual Transition Temperatures

We report synthesis and characterization of a new class of side-chain liquid crystalline random terpolymers (SCLCP), its cross-linked network (SCLCN), and the corresponding shape memory properties. The SCLCP comprising three monomers, 5-{15-(cholesteryloxycarbonyl)-pentadecyloxycarbonyl}-bicyclo[2.2.1]hept-2-ene (NBCh15), 5-(acryloyl butoxycarbonyl)-bicyclo[2.2.1]hept-2-ene (NBBA), and poly(ethylene glycol) functionalized norbornene (NBPEG), is synthesized by ring-opening metathesis polymerization (ROMP) using Grubbs catalyst second generation, resulting in a random terpolymer. Each monomer provides a specific function in the terpolymer: (1) NBCh15 affords liquid crystalline properties, (2) NBBA is a cross-linkable unit, and (3) NBPEG acts as an internal plasticizer. The mesomorphic structure of the terpolymer investigated by X-ray diffraction (XRD) exhibits highly interdigitated smectic A (SmA) mesophase comprising cholesteryl ester mesogens. The acrylate end group in the NBBA undergoes cross-linking by curing at 120 degrees C, resulting in the SCLCN. Optimal cross-linking conditions are determined by monitoring gel fraction produced from different curing times. Thermal transitions including glass transition temperature (T-g) and clearing temperature (T-cl) before and after cross-linking are analyzed by differential scanning calorimetry (DSC). Linear viscoelastic properties of the SCLCN reveal three different relaxations associated with dynamic soft elasticity as well as T-g and T-cl. One-way shape memory cycles (IW-SMCs) of the SCLCN are programmed using (1) T-g, (2) T-cl, and (3) combined (T-g and T-cl) as a shape memory transition temperature (T-trans). The T-g-based SMCs exhibits excellent shape fixing (> 97%) and shape recovery ratio ( > 96%) with large strain ( > 150%). In the T-cl-based SMCs, the cooling induced elongation of strain is observed due to the development of interdigitated SmA mesophase. The shape fixing by interdigitated SmA is achieved during the T-cl-based SMCs unlike conventional shape fixing methods such as vitrification or crystallization. If both T-g and T-cl serve as T-trans for SMCs, the permanent shape is restored by two stages of shape recovery around T-g and T-cl. The dual T-trans (T-g and T-cl) inherent in this SCLCN allows for creating different types of SMCs and for the memorization of shape at two different temperature windows, thereby, programmed shapes by different mechanism would be recovered in a more precise manner.