Side-Chain Liquid Crystalline Polymers as Stimuli-Responsive Materials

Date of Completion

January 2011

Keywords

Chemistry, Polymer

Degree

Ph.D.

Abstract

Liquid crystalline (LC) materials are well-known for their commercial success in display technology ranging from cell phones, computer monitors to high-definition flat panel displays. Such technologies are based on the reorientation of LC mesogens in response to applied electric field, which results in a change in their optical properties. The orientation of LC can be also controlled by other means including heat, light and magnetic field due to their distinctive property of anisotropy. The intrinsic responsiveness together with self-assembled structures in a various mesophases (nematic, smectic, cholesteric and columnar) of LC allow for developing a new class of stimuli-responsive materials including LC lasers, sensors and artificial muscles/actuators beyond display applications. When LC molecules are linked to polymers, the resulting liquid crystalline polymers (LCPs) can not only exhibit responsiveness and mesophases from LC, but also retain polymeric viscoelasticity that provides processability and mechanical properties to the system. This dissertation focuses on the investigation of side-chain liquid crystalline polymers (SCLCPs) for two different types of thermally-responsive materials: (1) thermochromic polymers and (2) thermally-induced shape memory polymers (SMPs). ^ In the first part of the dissertation, synthesis and characterization of a series of SCLC type homopolymers (PNBCh-n) in which polynorbornene backbone comprises cholesteryl mesogenic side-chain are described. Between backbone and mesogen, varied length of flexible methylene spacer ( n = 0, 4, 5, 9, 10 and 15) is inserted, which leads significant changes in thermal, mesomorphic and optical properties of resulting homopolymers. Interestingly, cholesteric mesophases featured by selective light reflection property resulting from a periodic helical structure are exclusively observed in the specific length of spacer (PNBCh-9 and 10), while other homopolymer analogues only exhibit smectic mesophases. After extensive structural analyses by X-ray scattering, the extent of mesogen interdigitation as well as the motional decoupling between backbone and mesogen is proposed to be key structural parameters to access the cholesteric mesophase in PNBCh-9 and 10. The synthesis and structure property study of PNBCh-n will help elucidate the importance of molecular structure, particularly the length of spacers, on the formation of cholesteric mesophase and its importance in the design of thermochromic devices. ^ In the second part of the dissertation, we have explored smectic SCLC type linear (TP-n) and cross-linked random terpolymers (XL-TP- n) for shape memory applications. These terpolymers consist of polynorbornene backbones with three different functional side-groups (cholesteryl mesogens, amorphous PEG and butyl acrylate) and are synthesized via ring-opening metathesis polymerization (ROMP) with Grubbs catalyst 2nd generation (G2nd). Particularly, self-assembled smectic orderings in the nanometer scale are varied by the choice of methylene spacer length (n = 5, 10 and 15) between the backbone and the cholesteryl mesogen. Most importantly, this nanoscale smectic polymorphism in terpolymers significantly impacts the macroscopic shape memory behaviors as investigated by cyclic thermomechanical analysis. In addition, triple shape memory properties of these terpolymers are further explored, which allows a precise control of the shape recovery process as compared to conventional dual shape polymers. Overall, understanding and controlling the self-assembled nanostructure and morphologies in these terpolymers are critical to tailor the bulk shape memory response of materials and thereby the creation of interesting thermal actuators and morphing devices will be possible. ^

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