Sythesis And Properties Of Novel Side-On Main-Chain Liquid Crystalline Polymers

Liquid crystal polymers (LCPs) are a class of high performance polymers with excellent properties developed in the 1970s. LCPs products have high strength, high modulus, dimensional stability and other excellent properties because the molecule has a characteristic of spontaneous orientation. They are mainly used to produce special synthetic fibers and special engineering plastics. The liquid crystalline behavior of LCPs is usually divided into lyotropic LCPs and thermotropic LCPs while they can also be divided into the main chain LCPs (MCLCPs) and side chain LCPs according to their chemical structure. In this paper, we design and synthesize a series of X-shaped mesogenic monomers, bearing two terminal-alkene tails. Starting from these X-shaped monomers, acyclic diene metathesis polymerization and thiol-ene polyaddition are applied for the first time to prepare two series of side-on main-chain liquid crystalline polymers respectively. We also describe a new system, poly (vinyl benzoate)-backbone MJLCPs which are synthesized by solution polymerization and bulk pHotopolymerization methods respectively. This thesis can be divided into three parts:1. A series of side-on MCLCPs have been synthesized by X-shaped mesogenic monomers via ADMET and thiol-ene click chemistry. The mesomorphic behviours and structure-property relationships of these new polymers are studied in detail by a combination of 1H NMR, GPC, TGA, DSC, POM and XRD experiments. To confirm the nature of the liquid crystalline phases for these side-on main-chain polymers exhibiting birefringent textures, simultaneous SAXS and WAXS were further performed. Polymer 8-8 and polymer 8-9 might have nematic phases. On the contrary, when the n-alkoxy tails of these side-on MCLCPs lengthen, Polymers 8-m (m=9-12) might have tilted SmC phases and the tilt angles (θ) are all perfectly around 380 according to the equation θ=cos-1(d/l). While the thiol-ene polymers 9-m could not be able to achieve liquid crystalline orders above room temperature.2. Furthermore, One major obstacle of utilizing these novel LCPs as some optical materials, is that the glass transition temperatures (Tg’s) of these side-on MCLCPs are usually below room temperature. To prevent the materials melting at room temperature, it is very important to find a way to get stabilized polymeric networks by cross-linking these LCPs. A side-on main-chain liquid crystalline elastomer fiber is prepared by cross-linking the liquid crystalline polymer using a UV-sensitive bifunctional benzopHenone crosslinker. In comparison with the pure polymer fiber’s monodomain alignment, the cross-linked elastomer fiber however shows non-aligned polydomain structure, probably due to the order competitions between the mixed crosslinkers, mesogens and polymer backbones.3. Previously reported polymer backbone structures of mesogen jacketed liquid crystalline polymers (MJLCPs) to date are limited to polyacrylate, polystyrene, polyacetylene, polysiloxane, polythiopHene and polynorbornene. In this article, we describe a new system, poly (vinyl benzoate)-backbone MJLCPs which are synthesized by solution polymerization and bulk photopolymerization methods respectively. Due to the low reactivity of vinylbenzoate group, we can not achieve a high degree of polymerization. Ironically, benefiting from this low reactivity disadvantage, we successfully prepare two loosely cross-linked MJLCP (xMJLCP) films homogeneous-aligned and homeotropic-aligned respectively for the first time, by UV-illumination on a unidirectional oriented mixture of vinylbenzoate monomers, crosslinkers and photo-initiators to perform polymerization and cross-linking at the same time, which are however extremely difficult for traditional much more reactive acrylate-type or styrene-type MJLCPs’ precursor monomers due to the inevitable spontaneous thermal-polymerization reactions occurring at high temperatures.