Synthesis and Properties of Novel Main-Chain Type Thermotropic Polyurethanes
| Autor: | Chien-Kung Lin, 林建功 |
|---|---|
| Rok vydání: | 2000 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 88 With the specific properties, high strength, ultra-high modulus and high performance, main-chain thermotropic liquid crystalline polymers are the important engineering plastics and have drawn a great deal of consideration. Polyurethanes are a broad class of polymers with various structures and wide applications. This article reports the syntheses and properties of some novel liquid crystalline polymers composed of mesogenic moieties and urethane linkage. First, two series of novel liquid crystalline polyurethanes were prepared: XDI/HBn and H6XDI/HBn. The chain length of alkoxy spacer (n=2, 6, 11) and the molecular structure of diisocyanate moiety were varied for study. All of the liquid crystalline polyurethanes exhibited monomorphism except that XDI/HB11 showed trimorphism. These polyurethanes with benzene ring urethane moiety (XDI/HBn) and with a longer alkoxy chain provided the stronger hydrogen bonding and showed the higher transition temperatures. Second, 4,4’-bis(w-hydroxyalkoxy)-a-methylstilbene with various spacer length (HMSn, where “n” is the carbon number of the hydroxyalkoxy group) and two series of novel thermotropic liquid crystalline polyurethanes containing the mesogenic core of 4,4’-bis(w-hydroxyalkoxy)-a-methylstilbene were synthesized. One series was obtained by polyaddition reaction of 2,4-toluene diisocyanate (2,4-TDI) with HMSn, denoted to be TDIn, and the other was obtained by the reaction of 4,4’-diphenylmethane diisocyanate (MDI) with HMSn, denoted to be MDIn. All the polyurethanes except for TDI11, exhibited enantiotropic smectic phase. The transition temperatures and the temperature ranges of mesophases changed with the length of alkoxy sapcer and the kind of diisocyanate. The thermal degradation behaviours under nitrogen atmosphere depend upon the spacer length but nearly not upon the diisocyanate moiety. Polyurethanes with shortest spacer length showed one step degradation process, while the others exhibited two step degradation. The last part was to investigate the effects of various flexible spacers and bifurcated hydrogen bonding on the interaction and phase structure observed upon heating the thermotropic liquid crystalline polyurethanes, based on the biphenyl-4,4’-diol (mesogenic biphenol), flexible alkylene spacer, and 1,3-Bis(isocyanatomethyl)cyclohexane. The change of mesophase to isotropic phase is obvious without any heat process in this series of LCPUs. The mesophase region substantially increased along with increasing flexible spacer in backbone respects to thermodynamics stability. The existence of bifurcated hydrogen bonding in Mesogenic Polyurethane is supposed by the assignment of the IR absorption and wide angle X-ray spectra. This special hydrogen bonding results from both of mobile segment and substantial free stereo-hindrance, and in addition, the liquid crystalline moiety also affect the regularity of bifurcated hydrogen bonding in present study material. Liquid Crystal melting occurs when temperature excess over 150 oC and is accompanied by the disappearance of the other H-bonded peak respects to the substantial change in the frequency in IR spectra. However, the bifurcated H-bonds still exist that it maintains the regularity of structure at temperature of 200 oC, which evidenced from IR and X-ray spectra. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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