Synthesis And Characterization Of Dimers And Polymers

Liquid crystal dimers consist of two mesogenic units linked via a flexible alkyl chain.The dimers exhibit different mesophase and plentiful smectic phases,which are quite different to conventional low molar mass liquid crystals.The study of dimer has become one of the hot spots in the field of liquid crystal.The liquid crystals,which are introduced into the fluorin atom with small volume,high resistivity and low viscosity,display the properties of good intermediate phase behavior,low viscosity,higher resistivity,higher polarity,higher stability and quicker response speed.The fluorine atoms have been introduced into the terminal group of dimers to study the performance of fluorinated liquid crystal dimers,to develop the new fluorinated liquid crystal materials.Six intermediates have been synthesized in the second chapter of this dissertation.They are 4'-(4-trifluoromethylbenzoyloxy)-biphenyl-4-carboxylic acid(FA),2-hydroxypropyl 4'-((4-(trifluoromethyl)benzoyl)oxy)-[1,1'-biphenyl]-4 carbox-ylate(FA-PD),4-(undec-10-enoyloxy)benzoic acid(UBA),4-(allyloxy)benzoic acid(ABA),4-butoxybenzoic acid(BBA)and 4-propoxybenzoic acid(PBA).Six dimers have been synthesized with FA-PD and UBA,ABA,BBA,PBA,EBA and BA,respectively,which termed as 2-((4-(undec-10-enoyloxy)benzoyl)oxy)propyl 4'-((4-(trifluoromethyl)benzoyl)oxy)-[1,1'-biphenyl]-4-carboxylate(FA-PD-UBA),2-((4-(allyloxy)benzoyl)oxy)propyl 4'-((4-(trifluoromethyl)benzoyl)oxy)-[1,1'-biph-enyl]-4-carboxylate(FA-PD-ABA),2-((4-butoxybenzoyl)oxy)propyl 4'-((4-(trifluoro-methyl)benzoyl)oxy)-[1,1'-biphenyl]-4-carboxylate(FA-PD-BBA),2-((4-propoxy-benzoyl)oxy)propyl 4'-((4-(trifluoromethyl)benzoyl)oxy)-[1,1'-biphenyl]-4-carboxyl-ate(FA-PD-PBA),2-((4-ethoxybenzoyl)oxy)propyl4'-((4-(trifluoromethyl)benzo-yl)oxy)-[1,1'-biphenyl]-4-carboxylate(FA-PD-EBA)and 2-(benzoyloxy)-propyl 4'-((4-(trifluoromethyl)benzoyl)oxy)-[1,1'-biphenyl]-4-carboxylate(FA-PD-BA).The influence of bonding type of the terminal group and the length of the flexible terminal chain on the liquid crystal properties was investigated.These studies are beneficial to design and synthesize other fluorinated liquid crystal dimers.In the third chapter of this dissertation,two liquid crystalline monomers have been synthesized,which are 4-(undec-10-enoyloxy)phenyl 4-ethoxybenzoate(M1)and cholesteryl 4-allyloxybenzoate(M2).The polymers(P1?P7)have been synthesized by graft polymerization with M1,M2,and PMHS.The structures and properties of the obtained compounds were investigated by FT-IR spectroscopy,differential scanning calorimetry(DSC)and polarizing optical microscopy(POM)and X-ray diffractometer(XRD).Their structure property relationships were discussed in detail.All the synthesized compounds are consistent with the molecular design.The intermediates,FA and UBA,are smectic B LCs,FA-PD displayed smectic C and nematic LC properties in heating cycle and nematic LC properties in cooling cycle,ABA is a non-liquid crystal,BBA and PBA are nematic LCs.The LC dimers containing 1,2-propanediol(PD,[?]58928.0=-0.72 in THF)as the flexible space were synthesized keeping one of the mesogenic unit of the dimer fixed and changing the length and the type of bonding of flexible terminal chain of the other mesogenic unit of the dimers,which investigate the condition that is benefit to forming cholesteric phase.FA-PD-BA is a non-liquid crystal,whereas other five dimers displayed different mesophases.FA-PD-UBA is an enantiotropic chiral smectic A LC,FA-PD-ABA is a monotropic cholesteric LC,FA-PD-PBA,FA-PD-PBA and FA-PD-EBA are enantiotropic nematic LCs.The result indicated that the length and bonding type of the flexible terminal chain of the non-fluorinated mesogenic unit played an important effect on mesomorphic behaviour.The introduction of the terminal flexible chains is benefit to the formation of a liquid crystal phase,but the too long flexible chains tend to form a lamellar phase while is detrimental to the formation of the cholesteric phase.The introduction of allyloxy as the terminal chain is good for the formation of cholesteric phase.M1 is an enantiotropic nematic liquid crystal.P1 is a nematic liquid crystal polymer,P2?P7 are cholesteric liquid crystal polymers.The soft siloxane main chain lowers the temperature at which the liquid crystal phase appears,stabilizes the liquid crystal phase,and makes the polymer show a wider range of liquid crystal appears than the monomer.When the content of M1 is 71?85%,the Tg of the polymers are below 20.0?(19.6?,17.3?).Tg of the copolymers increased with the increase of the content of M2.P2 and P3 displayed cholesteric phase at room temperature,which lays the foundation for the application of cholesteric liquid crystal polymer at room temperature.The Td(?)of the P1?P7 are higher than 320.0?,showing the high thermal stability of the side-chain polymers.