Synthesis and properties of combined liquid-crystalline polymers with two symmetric mesogene units in the side-chain

Abstract

A series of combined liquid-crystalline polymers based on 4,4``-biphenyl, terephthalate with two symmetric mesogenic groups and methylene flexible spacers, denoted as P-n, have been prepared from 2,5-bis(n-(4``-methoxy biphenyl-4-yloxy)alkyloxy)terephthalyl chloride (n=2,6,10) and 4,4``-bis(6-hydroxyhexoxy) biphenyl [BHHBP] by condensation method. The thermal properties of polymers were characterized using DSC and its mesophase structure was confirmed by the polarized optical microscopic (POM) study and X-ray measurements. The polymers P-2 and P-6 show two endothermic peaks on heating and two exothermic peaks on cooling: at the first transition, the solid crystalline is turned into fluid liquid crystalline phase, and at the second transition, the materials entered into the isotropic phase. P-2 and P-6 with same or short length of methylene units in side chain spacer than that of main chain exhibited an enantiotropic liquid crystalline phase. In trimer and P-10 with longer length of methylene units in side chain spacer than that of main chain , they did not show the mesophase. All the polymers were crystalline at room temperature. P-2 and P-6 with even numbers of methylene units in spacers were transformed from crystalline solid directly into liquid-crystalline phase at 131.2℃ and 121.0℃ respectively, exhibited a smectic phase at 32℃ and 28℃ mesophase temperature. The X-ray diffraction pattern of the polymer P-6 exhibited a broad diffraction in small-angle region around q = 2.47 (50.9Å) indicating that mesophase formed by the polymer P-6 is smectic phase. This means that the side mesogenic groups are arranged perpendicular to the backbone in the mesophase and also tend to align themselves in layers.