Synthesis of Liquid Crystalline Polyurethane Containing Zinc Complex

Abstract

Thermotropic liquid crystalline zinc complex (ZnL₁) and its hexamethylene and phenyl derivatives, namely ZnL₂ and ZnL₃, have been synthesized. Characterizations were carried out using FTIR spectroscopy, NMR spectroscopy, elemental analysis and X-ray crystallography. Polyurethanes based on hexamethylene diisocyanate (PU₁ZnL₁ and PU₂ZnL₁) and toluene diisocyanate (PU₃ZnL₁ and PU₄ZnL₁), that contained ZnL₁ unit in their main chain have been synthesized. Characterization of polyurethanes were carried out using FTIR spectroscopy. Their thermal properties were studied by differential scanning calorimetry and thermogravimetric analysis. Liquid crystalline properties of ZnL₁, its derivatives and polyurethanes were examined by using polarizing optical microscope. The results showed that ZnL₁ exhibited liquid crystallinity in the temperature ranges from 220-230°C. While ZnL₂ exhibited liquid crystallinity in the temperature ranges from 120-200°C because of long alkyl groups that increased flexibility to the structure. ZnL₃ exhibited liquid crystallinity in the temperature ranges from 145-200°C. This is due to an introduction of two phenylene units to the rigid structure of ZnL₁ that resulted in the reduction of the chain packing and the chain mobility. Therefore, ZnL₃ exhibited mesophase in the shorter temperature ranges as compared with ZnL₂. PU₁ZnL₁ and PU₂ZnL₁ exhibited liquid crystallinity in the temperature ranges of 170-220°C and 170-200°C, respectively. PU₃ZnL₁ and PU₄ZnL₁ did not show liquid crystalline phase. This might be due to the aromatic unit of toluene diisocyanate that hindered an anisotropic orientation of the mesogenic main chains.