Synthesis of poly(ethylene glycol) based liquid crystalline mesogen and investigation of liquid crystalline properties

Liquid crystals; between the liquid and the solid phase, exhibit the properties of both the mobility of the molecules and the anisotropic properties of the crystalline solid phase. Liquid crystals are divided into two classes, called lyotropic and thermotropic, depending on the orientation of the molecules in the mesophase. Thermotropic liquid crystals; they are divided into subgroups as low molecular weight, discotic (disk shaped) and calamitic (rod) and high molecular weight (polymeric) main chain liquid crystal polymers and side chain liquid crystals. Click chemistry has gain attention in last decade due to applicability of the reactions in wide range of areas. Side chain liquid crystals can also be prepared via click chemistry reactions. In this thesis, two series of a new poly (ethylene glycol) (PEG500 and PEG750) based rod-like liquid crystal linked via 1,4-disubstituted 1,2,3-triazole ring was synthesized using click chemistry. Firstly, azide-terminated poly (ethylene glycol)'s were synthesized and then, reacted with prepared cyanobiphenyl based liquid crystal mesogen having terminal alkyne moiety. Synthesis of azide terminated poly(ethylene glycol) (PEG) was performed through two-step modification reaction; the first step is sulphonation of hydroxyl functional group of PEG and the second step azidation of tosyl functional group. The characterization of synthesized products was obtained by using Fourier-transform infrared (FT-IR) and 1H-NMR analysis. Liquid crystalline properties and mesophases were investigated by using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Reaction steps were analyzed by using 1H-NMR and FT-IR spectroscopy. Results have showed that reactions were successfully obtained. POM studies showed that prepared cyanobiphenyl based liquid crystal mesogen does not show any liquid crystalline phase during heating and cooling cycles. Phase transition from crystal to liquid and liquid to crystal phases was observed at high temperature. On the other hand, PEGylated-cyanobiphenyl liquid crystalline mesogens have showed liquid crystalline mesophases at very low temperatures. PEGylated-cyanobiphenyl liquid crystalline mesogen of PEG500 exhibited liquid crystalline mesophase at -60°C and liquid crystal phase transition to isotropic phase at -7 °C during heating cycle, while PEGylated-cyanobiphenyl liquid crystalline mesogen of PEG750 showed that two different liquid crystalline mesophase.