Synthesis and characterization of new liquid crystalline polyurethane copolymer

Abstract

Polyurethanes (PU) are polymers synthesized by the reaction of polyols with isocyanates, producing materials that range from soft and flexible to highly rigid depending on their chemical structure. Their physical properties, such as heat resistance, hardness, and flexibility, are strongly influenced by the type of monomers used, while the (-NH-CO-O-) bonds in their backbone provide chemical stability and mechanical strength. Therefore, polyurethanes have a wide range of applications, extending from coatings and elastomers to foams and high‑performance engineering materials Liquid crystal (LC) compounds exhibit an intermediate state of matter, combining the fluidity of liquids with the long-range molecular order of crystalline solids. This unique dual nature allows LC materials to form highly ordered structures while maintaining mobility, making them attractive for advanced functional applications. When incorporated into polymer backbones, LC monomers can significantly enhance thermal, mechanical, and morphological properties. The incorporation of mesogenic groups into polymer structures is intended to improve molecular order and introduce liquid crystalline behavior, while long alkyl chains are often employed to lower the glass transition temperature (Tg), enhancing flexibility and processability. These chains act as flexible spacers, reducing steric hindrance and improving molecular packing, which facilitates the alignment of mesogenic units. By promoting this alignment, long alkyl chains contribute to the formation of ordered phases and help maintain their stability over a broader temperature range This structural design concept combines the flexibility provided by alkyl segments with the orientation of liquid crystalline groups to produce polymers with improved thermal and morphological characteristics. In this study, a side-chain liquid crystal polyurethane copolymer was synthesised. For this purpose, two different monomers, M1-diol (0.5 mmol, octadecyl-2,2-bis(hydroxymethyl)propionate) and M2-diol (0.5 mmol, 8-(4-cyanobiphenyl-4′-oxy) octyl-2,2-bis(hydroxymethyl)propionate) with HMDI (1 mmol, hexamethylene diisocyanate) in the presence of dibutyltin dilaurate as the catalyst. This study was designed based on our research group's previous work on PU synthesis using M2-diol [1].The aim of the thesis is to investigate the effect of incorporating long alkyl chains into the polyurethane structure on its liquid crystalline properties. The reference study [1].reported that the polyurethane showed mesophase formation during both heating and cooling; however, the liquid crystalline phase transitions were not clearly identified. The transition from the liquid crystalline phase to the isotropic state could not be detected by POM because the material began to decompose before reaching the isotropic phase. Additionally, the DSC thermograms did not display thermal transitions that could support the POM observations. The DSC thermograms obtained in this study show distinct thermal transitions for the polyurethane copolymer. The melting of the crystalline domains and the subsequent transition from the liquid crystalline phase to the isotropic state are clearly observed. These results are supported by POM images, which display a well defined nematic texture within a narrow temperature interval and a direct transition to the isotropic phase. Upon cooling, both the isotropic to mesophase transition and crystallization are detected, and POM confirms the presence of a stable nematic mesophase over a wide range. The results, supported by characterization studies, confirmed that these long alkyl chains reduced crystallization and improved the liquid crystalline behavior in the polyurethane copolymer. The chemical structure characterization was carried out using FTIR and HNMR spectroscopy, while the thermal properties were examined by differential scanning calorimetry (DSC), and the identification of liquid crystal mesophase structures and transition temperatures were performed using polarized optical microscopy (POM) and DSC, respectively.