Azomethines esters

New Liquid Crystalline Poly(azomethine esters) Derived from PET Waste Bottles... 

In this chapter, the new thermotropic liquid crystal poly(azomethine esters) were designed and synthesized as T-shaped azomethine bisphenols containing azomethine linkage (-CH=N) attached to different halogens and were reacted with terephthaloyl chloride that has been obtained from the regenerated terephthalic acid of PET waste bottles. The effect of the azomethine linkages and the type of the substituents (halogens) on the liquid crystalline behavior, thermal stability, as well as the crystalline states of these polymers were further studied and analyzed. 

Terephthalic acid (TPA) was converted into terephthaloyl chloride by using the same procedure as described by Li Zifa et al. (1996). 0.01 mol of terephthaloyl chloride was dissolved in 300 mL THF, an equimolar quantity of compotmd, I (a) was dissolved in 200 mL THF followed by the addition of 5 mL pyridine. The terephthaloyl chloride solution was taken in a dropping funnel and added dropwise to a mixture of 0.01 mol azomethine bisphenol in 200 mL THF and 5 mL of pyridine at 0 °C for 2 h. The reaction mixture was stirred for 3 h at room temperature and then refluxed for 3 h. The reaction mixture was poured into methanol to precipitate and afterwards filtered. All the obtained poly(azomethine esters) were washed with diethyl ether and methanol and finally dried in a vacuum oven at 75 °C for 24 h. The polymers (b-d) were synthesized using the same method as described in this procedure. The results were summarized in Table 12.1. 

New thermotropic liquid crystalline poly(azomethine esters) containing azomethine linkages were prepared by polycondensation reaction of azomethine bisphenols with terephthaloyl chloride, which was produced from terephthalic acid. Terephthalic... 

Table 12.1 Inherent viscosity, yields and CHN data for LC poiy(azomethine esters) 3(a-d)...

Solubility tests carried out on the polymers revealed that all the LC poly (azomethine esters) were insoluble in common organic solvents such as chloroform, THF and acetone. This leads to the difficulty in their molecular weight determination by gel permeation chromatography (GPC). The inherent viscosities (r ii ) of the polymers 3 (a-d) were as given in Table 12.1. The low inherent viscosities may be attributed to the poor solubility of the polymers in the polymerization solution. Precipitate was also formed during the polymerization which hinders further propagation of the polymer chain and thus leads to lower molecular weight. 

Differential Scanning Calorimetry (DSC) and polarizing optical microscope (POM) were used to study the mesophases of the liquid crystalline poly(azomethine esters). The mesophase confirmation and the temperature data of poly(azomethine esters) were summarized in Table 12.2. The thermal behaviors of the polymers 3 (a-d)... 

Polarized optical microscopy (POM) was performed to confirm the transition temperatures obtained by DSC studies and to find out the type of mesophases present in the synthesized polymers, the crystal phase to nematic phase transition temperature (Cr-N), and nematic phase to isotropic phase (N-1) temperature. It was found that the transition temperatures of the polymers are almost the same as those obtained in DSC. The POM results revealed that the poly(azomethine esters) 3(b-d) exhibit nematic liquid crystal phases and the images were shown in Fig. 12.5. 

In the present work, terephthalic acid was successfully regenerated from PET waste bottles via saponification process in high yield and purity. The new liquid crystalline poly(azomethine esters) based on terephthalic acid were prepared and characterized successfully using important and vital spectroscopic methods. The combining results that obtained from DSC, POM and XRD studies were confirmed the liquid crystallinity of the poly(azomethine esters). AU the polymers exhibit nematic mesophases except that for polymer 3(a). Furthermore, TGA results also confirmed that the new synthesized polymers possess high thermal stability. 

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