INTRODUCTION:

Polymers incorporating mesogenic moieties in the main chain are known to exhibit liquid crystalline behavior. Almost all pure compounds exhibiting thermotropic mesomorphism have pronounced anisotropy of shape. The majority of liquid crystalline substances are based on “lath-like” or “rod-like” molecules. A new list of technological applications of liquid crystalline polymers has generated recent attention for their good thermal stability and excellent mechanical properties¹.The texture, mechanical strength and thermal stability of the polymer was completely dependent on the chemical structure and molecular weight of the monomers, thus fully aromatic monomers show good liquid crystalline property². In the past decades many researchers have developed many LCP’s with considerable efforts on modification of the monomeric structure to reach more practical applications in industries^3-5.

Incorporation of aromatic monomers to the polymer forms rigid structure in the main chain of the polymer and also increases the melting, decomposition temperature and mechanical strength of the polymers⁶. Number of methods has been used to synthesize polyesters, but the simple and the best method to synthesis LCP’s is direct polycondensation ^7-8, because this technique was simple to prepare LCP’s without catalyst and minimizing the usage of other toxic chemicals⁹.

In this work, we describe the synthesis of novel LCP using 4,4’-biphenol using direct polycondensation reaction in the presence of 1,4-dihydroxy naphthalene and isophthaloyl chloride. Fully aromatic polyesters have found very few commercial applications, due to their high crystallinity, they are more difficult to process. Surfing of literature implies that less quantum of work has been reported. Hence the scope of present investigation is important as these findings provide a closer insight into the application of polymerization techniques in designing high performance polymers.

EXPERIMENTAL PROCEDURE:

Materials and method

All the chemicals used in this investigation namely isophthaloyl chloride, 4,4’-biphenol, 1,4-dihydroxy naphthalene were of high purity and were obtained from Merck and were used without further purification. Purification of 1,2-dichloro benzene used as solvent in this process was done by keeping it overnight in anhydrous calcium chloride then filtered and purified using fractional distillation method at the temperature of 180°C and finally separated. Petroleum ether was also dried in presence of anhydrous calcium chloride and finally distilled. Other solvents like acetone, chloroform, tetrachlorocarbon, o-chloro phenol, DMSO and methanol all in Analar grade were used, to check the solubility of the polymer. Special grade CDCl₃ was used for recording Nuclear Magnetic Resonance spectrum of the polyester.

Characterization of random copolyester

The FTIR spectrum of the copolyester from 4000-400 cm^-1was reported using Perkin-Elmer 1600 series spectrophotometer with the samples incorporated in KBr pellets.

The UV-Visible spectral analysis was performed on Shimadzu-UV-160A spectrophotometer using acetone solution.

The ¹H NMR spectra were recorded on JEOL GSX 400 FT-NMR spectrometer operating at room temperature. Samples for analysis were prepared by dissolving about 10mg of the copolyester in 5ml of spectral grade CDCl₃ solvent.

TGA was done on a Perkin-Elmer 7 series Thermal Analysis system fitted with a data station. The sample was heated from 50 to 800 ^oC in Nitrogen atmosphere at a heating rate of 20°C min^-1 and weight loss of the sample at different temperatures was recorded.

A Metler FP84 HT hot stage was used for DSC measurements. 5-10 mg sample was heated at a scan rate of 20°C min^-1 during the cycle.

The X-ray diffraction pattern was obtained from a Philips-XPERT-PRO diffraction unit. CuKα target and Ni filter were used and 5^o to 40^o scattering angles (2θ) were chosen. The degree of crystallinity and the corresponding d spacing values were calculated.

Synthesis of Random Copolyester (P3BNI)

Polymerization reaction, direct polycondensation of isophthaloyl chloride, 4,4’-biphenol, 1,4-dihydroxy naphthalene in the ratio of 3:2:1 taken in a reaction flask with 150ml of 1,2-dichloro benzene was refluxed at 130° to 150°C for 25 hours in inert Nitrogen atmosphere with constant stirring. The content was cooled and extracted with petroleum ether and refrigerated overnight. Filtered crude sample was dissolved and evaporated in minimum amount of acetone to obtain pure polyester. The sample was dried in vacuum with phosphorous pentoxide, produced a yield of 72 %.

Scheme for synthesis of copolyester (P3BNI)

RESULTS AND DISCUSSION:

Solubility

The solubility of polymers is an important physical property to be considered for polymer processing. In general, the solubility of polyester depends upon the rigidity of the polymer backbone, molecular weight and dipolar interaction between the polymers and solvent molecules. The solubility of the random copolyester synthesized in the present work was tested qualitatively in a variety of organic solvents.

About 10mg of copolyester was taken in small stopper test tube containing 5ml of the solvent and was kept for 24 hours with occasional shaking. If the polyester was insoluble in cold, the mixture was slowly heated up to boiling point of the solvent. The process was done with various solvents qualitatively to check the relative solubility of the polymer in the solvents and was noted.

Name of Polyester	Acetone	CHCl₃	CCl₄	m-cresol	o-chloro phenol	D MF	Methanol
P3BNI	+++	++	+	+	+++	++	++

+++ - Freely soluble

++- Soluble

+ - Sparingly soluble

FTIR spectrum

The FTIR Spectroscopy is one of the important tools to identify the functional groups present in the synthesized copolyester. The IR spectrum of the synthesized polyester showed the following characteristic absorption bands as shown in Figure 1. A band observed at 1747.51 cm^-1corresponds to the carbonyl stretching of the ester group in the polyester. The bands at 1072.42 and 1232.51 cm^-1may be dueto ester C–O stretching and indicate the ester linkage present in the polymer. The band arising from characteristic aromatic C-H stretching absorptions was present at 3068.75cm^-1. The absorption peak noted at 713.66 cm^-1 and at 808.17 cm^-1 was attributed to the aromatic C-H bending vibration. The band observed at 1600.92 cm^-1 was due to aromatic C=C stretching vibration. The complete analysis of IR spectra of various polymers has been established with detailed structural information given by several workers^10-11.

Figure.1 FTIR Spectrum of P3BNI

UV-visible spectra

The synthesized copolymer shows possible π-π* electronic transition which was recorded in acetone solution due to the presence of aromatic chromophore in the Spectrum given in Figure 2. The absorption maxima identified are not influenced by the length of spacer of the polymer chain but are influenced by the condensed rings present in the polyester.

Figure 2. UV - Vis Spectrum of P3BNI

¹H NMR spectrum

The ¹H-NMR spectrum of the synthesized polyester was obtained in CDCl₃ solution with TMS as reference and is presented in Figure 3. The ring protons of isophthaloyl moiety are highly deshielded and hence have high d values. Similarly, the protons of 4,4’-biphenol and 1,4-dihydroxy naphthalene rings present in the polymer chain also absorb at down field. In the case of polyester, the spectrum shows the presence of aromatic protons in the range of 6.9 - 7.4ppm.

Figure 3. ¹H NMR Spectrum of P3BNI

Thermo gravimetric Analysis (TGA)

The thermal behaviour of the copolyester was evaluated by TGA in Nitrogen atmosphere at a heating rate of 20°Cmin^-1. It can also be used to determine the kinetic parameters of degradation of copolyester. The continuous weight loss curve for the thermal degradation of the copolyester is provided in Figure 4. The TGA curve shows a small weight loss initially which may be due attributed to loss of moisture and entrapped solvents. The observed thermogram suggests that the copolyester undergoes three stage degradation. The first degradation stage is slow and stops at around 150°C with 3.771% of degradation. The second stage of degradation stops around 400°C with approximately around 13.75% of the copolyester getting degraded. In the third stage degradation, mass loss was rapid and it stops at around 780°C with 45.66% of degradation and 36.82% residue remained after decomposition. This may be due to the aromatic mesogens present in the copolyester. It is evident from the thermogram that the copolyester P3BNI degraded in a wide range of temperatures.

Figure 4. TGA -Thermogram of P3BNI

Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry is used to detect the phase transitions in the random copolyester. The DSC thermogram was obtained in Nitrogen atmosphere for the copolyester and presented in Figure 5. The thermogram contains sharp as well as broad peaks at characteristic temperatures indicating phase transitions, before melting. The presence of more than one endotherm in the thermogram revealed that the copolyester undergoes more than one phase transition process when subjected to thermal treatment. This thermal behavior is due to the nature of the spacer which links the main chain¹². Polymers showing high T_g and T_m values contain rigid 1, 2-phenylene spacer and polymers exhibiting low T_g and T_m values contain flexible spacer which dissociates the long range order of the mesogenic groups of the main chain and also decouples the motions of the mesogenic moiety from those of the polymer backbone. The glass transition temperature of the copolyester determined from the heating scan of DSC analysis is found in the range 100° to 120°C. The endotherm at 241.33°C reveals the mesophase transition preceded by the onset of melting at 214.38°C.

Figure 5. DSC -Thermogram of P3BNI

Scanning Electron Microscopic Studies(SEM)

SEM investigation was carried out for the polyester at different locations on the surface of the film with diverse magnifications. SEM studies investigate the recognizable textures of the LC polyester at moderately high temperature during a time period at which the polymer may decompose and gives the information about the structure of the polyester film. The SEM micrographs are shown in Figure 6(a) and 6(b) reveals the extremely orientated identical distribution and constant microstructure, which leads to excellent mechanical properties of the polyester surface. It is the evidence of crystallinity in the polymeric material and it indicates the long range orientation order in polymer.

Figure 6(a), 6(b). SEM images of P3BNI

Wide angle X-ray Diffraction

XRD pattern was recorded in XPERT-PRO, Philips at voltage of 40kv and current at -30mA, scanning speed of 7.5 10.3 % at radiation target CuKα was Ni filtered. (λ=1.540Å) the range of scattering angle 2θ was 5-40°. X-ray diffraction pattern was obtained at room temperature and mathematically elaborated using hpert high score soft ware. Diffractogram is given in Figure 7. The data is provided in the table 1 given below.

Figure 7. X-ray Diffraction Pattern of P3BNI

Table 1. X-ray Diffraction values of P3BNI

Pos. [°2Th.]	Height [cts]	FWHM Left [°2Th.]	d-spacing [Å]	Rel. Int. [%]
11.5944	24.12	1.5744	7.63245	9.22
19.2103	261.73	0.492	4.62034	100
23\.8593	152.4	0.984	3.72955	58.23
28.5688	56.89	1.1808	3.12456	21.74

Gel permeation chromatography

Gel permeation chromatography (GPC) is a type of size exclusion chromatography (SEC), that separates analytes on the basis of size. The technique is often used for the analysis of polymers. GPC yields the complete molecular weight distribution and the polydispersity of synthetic polyesters. The Polymer (P3BNI) was subjected to GPC analysis by Shimadzu LC solution Analysis with an Injection Volume of 100 µL. The auto-scaled chromatogram is presented in Figure 8 and the peak table 2 is followed by GPC results in table 3. Polydisperse systems display an array of chain lengths which broaden the molecular weight distribution. Polydispersity index (PDI) is used as a measure of broadness of molecular weight distribution. The larger the PDI, the broader the molecular weight. PDI of a polymer is calculated as the ratio of weight average by number average molecular weight. Information on PDI is required for improved selection of polymers for an application¹³. For monodisperse polymers, the PDI is 1. The best controlled synthetic polymers used for calibrations have PDI of 1.02–1.10. Step-growth polymerization reactions typically yield values of Mw/Mn of around 2.0.

Figure.8 GPC Thermogram of P3BNI

Table 2. Peak Table

Peak#	Ret. Time	Area	Height	Area %	Height %
1	9.008	1045950	21438	72.598	53.697
2	9.417	326482	13520	22.661	33.863
3	10.113	68311	4966	4.741	12.440
Total		1440743	39925	100.000	100.000

Table 3. GPC results

GPC Results	Peak 1	Peak 2	Peak 3
Number Average Molecular Weight(Mn)	1401	470	836
Weight Average Molecular Weight(Mw)	1855	490	1469
Average Molecular Weight(Mz)	2551	509	2379
Z+1 Average Molecular Weight(Mz1)	3398	527	3349
Mw/Mn	1.32424	1.04315	1.75753
Mz/Mw	1.37513	1.03913	1.61866

Anti-Cancer Activity

Cell line tested were procured from NCCS Pune HeLa (Cervical carcinoma). Cells were cultured in a tissue culture flask in Dulbecco's Modified Eagle Medium (DMEM) supplemented with 10% FBS (Fetal Bovine Serum) and 1% Antibiotic/ Antimycotic Solution. The anti-proliferative effects of the polyester samples were tested against HeLa (Cervical carcinoma) cell line using MTT assay. Cells were trypsinized with 1X trypsin and the cell density was counted with the help of Haemocytometer. The cells were then seeded in 96 wells at a required density and cultured for 24 h in DMEM with 10% FBS and 1% antibiotic solution at 37º and 5% CO₂. The compound was solubilized in DMSO (with the final concentration of DMSO &salt; 0.5%) The test samples were added to each culture well at a concentration of 50µM and incubated for 24 hours. 20µl of MTT (5 mg/ml) was added to each culture well and incubated for 4hours. The supernatant was then removed by aspiration, and the precipitates were then solubilized in DMSO. The extent of the reduction of MTT was quantified by measuring the absorbance at 570 nm.

In HeLa cell line, compounds of 50, 60 µg / ml concentration confirmed the percentage viability in the range of 75 to 80 %, whereas the compounds of 70 µg / ml concentration showed the maximum percentage of cell viability inhibition range of 65 to 75 %. The remaining compound corresponds to the concentration of 80, 90 and 100 µg / ml showed the least cell viability inhibition in the range of 90 to 125 %. Among these different concentrations, the compound corresponds to 70 µg / ml may act as potential drug against the HeLa cell line. The polyester in the conc. of 100 and 90 µg/ml contorted the highest cell viability, while the remaining concentration of the same compound showed 76 -96 % of sustainable cells.

Figure.9 Anti Cancer Activity of P3BNI

CONCLUSION:

A novel thermally stable liquid crystalline polyester containing 4,4'-dihydroxybiphenyl moiety has been synthesized. The reaction pathway involving solution polycondensation technique was used. The copolyester was soluble in common solvents like acetone and chloroform and was further analyzed by different techniques. The chemical structure of the synthesized polyester was confirmed by UV – Vis, IR and ¹H NMR spectral values which are in accordance with functional group-ester linkage, protons of the polymer and the nature of mesogens present. The degradation of the polyester was investigated and the thermal analysis reveals the glass transition temperature and melting mesophase formation temperature which are useful in determining the liquid crystalline nature of the polyester, which was further investigated by SEM micrograph and XRD pattern providing useful information regarding the film surface of the polymer structure. The molecular weight has a drastic influence on the phase transition temperature of the polyester and was determined using GPC. The chromatogram revealed three peaks with Mw/Mn values ranging from 1.04315 to 1.75753. The anti-proliferative effects of the polyester sample tested against HeLa (Cervical carcinoma) cell line using MTT assay reveals increased cell viability with increasing concentration. The thermotropic liquid crystalline random copolyester should be potential candidate for future technological and industrial applications.

REFERENCES:

1. B.K. Chen, S.Y. Tsau, J.Y. Cen, Bor-Kuan Chen, Sun-Yuan Tsay, Jun-Yuan Chen, Synthesis and properties of liquid crystalline polymers with low Tm and broad mesophase temperature ranges, Polymer, Volume 46, Issue 20, 2005, Pages 8624-8633.

2. Chung, T., The recent developments of thermotropic liquid crystalline polymers. Polym. Eng. Sci., Volume 26, Issue 13, 1986, Pages 901-919.

3. Elsner G., Riekel C., Zachmann H.G., Synchrotron radiation in polymer science, Characterization of Polymers in the Solid State II: Synchrotron Radiation, X-ray Scattering and Electron Microscopy. Advances in Polymer Science, Volume 67, 1985, Pages 1-57.

4. Akemi Nakai, Toshio Shiwaku, Wei Wang, Hirokazu Hasegawa, Takeji Hashimoto, Phase-separated structures formed in polymer mixtures containing a thermotropic liquid crystalline copolyester as one component, Polymer, Volume 37, Issue 11, 1996, Pages 2259-2272.

5. R.W. Lenz, Synthesis and Properties of Thermotropic Liquid Crystal Polymers with Main Chain Mesogenic Units, Polymer J., Volume 17, 1985, Pages 105-115.

6. A. A. Collyer, Thermotropic liquid crystal polymers for engineering applications, Materials Science and Technology, Volume 5, Issue 4, 1989, Pages 309-322.

7. Jackson, W. J. and Kuhfuss, H. F., Liquid crystal polymers. I. Preparation and properties of p-hydroxybenzoic acid copolyesters. J. Polym. Sci. A Polym. Chem., Volume 34, Issue 15, 1996, Pages 3031-3046.

8. A.H. Khan, J.E. McIntyre, A.H. Milburn, Transitions in linear thermotropic polyesteramides, Polymer, Volume 24, Issue 12, 1983,Pages 1610-1614.

9. Yamazaki, N., Matsumoto, M. and Higashi, F., Studies on reactions of the N-phosphonium salts of pyridines. XIV. Wholly aromatic polyamides by the direct polycondensation reaction by using phosphites in the presence of metal salts. J. Polym. Sci. Polym. Chem. Ed., Volume 13. Issue 6, 1975, Pages 1373-1380.

10. Akihiro Abe, Configurational aspects of the odd-even effect in thermotropic liquid crystalline polyesters, Macromolecules, Volume 17, Issue 11, 1984, Pages 2280-2287.

11. Yiwang Chen, Yan Yang, Jiying Su, Licheng Tan, Yan Wang, Preparation and characterization of aliphatic/aromatic copolyesters based on bisphenol-A terephthalate, hexylene terephthalate and lactide mioties, Reactive and Functional Polymers, Volume 67, Issue 5, 2007, Pages 396-407.

12. Krigbaum, W. R., Asrar, J., Toriumi, H., Ciferri, A. and Preston, J., Aromatic polyesters forming thermotropic smectic mesophases. J. Polym. Sci. B Polym. Lett. Ed., Volume 20, Issue 2, 1982, Pages 109-115.

13. Anshuman Shrivastava. Introduction to Plastics Engineering, Editor(s): Anshuman Shrivastava, In Plastics Design Library, Introduction to Plastics Engineering, William Andrew Publishing, 2018, Pages 1-16.

14. Elango, S. Guhanathan, 2,3 butanediol based liquid crystalline random copolyester, Synthesis and characterization, Int. Journal of the Physical Sciences.,(2010).

Received on 01.12.2019 Modified on 31.12.2019

Asian J. Research Chem. 2020; 13(2):103-108.

DOI: 10.5958/0974-4150.2020.00021.8