1

Chapter 12

New Liquid Crystalline Poly (azomethine esters) Derived

from PET Waste Bottles

Issam Ahmed Mohammed and Nurul Khizrien A. K.

Abstract Nowadays, the disposal of non-biodegradable polyethylene terephthalate (PET)

bottles is considered as an environmental issue faced by both developing and underdeveloped

countries of the world. This research covers the historical development and the impact of

structural concept of polyester and polyethylene terephthalate (PET) that contributes to the

regeneration of terephthalic acid (TPA) from PET waste bottles to replace petrochemical

resources and consequently act as a component in the production of liquid crystalline

polymers focusing on liquid crystalline poly (azomethine esters). The experimental section

covers four T-shaped azomethine bisphenols prepared via the reaction of 2,5-

dihydroxybenzaldehyde with different aromatic amines, namely aniline, p-iodoaniline, p-

bromoaniline and p-chloroaniline and subsequently reacted with TPA to produce the new

thermotropic liquid crystalline poly (azomethine esters). The structures of the terephthalic

acid, azomethine bisphenols and poly (azomethine esters) were confirmed by FTIR, 1H NMR,13C NMR spectroscopy and elemental analysis (CHN). The liquid crystalline behaviors of

these polymers were studied by differential scanning calorimetry (DSC), polarizing optical

microscope (POM) and x-ray diffraction (XRD). All the synthesized poly (azomethine esters)

possesses good thermal stability and exhibit nematic phases except for the polymer that

derived from aniline did not show any mesophase.

Keywords Synthesis • Liquid crystalline • Poly(azomethine esters) • PET wastebottles • Nematic mesophaseIssam A. M. (*)Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur,Malaysia. E-mail: issam_usm@yahoo.comNurul Khizrien A. K.School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia.

2

Liquid crystal

The existence of liquid crystal was known when the Austrian chemist Friedrich Reinitzer

discovered a strange phenomenon of two distinct melting points on a cholesterol substance

back in 1888. The solid crystal melted into cloudy liquid at 145.5°C until the cloudiness

disappeared and changed to a clear transparent liquid at 178.5°C. The term “liquid crystal”

was introduced by Otto Lehmann when he discovered in 1890 that ammonium oleate and p-

azoxyphenetole showed turbid states between the extremely crystalline and the truly isotropic

fluid state. This phenomenon was challenged for almost half of a century by some scientists

because they previously knew of three states of matter, but the concept was finally accepted

and supported through conclusive experiments and theories.

Pierre-Gilles de Gennes, a French physicist found attractive agreement between liquid

crystals, superconductors and magnetic materials in the 1960s and in 1991. He was awarded

the Nobel Prize in Physics for discovering and developing methods for studying order

phenomena in liquid crystals. George William Gray, a British chemist thereafter made some

fundamental contributions, which then profoundly influenced the modern development of

liquid crystal science. Until the early 1970s, new types of liquid crystalline states were

discovered and applied especially in the display industry.

Liquid crystals are phenomenal organic materials with regard to the molecules exhibiting

intermediate phases, which flow like liquids as well as possess the characteristic of ordered

crystals. The materials form regions of highly ordered structure while in the liquid phase yet

the degree of order is rather less than a regular solid crystal and possessing unique direction-

dependent properties because the fact that the molecules are anisotropically oriented.

Tremendous progress in liquid crystal research has resulted in the expansion of new

discoveries, knowledge and applications, for instance, the market for lightweight and low

power electronic devices was rising exponentially, fueled up by the application of displays,

electro-optical devices, organic light emitting diodes, photovoltaic devices, organic field

effect transistors, biosensor, temperature and pressure sensors (Berdague et al. 1993).

Liquid crystalline polymers (LCP) comprises of aromatic rigid mesogenic groups such as

benzene ring or biphenyl group with a flexible spacer of a range of linkages such as ester,

ether, azo or imine groups and are known to exhibit liquid-crystalline (LC) behavior with

positional and orientational orders in LC phase (Issam et al. 2010) upon heating or in

3

concentrated solution. These structures if built into polymer chains can uphold mesophase

formation with changes in temperature (thermotropic LC) or at certain concentrations in a

suitable solvent. Liquid-crystalline mesophase can be formed by polymers with mesogenic

moieties in the main chain (Issam et al 2012) or mesogenic units bonded to main chain known

as side groups (Tomasz Ganicz et al 2009). As it is known, mesophase formation is generally

depends on the aliphatic spacer length and substituent on the mesogen ring.

In general, LCP with completely aromatic polymers had attractive mechanical properties at

high temperatures, exceptional chemical resistance and inherent flame retardant as well as

good weatherability. Among successful polymers around the world, aromatic polyesters and

polyazomethines are acknowledged as a substantial class of high-performance materials

because of their excellent thermal stability, good mechanical properties, and environmental

resistance but more particularly, as promising materials for coating, electrical, optical,

photonic, and magnetic application (Atta et al 2005).

Liquid Crystal Polyesters

Polyesters are made either by reaction of a dicarboxylic acid with a diol or by the self-

condensation of a hydroxyl acid. Polyesters have gained significant attention in the past

decade as their various adaptable structures intensify their wide applications such as coating

materials, microfibers for outdoor wear and sportswear, industrial fibers, clear casting,

polyester concrete and composites (Issam et al. 2011). Polyesters represent the majority of the

prepared liquid crystal polymers (LCP) from the concise history of the development of the

first-generation LCP. The basic structures in liquid crystal polyesters are benzene rings

attached at para positions through ester groups, which also represent the most important class

of aromatic LCP. Liquid crystal polyesters are generally called melt liquid crystal type

(thermotropic liquid crystal) polymers, and have been remarkably excellent to melt fluidity

due to their peculiar behavior.

Melt solution and slurry polycondensation are the usual synthetic routes used for the

synthesis of aromatic LC polyesters and the methods utilized are (1) the polycondensation of

terephthalic acid diesters and aromatic diols, (2) the polycondensation of terephthalic acid and

acetate of aromatic diols with the addition of transesterification catalysts, and (3) the

polycondensation of aromatic diols and aromatic diacid dichlorides (Atta et al. 2005, 2007).

The next development in liquid crystal polyesters was the preparation by polycondesation

based on terephthalic acid (TPA) and hydroquinone (HQ) or p-hydroxybenzoic acid (HBA).

4

The polyesters are insoluble with very high melting temperatures of 600°C for poly

(TPA/HQ) and 610°C for poly (HBA), which are by far too high to obtain stable liquid

crystalline phases for melt processing. In 1972, Economy and coworkers patented several

copolyester compositions, and one of these are the copolymerization of poly (4-

hydroxybenzoic acid) (PHB) with 4,4’-dihydroxybiphenyl (BP) and terephthalic acid (TPA)

due to the need for lower melting, melt-processable polymers. Considerable synthetic efforts

have been attempted in order to decrease the melting temperatures of aromatic LC polyesters

while retaining LC properties. The copolyester structure was tailored by partial substitution of

TPA with isophthalic acid to produce a melt-spinnable material.

The formation of thermotropic LC phases reduces the melt viscosities during processing

and melt-processable LCP could be obtained by lowering the chain regularity of the main

chain while maintaining the chain stiffness by random copolymerization or the use of other

aromatic comonomeric units. Jackson and Kuhfuss (1976) produced X7G fibers at Eastman-

Kodak by progressively increasing the rigidity of polyethylene terephthalate (PET), a

commercial, thermoplastic polymer by the introduction of increasing amount of HBA units in

the flexible macromolecule. Majority of aromatic thermotropic liquid crystal polyesters are

known to be excellent in small-thickness moldability, heat resistance, mechanical strength,

dimensional stability and the like; since the polyesters can be adapted to soldering

temperatures about 240 to 260° C. The thermal stability of liquid crystal polymer with low

branching coefficient is better than that of the dumbbell-shaped. As there are adamant

concerns for biodegradable polymeric materials relating to environmental awareness, several

biodegradable aliphatic polyesters have been developed and some of them are now

commercially handy because of their good biodegradability but still constitute serious

obstacles due to their poor thermal and mechanical properties (Tserki et al 2006).

Polyethylene terephthalate (PET)

Polyethylene terephthalate (PET) is a semicrystalline thermoplastic polyester used in the

preparation of variety of products such as fibres, filaments, sheets and soft drink bottles

(Shukla 2003) and (Shukla and Harad 2006). PET is one of the best-known consumer plastic

for high softening-point blow-moulded bottles and the world demand for PET bottles have

been increasing due to its high mechanical strength, light weight, low permeability to gases,

relatively low manufacturing cost and its non-toxicity (Karayannidis et al. 2005).

5

Almost 104 million tones of post-consumer PET bottles are being disposed and dumped

indiscriminately onto bare lands and water, which thus constitute a serious impediment to the

environment. PET does not create any direct hazard in the environment, but due to the

increasing plastic’s waste and its high resistance to the atmosphere and biological agents, it

may be considered as pollutant material (Sinha et al. 2010). Thus, recycling of PET bottles is

a common environmentally friendly procedure used to reduce plastic waste and also

contribute to the conservation of raw petrochemical products and energy. As there is urgent

demand for PET waste conversion, many scientists have converted PET waste to other

valuable for raw materials (Issam, 2014) and besides react with other monomers to produce

new polymers such as unsaturated poly (ester-urethane) (Issam et al. 2011). As terephthalic

acid (an aromatic diacid) is used for the preparation of various types of polyesters, including

their liquid crystals, it is worth noting that used is predominantly obtained from petroleum

resources that are valuable and non-renewable (Atta et al.2005). The ultimate aim of this

research is to regenerate TPA from PET waste bottles to be used as a new raw material in the

preparation of other polymers. This will lead to minimizing dependency on the petrochemical

resources as well as to reduce the amounts of waste. This work thus uses TPA as one of the

main reactants in producing liquid crystalline poly (azomethine esters).

Liquid Crystal Poly (azomethine esters)

Liquid crystal poly (azomethine esters) consists of a rigid mesogenic moiety (e.g., benzene

ring, biphenyl group) and an azomethine linkage (-CH=N-) in the main chain or side chain.

They are of the interest due to their unique performance properties namely, high mechanical

strength, high-thermal stability, environmental resistance, and are promising materials in

photonic and opto-electronic applications (Vasanthi et al. 2013).

This statement is substantiated by the fact that the incorporation of azomethine groups in

the main chain LC improves the thermal properties, and the polymer formed mesomorphic

phase on heating (thermotropic liquid crystal polymer). However, the drawbacks from their

high melting points and poor solubility minimize their practical applications by conventional

methods. In order to enhance their processability, several approaches have been attempted

such as introduction the flexible aliphatic segments into the main chain, or via the bulky

lateral substituents. Issam et al. (2010) reported that the azomethine group with a flexible

spacer is very useful in exhibiting LC properties. They can be synthesized by forming the

linking groups between rigid core and the flexible spacer (-O- or -OOC-) in the

6

polymerization step or alternatively; polymers are obtained as polyethers (-O- linkages

formed in the polymer synthesis) or polyesters (-OOC- linkages formed in the polymer

synthesis). Rapid progress in the field of polymer science creates various modified

polyazomethines with the sole aim of decreasing their melting temperature, enhance their

solubility and to develop mesomorphism such as poly(azomethine-esters), poly (azomethine-

ethers), poly(azomethine-carbonates), poly(amide-azomethine-esters), poly (acrylate-

azomethines), thermosetting polyazomethines and poly(azomethine-sulfones) (Aslan et al.

1997). Among these polymers, thermotropic liquid crystals poly (azomethine esters) has

received greater technological importance due to their high thermal stability and good

processability. Because of these valuable properties, great efforts have been concentrated on

the synthesis of new compounds with enhanced properties. Previous studies have shown that

poly (azomethine esters) integrated the beneficial properties of both polyazomethines and

polyesters, due to the contribution of the anisotropic azomethine rigid cores and isotropic

ester moieties. It has been reported that the ester group increases the electrical conductivity of

copolymers in poly (azomethine-esters) in two ways i.e. by increasing the flexibility of the

chain and making the charge carrier movement more along the chain (Han et al. 1997). Marin

et al. (2006) investigated the usefulness of aliphatic–aromatic polyazomethines with ester

groups for opto-electrical applications by synthesizing polyazomethines with various

structures of the dialdehyde moiety. By varying the dialdehyde structure, they managed to

control the conjugation along the polymer backbone and consequently, control the optical and

electrical properties of the polymers.

In order to investigate the role of the sub-unit of dialdehyde, groups such as thiophene,

carbazole, stilbene or phenylene were introduced to the polyazomethine main chain. Poly

(azomethine esters) with stilbene bonds showed promising opto-electronic properties due to

the lowest value of the optical energy gap (Eg= 2.28 eV) and the highest thermoluminescent

intensity (I=42). They also suggested there was a new application potential for

polyazomethines by exploiting their liquid-crystalline and transport properties toward stable

organic opto-electronic devices, for example, in OLEDs and solar cells.

Marin et al. (2006) successfully synthesized novel functional thermotropic liquid

crystalline polyazomethine derivatives by incorporating new properties of luminescence;

fluorene and/or oxadiazole chromophoric units. The polymers showed an impressive

mesomorphic state controlling the order degree of the semicrystalline state while being

viscous fluids. The semiflexible polyazomethines have high thermal stability, good

mechanical properties and good solubility in polar solvents thus relevant for opto-electronic

7

material design with the aim of thermostable films with good mechanical properties.

Afterward, (Marin et al. 2013) synthesized thermotropic liquid crystalline dimers containing

azomethine and pyrrole, indole, or diphenyl chromophoric moieties.

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.

Materials and Methods

Materials

All the chemicals were commercially obtained and used without further purification. PET

bottles of Spritzer (Malaysia) mineral water were shredded into flakes. Sodium hydroxide AR

(Chem.AR), Ethylene glycol (Aldrich Chem. Co.) and sulphuric acid (Mallinckrodt Chem.

Co.) were used as purchased. Aniline, p-iodoaniline, p-bromoaniline and p-chloroaniline were

purchased from (Aldrich Chem. Co.). 2,5-dihydroxybenzaldehyde, absolute ethanol,

methanol, 1-butanol, dimethylsulfoxide (DMSO), ethyl methyl ketone and diethyl ether

(Fluka Chem. Co.).

Instrumentation

FTIR spectra for the monomers and polymers were recorded on a Perkin-Elmer 2000 FTIR

spectrophotometer using KBr pellet method. 1H NMR and 13C NMR spectra were recorded on

a Bruker 400 MHz NMR spectrometer, Dimethylsulfoxide (DMSO-d6) was used as the NMR

Solvent and tetramethylsilane (TMS) used as internal reference. Differential Scanning

Calorimetry (DSC) was performed on a Perkin-Elmer DSC7 series in nitrogen atmosphere at

a heating rate of 10oC/min. Polarizing optical microscope (POM) was measured in Nikon

Eclipse E 600 connected to a Linkam THMS 600 heating stage. The X-ray diffractograms

were recorded using Siemens Model D-5000 diffractometer at room temperature with nickel–

filtered CuK radiation. Thermogravimetric analysis (TGA) was performed on a Perkin-

Elmer TGA7 series in nitrogen atmosphere at a heating rate of 20oC/min. Inherent viscosities

8

of polymer solutions (0.2 dL/g in DMSO) were determined at 30oC in an Ubbelohde

viscometer.

HO OH

CH

N

X

HO OH

CHO

X NH2- H2O

1 (a-d)

Ia: X = -H 1 (a-d)

Ib: X = -Cl

Ic: X = -Br

Id: X = -I

Scheme 12.1 Synthesis of T-shaped azomethine bisphenols 1 (a-d)

C C OH

O

HO

O

C C Cl

O

Cl

O

SOCl2

Pyridine

C C OH

O

HO

O

C C

OO

CH2CH2O OH2CH2CONaOH

n

PET

2

9

HO OH

CH

N

X

C C Cl

O

Cl

O

- 2 HCl

C C O

OO

O

CH

N

X

n

1 (a-c)

2

3 (a-d)

3a: X = H

3b: X = Cl

3c: X = Br

3d: X = I

Scheme 12.2: Synthesis of poly (azomethine esters) 3 (a-d)

10

Monomer synthesis

Synthesis of azomethine bisphenol 1 (a)

A solution of 1.38g (0.1 mol) 2,5-dihydroxybenzaldehyde in 30 ml of absolute ethanol was

added dropwise into a mixture of 1.07g (0.1 mol) of aniline in 30 ml absolute ethanol

containing few drops of acetic acid in a 500ml flask. The mixture was refluxed for 6 h with

continuous stirring. 2,5-dihydroxybenzelidene aniline was separated as a precipitate, washed

several times with diethylether, and dried in a vacuum oven at 70°C for 24 h. Final

purification was carried out by re-crystallization from 1-butanol to give reddish crystals;

Melting point (˚C): 150.5-153. Yield: 88 %. FTIR (KBr) (cm-1): 3407 (-OH stretching), 1622

(-C=N stretching). 1H NMR (DMSO-d6) (ppm): 8.36 (1H, -CH=N-), 7.46 - 6.79 (8H,

aromatic). 13C NMR (DMSO-d6) (ppm): 163.7 (-CH=N), 135.0 – 115.8 (aromatic C=C),

152.8 (-C-N, aromatic). Elemental analysis: Calc. for C13H11NO2: C, 73.23; H, 5.20; N, 6.57;

Found: C, 73.18; H, 5.33; N, 6.59.

Synthesis of azomethine bisphenol 1 (b)

The monomer was synthesized using the same procedure as in the above except that aniline

was substituted by p-chloroaniline. Melting point (˚C): 181-183. Yield was 79%. FTIR (KBr)

(cm-1): 3350 (-OH stretching), 1620 (-C=N stretching). 1H NMR (DMSO-d6) (ppm): 8.0 (1H,

-CH=N-), 7.26 - 7.83 (7H, aromatic). 13C NMR (DMSO-d6) (ppm): 163.9 (-CH=N), 151.9 –

112.3 (aromatic C=C). Elemental analysis: Calc. for C13H10NO2Cl: C: 63.03, H: 4.04, N:

5.65; Found: C, 63.10; H, 4.05; N, 5.58.

Synthesis of azomethine bisphenol 1 (c)

The same procedure as in the above was adopted except that aniline was substituted by p-

bromoaniline. Melting point (˚C): 192-193.5. Yield was 82 %. FTIR (KBr) (cm-1): 3345 (-OH

stretching), 1621 (-C=N stretching). 1H NMR (DMSO-d6) (ppm): 8.86 (1H, -CH=N-), 7.25 -

7.93 (7H, aromatic). 13C NMR (DMSO-d6) (ppm): 164.9 (-CH=N), 152.3 – 110.8 (aromatic

C=C). Elemental analysis: Calc. for C13H10NO2Br: C, 53.44; H, 3.42; N, 4.79; Found: C,

53.40; H, 3.46; N, 4.73.

Synthesis of azomethine bisphenol 1 (d)

The same procedure as in the above was adopted except that aniline was substituted by p-

iodoaniline. Melting point (˚C): 197-198. Yield was 78%. FTIR (KBr) (cm-1): 3355 (-OH

stretching), 1622 (-C=N stretching). 1H NMR (DMSO-d6) (ppm): 8.53 (1H, -CH=N-), 7.21 -

11

7.89 (7H, aromatic). 13C NMR (DMSO-d6) (ppm): 163.9 (-CH=N), 152.0 – 114.4 (aromatic

C=C). Elemental analysis: Calc. for C13H10NO2I: C, 46.03; H, 2.95; N, 4.13; Found: C, 45.99;

H, 2.98; N, 4.17.

Regeneration of terephthalic acid (TPA) from PET waste bottles

Sodium Hydroxide pellets were first dissolved in ethylene glycol at a temperature of between

60°C to 100°C. PET flakes were added to the mixture when NaOH pellets were partially

dissolved. The ratio of PET flakes to NaOH pellets to ethylene glycol was 1:2:1.5,

respectively. The mixture was well mixed and heated at 190°C to 200°C in a stainless steel

container and the temperature was maintained for about 20 minutes until the mixture turned

milky. The mixture was subsequently stirred continuously and after 20 minutes, the mixture

was cooled to room temperature. Distilled water was afterwards added to dissolve all white

precipitates. It was then stirred and filtered through a coffee-filter. When the mixture turned

clear yellow, a few drops of 5M H2SO4 were added to this solution until pH 2. A white

powder appeared and was filtered using Buchner funneled (under suction) while ethylene

glycol was collected as the filtrate. The white precipitate (TPA) was collected and washed

several times with water and finally dried in a vacuum oven for 24 h at 60°C.

Preparation of L.C Poly (azomethine esters) 3 (a-d)

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 300ml THF, an equimolar quantity of compound, I (a) was dissolved in 200ml

THF followed by the addition of 5ml pyridine. The terephthaloyl chloride solution was taken

in a dropping funnel and added dropwise to a mixture of 0.01 mol azomethine bisphenol in

200ml THF and 5ml of pyridine at 0oC 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 75oC for 24 h. The

polymers (b-d) were synthesized using the same method as described in this procedure. The

results were summarized in Table1.

12

Results and Discussion

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 acid (TPA) was

regenerated from PET waste bottles via saponification process, and the chemical structure of

the TPA was confirmed by FTIR and 1H NMR spectroscopy. FTIR spectrum of TPA showed

the presence of C=O band at 1685 cm-1 and the -C = C- stretching of the aromatic ring

appeared at 1574 cm-1 and 1510 cm-1. The asymmetrical stretching of -OH group appeared at

3064 cm-1 while the symmetrical stretching band appeared at 2551 cm-1. The absorption

bands at 1424 cm-1 and 937 cm-1 were attributed to the C-O stretching and C-O-H

deformation, respectively. Furthermore, 1H NMR spectrum of terephthalic acid (Figure 12.1)

showed a broad peak centered at 13.3 ppm, corresponding to the proton of hydroxyl group

and a strong singlet peak at 8.1 ppm due to the protons of aromatic ring.

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

Compound CHN Yield (%) Inherent viscosity,

ηinh (dl/g)Calculated (%) Found (%)

3a C:73.46, H:3.79,

N:4.08

C:72.52, H:3.65,

N:4.10

80 0.43

3b C:66.75, H:3.17,

N:3.70

C:66.19, H:3.18,

N:3.54

76 0.50

3c C:61.03, H:2.90,

N:3.39

C:59.99, H:2.98,

N:3.23

82 0.52

3d C:53.74, H:2.56,

N:2.98

C:52.69, H:2.47,

N:2.91

72 0.49

13

Fig. 12.1. 1H NMR spectrum of terephthalic acid.

The preparation of T-shaped azomethine bisphenols 1 (a-d) from

2,5-dihydroxybenzaldehyde and p-substituted aniline was outlined in Scheme 1. The

structures of azomethine bisphenols were confirmed by FTIR, 1H and 13C NMR spectroscopy

and elemental analysis (CHN). The FTIR spectra of

the monomers I (a-d) showed sharp absorption bands at 1620-1622 cm-1 which was assigned

to the -CH=N- stretching and confirms the formation of azomethine linkage. The absorption

bands at 3345-3407 cm-1 were due to the stretching vibrations of the hydroxyl groups. Further

confirmation was carried out by 1H NMR spectroscopy of monomer 1 (b) (Figure 12.2)

which showed the presence of azomethine proton (-CH=N-) at 8.86 ppm. The characteristic

singlet peak which centered at 9.12 ppm was attributed to the proton of the phenolic (Ph-OH)

group.13C NMR spectrum of monomer 1 (b) showed peak at 163.11 ppm, which was due to the

azomethine carbon (-CH=N-) and thus confirms the formation of azomethine linkage. In

addition, the carbons of the aromatic rings appeared at 112.3 to 131.2 and 148.7, 149.8 and

151.9 for C-N and C-O, respectively. Elemental analysis (CHN) data also supported the

formation of the expected monomers and confirmed the calculated data.

14

Fig. 12.2. 1H NMR spectrum of monomer 1 (a)

Fig. 12.3. 1H NMR spectrum of polymer 1 (a)

The general polymerization reactions of the LC polymers 3(a-d) were as outlined in Scheme

2. The structures were confirmed by CHN, FTIR and 1H and 13C NMR spectroscopy. The

FTIR spectra of polymers showed characteristic absorption bands in the range 1681-1711 cm–

1 due to the carbonyl stretching (-C=O) of ester linkage, and in the range 1619-1622 cm–1 due

to the azomethine (-CH=N-) linkage. The other absorption bands which represent the

aromatic ring appeared in range of 1508-1603 cm–1.

In the 1H NMR spectrum of polymer 3(a) shown in Figure 12.3, we notice the disappearance

of the singlet peak at 9.12 ppm for -OH group and the appearance of two new peaks at 8.09

15

and 8.1 ppm were due to the protons in aromatic ring of the ester groups. Moreover, the

multiplet peaks found at 7.51-8.21 ppm were attributed to the aromatic protons of the

monomer. Furthermore, the proton of azomethine group appeared at 8.75 ppm. In the 13C

NMR spectrum, the new peak which is centered at 182 ppm is assigned to carbon of the

carbonyl groups of the ester linkages. Other peaks for carbons of azomethine at 176.07 and

aromatic rings at the range of 121.11-152.13 confirmed the chemical formula of polymer 3(b).

In addition to the structural confirmation via the spectroscopy analysis, the elemental analysis

data (Table 1) also supported the formation of the expected polymers 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 (ηinh) 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.

Thermotropic LC properties of the polymers 3 (a-d)

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) were investigated using DSC (Figure

12.4). The samples were heated from room temperature to 400°C at the rate of 10°C/min

followed by cooling to room temperature and reheating the samples to 320°C also at the rate

of 10°C/min. It is apparent in the DSC curve that Tg for polymer 3(b) was 147oC while the Tm

and Ti were centered at 187 and 309oC, respectively. The sample was held for one minute and

then cooled. On cooling, one exothermic peak was observed at 200°C arising from

crystallization and the second run was carried out by reheating of the polymer at the same

condition. The melting points (Tm) for polymers 3(c) and 3(d) appeared at 174oC and 167oC

respectively, whereas the Ti for these polymers found at 300oC and 296oC, respectively.

However, the DSC curve for polymer 3(a) showed only melting point at 364oC without any

sign for Ti. The high Tm reflects the high thermal stability of the LC poly (azomethine esters)

obtained.

16

Fig. 12.4. DSC curves of poly (azomethine esters) 3 (a-d)

Table 12.2. DSC and POM data of LC poly (azomethine esters) 3(a-d)

PolyestersDSC

Cr-N (°C) N-I (°C)

Tm(°C) Ti (°C) T (°C)

3a 364 - - - -

3b 186 300 114 188 300

3c 174 300 126 174 303

3d 167 296 129 169 298

17

It may be observed from Table 12.2 that polymer 3(d) with the iodine substituent group gives

lower melting point (Tm) compared to the polymers with chlorine and bromine [3(b) and 3(c)]

and also with polymer 3 (a), which is based on aniline. This may be attributed to the size

difference which causes lack of intermolecular interaction as the size increases.

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-I) 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 Figure 12.5.

3 (b)

18

3 (c)

3 (d)

Fig. 12.5. Polarizing photomicrograph of polymer 3 (b-d)

XRD diffraction studies was also carried out to analyze the crystallinity of the polymers 3(a-d) and was performed at room temperature. To avoid the trace of the solvents, powder

19

samples were placed on a zero-background quartz plate, placed for 20 sec. in an ovenpreheated to 110 ºC, quenched to room temperature using a N2 purge, and then analyzed in thediffractometer. The XRD patterns are shown in Figure 12.6. All the polymers show similarmultiple sharp diffraction peaks in the range of 2 = 11.0 to 30.0o which indicates thecrystalline to semi-crystalline nature of the polymers. This may be due to the slight differencein the chemical structures of these polymers. Furthermore, the results of the x-ray diffractionwere in agreement with the POM textures, and this confirmed that the synthesized polymers3(b-d) were in liquid crystal phases (nematic mesophase).

Fig. 12.6. XRD pattern of poly (azomethine esters) 3 (a-d)

20

Thermal stability and degradation of the polymers 3(a-d) were analyzed based on the

thermograms obtained from TGA at constant heating rate of

20oC min–1 in the temperature range of 30 to 800oC under nitrogen atmosphere.

Table 12.3 TGA data of LC poly(azomethine esters) 3(a-d)

Equal weights of all the polymer samples were used to eliminate the mass effect.

Enhancement of thermal stability of the polyesters observed in the present studies can be

attributed to the delocalization characteristic of azomethine linkage. The results imply that the

presence of azomethine linkage in the backbone of polyester is ideally suited to produce good

thermal stability and high yield. The temperature required for 10% degradation (T10) and the

amount of chars formed at 800oC were summarized in Table 12.3. It is evident to the TGA

curves that two steps of degradation have occurred. The first step at the range of 320-329oC

was assigned to the cleavage of ester groups, whereas the second step at the range of 450-

480oC, which is attributed to the degradation of azomethine and aromatic rings.

LC polymers Degradation temperature at weight loss, °C

10% 20% Residue at 800 °C

3a 380 410 25

3b 335 363 15

3c 343 375 17

3d 330 358 13

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Conclusion

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). All 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.

Acknowledgement

The authors would like to thank University of Malaya for their support throughout this work.

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