Investigations on Preparation and Characterization of Certain Copolyesters

Polymers have contributed tremendously to human life from simple household items to high technological applications. The amount of plastic wastes has increased all over the world, which threatens to cause a serious environmental problem and has become a matter of great concern to the people throughout the world including scientists from different countries. Some steps such as reducing the use of plastic products and recycling the plastic waste have been taken to solve this problem. One of the feasible ways is to develop biodegradable plastics 1.Therefore, a series of biodegradable polymers were developed in the past decades, most of which were aliphatic polyesters, such as poly(lactic acid) (PLA), poly(ε-caprolactone) (PCL), poly(3-hydroxybutyrate) (PHB), poly(butylene succinate) (PBS), poly(propylene succinate) (PPSu), poly(propylene adipate) (PPAd) , poly(ethylene succinate) (PES),and poly(propylene sebacate) (PPSe) 2-7 . Much attention is being paid to aliphatic polyesters because of their degradability 8,9. But the limitation of these polyesters is due to low thermal and mechanical properties. Copolymerization 10,11 and blending 12,13 with aromatic polyesters to modify the properties of aliphatic polyesters are better choices in both mechanical and thermal aspects. Meanwhile, a study shows that there is degradability for aromatic polyesters when they are copolymerised with aliphatic polyesters 14. In recent years thermotropic liquid crystalline polyesters have received a great deal of attention 15,16. Some interesting attempts were made in the 1980’s to synthesize thermotropic random copolymers by introducing two different lengths of flexible spacers in the main-chain 17,18. Liquid crystalline aliphatic-aromatic random copolyesters are becoming increasingly important as they are used as structural materials in various fields whose particular properties make them ideally suited to high precision mouldings, as well as for high performance fibres. Bearing in mind the high commercial potential of aliphatic-aromatic copolyesters and their interesting properties, the work focused on the synthesis and characterization of certain random aliphatic-aromatic copolyesters 19. The structure of their repeating units and the effect of copolymer composition on the physical and thermal properties, optical polarising microscopic studies as well as degradation were investigated in alkali medium.


INTRODUCTION
Polymers have contributed tremendously to human life from simple household items to high technological applications.The amount of plastic wastes has increased all over the world, which threatens to cause a serious environmental problem and has become a matter of great concern to the people throughout the world including scientists from different countries.Some steps such as reducing the use of plastic products and recycling the plastic waste have been taken to solve this problem.One of the feasible ways is to develop biodegradable plastics 1 .Therefore, a series of biodegradable polymers were developed in the past decades, most of which were aliphatic polyesters, such as poly(lactic acid) (PLA), poly(ε-caprolactone) (PCL), poly(3-hydroxybutyrate) (PHB), poly(butylene succinate) (PBS), poly(propylene succinate) (PPSu), poly(propylene adipate) (PPAd) , poly(ethylene succinate) (PES),and poly(propylene sebacate) (PPSe) [2][3][4][5][6][7] .
Much attention is being paid to aliphatic polyesters because of their degradability 8,9 .But the limitation of these polyesters is due to low thermal and mechanical properties.
Copolymerization 10,11 and blending 12,13 with aromatic polyesters to modify the properties of aliphatic polyesters are better choices in both mechanical and thermal aspects.Meanwhile, a study shows that there is degradability for aromatic polyesters when they are copolymerised with aliphatic polyesters 14 .In recent years thermotropic liquid crystalline polyesters have received a great deal of attention 15,16 .Some interesting attempts were made in the 1980's to synthesize thermotropic random copolymers by introducing two different lengths of flexible spacers in the main-chain 17,18 .Liquid crystalline aliphatic-aromatic random copolyesters are becoming increasingly important as they are used as structural materials in various fields whose particular properties make them ideally suited to high precision mouldings, as well as for high performance fibres.Bearing in mind the high commercial potential of aliphatic-aromatic copolyesters and their interesting properties, the work focused on the synthesis and characterization of certain random aliphatic-aromatic copolyesters 19 .The structure of their repeating units and the effect of copolymer composition on the physical and thermal properties, optical polarising microscopic studies as well as degradation were investigated in alkali medium.and distilled under reduced pressure.Titanium tetra butoxide (Ti (OBu) 4 ), used as catalyst, purchased from Lancaster was used as such.All the other materials and solvents used were of analytical grade.

Synthesis of Copolyesters
The copolyesters were synthesized by two step melt poly condensation method, because in this method, the chemical reaction is simple, non-toxic and less expensive compared with other synthetic methods.As an example, the synthesis of poly (butylene succinate -co-butylene Terephthalate) PBSBT is described.The polycondensation flask is a three neck flask equipped with a condenser, nitrogen inlet and a thermometer.To stir the reaction mixture a magnetic stirrer was used.The reaction mixture consists of 1, 4 Butane diol, Succinic acid (Lancaster AR grade) and Terephthalic acid taken in the ratio 2:1:1, which means 2 moles of reactant react with each one mole of diacids to give a linear polyester.The reaction mixture was purified with nitrogen and heated in an oil bath.The purpose of using nitrogen gas is to maintain inert atmosphere.Due to this, degree of polymerization increases; so does the molecular weight.The temperature of the reaction mixture was raised to 150°C in 20min.Then the temperature was gradually raised in steps of 10°C every minute to the reaction temperature of 210°C to remove water being the esterification by product as there is a possibility of decomposition of monomers which leads to decrease in yield.When water ceased to be generated, a predetermined amount of titanium tetra butoxide (Ti(OBu) 4 ) (0.1mmol) catalyst was added to the reaction mixture.Subsequently, the pressure of the reaction system was lower than 0.5mmHg.The reaction was terminated when the mechanical stirrer stopped rotating.The resulting mixture was crude copolymers and dissolved in chloroform as the synthesized polymer shows more solubility in polar solvents such as chloroform and hence used as solvent.Then it was poured into excess of dry cold methanol to purify the polyester.The precipitated copolyesters were dried in a dessicator and used for further characterization.

Characterization Methods
Chemical structure of pure copolyesters synthesized was investigated by FTIR measurements which were performed using KBr pelleted samples with a Perkins Elmer 200 FTIR spectrophotometer with a resolution of 4cm -1 in the range of 400 -4000 cm -1 . 1 H NMR spectra were recorded on AV 3500MHz Spectrometer by using 7% wt of CDCl 3 mixed solvent. 13C NMR spectra were recorded at 300-600 MHz in deuterated chloroform as solvent.The DSC scans were recorded at a heating rate of 10 o c/min using Perkin-Elmer Pyris I analyzer.Indium was used as the calibration standard.A Siemens D 500 diffractometer with CuK α filter radiation was used for assessing crystallinity of the polymers.The samples were scanned over the range of 2θ angle, from 10° to 90°.The morphology of the polyester film was investigated by Scanning Electron Microscope before and after degradation by keeping in 0.1N NaOH solution at regular time intervals.Tetrahydrofuran was used as eluent.Polystyrene standard was used for calibration with a flow rate of 1ml/min.Concentration of the mobile phase was 0.005g/5ml.The optical texture of the melts was examined on a hot stage (Mettler FP-5) attached to a polarizing microscope.

Fourier -Transform Infrared (FTIR) Spectroscopy
FTIR spectra of copolymers derived from PBSBT and PBSeBT are shown in Figure (1a&1b.Band characteristics of aromatic esters were observed between 1719.45cm - and 1717.59cm -1 (C=O stretching) and between 730.84cm -1 and 727.96 cm -1 (ring C-H out of plane bending).The C-H symmetric stretching of aliphatic-CH 2 -group was observed at 2953.49-2930.39cm -1 .Strong vibrational modes observed at 1164.59 -1171.59cm -1 were associated with C=O stretching modes of esters.

13 C NMR spectral data of random copolyester
The chemical shift values obtained from 13 C NMR Spectra of the copolyesters PBSBT and PBSeBT are as follows: A singlet at δ= 133.97-134.07ppm was due to quartenary carbon atoms of terephthalate.A doublet at δ = 172.30-173.84ppm was due to carbonyl carbon atom

Differential Scanning Calorimetry (DSC)
DSC analysis was used to find the glass transition temperature (T g ) of synthesized polymers are shown in Figs 4a &4b.Glass transition temperature of any polymer is the temperature at which it becomes soft and rubbery in nature on heating.That is to say, the temperature above which it is hard and glass and below which it is rubbery is called the glass transition temperature (T g ).
From the above Table 1 it is clear that glass transistion temp (T g ), Melting temperature (T m ) Decomposition temperature (T d ) and heat of fusion ∆Hm increase with increase in chain length of polyesters.The glass transition temperature (T g ) of polyester is closely related to the flexibility of the chain.Due to high barrier of bond rotations T g value is high for PBSeBT.

X-ray Diffraction Analysis
X-ray diffractogram of the synthesized polymers are shown in Figs 5a &5b.Gaussian curves are used to describe the amorphous phase and crystal reflections of diffractogram 20,21 .The diffractograms reveal that the degree of crystallinity is higher for PBSBT than for PBSeBT which is supported by increase in the melting point obtained from DSC measurements for polymers.This indicates that the crystallinity of the polyester decreases with the increase in the length of flexible segments.Thus, PBSeBT was found to have low crystallinity and the value obtained is tabulated as shown in Table .2.The percentage crystallinity of copolyester was calculated by Debye Scherre's equation.

Scanning Electron Microscopy (SEM)
The surface morphology of the copolyester was examined through scanning electron microscope (SEM) and the weight loss due to degradation is shown in Table .3.The SEM images in Figs.6a & 6 b show rough surface with cavities on the film which is ample evidence for the degradation property for the synthesized copolyesters.
Thus it is clear from the above Table that the rate of degradation in alkaline medium decreases with increase in the chain length of the copolyester.Percentage of weight loss is calculated with time and is graphically represented below as shown in Fig. 7.

Gel Permeation chromatoghraphy (GPC)
Gel permeation chromatography has its greatest value for measuring the molecular weight and polydispersity of synthetic polymers.This method depends on distribution coefficients, relating the selective distribution of an analysis between the mobile phase and stationary phase.It gives an idea of number average and weight average molecular weight.If molecular weight is more, processability is early.The chromatograms in Figs.8a &

Optical Polarized Microscopy (OPM)
Thermotropic liquid crystalline polyesters with low T g values followed by a sharp transistion to a mesophase with as low T m value as possible are attracting current interest due to their use in reprographic appllications.The widely employed technique to identify the mesophases in thermotropic liquid crystalline polymers is OPM studies.
OPM and DSC were used to determine the mesogenic transistion temperature (T m ) as well as the mesophase to the isotropic transition temperature (T i ).From Table.5, it is observed that the incorporation of the spacers and increasing their length result in increase in the range of mesophase temperatures.The texture examination has become a very useful tool in the determination of the type and nature of the polymeric liquid crystals.OPMs of the synthesized    for polymers possessing desired mechanical, thermal and optical properties [22][23][24] .Nematic texture was observed in the heating as well the cooling cycles OPM.Hence, these copolyesters are commonly termed liquid crystalline glasses.

CONCLUSION
The Aliphatic-aromatic random Liquid crystalline biodegradable Copolyesters were synthesized from sebacic acid/ succinic acid, Terephthalic acid and 1,4-butanediol by a two-step melt polycondensation, transesterification in the bulk using Titanium tetra butoxide as catalyst at 210 o C under reduced pressure.IR and NMR spectral data were in agreement with the functional group and nature of mesogens present.Thermal methods of analysis (DSC) helped determine the glass transition temperature, melting temperature, and decomposition temperature of the copolyesters.The biodegradation of these polyesters was carried out in alkali medium using the 0.1N NaOH.The results clearly indicated that the length of the polymer chains would be the major controlling factor on the biodegradability of polyesters.The highest degradation rate in alkali medium was observed for PBSeBT, which could be due to its lower crystallinity and melting temperature.Thus, the degradation of polyesters was influenced by the polyester composition, polyester properties such as molecular weight, melting temperature, crystallinity, and the glass transition temperature.Thermotropic liquid crystalline copolyesters synthesized in the present study showed nematic texture.So the synthesized copolyesters are commonly known as liquid crystalline glasses and this kind of polyester can also be used in biomedical field.
NMR is used to study the structure of repeating units and nature of proton present in the polymer chain.The chemical shift values observed from 1H NMR spectra of the copolyesters of PBSBT and PBSeBT are as follows: A singlet at δ = 8.1-8.06 ppm was due to aromatic protons of Terephthalic group, multiplet at δ = 4.02-4.05ppm was due to the protons of -CO-OCH 2 -group, and multiplet at δ = 1.48-1.34ppm was due to methylene protons of 1,4 butane diol (Figs.2a & 2b).