SYNTHESIS AND CHARACTERIZATION OF CERTAIN PHOTOCROSSLINKABLE RANDOMCOPOLYESTERS FROM

4,4′-OXYBIS(BENZOIC ACID) N. Malathy1 , D. Roop Singh2

1. Teacher Research Fellow under University Grants Commission– Faculty Development Programme.

2. Post – Graduate & Research Department of Chemistry, Presidency College, Chennai, Tamil Nadu – 600 005.

Abstract A series of new thermotropic liquid crystalline random copolyesters with ethereal linkages were prepared from a potential mesogenic 4,4′-oxybis(benzoic acid) by direct polycondensation with different aliphatic and aromatic diols in pyridine solution. Diphenylchlorophosphate (DPCP) was employed as the condensation agent. Viscosity measurements, FT IR, 1H and 13C NMR spectral data were used for investigating their structural features. The thermal phase transitions and thermotropic liquid crystalline (TLC) behaviour of these polymers were investigated by differential scanning calorimetry (DSC) and optical polarizing microscopy (OPM). Scanning electron microscopy (SEM) coupled with UV irradiation experiments were used to establish the photocrosslinkability of these polyesters. Interestingly, these copolyesters containing arylidene keto moiety in the main chain exhibited nematic mesophase and were photocrosslinked upon UV irradiation. Key words : Thermotropic liquid crystalline polyesters, photocrosslinking. INTRODUCTION Thermotropic liquid crystalline polymers (TLCP) continue to draw attention primarily due to their wide range of applications from ultrahigh strength fibres to nonlinear optical (NLO) devices [1-3]. Polyesters synthesized from 4,4′-OBBA have been reported to possess intriguing anisotropic, mechanical, electrical and optical properties suitable for technological applications [4]. They are widely used in information storage devices, non-linear optics and UV shielding materials. Photocrosslinkable polymers have attracted considerable attention for their applications in surface coatings, printing inks, printing plates, photo-resists due to their faster curing, high thermal stability and superior chemical resistance [5]. Extensive work has been carried out in recent years on the synthesis and characterization of photocrosslinkable polymers having photocrosslinkable moiety in the main chain or as a pendant group in the polymer backbone [6]. Photocrosslinkable polymers are potential candidates for anisotropic network systems such as elastomers and thermosets. In this paper, the preparation and characterization of four random copolyesters by direct polycondensation of nonlinear mesogen 4,4′ -OBBA with certain aliphaticdiols and arylidenediols is reported. All of these random copolyesters were characterized with a variety of experimental techniques including viscosity measurements, qualitative solubility tests, spectral studies (FTIR, 1HNMR, 13CNMR), optical polarizing microscopy (OPM) and studies on photocrosslinking behaviour by UV irradiation experiments. The morphology of the polymer was investigated by scanning electron microscopy (SEM). EXPERIMENTAL Materials

Pyridine (Merck, 99% pure) used as polymerization medium, was refluxed over potassiumhydroxide and distilled (b.p : 1150C). Lithium chloride anhydyous (Aldrich, Analar) was dried in vacuum. Methanol (b.p. 650C) was purified by refluxing over

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quicklime and distilled before use. Vanillin (Aldrich,Analar), p-hydroxybenzaldehyde(Aldrich), cyclohexanone(Aldrich) cyclopentanone(Aldrich) were used as such in the synthesis of arylidenediols.

4,4′–oxybis(benzoicacid), 1,4–butanediol, 1,3-propanediol, 1,5 - pentanediol and diphenylchlorophosphate were purchased from Aldrich Chemicals and used without further purification.

Preparation of arylidenediols Arylidene diols used in the synthesis of the polyesters were prepared by the condensation of the respective ketone with aromatic hydroxyl aldehydes in the mole ratio 1:2. Bis(4-hydroxybenzylidene)cycloalkanones and bis(4-hydroxy3-methoxybenzylidene)cycloalkanones were synthesized by the method reported by Arumugasamy [7, 8]. Synthesis of copolysters All the four copolyesters were prepared by direct polycondensation of two diols and one diacid in the respective mole ratio 1:1:2 in pyridine solution using diphenylchlorophosphate (DPCP) as condensation agent[9,10]. This method gives the polyesters in high yield and avoids the tedious preparation of acidchlorides. CHARACTERIZATION The inherent viscosity of the copolyesters was determined in N,N-dimethylacetamide (DMAc)solution at 30°C using an Ubbelhode viscometer. The solubility of these polyesters was tested in various solvents qualitatively. The polyesters reported here were soluble in solvents such as DMSO,DMAc,DMF,THF and partially soluble in n-butanol,chloroform and ethylacetate.

Regarding spectral characterization of the copolyesters, IR spectra were recorded using Nicolet 510 FTIR analyzer with their neat films in KBr pellets. The 1 H NMR and 13C NMR spectra were recorded with JEOL GSX – 400 MHz instrument in DMSO-d6/CDCl3 solvent with TMS as internal reference. In thermal analysis, DSC thermograms were recorded in Dupont 2910 differential scanning calorimeter using 5mg samples under nitrogen atmosphere at a heating rate of 10°C/min. The thermotropic liquid crystalline behaviour of these copolyesters was detected in Olympus polarizing microscope equipped with a pair of crossed polarizers and Mettler hot stage. The morphology of the sample was investigated using Hitachi S-3000 Hz scanning electron microscope. The rate of photocrosslinking was measured in DMAc solution with UV – visible spectrophotometer Elico SL-159 by UV irradiation from a 160W medium pressure mercury lamp at different time intervals.

RESULTS AND DISCUSSION All the four copolyesters synthesized from 4,4′ -oxybisbenzoic acid are more soluble. The presence of ether group in the main chain of polyester enhances the solubility in organic solvents and hence facilitates processing [11]. The percentage yield of the polyesters and their inherent viscosity values determined in N,N-dimethylacetamide (DMAc) solutions at 303K are given in Table 1. It is observed that ηinh values of the polymers derived using aliphatic diols are lower than that of those derived from arylidene diols. This may be attributed to the combined effect of higher molecular weight and the rigidity of arylidene

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diols. It is possible that the presence of oxybis group in the main chain and spacer decreased the inherent viscosity values.

Table 1. Polymer code and properties of random copolyesters

S.No Common Diol Varying Diol Diacid Polymer Code Yield% ηinh dL/ Tg°C

1 1,4-BD 1,3-PR 4,4′-OBBA EBPR 60 0.42 55.1

2 1,4-BD 1,5-PE 4,4′-OBBA EBPE 62 0.51 57.9

3 1,4-BD BHCH 4,4′-OBBA EBBH 82 0.60 60.2

4 1,4-BD BVCP 4,4′-OBBA EBVP 77 0.69 62.4 1,4 − BD : 1,4 – Butane diol 1, 3 – PR : 1, 3 – Propane diol 1,5 – PE : 1,5 – Pentane diol BHCH : Bis(4-hydroxybenzylidene) cyclohexanone BVCP : Bis(4-hydroxy 3-methoxybenzylidene) cyclopentanone 4,4′-OBBA : 4,4′-oxybis(benzoic acid) ηinh dL/g - inherent viscosity, Tg°C - glass transition temperature SPECTRAL CHARACTERIZATION The IR spectra presented in figures1(a) and 1(b) of random copolyesters showed characteristic absorptions at 1710 – 1750 cm-1 due to the ester C = O stretching and 1200 – 1300 cm-1 due to the ester C – O stretching, indicating that the linkage is present in all the polymers synthesized. It may be noted that the absorption at ν =1630 – 1670 cm-1 present in EBBH and EBVP indicates that arylidene keto moiety is incorporated into the polymer chain.

Fig.1 (a).Infrared spectrum of random copolyester EBPR

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Fig.1 (b).Infrared spectrum of random copolyester EBVP

The 1H NMR spectra of copolyesters presented in figures 2(a) and 2(b) indicate that the signals at δ 4.4 – 4.5 ppm show the presence of terminal methylene protons of 1, 4 – butanediol.

Fig.2 (a). 1H NMR spectra of random copolyester EBPR

Signal at δ 1.2 – 1.9 ppm indicates the central – CH2 protons of 1, 4 – butanediol and dicarboxylic acid. The aromatic protons of dicarboxylic acid show absorption at about δ 7.0 – 8.2 ppm. Since oxybisphenylene group is a bent one and due to delocalization of lone pair of electrons on ether oxygen, the protons are shielded and absorbed at signal at δ 1.2 – 1.9 ppm indicates the central – CH2 protons of 1, 4 – butanediol and dicarboxylic acid.

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Fig.2 (b). 1H NMR spectra of random copolyester EBBH

signal at δ 1.2 – 1.9 ppm indicates the central – CH2 protons of 1, 4 – butanediol and dicarboxylic acid. The aromatic protons of dicarboxylic acid show absorption at about δ 7.0 – 8.2 ppm. Since oxybisphenylene group is a bent one and due to delocalization of lone pair of electrons on ether oxygen, the protons are shielded and absorbed at upfield. The microstructure of the repeat units in the polymer chain can be identified satisfactorily using 13C NMR spectra presented in figures 3(a) and 3(b) though the 13C NMR spectra of the polymer is complex. The signals at δ =160 - 170 ppm in the 13C NMR spectra of EBBH indicated the carbonyl carbon of the ester group as well as the arylidene keto moiety. The aromatic carbon atoms are indicated by the signals at δ =110 – 140 ppm. Thus the proton decoupled 13C spectrum of the polymer synthesized in the present investigation indicates that the polymer chain contains ester group.The olefinic carbon atoms of the arylidene group absorbed at 142ppm. The methoxy carbon atom of the the vanillin group are indicated by the absorption at 75-80ppm. The copolymerization effect of these polyesters was attributed to their random placements along the polyester chain, which was also verified with 13C NMR spectroscopy

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Fig.3 (a). 13C NMR spectra of random copolyester EBPR

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Fig.3 (b). 13C NMR spectra of random copolyester EBBH

The UV – visible spectrum of the random copolyester EBPR has absorption maximum at 281nm,while the random copolyester EBBH and EBVP has two absorption maxima. The π-π* transition of aromatic rings can be distinguished from the π-π* transition of arylidene moiety by UV-visible spectra for EBBH and EBVP. The π-π* transition in aromatic group is indicated by the absorption at 270nm while λmax at about 330nm for EBBH and 403 nm for EBVP is due to π-π* transition in arylidene group.

THERMAL CHARACTERIZATION The thermal transition temperatures of the random coplysesters were determined from DSC thermograms (figure 4) and are listed in Table 1.

Fig.4.DSC thermograms of random copolyesters

It is observed that the Tg of the polyesters obtained using arylidene diol is higher than that of polymer derived using aliphatic diols. This may be due to the rigidity of arylidene moiety present in the polymer chain. It is also observed that the Tg value of vanillin based polyester EBVP is higher than that of EBBH which could be due to interlocking effect of the methoxy substituent present in the arylidene keto moiety. There are reports on such interlocking effects on the thermal properties of polymers by Lenz and coworkers [12]. They suggested that the interlocking effect depends on the size of the substituent. This was further supported by reported work of Kannappan and coworkers

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[13] by ultrasonic method. The magnified optical polarizing micrograph of the random copolyester EBBH is given in figure 5(b) which was taken on heating the sample at 203°C at the uniform rate of 4°C per minute.

Fig.5(a). Polarised Optical Micrograph of EBPR at 115°C on heating (X100)

Fig.5(b). Polarised Optical Micrograph of EBBH at 203°C on heating (X100 )

Marbled texture seen in this micrograph showed that this polyester exhibited nematic mesophase. The formation of mesophase is a reversible phenomenon as similar crystals could be seen for EBBH at 213°C in the micrograph on cooling the melt at uniform rate from 257°C [figure 5 (c)].

Fig.5(c).Polarised Optical Micrograph of EBBH at 213°C on cooling (X100)

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Fig.5(d). Polarised Optical Micrograph of EBVP at 193°C on heating (X100 )

The OPM studies also indicate that the copolymer EBVP exhibited liquid crystalline mesophase at 193°C [figure 5 (d)]. It formed a typical nematic appearance with Schlieren texture. Oxybiphenylene group is a non linear and flexible group which may be the reason for their nematic behaviour. Previous workers have identified nematic phase in simple polyesters containing arylidene keto moiety [14]. It was also observed that the Tm value of the polymer EBPR and EBPE is lower than that of EBBH and EBVP. Moreover the LC phase of EBPR [figure 5 (a)] and EBPE is also less pronounced when compared to EBBH and EBVP.

PHOTOCROSSLINKING STUDIES Polymers with arylideneketo moiety in the main or side chain are observed to be crosslinked by UV irradiation. These moieties in the polymers function as mesogens as well as photoactive centres. Studies reveal two kinds of photoreactions that proceed in these polymer system, namely photoisomerization and photocrosslinking [14]. The favourability of these two reactions depends on the mobility of the polymer chains. Studies were made on the kinetics of photocrosslinking of four random copolyesters EBPR, EBPE, EBBH and EBVP by UV spectral analysis. The rate of photocrosslinking was measured in DMAc solution of concentration 0.01g dL-1 after UV irradiation from a 160 W medium pressure mercury lamp at different time intervals. The absorbance (At) decreased with time of irradiation indicating that there is steady rate of photocrosslinking in these polyesters. Figures 6(a), 6(b) and 6(c) represent the UV spectra of EBPR, EBBH and EBVP respectively.

Fig.6 (a).UV-visible spectra of EBPR on UV irradiation at different time intervals (min )

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Fig.6 (b).UV-visible spectra of EBBH on UV irradiation at different time intervals (min )

Fig.6 (c).UV-visible spectra of EBVP on UV irradiation at different time intervals (min )

The rate of crosslinking of EBPR and EBPE is lower than that of EBBH and EBVP (figure 7).

Fig.7. Relative change in the UV spectral intensity with photolysis time for the random copolyester EBPR,EBBH and EBVP

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The higher rate of crosslinking in polymers EBBH and EBVP may be because of the olefinic bond of the arylidene keto moiety. Between EBBH and EBVP, EBBH has higher rate of crosslinking than EBVP, which may be because of the methoxy substituent present in the polymer EBVP which physically hinders the polymer chain aligning closer to each other and renders the photocrosslinking difficult. There are reports on such hindering effect created by methoxy substituent on the process of photocrosslinking of copolyesters by Kannappan and coworkers [15].

Fig.8.(a) Photocrosslinking of EBVP on UV irradiation.

Plot of log(Ao/At) against time is linear [ Fig 8(a), 8(b) and8(c) ] which shows that the photocrosslinking follows first order kinetics with a rate constant of 1.31X10-4s-1 (EBVP), 1.036X10-4s-1 (EBPR) and 9.53X10-5s-1 (EBBH).

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Fig.8.(b) and Fig.8.(c) .Photocrosslinking of EBPR and EBBH on UV irradiation.

The photocrosslinking behaviour investigated by SEM studies are presented in figures 9 (a) and 9 (b) taken before and after UV irradiation for EBBH. It is evident from these photographs that UV irradiation causes photocrosslinking in the polyester EBBH .

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Fig.9 (a). SEM Photomicrograph of random copolyester EBBH before UV irradiation.

Fig.9 (b). SEM Photomicrograph of random copolyester EBBH after UVirradiation.

CONCLUSION

All the four random copolyesters contain structurally different repeating unit and the main aim of the investigation is to study the influence of the microstructure on thermal properties and photocrosslinking of these polymers. The random copolyester which contain arylidene keto moiety, can function both as a mesogen and photo-active chromophore. These polyesters with photo-active chromophores can be crosslinked by UV irradiation, which was eatabilished by SEM investigation. Oxybiphenylene group is a bent and flexible group and this may be the reason for the nematic behaviour of the polyesters derived from 4,4’-oxybis(benzoic acid).

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ACKNOWLEDGEMENT The authors are thankful to the University Grants Commission of India for permitting and supporting research by Faculty Development Programme.

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