INTRODUCTION:

Thermotropic liquid crystalline (TLC) behaviour of polymeric materials is of considerable current interest, because of their potential applications as high performance materials¹. One of the basic requirements in the synthesis of thermotropic liquid crystalline polymers (TLCPs) is to reduce their melting transition temperature, T_m while still sustaining their liquid crystalline behaviour. Some of the general methods to reduce T_mand to improve solubility are incorporation of flexible spacers between aromatic rings, structural modifications like bends, crankshaft-shaped, kinks etc.,². Adopting random copolymerization is another effective approach in decreasing the T_m value of aromatic polyesters. Recently, attempts have been focused on the synthesis of flexible liquid crystalline-cum-photocrosslinkable polymers (LC-PCPs). These polymers contain photocrosslinkable moieties either in the main chain or as a pendant group in the polymer backbone.

The photocrosslinkability of the polymers is due to the presence of photocrosslinkable moieties like azopolyesters³, arylidene esters⁴, arylidene ketones, siloxanes⁵, cinnamate esters⁶ and acrylate esters⁷. Photocrosslinkable polymers find potential applications in the preparation of photocurable coatings, photocrosslinked hydrogels, optical lithographic materials, photosensitizers and photoresists^8,9. A series of flexible LC-PCPs have been synthesized with arylideneketo moiety^9,10. The unusual liquid crystalline behaviour exhibited by these polymers has also been investigated¹¹.

The present work deals the synthesis and characterization of certain flexible photocrosslinkable polyesters containing arylidene moiety. The twelve new copolyesters were synthesized from dicarboxylic acids such as terephthalic acid, isophthalic acid, 4,4¢-oxybis(benzoic acid) with four arylidenediol monomers and a common monomer diol 2-hydroxyethyl-4-hydroxybenzoate using diphenylchlorophosphate(DPCP) as the condensing agent. The liquid crystalline properties and the photocrosslinking behaviour of these polymers were investigated with respect to the change in the mesogenic units.

MATERIAL AND METHODS:

Materials:

Terephthalic acid (TA) (98%), isophthalic acid(IA) (99%), 4,4¢-oxybis(benzoic acid) (OBBA) (99%), 4-hydroxybenzaldehyde (98%), vanillin (99%), 4-hydroxy benzoic acid (99%) and diphenylchlorophosphate (DPCP)(99%) are purchased from Sigma Aldrich and were used as supplied. Lithium chloride anhydrous (Merck, GR) was dried at 130ºC under vacuum for 4 h and at 180ºC for 10 hours. 2-chloro-1-ethanol (SRL, India), potassium bicarbonate(Merck, India) were used as received without further purification. Cyclohexanone and cyclopentanone purchased from SRL, India were freshly distilled at their boiling points 155ºC and 131ºC respectively. Pyridine (Merck 99% pure) was refluxed over potassium hydroxide and distilled (b.p.115ºC). N,N-Dimethylacetamide (DMAc) (Merck, India) was dried over anhydrous copper sulphate and distilled under reduced pressure (b.p. 65 ºC/10mm).Dimethyl sulphoxide (Merck, India) was dried over anhydrous calcium sulphate overnight and then distilled under reduced pressure. The fraction distilling at 75-76 ºC/12mm Hg was collected (b.p. 75-76 ºC/12mm Hg). N,N-Dimethyl formamide was stirred with freshly dried copper sulphate overnight under reduced pressure. The fraction boiling at 75 ºC/12mm Hg was collected (b.p. 75-76ºC/12mm Hg).

Preparation of monomers:

Preparation of arylidenediols:

Arylidenediols used in the synthesis of the polyesters were prepared by the condensation of respective ketone with aromatic hydroxy aldehyde in the mole ratio 1:2 as reported by Arumugasamy^12,13.(Scheme 1)

Scheme 1:Synthesis of arylidenediols

The synthesis of Bis[4-hydroxy(benzylidene)]cyclopentanone (BP) is described as a representative case. 18.5g (0.15mol) 4-hydroxybenzaldehyde and 6.31g (0.075mol) cyclopentanone were dissolved in 75 ml of dry methanol and maintained in ice cold condition. To this mixture, catalytic amount of con. sulphuric acid was added slowly drop by drop. An exothermic reaction took place and the reaction mixture turned bright yellow, dark green and finally pink. The product was filtered, washed several times with distilled water and dried. The crude monomer was recrystallised twice from methanol to yield yellowish green crystals. Yield was 87%. FT-IR (KBr): 1668 (cyclopentanone C=O), 3295 (OH) and 1597 cm^-1(C=C) ¹H-NMR (DMSO-d₆): δ 10.05(s,2H,OH), 7.53 to 6.87 (m,2H, —CH=, 8H aromatic) 3.01(s,4H, β CH₂cyclopentanone). All the other arylidene diolsbis[4-hydroxy(benzylidene)]cyclohexanone(BH), bis[4-hydroxy (3-methoxybenzylidene)]cyclopentanone,(VP), bis[4-hydroxy(3-methoxybenzylidene)] cyclohexanone (VH)were prepared by a similar procedure and obtained in good yield ranging from 85 to 90%.

Preparation of 2-hydroxyethyl-4-hydroxybenzoate(D):

2-Hydroxyethyl-4-hydroxybenzoate (D) was prepared according to a typical procedure¹⁴ as shown in Scheme 2.A mixture of 4-hydroxybenzoicacid 16.6g(0.12mol), anhydrous potassiumbicarbonate12g(0.12mol), 2-chloro-1-hydroxyethane 9.7g(0.12mol) and 80ml of dry DMSO was heated at 80ºC with stirring. After 5h, the solution was poured in 200ml of cold water saturated with sodium chloride and kept at 5ºC overnight. The product was filtered, washed with cold water and dried. The residue was crystallized from distilled water twice. (m.p. 139ºC). Yield was 61%.¹H-NMR (DMSO-d₆): 10.3δ(OH), 7.8δ(dd,4H aromatic meta to OH) 6.8δ(dd, aromatic ortho to OH), 4.9 δ (t,OH) 4.2 δ (t, COOCH₂), 3.7 δ(t,CH₂OH).

Scheme 2: Preparation of 2-hydroxyethyl-4-hydroxybenzoate(D)

Synthesis of copolyesters:

All the twelve copolyesters were prepared by direct polycondensation of two diols and one diacid in the mole ratio 1:1:2 using DPCP as the condensing agent in pyridine¹⁵.

A typical procedure for the synthesis of random copolyester OBPD is as follows: Diphenylchlorophosphate(13mmol) was added to 1.2912g(5mmol) oxybis(benzoic acid) in pyridine(10mL), stirred for 15 min and treated with a solution of 0.4250g LiCl(10mmol)in 10mL pyridine . The mixture was stirred continuously at room temperature for 30 min. The reaction mixture was slowly heated and maintained at 120ºC for 20 min. To this mixture, 0.73g (2.5mmol) of diol BP in 5ml pyridine and 0.455 g(2.5mmol) of diol D in 5ml pyridine were added drop wise simultaneously over a period of 20 min with constant stirring. The mixture was maintained at 120ºC with continuous stirring for 3hrs. The solution was cooled to room temperature and poured into 500 ml water/methanol (1/1,v/v) and the polymer was precipitated. The product was filtered, washed with hot methanol and dried at 50º C in vacuum.

CHARACTERIZATION:

The inherent viscosities of the copolyesters were determined at a concentration of 0.1 g dL^-1 in dimethylformamide with an Ubbelohde viscometer. The solubility of the polymers was determined using 0.005 g of the polymer in 1 ml of the solvent. Infrared spectra of the polymer samples were recorded by Perkin Elmer Spectrum One FT-IR Spectrometer using KBr pellet from 450cm^-1 to 4500cm^-1. High resolution NMR spectra were recorded on a BrukerBiospin FT-NMR spectrometer operating at 400MHz for ¹H and 100 MHz for ¹³C nucleus in DMSO solvent and with TMS as internal reference. DSC thermograms were recorded with a DSC Q 20 V24.2 Build 107(Universal V4.5A TA Instruments) at the scan rate of 10ºC/min under nitrogen atmosphere. X-ray diffraction measurements on powder samples were carried out using the Rigaku Mini flex II Desk top X-ray diffractometer with a source of Cu K-alpha radiation. The thermotropic liquid crystalline behaviour of the polymer samples was detected on a hot stage Olympus BX50 polarizing microscope. The magnification was 10X and the heating rate was maintained at 10ºC/min. SEM photomicrographs of the polyesters were recorded using Hitachi S-3400 SEM instrument. UV spectral studies was carried out with Elico SL 159 UV spectrophotometer.

RESULTS AND DISCUSSION:

There are a few reports on the synthesis and characterization of LC-PCPs containing arylidene moiety and there arises a need for extensive investigation of their TLC behaviour¹¹. All the copolyesters in the present work were prepared successfully by direct solution polycondensation using diphenylchlorophosphate in pyridine medium. This method avoids the preparation of acid chlorides. All the copolyesters were yellow amorphous solids. The inherent viscosities and the percentage yield were summarized in table 1. The inherent viscosity values ranging from 1.25 to 1.68 dL/g , which showed that the copolyesters have high molecular weight. In a given series, the polyesters with the rigid cyclopentanone ring had higher inherent viscosities than the polyesters with cyclohexanone ring. All the copolyesters were soluble in solvents like chlorophenol, sulphuric acid, dimethylacetamide, dimethylformamide, acetone, dimethylsulphoxide and not soluble in chlorinated solvents like chloroform. The enhanced solubility of the copolyesters in cheaper organic solvents is due to the presence of arylideneketo moiety and a flexible spacer in the D monomer.

SPECTRAL CHARACTERIZATION

The representative FT-IR spectra of polyesters TBHD and OVHD are shown in the Fig.1 and Fig.2. FT-IR(KBr) for TBHD: 3421(overtone of C=O)¹⁶, 2924, 2853(aliphatic), 1734 (ester carbonyl), 1665 (C=O of cyclohexanone ring), 1599 (C=C aromatic) and 1262 to 1015 cm^-1 (C—O—C of ester). FT-IR(KBr) for OVHD: 3374(overtone of C=O), 2930, 2834(aliphatic), 1742,1719 (ester carbonyl), 1690 (C=O of cyclohexanone ring), 1594 (C=C aromatic) and 1246 to 1012 cm^-1 (C—O—C of ester).

The structural units in the polyester were identified by ¹H and ¹³C spectra. The representative ¹H NMR spectra of polyesters TBHD and OBPD recorded in DMSO-d6 are exposed in Fig. 3 and Fig.4. The ¹³C NMR spectra of two typical random copolyesters TBHD and OBPD are shown in Fig.5 and Fig.6 respectively. The assignment of signals in ¹H NMR spectrum of TBHD is given in Fig.3. The protons of cyclohexanone appeared at 1.75 and 2.87 δ. The benzylidene aromatic protons appeared in the region of 6.85 to 7.42 δ. The aromatic protons of terephthaloyl and D monomeric units appeared in the region of 7.63-8.30 δ. A singlet at 4.668δ is assigned to the OCH₂ protons connected to the benzoyl group of common diol D. This shows the incorporation of all the three monomers in the polymer backbone. In the ¹³C NMR spectra of OBPD as shown in Fig.6, the presence of ester carbonyls was indicated by the presence of signals at 159 to 166 ppm in the polymer backbone. The C=O group of the cyclohexanone moiety appeared at 195 ppm. The aromatic carbon atoms are indicated by the signals in the range of 116-136 ppm. The methylenic carbon of cyclopentanoneand methylenic carbons of D monomeric unit appeared at 26 ppm and 63 ppm respectively.

Fig.1: IR spectrum of TBHD

Table 1. Polymer Code, Comonomers, Yield, Inherent Viscosity(η_inh ), and FT-IR data of Random Copolyesters

Polymer code	Monomers	Yield(%)	*η_inh* (dL/g)	FT-IR Data
Polymer code	Monomers	Yield(%)	*η_inh* (dL/g)	CH₂	C=O	C=C	C—O—C
TBHD	TA+D+BH	64	1.31	2924, 2853	1734	1599	1262, 1161, 1070
TBPD	TA+D+BP	61	1.47	2923,2853	1739	1597	1260, 1164, 1071
TVHD	TA+D+VH	63	1.42	2933	1732	1590	1260, 1159, 1067
TVPD	TA+D+VP	60	1.57	2923	1718	1585	1222, 1163, 1031
IBHD	IA+D+BH	63	1.36	2941, 2841	1737	1594	1242, 1162, 1065
IBPD	IA+D+BP	61	1.53	2915	1735	1597	1239, 1167, 1082
IVHD	IA+D+VH	62	1.53	2934, 2835	1746	1581	1254, 1161, 1060
IVPD	IA+D+VP	58	1.74	2936, 2839	1724	1586	1218, 1124, 1060
OBHD	OBBA+D+BH	74	1.25	2943	1739	1595	1247, 1162, 1069
OBPD	OBBA+D+BP	71	1.42	2913	1738	1597	1248, 1165, 1079
OVHD	OBBA+D+VH	68	1.63	2930,2834	1742	1594	1246, 1161, 1067
OVPD	OBBA+D+VP	70	1.68	2923, 2835	1729	1582	1244, 1170, 1038

Fig.2: IR spectrum of OVHD

Fig.3: ¹H NMR Spectrum of TBHD

Fig.4: ¹H NMR Spectrum of OBPD

Fig.5: ¹³C NMR of TBHD

Fig.6: ¹³C NMR Spectrum of OBPD

THERMAL CHARACTERIZATION:

The thermal transition temperatures of the twelve polymers were determined by DSC thermograms(Fig.7, Fig.8), and are listed in Table 2.

Fig.7: DSC curves of polymers containing cyclopentanone ring

Fig.8: DSC of polymers containing cyclohexanonering

It may be pointed out that the transition and melting temperatures of the polyesters in the present investigation were lower than the fully aromatic polyesters. The copolymerization with arylidenediols and D monomer significantly reduced the T_g and T_m values. The phase transition temperatures of the copolyesters are structurally dependent. The polyesters prepared from 4,4¢-oxybis(benzoic acid) have lower glass transition temperature than the polyesters derived from terephthalic acid and isophthalic acid. This may be attributed to the bent and flexible nature of oxybisbenzoyl unit.

Table 2 Phase transition temperatures of the polyesters determined by DSC

Polymers	Differential Scanning Calorimetry (DSC)
Polymers	T_g	T_K→K(º C)	T_m(º C)	T_i(º C)	ΔT
TBHD	62	116	220	261	41
TBPD	74	176	233	308	75
TVHD	56	130	184	213	29
TVPD	69	-	170	215	45
IBHD	70	-	272	322	50
IBPD	71	239	262	311	49
IVHD	-	176	205	239	34
IVPD	70	136	169	202	33
OBHD	52	-	201	261	60
OBPD	61	252	268	300	32
OVHD	-	-	188	236	48
OVPD	-	-	197	216	19

The DSC data also showed that the melting and isotropisation temperatures of polyesters with VP and VH arylidenediols are lower than that of polyesters with BP and BH arylidenediols. This is due to the presence of methoxy substituents in vanillin based polyesters reduce the coplanarity of adjacent mesogenic groups¹⁷. The 4-hydroxybenzaldehyde based polyesters maintain broader liquid crystalline range(ΔT) than vanillin polyesters. This is attributed to the coplanar geometry in the BH and BP based polyesters which would favour a more effective molecular packing. The polyesters having cyclopentanone moiety have higher glass transition and melting temperatures than the polyesters with cyclohexanone moiety. This may be due to the rigidity caused by the presence of cyclopentanone moiety in the polymer backbone. In seven polyesters an endothermic peak was observed before melting temperature. This may be due to crystal to crystal transition resulted from the different crystalline polymorphs¹⁸.

The TLC behaviour was also studied with hot stage optical polarized microscopy at 10X and the representative photographs are shown in Fig. 9. The polyesters exhibited the property of stir opalescence on melting, which indicated the liquid crystallinity. The polyesters exhibited characteristic with high birefringence or grainy texture.

Fig.9: Optical Polarized Micrographs for (a)OBHD at 205 °C (b)OBPD at 288 °C (c) OVHD at 190 °C (d) OVPD at 212 °C

PHOTOCROSSLINKING STUDIES:

The polyesters prepared contain arylidene moiety, which functions as photo-active chromophore which will undergo photocrosslinking by UV irradiation. The photocrosslinking behaviour of the polymers was established by recording UV visible spectra in DMAc solution[0.01g dL^-1] successively after UV irradiation from a 160 W medium pressure mercury lamp at regular time intervals. The changes in the UV spectral pattern on irradiation of a typical polymer TBPD is shown in Fig. 10. The absorbance A_tat 360 nm indicates p®p* transition of the exocyclic double bond of the arylidene mesogenic unit in the polymer backbone. The decrease in absorbance observed with increase in the time of irradiation proved that there is steady rate of photocrosslinking. The photocrosslinking is attributed through 2+2 cycloaddition of carbon-carbon double bond in the arylidene moiety present in the polymer backbone¹⁹.

Fig. 10 UV spectral pattern on UV irradiation of TBPD

The relative reactivity A₀−A_t/A₀of some of the polymers is plotted against the time of irradiation, where A₀ is absorbance before irradiation and A_t is absorbance after irradiation for time t (Fig. 11). It is observed that the polyesters containing cyclopentaone ring were found to be more photoreactive than polyesters containing cyclohexanone ring. The lesser size of cyclopentanone ring provides favourable geometry for cycloaddition reactions.

Fig.11 Relative reactivities of some polymers

The photocrosslinking behaviour was further substantiated by SEM studies. About 2mg of the finely powdered polyester samples were exposed to UV irradiation under 160 W medium pressure mercury lamp in air for 5 h and the SEM photomicrographs before and after UV irradiation were taken and shown in Fig.12. It is evident from these photographs that there is photocrosslinking in the polyester after UV irradiation.

Fig. 12: Scanning Electron Micrographs of (a) TBHD before UV irradiation (b) TBHD after UV irradiation (c) TBPD before irradiation (d) TBPD after irradiation

X RAY DIFFRACTION STUDIES:

X-ray diffraction patterns of some of the polymers are shown in Fig.13. The patterns show 2q values in the range of 5 to 32, which is characteristic of polyesters. X-ray diffractograms indicated that all the polymers show some degree of crystallinity in the amorphous background. This is due to the presence of polar groups like C=O, C=C, which induces some order between adjacent chains of the polymer²⁰. On comparison between the polymers, the 4-hydroxy benzaldehyde based polymers have higher degree of crystallinity than the vanillin based polymers. The presence of methoxy substituent in vanillin based polymers caused some hindrance between repeating units leading to unsymmetrical orientation and reduced the crystallinity.

Fig. 13: X-ray diffraction patterns of some polyesters

CONCLUSION:

Twelve new photocrosslinkable liquid crystalline copolyesters were synthesized successfully by polycondensation with diphenylchlorophosphate and characterized spectroscopically. The polyesters were soluble in commonly available organic solvents due to the presence of arylidene moiety and short length methylenic spacer in D monomer. The high inherent viscosity values established the high molecular weight of the polyesters. The copolyestersexhibited anematic or grainy texture as revealed by HOPM studies. The melting point and isotropization temperature of copolyesters with BH and BP arylidenediols were higher than that of the copolyesters with VH and VP arylidenediols. The copolyesters containing BH and BP units show broad liquid crystalline range. UV spectral and SEM studies established thephotocrosslinking behaviour of polyesters on irradiation by UV light. All the polyesters are crystalline with amorphous nature.

ACKNOWLEDGEMENTS:

The authors are grateful to the University Grants Commission, New Delhi for the financial assistance. Two of the authors S. Vasanthi and J. Arul Moli thank UGC, New Delhi for the award of Teacher Fellowships. We gratefully acknowledge Central Leather Research Institute, Chennai, for providing OPM facilities.

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Received on 14.06.2011 Modified on 23.06.2011

Asian J. Research Chem. 4(8): August, 2011; Page 1343-1349