effect of molecular architecture and size of mesogen on phase behavior and photoactive properties of...

Effect of Molecular Architecture and Size of Mesogen onPhase Behavior and Photoactive Properties of PhotoactiveLiquid Crystalline Hyperbranched Polyester EpoxiesContaining Benzylidene Moiety

V. SRINIVASA RAO, A. B. SAMUI

Polymer Division, Naval Materials Research Laboratory, Ambernath-E, Thane–Dist, Mumbai-421506, India

Received 18 August 2007; accepted 21 September 2007DOI: 10.1002/pola.22405Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A series of photoactive liquid crystalline linear and hyperbranched poly-ester epoxies were synthesized by polyaddition of photoactive bis benzylidene alka-none diol monomers and terephthalic acid and trimesic acid respectively with goodyield. The effect of molecular architecture (linear and hyperbranched), size of meso-genic unit (cyclic and acyclic units) on the physicochemical, thermal, mesogenic, andphotoactive properties of hyperbranched polymers were studied and compared.Degree of branching of hyperbranched polymers was found to be in the range of0.46–0.49. Monomers containing cyclic moieties only exhibited nematic mesophase,while all polymers exhibited typical nematic mesophase. Intermolecular photo cyclo-addition reaction was studied by ultraviolet–visible spectra (UV–vis) and NMR spec-troscopy and photo viscosity measurement of UV irradiated polymer solutions. Fasterphoto induced behavior of hyperbranched polymers containing acyclic alkanone moi-ety, as compared to polymers containing cycloalkanone moieties, was observed. Thechange in the refractive index was found to be in the range of 0.02–0.024. Substantialvariation of refractive index indicates that this polymer could be used for optical re-cording. All the polymers were also found to be fluorescent in nature. VVC 2007 Wiley

Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 552–563, 2008

Keywords: benzylidene polymers; hyperbranched polymers; liquid crystalline epoxy;liquid-crystalline polyesters; liquid-crystalline polymers; photoactive polymers; photo-reactive effects; polyesters; refractive index; synthesis

INTRODUCTION

In recent years, dendrimers with different func-tionalities gained attention of both researcherand industrialist because of their physicochemi-cal properties, such as high solubility, low solu-tion, and melt viscosity and absence of entangle-ments, high density of peripheral functionalities,

symmetrical shape, and monodispersity.1–4

Although known to posses a less ordered struc-ture, hyperbranched polymers have many simi-larities in structure and properties with den-drimers and can usually be synthesized by sim-ple, more economical, and less time consumingone-step polymerization.5–8

In this domain, photoaddressable dendriticliquid crystalline polymers (PADLCPs) form anew type LCPs with physical properties fallingbetween macromolecular compounds and low-molar-mass liquid crystals. They usually com-bine low viscosity, rapid response, and strong

Correspondence to: A. B. Samui (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 552–563 (2008)VVC 2007 Wiley Periodicals, Inc.

552

anisotropic optical properties even with weakexternal fields (optical, electric and magneticfields) similar to low-molar-mass liquid crystalsand easy processability, stability of the storedinformation, and possible data recording on thethin polymer films similar to macromolecularcompounds.9–12 The photochromic event can bringconfigurational and conformational changes inmolecular level which leads to structural changesin the macroscopic level accompanied by changesin other properties,13–17 such as refractive index,birefringence, etc. The LCPs offer an additionaladvantage that the induced variations can befrozen in a glassy state.

Frechet et al. reported the preparation ofhyperbranched polyethers18–21 and hyper-branched polyester with epoxide ends and inter-nal hydroxyl functionalities, from ABn and A2 þB3 monomers by proton transfer polymerization.Very recently, Nishikubo et al. synthesizedhyperbranched epoxy polyesters from 1, 3, 5,benzene tricarboxylic acid and diglycidyl ether ofbisphenol-A by A2 þ B3 synthetic approach.22

After preliminary work by Kim23 on liquid crys-talline hyperbranched polymer (HBLCPs), syn-thesized by ABn approach, the revolutionarywork in this field was carried out by Percec etal.,24–32 Shibaev et al.,11,12,33 and others.17–20

Shibaev’s group11 reported the first photorespon-sive carbosilane dendrimers of various genera-tions bearing liquid-crystalline cinnamoyl groupsat the periphery. Their study revealed that thecinnamoyl molecules underwent two types ofphotoprocesses, E–Z isomerization and [2þ2]photocycloaddition, leading to the formation ofthree-dimensional networks. Wang et al.34 re-ported the photoresponsive behavior of cinnamoyl-shell-modified poly (amido amine) dendrimers,which dimerized under ultraviolet (UV) light.

Mostly, photoactive LCP are synthesized bycopolymerization of mesogenic and photochromicmonomers. In this approach there is a possibi-lity of inhomoginity even after careful control ofconcentration and selection of monomers withsuitable reactivity ratios. Further it is needed tochoose longer spacer between main chain andchromophore to incorporate more concentrationof photochromic groups without disruption ofmesophase.33 These drawbacks can be overcomeby choosing a moiety which can act as bothmesogen and chromophore. There is only limitednumber of reports on such type of photorespon-sive LCPs; among those azobenzene and Bisbenzylidene cycloalkanone based polymers are

the best examples.35,36 In general, the photoiso-merization of the E/Z configuration of the azo-benzene group causes photochromic variationsin the visible range, leading to color changes inthe polymer materials. However, the photoin-duced Z-form azobenzene gradually returns tothe E-form even at room temperature.37–41 Toovercome the defects of thermal instability ofazobenzene, photochromic compounds with C¼¼Csegments were synthesized.42–44 Bis benzylidenecycloalkanone, a notable mesogenic and photoac-tive molecule, was reported in wide range ofpolymeric backbone. Borden et al.45 introducedthis moiety into the polymer back bone andlater other groups Gangadhara and Kishore,46

Kannan and coworkers,47–49 and Aly et al.50

studied this moiety well in polymer backboneand side chain. Our group introduced this photo-active mesogen into hyperbranched architectureand compared their properties with correspond-ing linear analogues.36,51,52 A first review onsynthesis and application of polymers having bisbenzylidene moiety was compiled by ourresearch group.53 Here we report the effect ofmolecular architecture (linear and hyper-branched), structure (cyclic and acyclic nature)of bis benzylidene alkanone units on the phasebehavior and photo responsive properties. To thebest of our knowledge this is the first report onphotoactive liquid crystalline hyperbranchedpolyester containing cyclic and acyclic bis ben-zylidene alkanone units.

EXPERIMENTAL

Materials

Trimesic acid (1, 3, 5-benzene tri carboxylic acid;TMA; Acros organics), Terephthalic acid (Acrosorganics), 4-hydroxybenzaldehyde (98%; Fluka,Switzerland), tetra-n-butyl ammonium chloride(TBAC, Lancaster), and sodium hydroxide (99%,SD Fine chemicals, India) were used withoutfurther purification. Acetone, cyclohexanone andepichlorohydrin, dimethylformamide (DMF), anddimethyl sulfoxide (DMSO) (Sd. Fine Chemicals,India) were dried with calcium hydride (AR)(Sd. Fine Chemicals, India) and purified by dis-tillation before use as reported.54

Techniques

FTIR spectra were recorded on Perkin–Elmer1600 series Fourier Transform infrared spectro-

PHOTOACTIVE LIQUID CRYSTALLINE POLYESTER EPOXIES 553

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

photometer using KBr pellets. UV–visible spec-tra (vis) were taken on a Cary 500 Scan UV–vis-NIR spectrophotometer. 1H and 13C NMR spec-tra were recorded on a 500 MHz Bruker-FTNMR spectrometer using DMSO-d6 as solvent.Chemical shifts were measured using tetramethyl silane as an internal standard (TMS).Thermo gravimetric analyzer (TGA) (TA instru-ments His Res TGA 2950) was used for thermalanalysis with a heating rate of 20 8C /min in N2

atmosphere. Differential scanning calorimeter(DSC) (TA instruments) was used to determinethe thermal transitions. Heating/cooling rateused for all DSC analysis was 5 8C/ min. A LeicaDMLD, optical polarizing microscope with imageanalyzer equipped with LINKAM TMS 94 hotstage and LINKAM LNP controlling unit wasused to observe the thermal transition and liq-uid crystalline state. Refractive indices of poly-mer thin films were measured using FilmetricsF20, thin-film analyzer. Inherent viscositieswere measured with an Ubbelohde Viscometerat 30 8C (0.5 g/dL) in DMSO. Photolysis of thepolymer was carried out in DMSO solution at30 8C. The sample was irradiated with mediumpressure 100 W mercury lamp by discontinuousmode from a distance of 10 cm for various timeintervals. Irradiated solution was subjected tospectral analysis, viscosity measurements, andfluorescence analysis. Fluorescence spectra wererecorded on Perkin–Elmer luminescence spectro-photometer.

Synthesis

Synthesis of Monomers

Synthesis of Photoactive Diol. Synthesis andcharacterizations of photoactive bis benzylidenediols {1,3-bis (4-hydroxybenzylidene)-acetoneand 2,6-bis (4-hydroxybenzylidene)-cyclohexa-none} were described in our earlier communica-tion.36,55 In a typical recipe, a mixture of 24.4 g(0.2 mol) of 4-hydroxybenzaldehyde, 12.35 g (0.2mol) of boric acid and 100 mL of concentratedhydrochloric acid were charged into a thor-oughly dried 500 mL 3-necked flask equippedwith mechanical stirrer and condenser. Theflask was cooled to 0 8C and 5.81 g (0.1 mol) ofacetone was added for 1 h with vigorous stir-ring. After completion of the addition, the reac-tion was continued for 24 h at room temperatureand poured into 2 L of chilled water. The pre-cipitated yellow product was filtered, dried, and

recrystallized from a methanol/water mixture(1:1 v/v). The product was dried at 50 8C in avacuum oven for 24 h. A single product spot wasconfirmed by thin-layer chromatography withethyl acetate and hexane (3:1 v/v) as an eluent.The product obtained was 1, 3-Bis (4-hydroxy-benzylidene)-acetone (BA).

Yield: 75%. mp: 245–246 8C.ELEM. ANAL. (C17H1403) (266.28): Calcd C,

76.67%; H, 5.29%. Found: C, 75.90%; H, 5.46%.FTIR (KBr, cm�1): 3256 (��OH), 1642

(C¼¼O, ketone), 1596 (C¼¼C, benzylidene), 1568,1508 cm�1 (aromatic).

1H NMR (DMSO-d6, d ppm): d ¼ 10.04 (s,2H, Ph��OH), 7.66 (d, J ¼ 15.5 Hz, 2H,¼¼CH��), 7.63 (d, J ¼ 8.0 Hz; 4H; aromatic),7.11 (d, J ¼ 16.0 Hz, 2H; Ph��CH¼¼), 6.84(d, J¼ 8.0 Hz; 4H; aromatic).

13C NMR (DMSO-d6, d ppm): d ¼ 188.61(C¼¼O), 160.58, 130.67, 128.48, 124.26 (aromaticring carbon), 142.30, (��CH¼¼, a to C¼¼O),115.49 (��CH¼¼).

2, 6-Bis (4-hydroxybenzylidene)-cyclohexanone(BCH) BCH was synthesized by following thesame procedure as in the case of BA using cyclo-hexanone in place of acetone.

Yield: 75%. mp: 292–295 8C.ELEM. ANAL. (C20H180 3) (306.34): Calcd C,

78.41%; H, 5.92%. Found: C, 77.90%; H, 5.90%.FTIR (KBr, cm�1): 3256 (��OH), 1646

(C¼¼O, ketone), 1582 (C¼¼C, benzylidene),1568,1508 cm�1 (aromatic).

1H NMR (DMSO-d6, d ppm): d ¼ 9.95 (s,2H, Ph��OH), 7.54 (s, 2H, ��CH¼¼), 7.42 (d, J ¼8.7 Hz; 4H; aromatic), 7.39 (d, J ¼ 8.7 Hz, 4H;aromatic), 2.86 (t, 4H, b to C¼¼O), 1.72 (m, 2H, cto C¼¼O).

13C NMR (DMSO-d6, d ppm): d ¼ 188.49(C¼¼O), 158.34, 133.30, 132.48, 126.47 (aromaticring carbon), 135.82 (a to C¼¼O), 115.54(��CH¼¼), 27.99 (b to C¼¼O), 22.55 (c to C¼¼O).

Synthesis of Photoactive Diepoxides

Synthesis and characterizations of photoactivebis benzylidene diepoxides {1,3-bis (4-epoxypro-poxybenzylidene)-acetone (BAEP) and 2,6-bis (4-epoxypropoxybenzylidene)-cyclohexanone} weredescribed in our earlier communication.55,56 In atypical recipe, mixture of 0.02 mol (5.32 g) ofBA, 0.6 mol (57.50 g) of epichlorohydrin and0.0006 mol (0.17 g) of TBAC, were charged intoa thoroughly dried 500-mL 3-necked flaskequipped with stirrer, condenser, and nitrogen

554 RAO AND SAMUI


purge. While stirring, the mixture was heated toreflux for 60 min. Then, 1.64 g of NaOH (15%aqueous solution) was added drop wise and heldat reflux for 30 min. Stirring was continued fur-ther for 90 min. The volatile compounds wereevaporated at 60 8C and the residue was pouredinto large amount of distilled water. The precipi-tated monomer was filtered, washed with dis-tilled water and dried at 50 8C in a vacuumoven for 24 h. Further, recrystallisation fromchloroform yielded yellow crystals. Single pro-duct spot was confirmed by thin-layer chroma-tography using a mixture of chloroform andn-heptane (5:1 v/v) as eluant. The product ob-tained was BAEP.

Yield: 72%. mp: 127–128 8C.ELEM. ANAL. (C23H2205)1 (378.40): Calcd C,

72.99%; H, 5.86%. Found: C, 73.18; H, 5.95.FTIR (KBr, cm�1): 1658 (C¼¼O, ketone),

1598 (C¼¼C, benzylidene), 1564, 1508 (aromatic),1027(C��O��C), 913 cm�1 (epoxy).

1H NMR (DMSO-d6, d ppm): d ¼ 7.73 (d, J ¼8.5 Hz, 4H, aromatic), 7.70 (d, J ¼ 15.5 Hz; 2H;¼¼CH��), 7.18 (d, J ¼ 16.0 Hz, 2H; Ph��CH¼¼),7.03(d, J ¼ 8.5 Hz; 4H; aromatic), 4.40 (dd, 2H,CH2 of glycidyl unit), 3.88(dd, 2H, CH2 of glycidylunit), 3.35 (m, 2H, CH of epoxy), 2.84 (m, 6H,CH2 of epoxy), 2.70 (dd, 2H, CH2 of epoxy).

13C NMR (DMSO-d6, d ppm): d ¼ 188.61(C¼¼O), 160.58, 130.67, 128.19, 124.26 (aromaticring carbon), 142.46 (��CH¼¼, a to C¼¼O), 115.49(��CH¼¼), 69.51, 50.03, 44.18 (CH2 of glycidylunit, CH and CH2 of epoxy ring).

2,6-bis (4-epoxypropoxybenzylidene)-cyclohexa-none (BCHEP).

BCHEP was synthesized by following thesame procedure as in the case of BAEP usingBCH in place of BA.

Yield: 72%. mp: 125–129 8C.ELEM. ANAL. (C26H2605)1 (418.46): Calcd C,

74.62%; H, 6.26%. Found: C, 73.78; H, 6.07.FTIR (KBr, cm�1): 1658 (C¼¼O, ketone),

1598 (C¼¼C, benzylidene), 1564, 1508 (aromatic),1027(C��O��C), 913 cm�1 (epoxy).

1HNMR (DMSO-d6, d ppm): d ¼ 7.58 (s, 2H,��HC¼¼), 7.52 (d, J ¼ 9.1 Hz, 4H; aromatic),7.04 (d, J ¼ 8.5 Hz, 4H; aromatic), 4.40 (dd, 2H,CH2 of glycidyl unit), 3.88(dd, 2H, CH2 of gly-cidyl unit), 3.36 (m, 2H, CH of epoxy), 2.88 (m,6H, CH2of epoxy coupled with ��CH2 b toC¼¼O), 2.74 (dd, 2H, CH2 of epoxy), 1.72 (s, 2H,c to C¼¼O).

13CNMR (DMSO-d6, d ppm): d ¼ 188.61(C¼¼O, ketone), 158.69, 134.36, 132.29, 128.26

(aromatic ring), 135.39 (a to C¼¼O), 114.66(��CH¼¼), 69.07, 49.60, 43.78 (CH2 of glycidylunit, CH and CH2 of epoxy ring), 27.92 (b toC¼¼O), 22.47 (c to C¼¼O).

Synthesis of Photoactive Linear andHyperbranched Polymers

Synthesis of Photoactive Liquid Crystalline Lin-ear Polyester Epoxy Containing BenzylideneMoiety. Photoactive liquid crystalline linearpolyester epoxies with pendant hydroxyl groupswere synthesized by polyaddition of photoactivebis benzylidene alkanone biepoxy monomer(BAEP and BCHEP) with terephthalic acid (ace-tone based polymer abbreviated as LAPE andcyclohexanone based polymer abbreviated asLCHPE) using the method reported by Nishi-kubo and coworkers.57 In a typical recipe, mix-ture of 2.30 g (0.0061mol) BAEP, 0.998 g (0.0060mol) terephthalic acid, 0.085 g of TBAC (5 mol%), and 60 mL of DMF were charged in to a100 mL thoroughly dried three-necked flaskequipped with stirrer, condenser, and N2 inlet.The mixture was stirred at 110 8C for a periodof 24 h. The resulting mixture was poured in toexcess amount of chilled water. Precipitatedproduct was filtered and reprecipitated threetimes, dried at 50 8C in vacuum for 16 h (yield77%) [Scheme 1(a)].

LAPEYield: 78%.FTIR (KBr, cm�1): 3420 (��OH), 1717

(C¼¼O, ester), 1641 (C¼¼O, ketone), 1597 (exocy-clic C¼¼C), 1569, 1509 (aromatic), 1035(C��O��C), 915 (epoxy).

1H NMR (DMSO-d6, d ppm): d ¼ 8.11 (m,4H, aromatic terephthalic acid unit), 7.74 (m,6H, ��CH¼¼ coupled with aromatic protons ofdiepoxy unit), 7.21 (d, J ¼ 16.00 Hz, 2H;Ph��CH¼¼), 7.05 (d, J ¼ 7.5 Hz, 4H; aromaticprotons of diepoxy unit), 5.55 (br s, 2H, OH),4.50–4.00 (m, 12H, ��O��CH2��CH(OH)��CH2��O�� and CH2 of glycidyl unit), 3.90 (dd,4H, CH2 of glycidyl unit), 3.46 (d, 2H, CH, epoxyring), 2.80 (s, 2H, CH2 of epoxy ring), 2.70 (s,2H, CH2 of epoxy ring).

13CNMR (DMSO-d6, d ppm): d ¼ 188.58(C¼¼O; ketone), 165.43 (C¼¼O, ester), 160.84,129.46,127.98, 124.15 (aromatic ring carbons ofphotoactive diepoxy), 134.60, 134.07, 131.29,130.68 (aromatic ring carbons of terepthalic-acid), 143.11 (carbon a to C¼¼O), 115.46 (��CH¼¼),69.77, 67.17 (O��CH2��CH(OH)��CH2��O��),



Scheme 1. Reaction scheme for synthesis of linear and hyperbranched polymer.(b) Structure of hyperbranched polymer.

556 RAO AND SAMUI


68.98, 50.05, 44.19 (CH2 of glycidyl units andCH and CH2 of epoxy ring).

LCHPEYield: 72%.FTIR (KBr, cm�1): 3420 (��OH), 2982 (ali-

phatic), 1718 (C¼¼O, ester), 1654 (C¼¼O, ketone),1596 (C¼¼C), 1568, 1509 (aromatic) 1034(C��O��C), 915 cm�1 (epoxy).

1HNMR (DMSO-d6, d ppm): d ¼ 8.14 (s, 4H,aromatic terephthalic acid unit), 7.60 (s, 2H,��CH¼¼), 7.50 (d, 9.00 Hz, 2H; aromatic diepoxyunit), 7.05 (d, J ¼ 9.0 Hz, 2H; aromatic diepoxyunit), 5.60 (broad, OH), 4.50–4.00 (m, 12H,��O��CH2��CH(OH)��CH2��O�� and CH2 ofglycidyl unit), 3.90 (m, 2H, CH2 of glycidyl unit),3.46 (m, 2H, CH, epoxy ring), 2.86 (br, 6H, b toC¼¼O and CH2 of epoxy ring), 2.70 (br, 2H, CH2,epoxy ring), 1.72 (m, 2H, c to C¼¼O).

13CNMR (DMSO-d6, d ppm): d ¼ 189.02(C¼¼O; ketone), 165.42 (C¼¼O; ester), 159.37,134.04, 132.67, 128.82 (aromatic ring carbons ofphotoactive diepoxy), 134.60, 130.01 (aromaticring carbons of terephthalic acid), 135.84 (carbona to C¼¼O), 115.10 (��CH¼¼), 69.67, 67.15 (O��CH2��CH(OH)��CH2��O��), 68.98, 50.05, 44.19(CH2 of glycidyl units and CH and CH2 of epoxyring), 28.33 (carbon b to C¼¼O), 22.87 (c to C¼¼O).

Synthesis of Photoactive Liquid CrystallineHyperbranched Polyester Epoxy ContainingBenzylidene Moiety. Photoactive liquid crystal-line hyperbranched polyester epoxies with pend-ant hydroxyl groups (HCHPE) were synthesizedby polyaddition of photoactive bis benzylidenealkanone biepoxy monomer (BAEP and BCHEP)with trimesic acid (acetone based polymer abbre-viated as HAPE and cyclohexanone based poly-mer abbreviated as HCHPE) using the methodreported by Nishikubo and coworkers57 In a typ-ical recipe, mixture of 6.05 g (0.016 mol) of pho-toactive biepoxide (BAEP), 2.10 g (0.010 mol) oftrimesic acid, 0.14 g of TBAC (5 mol %), and 100mL of DMF were charged into 250-mL three-necked flask equipped with stirrer and a con-denser. The mixture was stirred at 110 8C for24 h under N2 atmosphere. Resulting mixturewas poured into 2 L-chilled water. The precipi-tated polymer (three times) was washed withdis tilled water, filtered, dried at 50 8C in vacuumfor 16 h. Reaction is shown in Scheme 1(a,b).

HAPEYield: 78%.FTIR (KBr, cm�1): 3402 (��OH), 1723

(C¼¼O, ester), 1667 (C¼¼O, ketone), 1597 (ben-

zylidene, C¼¼C), 1570, 1509 (aromatic), 1035(C��O��C), 915 (epoxy).

1HNMR (DMSO-d6, d ppm): d ¼ 8.76, 8.72,8.68 (br,3H, aromatic protons of trimesicacid),7.69 (s, 6H, ��CH¼¼ coupled with aromatic di-epoxy unit), 7.15 (d, 2H; Ph��CH¼¼), 7.02 (d,4H; aromatic diepoxy unit), 5.42 (br s, 2H, OH),4.45–4.00 (br m, 12H, ��O��CH2��CH(OH)��CH2��O�� and CH2 of glycidyl unit), 3.94 (brs, 2H, CH2 of glycidyl unit), 3.47 (d, 2H, CH, ep-oxy ring), 2.90 (s, 2H, CH2 of epoxy ring), 2.72(s, 2H, CH2 of epoxy ring).

13CNMR (DMSO-d6, d ppm): d ¼ 188.54(C¼¼O; ketone), 164.60 (C¼¼O; ester), 160.78,130.18, 127.96, 124.08 (aromatic ring carbons ofphotoactive diepoxy), 134.20, 131.28, 130.78 (ar-omatic ring carbons of trimesicacid), 143.04(carbon a to C¼¼O), 115.41 (��CH¼¼), 69.76,67.12 (O��CH2��CH(OH)��CH2��O��), 68.98,50.05, 44.19 (CH2 of glycidyl units and CH andCH2 of epoxy ring).

HCHPEYield: 76%.FTIR (KBr, cm�1): 3397 (��OH), 1724

(C¼¼O, ester), 1654 (C¼¼O, ketone), 1596 (exocyc-lic C¼¼C), 1507 (aromatic), 1031(C��O��C), 915(epoxy).

1H NMR (DMSO-d6, d ppm): 8.72, 8.68, 8.63(br,3H, aromatic protons of trimesicacid), 7.60–7.40 (m, 4H, aromatic diepoxy unit coupled with2H, ��CH¼¼ unit), 7.00 (d, 4H, aromatic protonsof diepoxy unit; J ¼ 9.0 Hz), 5.78–5.68 (s, 2H,��OH), 4.52–4.00 (m, 12H, ��O��CH2��CH(OH)��CH2��O�� and CH2 of glycidyl unit), 3.90–3.72 (m, 2H, CH2 of glycidyl unit), 3.45 (m, 2H,CH of epoxy ring), 2.92 (d, 4H, CH2 b to C¼¼O),2.86–2.70 (m, 4H, CH2 of epoxy ring), 1.70 (s,2H, c to C¼¼O).

13C NMR (DMSO-d6, d ppm): 189.00 (C¼¼O,ketone), 164.00 (C¼¼O, ester), 159.00, 135.30,133.00, 132.90,132.40, 131.99,128.00 (aromaticring carbons), 130.30 (carbon a to ketone),115.00 (��CH¼¼ unit), 70.30, 67.10 (��O��CH2��CH(OH)��CH2��O��), 69.60, 50.60, 44.10(CH2 of glycidyl units and CH and CH2 of epoxyring), 27.90 (carbon b to C¼¼O), 22.40 (c toC¼¼O).

RESULTS AND DISCUSSION

There have been continuing efforts to establishstructure–property relationships for varioustypes of thermotropic LCPs. There are many



papers describing LC properties of HBLCPs par-ticularly Percec et al., who reported differenttypes of dendritic polymers revealing thermo-tropic LC properties. The present investigationthrows light on designing structure of photoac-tive HBLCPs. Photoactive liquid crystalline lin-ear and hyper branched polyester epoxies weresynthesized by polyaddition of photoactive bisbenzylidene alkanone diol monomers and ter-ephthalic acid and trimesic acid respectivelyusing nucleophilic catalyst (TBAC catalyst) with>70% yield. Gelation was prevented by monitor-ing the temperature, time and concentration ofmonomers and catalyst (5 mol % Bu4NCl inDMF at 110 8C for 24 h).

All polymers are fully soluble in DMF, DMSO,chloroform, DMAC, and THF, partially soluble inmethanol and further, solubility of hyper-branched polymers was found to be more thanthe linear one. Inherent viscosities of polymerswere found to be in the range of 0.16–0.37 dL/g(Table 1). FTIR spectroscopy studies confirmstructure of hyperbranched and linear polymers.The absorption peaks around 1720, 1650, 1034,and 915 cm�1 arising from ester, ketone, ether,and epoxy groups, respectively, confirms theincorporation of both monomers into the polymerstructure. Further, 1H and 13C NMR spectralvalues provided additional information about thestructure of these hyperbranched polymers. Thepeaks appearing at d 3.4 (epoxy��CH), 2.8 and2.7 ppm (epoxy��CH2) indicate the presencefree epoxy end groups.58,59 The signal due to��O��CH2, ��CH��OH, ��CH2��OCO�� units

from epoxy ring opening appear between d 4.5–4.0 ppm. The peak at d 5.5 ppm can be attributedto the hydroxyl group generated due to ringopening reaction of epoxy ring. The doublets at7.7 ppm and 7.2 ppm with coupling constant (J)16.0 Hz correspond to trans position of doublebond (Ph��CH¼¼CH��) of monomer (BA) and poly-mers (LAPE andHAPE).

13C NMR provides additional insight into thestructure of these hyperbranched polymers. Theepoxide end group resonances are observed atd 50.0 (CH) and 44.1(CH2) ppm. All signals upfield from theses epoxides correspond to methyl-ene and methine groups (d 69.7 and 67.1 corre-sponds to ��O��CH2��CH(OH)��CH2��O��)resulting from incorporation of diepoxy unitsinto the polymer backbone. Degree of branching(DB) was estimated by using the methoddescribed by Frechet and coworkers60 and Freyand Holter61 from 1H NMR. The integral valuesof dendritic, linear, and terminal unit protonresonances of trimesic acid (around 8.72, 8.68,8.63 ppm, respectively), were used for the esti-mation of DB. The DB values are found to be inthe range of 0.46–0.49 which agree well withgeneral scale for the hyperbranched polymersmade by one-pot procedure.62

Thermal and Mesogenic Properties

Thermal and mesogenic properties of monomersand polymers are summarized in Table 1. TGA

Table 1. Physicochemical, Thermal, and Mesogenic Properties of Monomers and Polymers

Monomer/Polymer

Molecular Weightsa

ginhb (dL/g)

Thermal Transitions

DSCc POMd

Mn Mw PDI Tg (8C) Tm (8C) Ti (8C) Tm (8C) Ti (8C)

BAEP – – – – – – 131 – 128BCHEP – – – – – 143 174 130 165LAPE 3519 5829 1.6 0.37 106 201 –e 190 230LCHE 3082 4370 1.4 0.22 81 204 258 200 253HAPE 3345 5460 1.6 0.18 155 237 –e 240 –e

HCHE 4051 5965 1.4 0.16 87 259 –e 260 –e

a Determined by GPC in THF with an RI detector. Mn ¼ numer average molecular weight; Mw ¼ weigh average molecularweight; PDI ¼ polydispersity index.

b Inherent viscosity measured at 30 8C with a polymer solution (Concentration ¼ 0.5 g/dL in DMSO).c Transition temperature identified from second heating cycle with DSC (at a heating rate of 5 8C/min under N2 atm).d Transition temperature identified with POM at a heating rate of 5 8C/min.e Not identified as the isotropization is accompanied by degradation.

558 RAO AND SAMUI


indicates higher thermal stability of hyper-branched polymers because of their compactstructure. Glass transition temperature (Tg)values of LAPE, LCHPE, HAPE, and HCHPEare found to be 106, 81, 155, and 87 8C, respec-tively. In general, transition temperatures ofpolymer are dependent on the structures of themesogens (hard segments) and the spacer units(soft segments). In the present study we havestudied the transition temperatures of polymershaving different mesogenic units with differentarchitectures (hyperbranched and linear) andsimilar spacer length. The spacers are gener-ated through ring opening of epoxy group. Poly-mers containing acyclic moiety exhibit higherglass transition temperature (Tg) values due totheir more packing ability compared to the poly-mers containing cyclic moiety. Hyperbranchedpolymers show higher glass transition tempera-ture (Tg) values compared to their linear ana-logues because of their compact structure.Monomer BCHPE showed mesomorphic transi-tion from crystalline state at 143 and at 130 8Cin DSC and POM studies, respectively. BAEPdoes not show mesophase probably due to bis-benzylidene mesogen generated from acetone isnot rigid enough to form LC phase. Polyestersmade from it shows mesophase since when itcombines with aromatic diacid having no spacerit gains required rigidity to form LC phase.LAPE, LCHPE, HAPE, and HCHPE exhibitendotherms at 201, 204, 237, and 259 8C,respectively, which can be assigned to meso-morphic transition from crystalline state. It canbe observed that mesomorphic transition tem-peratures for cycloalkanone based polymers arehigher than that of acetone based polymers.The expected reason for the opposite trendobserved in glass transition and transition tomesomorphic state from crystalline state isthat below glass transition temperature theeffect of packing ability and above the glasstransition the effect of mesogen rigidity seemsto play predominant role in deciding the transi-tion temperature. LC to isotropic transition forboth cyclic and acyclic moieties is observed onlyin the linear architecture (LAPE, LCHPE) andnot observed in hyperbranched architecture asLC phase extend to higher temperature accom-panying degradation. Typical nematic meso-phase of monomer and polymers, confirmed inPOM study more or less in the same regions asobserved in the DSC, are shown in Figure1(a,b).

Photoresponsive Properties

Hyperbranched polymers are studied by UV–visspectroscopic technique for their photoactivebehavior in DMSO. Despite different architec-ture the absorption maxima of both linear andhyperbranched polymers are found to be around360 nm corresponding to p–p* transition of theolefinic double bond of the photoactive units inpolymer backbone. By irradiating the polymersolutions with UV light of wavelength 365 nmat room temperature, decrease in intensity aswell as decrease in the wavelength of absorb-ance (blue shift in the wavelength) are observed

Figure 1. POM photomicrographs of (a) BCHEP(130 8C), (b) LAPE (190 8C) (mag. 1003).



for all polymers as the time of irradiationincreases [Fig. 2(a)]. Photoactivity leads to rup-ture of the benzylidene C¼¼C bond followed bydimerization via 2p þ 2p intermolecular cycload-dition reaction.36 Polymer containing acyclicmoiety (HAPE and LAPE) showed faster rate ofphotocyclisation as compared to polymers con-taining cyclohexanone [Fig. 2(b)]. Further,hyperbranched polymers showed faster rate ofphotocycloaddition as compared to their corre-sponding linear analogues. This is because ofless steric hindrance for the acyclic benzylidenemoiety (HAPE and LAPE) compared to thehighly rigid cyclic moieties (HCHPE andLCHPE) and high proximities of photoactive

moieties in hyperbranched architecture com-pared to linear architecture facilitating thephotocycloaddition. The same trend has beenobserved for the polymer films coated on glassslide [Fig. 3(a,b)]. Irradiated polymer sampleswere further characterized by performing NMRmeasurements in DMSO-d6. Formation of newpeaks is observed around 1.9 ppm and 2.7 ppm(LCHPE; Fig. 4) in 1H NMR spectrum withgrowing intensity with time of exposure (0–120min). This corresponds to formation of cyclobu-tane ring due to cycloaddition reaction in bothpolymers. This behavior is also confirmed bystudying the change in inherent viscosity (inDMSO) by using Ubbelohde viscometer at 30 8C

Figure 2. Photocrosslinking study of polymer solu-tions (a) UV–vis spectra of HAPE solution irradiatedfor different time intervals (b) Rate of photocycloaddi-tion of polymer solutions. [Color figure can be viewedin the online issue, which is available at www.interscience.wiley.com.]

Figure 3. Photocrosslinking study of polymer films(a) UV–vis spectra of HAPE film irradiated for differ-ent time intervals (b) Rate of photocycloaddition ofpolymer films. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

560 RAO AND SAMUI


at a concentration of 0.5 g/dL under UV expo-sure. There is increase in inherent viscosity ofall polymers upon UV light exposure and trendis similar to photocycloaddition (Fig. 5). Theseresults support the spectral studies, which arediscussed earlier and previous reports on similarclass of polymers.36,63,64

The variation of refractive index in polymerswith irradiation is also studied. All four poly-mers show decrease in refractive index. HAPEshows greater decrease from 1.474 to 1.450 (Dn¼ �0.024) compared to other polymers films(Fig. 6). The trend in variation of refractiveindex is to that observed in photocrosslinkingstudy. The change in the refractive index isobserved due to large change in the molecularpolarizability during phtocrosslinking whichleads to large variation in contribution from

bond refraction to the total molar refractionaccording to Lorentz–Lorenz equation.65,66 Opti-cal path difference, which is the difference inthe product of the refractive index and filmthickness, is the essential figure of merit for thediffraction efficiency for the phase type holo-graphic recording. In the present systemdecrease in refractive index contribute todecrease in the optical path difference. Since theoptical path difference between irradiated andunirradiated parts is several percentage of theoriginal optical path length, application of thismechanism is expected to contribute to the im-provement of efficiency of holographic recording.65

Fluorescence Study

All polymers are fluorescent in nature becauseof presence of photochromic bisbenzylidenecycloalkanone units. Fluorescence spectra ofpolymers are recorded in DMSO at various timeintervals under exposure to UV light (365 nm).A fluorescence maximum is observed at 575 nmfor HAPE and the fluorescence intensity isfound to decrease with time of irradiation. Therate of change in fluorescence intensity of HAPEis shown in Figure 7 (Where I0 and It are fluo-rescence intensity before and after UV irradia-tion for a time period ‘‘t-minutes’’ respectively,the rate of decrease is shown as (I0�It)/I0 vs.time). Similar fluorescence intensity changesupon irradiation with 365 nm light are observedfor other polymers. The decrease in the fluores-cence intensity upon irradiation with 365 nmlight can be explained in terms of competition in

Figure 4. 1H NMR spectrum of LCHPE in DMSO-d6 during photolysis.

Figure 5. Rate of change of inherent viscosity underUV light irradiation. [Color figure can be viewed inthe online issue, which is available at www.interscience.wiley.com.]

Figure 6. Change in refractive Index of polymerfilms during photolysis. [Color figure can be viewed inthe online issue, which is available at www.interscience.wiley.com.]



the excited state to convert to dimer by cycliza-tion channel and to deactivate to the relaxedstate by emitting fluorescence.67 With anincrease in the time of irradiation the number ofreacted molecules (dimmers) increase and thenumber of unreacted excited molecules decreaseresulting in decrease in fluorescence intensity.Increase in photoviscosity of the UV-light irradi-ated samples support the dimerization asexplained under photoresponsive properties.

CONCLUSIONS

A series of novel photoactive liquid crystallinelinear and hyperbranched polyester epoxieswere synthesized by polyaddition reaction ofphotoactive bisbenzylidene cycloalkanone diolmonomers with terephthalic acid and trimesicacid, respectively, characterized and structureproperty relation studied thoroughly. Monomerscontaining cyclic moieties only exhibited nematicmesophase, while all polymers showed typicalnematic mesophase. Upon irradiation of polymersolutions with UV light all polymers underwentintermolecular photo cycloaddition reaction.Rate of photo cycloaddition for hyperbranchedpolymers containing acyclic alkanone moietywas found to be higher than that of polymerscontaining cycloalkanone moieties. Higher pho-toacitvity indicated their potential application asphotoresponsive functional materials. All thepolymers were also found to be fluorescent innature. The change in the refractive index wasfound to in the range 0.02–0.024. Substantialvariation of refractive index indicates that thispolymer could be used for optical recording. Fur-ther, study of diffraction efficiency will be car-ried out to asses the applicability of these poly-

mers for holographic recording in forthcomingcommunications.

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