structure–property relationship of photoactive liquid crystalline polyethers containing...

Structure–Property Relationship of Photoactive LiquidCrystalline Polyethers Containing Benzylidene Moiety

V. SRINIVASA RAO, A. B. SAMUI

Polymer Division, Naval Materials Research Laboratory, Thane, Mumbai-421506, India

Received 25 October 2008; accepted 15 January 2009DOI: 10.1002/pola.23303Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A series of main chain photoactive liquid crystalline polyethers, contain-ing rigid bisbenzylidene photoactive mesogen and flexible methylene spacers, weresynthesized by polycondensation of bisbenzylidene diols and dibromoalkanes. Thepolyethers were characterized with 1H NMR, gel permeation chromatography (GPC),differential scanning calorimeter (DSC), thermo gravimetric analyzer (TGA), andpolarized light optical microscopy. The individual and combined effects of spacerlength and number of methoxy substituents on mesogenic and photoactive propertieswere investigated. Both first order and second order transition temperaturesdecreased with increased spacer length and the number of substituents. The com-bined effect of spacers and substituents drastically reduced the transition tempera-tures. All monomers and polymers showed mainly the smectic mesophase. In a fewcases, nematic droplets along with the smectic phase were observed. The width ofthe liquid crystalline phase reduced with an increasing number of methoxy substitu-ents on mesogenic unit. Variation of spacer length has a negligible effect on photocy-cloaddition. However, steric hinderance caused by the substituents decreased thephotoactivity as the number of substituents increased. Total energies of crosslinkeddimers calculated from modeling studies supported the above findings. Intermolecu-lar photocycloaddition was also confirmed by photoviscosity measurement. The re-fractive index change was found to be in the range of 0.017–0.031. VVC 2009 Wiley Periodicals,

Inc. J Polym Sci Part A: Polym Chem 47: 2143–2155, 2009

Keywords: benzylidene mesogen; liquid-crystalline polymers; molecular modeling;phase behavior; photoactive liquid-crystalline polymers; photoactive polymers;polyethers; structure–property relationship; thermal properties

INTRODUCTION

Liquid crystal oligomers/polymers with flexiblespacers have generated considerable research in-terest due to its possible fundamental and techno-logical exploitations. Systematic investigation ofliquid crystalline polymers started only afterintroduction of the spacer concept by Ringsdorf

and coworkers.1,2 Rigid mesogenic units wereonly used to design low molar mass liquid crys-tals/liquid crystalline polymers, considering suchstructures are prerequisite, until the introductionof concept of flexible rod-like mesogens based onconformational isomerism by Precec et al. Theyused flexible rod-like mesogens both in sidechain3,4 and main chain5–10 liquid crystallinepolymers and also suggested the classification ofrod-like mesogenic groups as rigid rod-like, semi-rigid/semiflexible rod-like, and flexible rod-likemesogens.11

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 2143–2155 (2009)VVC 2009 Wiley Periodicals, Inc.

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

2143

Liquid crystal dimers/oligomers and semi flexi-ble main chain polymers exhibit quite differenttransitional behavior when compared with con-ventional low molar mesogens.12,13 Since oligo-mers have many striking similarities with semi-flexible polymers, a range of monodisperse liquidcrystal oligomers (as model compounds for semi-flexible polymers) have been studied, by experi-mental investigation and by using molecular theo-ries, for their transitional properties to investi-gate how the liquid crystal properties evolve fromdimers to semiflexible polymers.14–17 Manyreports exist in the literature on liquid crystaldimers/oligomers highlighting the effect of struc-tural parameters, such as spacer length,18,19

even-odd effect,18,19 chemical nature of spacer(alkyl, siloxane,20 oligo ethylene oxide21), point ofattachment or linking position (para or meta) ofthe spacer to the mesogenic units,22 symmetricand nonsymmetric mesogens,23,24 and substitu-ents,12 on the transitional behavior. There arealso numerous reports on the liquid crystallinepolymer (both main chain and side chain) describ-ing the influence of structural parameters, suchas spacer length,25,26 multiple combinations offlexible spacers,27 mesogen length,28,29 even–oddeffect,30 interconnecting group between spacerand mesogen,31,32 chemical nature of spacer(alkyl, oligo ethylene oxide,33 siloxane29), polymerbackbone,34 symmetric and nonsymmetric meso-gens,35,36 substituents,37,38 flexible rod-like meso-gen and flexible spacer combination,39 and copoly-mer composition,40–44 on the thermal properties.Percec et al. reported a series of studies on liquidcrystalline polyethers/copolyethers based on con-formational isomerism by using 1-(4-hydroxy-phenyl)-2-(2-methyl-4-hydroxyphenyl)ethane/1-(4-hydroxy-40-biphenylyl)-2-(4-hydroxyphenyl) bu-tane and a,x-dibromoalkanes.45–48 The researchperformed in this area has been reviewed andorganized in several ways, to compare the effectsof structural parameters on polymer properties,by leading investigators (Finkelmann et al.,49 Shi-baev and Plate,50 McArdle,51 Percec et al.,51–53

Collings,54 Weiss and Ober,55 and others).The drawbacks of photoactive liquid crystalline

copolymers, such as inhomogeneity and selectionof monomers with suitable reactivity ratios56 andimbalance between the proportion of LC and pho-toactive moieties57 for required property, can beovercome by choosing a moiety that can act bothas mesogen and as chromophore. Different groupssuch as azobenzenes,58 stilbenes,59 cinnamates,60

bisbenzylidene,61–63 spirooxazine,64 or coumar-

ines65 are compatible with mesomorphic orderingand exhibit photoreactivity. To overcome thedefects of thermal instability of azobenzene,66–68

photochromic compounds with C¼¼C segmentswere synthesized.69,70 Borden71 introduced bis-benzylidene cycloalkanone into the polymer back-bone for their photoactivity study. Recently, ourgroup incorporated this photoactive mesogen intohyperbranched architecture and compared theirproperties with corresponding linear ana-logues.61,63,72,73 Interest in polyethers is increas-ing because of their intrinsically high flexibility,which enables these polymers to exhibit liquidcrystalline behavior even when the spacers arevery short.74,75 The inert polyether scaffold repre-sents an ideal system for further functionalizationdue to its stability toward both chemical reactionsand thermal stress.76 Further, the polyether back-bone exhibits good elastic and adhesive character-istics, which enables it to be used for designingmaterials for advanced technologies. The interest-ing properties of polyethers prompted us to intro-duce a bisbenzylidene photoactive mesogen intopolyether backbone to study the individual andcombined effect of the number of methoxy sub-stituents (0, 2, and 4) and varied spacer length (6,8, and 10 methylene units) on the mesogenic andphotoactive properties. To the best of our knowl-edge, other reports deal with the individualeffects of the nature of substituents and spacerlength on mesogenic and photoactive properties.However, no report is available in literature onsystematic study of both the experimental andcomputational modeling for combined effect of thenumber of substituents and the spacer length.

EXPERIMENTAL

Materials

4-hydroxybenzaldehyde (98%; Sigma-Aldrich),vanillin (3-methoxy-4-hydroxybenzaldehyde, 99%,Sd Fine chemicals, India), syringaldehyde (3,5-dimethoxy-4-hydroxybenzaldehyde, 98%, Sigma-Aldrich), 1,6-dibromohexane (96%, Sigma-Aldrich), 1,8-dibromooctane (98%, Sigma-Aldrich),1,10-dibromodecane (97%, Sigma-Aldrich), crownether (dibenzo 18-crown-6, 99%, Sd Fine chemi-cals), and potassium carbonate (99%, Sd Finechemicals) were used without further purification.Cyclohexanone, dimethylformamide (DMF), anddimethyl sulfoxide (DMSO) (Sd Fine Chemicals)were dried with calcium hydride (AR) (Sd Fine

2144 RAO AND SAMUI

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

Chemicals) and purified by distillation before useas reported.77

Techniques

FTIR spectra were recorded on a Perkin–Elmer1600 series Fourier Transform infrared spectro-photometer using KBr pellets. UV-visible spectrawere taken on a Cary 500 Scan UV-VIS-NIR spec-trophotometer.

1H NMR spectra were recorded on a 500 MHzBruker-FT NMR spectrometer using DMSO-d6 assolvent. Chemical shifts were measured using tet-ramethyl silane (TMS) as an internal standard.Molecular weights of polymers were determinedby GPC (polystyrene standards). TGA (TA instru-ments His Res TGA 2950) was used for thermalanalysis with a heating rate of 20 �C/min in N2

atmosphere. The thermal transitions were deter-mined by using DSC (TA instruments). Heating/cooling rate used for all DSC analysis was 5 �C/min. A Leica DMLD optical polarizing microscopewith image analyzer, equipped with LINKAMTMS 94 hot stage and LINKAM LNP controllingunit, was used to observe the thermal transitionand liquid crystalline state. Refractive indices ofpolymer thin films were measured using Filmet-rics F20, thin-film analyzer. Inherent viscositieswere measured with an Ubbelohde Viscometer at30 �C (0.5 g/dL) in DMSO. Photolysis of the poly-mers was carried out in DMSO solution at 30 �C.Irradiation of polymer samples was performedusing a Spectroline UV-lamp, model ENF-260C/FE by discontinuous mode from a distance of10 cm for various time intervals. Irradiated solu-tions/films were subjected to spectral analysis,viscosity, and refractive index measurements. Mo-lecular modeling studies were carried out usingAccelrys Materials Studio, version 4.2.

Synthesis

Synthesis of Photoactive Diol

Synthesis of photoactive bisbenzylidene diols wasdescribed in our earlier communication.62

2, 6-Bis(4-hydroxybenzylidene)cyclohexanone (BCH-0)

Yield: 75%. mp: 294–295 �C.ELEM. ANAL (C20H1803) (306.34): Calcd. C,

78.41%; H, 5.92%. Found: C, 77.90%; H, 5.90%.

FTIR (KBr/cm): 3256 (OH), 1646 (C¼¼O, ke-tone), 1582 (C¼¼C, benzylidene), 1568 and 1508/cm (aromatic).

1H NMR (DMSO-d6, d ppm): d ¼ 9.95 (s, 2H,PhAOH), 7.54 (s, 2H, ACH¼¼), 7.42 (d, J ¼ 8.7Hz, 4H, aromatic), 7.39 (d, J ¼ 8.7 Hz, 4H, aro-matic), 2.86 (t, 4H, b to C¼¼O), 1.72 (m, 2H, c toC¼¼O).

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 (ACH¼¼),27.99 (carbons b to C¼¼O), 22.55 (carbon c toC¼¼O).

2, 6-Bis(3-methoxy-4-hydroxybenzylidene)cyclohexanone (BCH-2)

BCH-2 was synthesized by following the sameprocedure as in the case of BCH-0 using vanillinin place of 4-hydroxybezaldehyde.

Yield: 73%. mp: 176–177 �C.ELEM. ANAL (C22H22O5) (366.39): Calcd. C,

72.11%; H, 6.05%. Found: C, 71.90%; H, 5.90%.FTIR (KBr/cm): 3374 (OH), 3075 (¼¼CAH),

2952 (CH3), 1638 (C¼¼O, ketone), 1578 (C¼¼C,benzylidene), 1513, 1466 (aromatic), 1256, 1036(PhAOAC).

1H NMR (DMSO-d6, d ppm): d ¼ 9.50 (s, 2H,PhAOH), 7.54 (s, 2H, ACH¼¼), 7.10 (s, 2H, aro-matic), 7.02 (d, J ¼ 8.5 Hz, 2H, aromatic), 6.83 (d,J ¼ 8.0 Hz, H, aromatic), 3.80(s, 6H, OCH3), 2.87(t, 4H, CH2 b to C¼¼O), 1.71 (m, 2H, CH2 c toC¼¼O).

13C NMR (DMSO-d6, d ppm): d ¼ 188.24(C¼¼O), 148.38, 148.34, 136.81, 134.56, 115.23,109.28 (aromatic ring carbons), 137.47(¼¼CHAPh), 126.25 ([C¼¼,a to C¼¼O), 56.73(OCH3), 28.04 (CH2 b to C¼¼O), 23.28 (CH2 c toC¼¼O).

2, 6-Bis(3,5-dimethoxy-4-hydroxybenzylidene)cyclohexanone (BCH-4)

BCH-4 was synthesized by following the sameprocedure as in the case of BCH-0 using syringal-dehyde in place of 4-hydroxybezaldehyde.

Yield: 77%. mp: 154–155 �C.ELEM. ANAL (C24H20O7) (420.4): Calcd. C,

68.56%; H, 4.79%. Found: C, 68.27; H, 4.54%.FTIR (KBr/cm): 3479 (OH), 3011 (¼¼CAH),

2936 (CH3), 1654 (C¼¼O, ketone), 1602 (C¼¼C,benzylidene), 1583, 1512, 1452 (aromatic), 1215,1068 (PhAOAC).

PHOTOACTIVE LIQUID CRYSTALLINE POLYETHERS 2145


1H NMR (DMSO-d6, d ppm): d ¼ 8.93 (s, 2H,PhAOH), 7.58 (s, 2H, ACH¼¼), 6.85 (s, 4H, aro-matic), 3.81 (s, 12H, OCH3), 2.95 (t, 4H, CH2 b toC¼¼O), 1.75 (m, 2H, CH2 c to C¼¼O).

13C NMR (DMSO-d6, d ppm): d ¼ 188.90(C¼¼O), 148.16, 136.90, 134.19, 109.02, (aromaticring carbons), 137.52 (¼¼CHAPh), 126.16 ([C¼¼, ato C¼¼O), 56.51 (OCH3), 28.29 (CH2 b to C¼¼O),23.01 (CH2 c to C¼¼O).

Synthesis of Photoactive Liquid CrystallinePolyethers Containing Benzylidene Moiety

Photoactive liquid crystalline linear polyetherswere synthesized by polycondensation of a photo-active bisbenzylidene diol and a dibromoalkane.In a typical recipe, a mixture of 2.44 g (0.01 mol)dibromohexane, 4.14 g (0.03 mol) K2CO3, 0.26 g(0.001 mol) 18-crown-6, and 30 mL dimethyl form-amide (DMF) were charged into a 100 mL thor-oughly dried three-necked flask equipped with adropping funnel, condenser, and N2 inlet. Whilestirring (using magnetic stirring bar), the mixturewas heated to 90 �C and then 3.06 g (0.01 mol) ofBCH dissolved in 40 mL of DMF was added usinga dropping funnel. The reaction was continued fora period of 24 h. The salts formed and remainingK2CO3 were removed by filtration. The volume ofthe filtrate was reduced to about 30 mL by distill-ing out DMF under reduced pressure, to whichexcess methanol was added to precipitate thepolymer product. The yellowish polymer was puri-fied by repeated precipitation from DMF intomethanol and water, respectively. The polymer

product was filtered and then dried at 60 �C invacuum for 16 h (Scheme 1).

Polyether 6, 0

FTIR (KBr/cm): 3060 (¼¼CAH), 2933, 2861 (CH2),1660 (C¼¼O, ketone), 1596 (C¼¼C, benzylidene),1563, 1506, 1469 (aromatic), 1251, 1017 (PhAOAC).

1H NMR (DMSO-d6, d ppm): d ¼ 7.57 (d, 2H,¼¼CHA), 7.49–7.40 (d, 4H, aromatic), 7.00–6.86 (d,J ¼ 6.5 Hz, 4H, aromatic), 4.02 (t, 4H, OCH2),2.86(s, 4H, CH2 b to C¼¼O), 1.83 (m, 2H, CH2 c toC¼¼O), 1.74 (m, 8H, CH2 b and c to CAO).

Polyether 8, 0

FTIR (KBr/cm): 3060(¼¼CAH), 2929, 2854 (CH2),1659 (C¼¼O, ketone), 1597 (C¼¼C, benzylidene),1563, 1507, 1469 (aromatic), 1251, 1027(PhAOAC).

1H NMR (DMSO-d6, d ppm): d ¼ 7.55 (br s, 2H,¼¼CHA), 7.48–7.32 (d, 4H, aromatic), 6.99–6.85 (d,J ¼ 6.5 Hz, 4H, aromatic), 4.01 (t, 4H, OCH2),2.85 (s, 4H, CH2 b to C¼¼O), 1.72 (br s, 2H, CH2 cto C¼¼O), 1.36 (m, 12H, CH2 b, c and d to CAO).

Polyether 10, 0

FTIR (KBr/cm): 3060 (¼¼CAH), 2923, 2850 (CH2),1660 (C¼¼O, ketone), 1597 (C¼¼C, benzylidene),1563, 1507, 1468 (aromatic), 1250, 1014(PhAOAC).

Scheme 1. Synthesis of polymers.

2146 RAO AND SAMUI


1H NMR (DMSO-d6, d ppm): d ¼ 7.57 (d, 2H,¼¼CHA), 7.48–7.39 (d, 4H, aromatic), 6.98–6.84 (d,J ¼ 6.5 Hz, 4H, aromatic), 4.00 (t, 4H, OCH2),2.85 (s, 4H, CH2 b to C¼¼O), 1.71 (br s, 2H, CH2 cto C¼¼O), 1.4–1.36 (br m, 16H, CH2 b, c, d and xto CAO).

Poyether 6, 2

FTIR (KBr/cm): 3071 (¼¼CAH), 2933, 2860 (CH3

and CH2), 1658 (C¼¼O, ketone), 1595 (C¼¼C, ben-zylidene), 1560, 1511, 1465 (aromatic), 1248, 1028(PhAOAC).

1H NMR (DMSO-d6, d ppm): d ¼ 7.57 (d, 2H,¼¼CHA), 7.11 (d, 2H, aromatic), 7.02 (d, J ¼ 6.5Hz, 2H, aromatic), 6.89 (d, J ¼ 6.5 Hz, 2H, aro-matic), 4.00 (t, 4H, OCH2), 3.81(br, 6H, OCH3),2.87(br s, 4H, CH2 b to C¼¼O), 1.72 (m, 2H, CH2 cto C¼¼O), 1.46 (br s, 4H, CH2 b to CAO), 1.22 (s,4H, CH2 c to CAO).

Polyether 8, 2

FTIR (KBr/cm): 3068(¼¼CAH), 2930, 2854 (CH3


1H NMR (DMSO-d6, d ppm): d ¼ 7.56 (d, 2H,¼¼CHA), 7.11 (d, 4H, aromatic), 7.01 (m, 2H, aro-matic), 3.98 (t, 4H, OCH2), 3.78 (s, 6H, OCH3),2.95(br s, 4H, CH2 b to C¼¼O), 1.70 (br s, 2H, CH2

c to C¼¼O), 1.39–1.21 (m, 12H, CH2 b, c and d toCAO).

Polyether 10, 2

FTIR (KBr/cm): 3070(¼¼CAH), 2924, 2850 (CH3


1H NMR (DMSO-d6, d ppm): d ¼ 7.56 (s, 2H,¼¼CHA), 7.11–6.93 (m, 6H, aromatic), 3.95 (m,4H, OCH2), 3.77 (br, 6H, OCH3), 2.87(t, 4H, CH2 bto C¼¼O), 1.80 (m, 2H, CH2 c to C¼¼O), 1.68 and1.38–1.22 (m, 16H, CH2 b, c, d and x to CAO).

Poyether 6, 4

FTIR (KBr/cm): 2993 (¼¼CAH), 2934, 2838 (CH3


1H NMR (DMSO-d6, d ppm): d ¼ 7.57 (s, 2H,¼¼CHA), 6.84 (s, 4H, aromatic), 3.87 (t, 4H,

OCH2), 3.78 (s, 12H, OCH3), 2.92 (br s, 4H, CH2 bto C¼¼O), 1.71 (s, 2H, CH2 c to C¼¼O), 1.62 (m, 4H,CH2 b to CAO), 1.44(m, 4H, CH2 c to CAO).

Polyether 8, 4

FTIR (KBr/cm): 2995(¼¼CAH), 2930, 2853 (CH3


1H NMR (DMSO-d6, d ppm): d ¼ 7.56 (s, 2H,¼¼CHA), 6.82 (s, 4H, aromatic), 3.85 (t, 4H,OCH2), 3.76 (s, 12H, OCH3), 2.91(br s, 4H, CH2 bto C¼¼O), 1.70 (br s, 2H, CH2 c to C¼¼O), 1.60 (m,4H, CH2 b to CAO), 1.40 (m, 4H, CH2 c to CAO),1.30 (m, 4H, CH2 d to CAO).

Polyether 10, 4

FTIR (KBr/cm): 2994(¼¼CAH), 2926, 2851 (CH3

and CH2), 1661 (C¼¼O, ketone), 1594 (C ¼¼C, ben-zylidene), 1576, 1500, 1457 (aromatic), 1254, 1064(PhAOAC).

1H NMR (DMSO-d6, d ppm): d ¼ 7.56 (s, 2H,¼¼CHA), 6.81 (s, 4H, aromatic), 3.85 (br s, 4H,OCH2), 3.77 (s, 12H, OCH3), 2.91(br s, 4H, CH2 bto C¼¼O), 1.70 (br s, 2H, CH2 c to C¼¼O), 1.59 (brs, 4H, CH2 b to CAO), 1.38 (br s, 4H, CH2 c toCAO), 1.26 (br d, 8H, CH2 d and x to CAO).

RESULTS AND DISCUSSION

In continuation to our efforts in investigating theinfluence of structural parameters on photoactiveliquid crystalline polymers, here we report theindividual and combined effect of spacer lengthand number of substituents on main chain photo-active liquid crystalline polyethers. Bisbenzyli-dene diols with two and four methoxy substitu-ents and without substituents were reacted withdibromoalkanes having three different evenspacer lengths (odd spacer lengths were avoidedto avoid even-odd effect in the interpretation ofindividual and combined effect of spacer lengthand number of substituents) to get main chainpolyethers. The synthesis of polymers is outlinedin Scheme 1. Molecular weights of the resultingpolymers were determined by GPC. The number-average molecular weights of the polymers are inthe range of 4000–4500 with polydispersity 1.3–1.4 and their inherent viscosities are found to bein the range of 0.33–0.43 dL/g (Table 1).



Monomers syntheses have been reported in ourrecent publication.54 The structures of synthe-sized monomers and polymers were identified byFTIR and 1H NMR spectroscopic techniques(Figs. 1 and 2). A drastic increase in the stretch-ing frequency of aliphatic alkane unit at 2930/cmand almost disappearance of absorption peak ofhydroxyl group indicate the incorporation of bothdiol and dibromoalkane units in the polymerbackbone.

Thermal Properties

The thermal transitions of polymers, identified byDSC (Fig. 3) and thermal optical polarized micro-scope, are summarized in Table 1. As a generalrule, the thermal properties of polymers areaffected mainly by the variation of molecularweight, structural rigidity, packing ability, interand/or intramolecular interaction, and free vol-ume. It is considered that the effect of differencesin molecular weights on thermal properties isnegligible, since the DP of all polymers is similar.It can be seen from Table 1 that the glass transi-tion temperatures decreases with increasingspacer length, since the flexibility of the backboneincreases segmental motions. As the number ofmethoxy substituents is increased the Tg valuesdecrease, which is the opposite trend to that

observed in our recent studies on polyesterepoxies containing susbtituents and in situ gener-ated aliphatic spacer of 3-carbon length alongwith hydroxyl group.55 Even though the rigidmesogenic unit and the methoxy substituents arethe same in both (earlier and present) cases, it issupposed that the flexibility achieved by large (6–10 aliphatic carbons) spacer length outweighs theinfluence of bulkiness of the substituents. Ineffect, the molecular segmental motions occur atlower temperatures. The combined influence ofspacer and substituents are very effective atreducing the glass transition temperature of thepolymer with flexible unit of 6-carbon length with-out substituent as it comes down from 116 �C to53 �C with increasing the flexible unit length to10-Carbon with four substituents.

In the present study, all polymers show liquidcrystallinity. This indicates that the bisbenzyli-dene moiety is able to tolerate the varied struc-tural parameters to retain liquid crystallinity.The trends in first order thermal transitions, thatis, mesogenic and isotropization temperatures, isexactly similar to that observed in the glass tran-sition temperature and the combined effect ofspacer length and number of substituents drasti-cally reduced the transition temperatures. Themonomers BCH-0 and BCH-2 show smectic Amesophase while BCH-4 shows nematic droplets

Table 1. Thermal and Physical Properties of polymers

Monomer/Polymer

ThermalTransitionsa (�C)and CorrespondingEnthalpy Changes

(kJ/mru) in Parentheses Mnb Mw

b PDIb ginhc (dL/g)

BCH-0 K283N295 Ti – – – –BCH-2 K130Sm177 Ti – – – –BCH-4 K 103Sm123N154 Ti – – – –Polyether 6, 0 Tg 116 Sm 247 (16.45) Ti 4200 5700 1.4 0.33Polyether 8, 0 Tg 90 Sm 191 (16.74) Ti 4100 5700 1.4 0.35Polyether 10, 0 Tg 85 Sm 154 (1.34) N 180 (1.02) Ti 4300 5600 1.4 0.40Polyether 6, 2 Tg 80 Sm 188 (16.87) Ti 4100 5600 1.4 0.39Polyether 8, 2 Tg 72 Sm 124 (5.57) N 161 (17.54) Ti 4400 5700 1.3 0.38Polyether 10, 2 Tg 60 Sm 111 (1.11) N 101 (4.21) Ti 4400 5900 1.3 0.41Polyether 6, 4 Tg 71 Sm 108 (12.47) Ti 4100 5700 1.4 0.39Polyether 8, 4 Tg 62 Sm 105 (11.08) Ti 4300 5500 1.3 0.37Polyether 10, 4 Tg 49 Sm 60 (2.32) Ti 4500 5800 1.3 0.43

aMelting temperature and glass transition temperatures are identified from first heating and first cooling cycles of DSC,respectively (at a heating rate of 5 �C/min under N2 atm).

bDetermined by GPC in THF with an RI detector. Mn ¼ number average molecular weight; Mw ¼ weight average molecularweight; PDI ¼ polydispersity index.

c Inherent viscosity measured at 30 �C with a polymer solution (Concentration ¼ 0.5 g/dL in DMSO).

2148 RAO AND SAMUI


Figure 1. 1H NMR of monomer BCH-4.

Figure 2. 1H NMR of Polyether 10, 4.



along with a smectic A phase. Polyethers withspacers having six and eight methylene units,except ‘‘polyether-8, 2,’’ show only smectic phaseswhereas polyether-8, 2 show nematic dropletsalong with a smectic phase (fine texture). Poly-

ethers with spacer length of 10 methylene unitsshow a nematic phase along with the smectic Aphase. The schlieren texture of polyether-10, 4observed by optical microscope is manifested inthe DSC thermogram. The width of LC phasereduces with increasing the number of methoxysubstituents on mesogenic unit. The transitiontemperatures and their corresponding enthalpyvalues along with the texture types are tabulatedin Table 1. The representative textures of mono-mers and polymers, obtained from polarized opti-cal microscope, are shown in Figures 4–6. Thethermal stability of polymers is evaluated by TGAin a nitrogen atmosphere. Polymers with two sub-stituents and without a substituent are stable upto 400 �C, whereas polymers with four substitu-ents are stable up to 260 �C (Fig. 7).

Photoresponsive Properties

The effect of spacer length and the number ofmethoxy substituents on photocrosslinking ofpolymer in solution and in film was studied by

Figure 4. POM photomicrographs of (a) BCH-2 (170 �C, 400�), (b) BCH-2 (170 �C,400�) annealed for 10 min, (c) BCH-2 (170 �C, 400�) annealed for 15 min, and (d)BCH-2 (120 �C, 400�). [Color figure can be viewed in the online issue, which is avail-able at www.interscience.wiley.com.]

Figure 3. Overlay of DSC of polyethers. [Color fig-ure can be viewed in the online issue, which is avail-able at www.interscience.wiley.com.]

2150 RAO AND SAMUI


Figure 5. POM photomicrographs of (a) BCH-4 (150 �C, 400�), (b) BCH-4 (140 �C,400�), (c) BCH-4 (125 �C, 400�), and (d) BCH-4 (115 �C, 400�).

Figure 6. POM photomicrographs of (a) Polyether 6, 2 (175 �C, 200�), (b) Polyether8, 2 (140 �C, 200�), (c) Polyether 10, 4 (55 �C, 400�), and (d) Polyether 10, 4 (80 �C,400�).

UV-vis spectroscopy and the findings are sup-ported by the change in photoviscosity, refractiveindex, and also by the molecular modeling. Theabsorption maxima of the polymer solutions and

films were measured before and after each irradi-ation period (i.e., 0, 15, 30, 60, and 120 min). Asignificant decrease in the absorption maxima forall the polymers with increasing irradiation (365nm) time indicates the photodimerization of ben-zylidene moieties (Fig. 8), which leads to the for-mation of cyclobutane units. Intermolecular pho-tocycloaddition was also confirmed by the 1HNMR measurement of irradiated polymer solu-tions in our previous report.54 No significantchange is observed in the rate of photocrosslink-ing with varying spacer length (6, 8, and 10).These results suggest that the conformationalfreedom within the observed range of spacerlength is more or less same, particularly in solu-tion (Fig. 9). While the substituents play a signifi-cant role on the rate of photocrosslinking, it canbe seen from Figure 9 that a substantial decreasein the rate of photocycloaddition with increa-sing number of the methoxy substituents atrigid units, that is, polymers without substitu-ents to the rigid unit show higher rate of

Figure 7. Combined TGA thermogram of poly-ethers. [Color figure can be viewed in the onlineissue, which is available at www.interscience.wiley.com.]

Figure 8. (a) UV-vis spectra of polyethers 10, 0 so-lution irradiated for different time intervals. (b) UV-vis spectra of polyethers 8, 4 film irradiated for differ-ent time intervals. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

Figure 9. Effect of substitution and spacer lengthon photocrosslinking of polymer solutions. [Color fig-ure can be viewed in the online issue, which is avail-able at www.interscience.wiley.com.]

Figure 10. Effect of substitution and spacer lengthon Photoviscosity of polymer solutions. [Color figurecan be viewed in the online issue, which is availableat www.interscience.wiley.com.]

2152 RAO AND SAMUI


photocycloaddition. The results suggest that thesteric hinderance of substituents prevents thechromophores to move close to each other leadingto a decrease in the rate of photocycloaddition. Toconfirm above observations, the irradiated poly-mer solutions were subjected to the measurementof inherent viscosity. There is an increase in pho-toviscosity of all the polymer solutions withincreasing irradiation time following similartrend as observed in the UV-vis spectroscopicstudy (Fig. 10). A study on the change in refrac-tive index of the irradiated polymer films providesadditional support to photocycloaddition of thechromophores. All the polymer films showdecrease in the refractive index following similartrend (Fig. 11) as observed above. This may bedue to large change in the molecular polarizabilityduring photocrosslinking resulting in a large vari-ation in contribution from the bond refraction tothe total molar refraction according to the Lor-entz-Lorenz equation.78,79 The change in the re-fractive index is found to be in the range of 0.017–0.031.

Molecular Modeling Study of Model Compounds

Among the more common uses of the atomisticsimulation tools in polymer science the predictionof total energy, to estimate the stability of the mol-ecule, is very important. Since oligomers/dimershave many striking similarities with the poly-mers, a range of liquid crystal dimers in theircrosslinked form (as model compounds for poly-mers) have been constructed using ‘‘Moleculebuilding tools in the Accelrys Materials Studio.’’The geometry of the initial structures of cross-linked dimers were optimized (energy minimized)by using the ‘‘Conjugate Gradient method’’ of

‘‘Discover molecular simulation program’’(Fig. 12). The energies of crosslinked dimers arecalculated using ‘‘Universal Force field,’’ ‘‘Atombased Summation’’ method of ‘‘Forcite module’’and the summary of energy data of all the ninemolecules with varied structural parameters aretabulated in Table 2. The ‘‘Universal Force field’’method calculates potential energy on the basis ofmolecular geometry and this energy comprisescontributions from the bonded (bond, angle, tor-sion, and inversion) and the nonbonded (vander-waals and elctrostatic) interactions. It can benoticed from the Table 2 that there is not muchvariation in the energy of the molecule withincreasing spacer length while there is anincrease in energy of the molecule with increasingnumber of substituents, which appears to be theresult of steric hinderance arising from the pres-ence of substituents. These can be correlated tothe experimental photocrosslinking studies of UV-vis spectroscopy, that is, the decrease in the rateof photocrosslinking with increasing number ofsubstituents is observed as the stability of result-ing molecule decreases.

Figure 11. Effect of substitution and spacer lengthon refractive index of polymer films. [Color figure canbe viewed in the online issue, which is available atwww.interscience.wiley.com.]

Figure 12. Three-dimensional structure of Dimer-6,4. [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com.]

Table 2. Total Energy of Dimers

No. ofMethoxy

Substituents

Total Enegy (Kcal)

6a 8a 10a

0 615 597 6202 699 684 26264 806 797 6801

aSpacer length.



CONCLUSIONS

Synthesis and characterization of a series of mainchain photoactive liquid crystalline polyetherscontaining rigid bisbenzylidene photoactive meso-gen and the flexible methylene spacers were car-ried out by a polycondensation of bisbenzylidenediols and dibromoalkanes. The individual and thecombined effects of spacer length and the numberof substituents on mesogenic and photoactiveproperties were investigated by using DSC, POM,UV-vis spectroscopy, and molecular modeling. Thetransition temperatures decreased with increasedspacer length and the number of substituents.The textures of polymers were identified as asmectic A mesophase and some of the monomersand polymers showed nematic droplets along withthe smectic phase. The rates of photocycloadditionof the polymer solutions and the films were deter-mined by using UV-vis spectroscopy. There waspractically no change in the rate of photocycload-dition with changing the spacer length. However,the steric hinderance caused by the substituentsdecreased the photoactivity. The intermolecularphotocycloaddition was confirmed by the photovis-cosity measurements of UV irradiated polymersolutions. The refractive index change was foundto be in the range of 0.017–0.031. Total energies ofthe crosslinked dimers calculated from molecularmodeling studies also confirmed the observedeffect of spacer length and substituent on photoac-tivity.

The authors thank J. Narayana Das, Director, NavalMaterials Research Laboratory for his encouragementand permission to publish this article and B. C. Chakra-borty, Head of Polymer Division for his continuous sup-port during this study. They also thank S. Das, PidiliteIndustries, India, for extending the GPC and DSCinstruments facility.

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