Synthesis and characterization of side-chain liquid crystalline homopolymers and block copolymers containing biphenyl4-ylthiophene and biphenyl-4-ylfluorene pendants

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Synthesis and characterization of side-chain liquid crystalline

homopolymers and block copolymers containing

biphenyl-4-ylthiophene and biphenyl-4-ylfluorene pendants

Kuan-Wei Lee, Hong-Cheu Lin

*

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, ROC Received 16 March 2007; received in revised form 23 April 2007; accepted 24 April 2007

Available online 29 April 2007

Abstract

A series of new liquid crystalline homopolymers (P1 and P2) and block copolymers (P3 and P4) composed of methacrylates containing pendant biphenyl-4-ylthiophene (M1) and biphenyl-4-ylfluorene (M2) units were synthesized by atom transfer radical polymerization (ATRP). The number-average molecular weights (Mn) of the homopolymer (P2) and diblock copolymers (P3 and P4) were in the range of

5153e8713 g mol1with polydispersity indices (PDIs) between 1.17 and 1.25. The thermal, mesogenic, and photoluminescence (PL) properties of all polymers were investigated. Except for the absence of mesogenic properties in block copolymer P4, polymers P1 and P3 possessed the smectic A phase and polymer P2 exhibited the nematic phase. Moreover, the mesomorphism and the layerd-spacing values of the smectic A phase in polymers P1 and P3 were confirmed and characterized by X-ray diffraction (XRD) patterns.

Keywords: Block copolymer; Atom transfer radical polymerization (ATRP); Mesogenic properties

1. Introduction

A wide variety of liquid crystalline (LC) copolymers[1e7] with optimized structures have been developed in recent years. In addition, numerous liquid crystalline (LC) block copoly-mers consisting of mesogenic blocks and isotropic blocks were synthesized via different types of living free-radical polymer-ization[8e11]and their phase behavior and morphology were characterized. In general, there are two kinds of LC block copolymers, i.e. main-chain and side-chain LC block copoly-mers, where mesogenic groups connected along the backbones are main-chain copolymers (such as copolyesters composed of p-hydroxybenzoic acid and poly(ethylene terephthalate), etc.) [12e14], and pendant mesogenic groups attached to the back-bones via flexible spacers are side-chain copolymers.

Among these LC copolymers, side-chain LC copolymers have attracted significant interests because of their liquid crys-talline behavior as low molecular mass pendant mesogens and their easy processing characteristic as polymers. Furthermore, side-chain LC polymers are often used in electro-optical appli-cations due to their lower viscosities and easier alignment ten-dencies than those of main-chain LC polymers. Several kinds of rigid cores, including azobenzene [15e17] and biphenyl units[18e23], were applied to mesogenic groups of side-chain LC polymers. In our previous research [24], side-chain LC polymers containing cyanoterphenyl units were also reported to possess mesogenic phases and high stabilities in thermal, chemical, and photochemical properties, but they had poor solubilities. Herein, in order to improve solubility, a series of new mesogenic homopolymers and block copolymers com-posed of methacrylates containing pendant biphenyl-4-ylthio-phene (M1) and biphenyl-4-ylfluorene (M2) groups were synthesized by atom transfer radical polymerization (ATRP), where block copolymers P3 and P4 were produced from sty-rene macroinitiator (SMi). Furthermore, the thermal, mesogenic,

* Corresponding author. Tel.: þ886 3 5712121x55305; fax: þ886 3 5724727.

E-mail address:[email protected](H.-C. Lin).

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and photoluminscent (PL) properties of all polymers were also investigated in this study.

2. Experimental 2.1. Measurements

1_{H NMR spectra were recorded on a Varian unity 300 MHz}

spectrometer using CDCl3andd6-DMSO solvents. Elemental

analyses were performed on a HERAEUS CHN-OS RAPID elemental analyzer. Transition temperatures were determined by differential scanning calorimetry (DSC) (PerkineElmer, model: Diamond) with heating and cooling rates of 5C/min. The mesomorphic properties were studied using a polarizing optical microscope (POM) (Leica, model: DMLP) equipped with a hot stage. Thermogravimetric analysis (TGA) was con-ducted on a Du Pont Thermal Analyst 2100 system with a TGA 2950 thermogravimetric analyzer at a heating rate of 10C/ min under nitrogen. Gel permeation chromatographic (GPC) analysis was conducted on a Waters 1515 separation module with chloroform as the eluant against a polystyrene calibration curve. UVevisible absorption spectra were recorded in dilute THF solutions (106M) on a HP G1103A spectrophotometer. Photoluminescence (PL) spectra were obtained on a Hitachi F-4500 spectrophotometer.

2.2. Materials

Unless otherwise specified, chemicals and solvents were of reagent grade and purchased from Aldrich, Acros, TCI, and Lancaster Chemical Co. Dichloromethane and THF were dis-tilled to keep it anhydrous before use. Pyridine was dried by refluxing over calcium hydride. The other chemicals were used without further purification.

2.3. Syntheses of monomers

The synthetic routes of monomers (M1 and M2) and the macroinitiator (SMi) are shown inScheme 1. Their synthetic details were described as follows.

2.3.1. 4-Bromo-40-octoxybiphenyl (2)

1-Bromooctane (11.6 g, 60 mmol), 4-bromo-40 -hydroxybi-phenyl (1) (10 g, 40 mmol), and potassium carbonate (16.6 g, 120 mmol) were dissolved in butan-2-one (100 mL) and re-acted under reflux for 24 h. After cooling to room temperature, the potassium salt was filtered off. The solvent was removed by rotavapor and the crude product was recrystallized from pe-troleum ether (bp: 35e60C) to yield a white solid (13.5 g, 93%). 1H NMR (ppm, CDCl3) d: 0.89 (t, J¼ 6.9 Hz, 3H),

1.29e1.47 (m, 10H), 1.80 (quintet, J¼ 6.6 Hz, 2H), 3.98 (t, J¼ 8.6 Hz, 2H), 6.99 (d, J ¼ 8.8 Hz, 2H), 7.40e7.54 (m, 6H)[24].

2.3.2. 40-Octoxybiphenyl-4-ylboronic acid (3)

4-Bromo-40-octoxybiphenyl (2) (5 g, 13.8 mmol) was dis-solved in THF (200 mL) and then n-butyllithium (8.9 mL,

2.5 M, 22.1 mmol) was added dropwise to react at 78_C.

The reaction mixture was maintained under this condition for 1 h. Furthermore, it was added dropwise to trimethyl borate solution (3.5 g, 33.2 mmol) at 78_{C. The solution was}

al-lowed to cool to room temperature overnight, and the final so-lution was acidified with 100 mL of 10% HCl soso-lution and stirred for 45 min at room temperature. Subsequently, the so-lution was washed with saturated sodium carbonate soso-lution and water, and then THF was removed. The crude product was extracted by diethyl ether and the organic layer was dried over magnesium sulfate. After removing the solvent by rotavapor, the resulting solid was washed with petroleum ether and briefly dried on filter to obtain a white solid (6.0 g, 80%). 1H NMR (ppm, d6-DMSO) d: 0.85 (t, J¼ 7.2 Hz, 3H), 1.24e1.41 (m,

10H), 1.71 (quintet,J¼ 6.6 Hz, 2H), 3.98 (t, J ¼ 6.6 Hz, 2H), 6.99 (d, J¼ 8.8 Hz, 2H), 7.56e7.62 (m, 4H), 7.83 (d, J ¼ 8.8 Hz, 2H), 8.03 (s, 2H) [24].

2.3.3. 2-(40-Octoxy-biphenyl-4-yl)-thiophene (6)

Compound 4 (3 g, 18.5 mmol), compound 3 (7.9 g, 24.1 mmol), and tetrakis(triphenylphosphine)palladium(0) (1.07 g, 0.93 mmol) were reacted in THF (200 mL) for 10 min, and then 130 mL of 2 M aqueous Na2CO3 solution

was added. The mixture was reacted and refluxed for 48 h. Af-ter reaction, the cooled solution was washed with dilute hydro-chloric acid (10%) and water, and dried over magnesium sulfate. The final solution was purified by column chromatog-raphy (silica gel, CH2Cl2/hexane 1:1) to yield a white solid

(4.8 g, 71%). 1H NMR (ppm, CDCl3) d: 0.87 (t, J¼ 6.9 Hz, 3H), 1.27e1.45 (m, 10H), 1.81 (quintet, J¼ 6.3 Hz, 2H), 3.97 (t,J¼ 6.6 Hz, 2H), 6.92 (d, J ¼ 9.6 Hz, 2H), 7.06 (dd, J ¼ 3.6 Hz, 1H), 7.26 (d,J¼ 5.4 Hz, 1H), 7.31 (d, J ¼ 3.6 Hz, 1H), 7.51 (m, 4H), 7.63 (d,J¼ 8.7 Hz, 2H). 2.3.4. 2-(40-Octoxy-biphenyl-4-yl)-9,9-diethyl-9H-fluorene (7)

Compound 7 was synthesized by means of analogous pro-cedures of compound 6 via Suzuki coupling reaction. Yield: 71%. 1H NMR (ppm, CDCl3) d: 0.34 (t, J¼ 6.6 Hz, 6H), 0.85 (t, J¼ 6.9 Hz, 3H), 1.27e1.45 (m, 10H), 1.83 (quintet, J¼ 6.6 Hz, 2H), 2.09 (m, 4H), 3.98 (t, J ¼ 6.3 Hz, 2H), 6.96 (d,J¼ 8.7 Hz, 2H), 7.29e7.35 (m, 3H), 7.54e7.75 (m, 10H). 2.3.5. 2-(40-Hydroxyl-biphenyl-4-yl)-thiophene (8) 2-(40-Octoxy-biphenyl-4-yl)-thiophene (6) (4.6 g, 12.6 mmol) was dissolved in dry chloroform (150 mL) under nitrogen and then boron tribromide (6.4 g, 25.2 mmol) was added dropwise to react at 78_{C. The mixture was allowed to warm up to}

room temperature and reacted for 24 h. The solution was washed with sodium hydroxide (1 M, 50 mL) and then was acidified with 10% HCl and stirred for 4 h. Finally, the suspension was filtered off and purified by column chromatography (silica gel, ethyl acetate) to yield a white solid (2.8 g, 89%). 1H NMR (ppm, d6-DMSO) d: 6.85 (d, J¼ 8.4 Hz, 2H), 7.14 (dd,

J¼ 3.9 Hz, 1H), 7.50 (d, J ¼ 4.0 Hz, 1H), 7.54 (d, J ¼ 3.6 Hz, 1H), 7.60 (m, 4H), 7.68 (d,J¼ 8.4 Hz, 2H), 9.56 (s, 1H).

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2.3.6. 2-(40 -Hydroxyl-biphenyl-4-yl)-9,9-diethyl-9H-fluorene (9)

Compound 9 was synthesized through analogous

proce-dures of compound 8. Yield: 87%. 1H NMR (ppm, d6

-DMSO) d: 0.35 (t, J ¼ 7.2 Hz, 6H), 2.07 (m, 4H), 6.58 (s, 1H), 6.94 (d, J¼ 8.4 Hz, 2H), 7.31e7.33 (m, 3H), 7.50e 7.76 (m, 10H).

2.3.7. 2-(40-(6-Hydroxyhexyloxy)-biphenyl-4-yl)-thiophene (10)

Compound 8 (2.5 g, 9.9 mmol), 6-chloro-1-hexanol (2.3 g, 12.9 mmol), K2CO3 (4.1 g, 29.7 mmol), and KI (20 mg)

were dissolved in 200 mL of DMF to reflux overnight. The re-action mixture was then cooled and poured into 200 mL of wa-ter and stirred for 2 h. The crude product was extracted with ethyl acetate and the organic layers were washed with a satu-rated aqueous solution of NaCl and water, and then the organic layer was dried over magnesium sulfate. After removing the solvent by rotavapor, the residue was recrystallized from abso-lute ethanol to give a colorless solid (2.4 g, 69%).1H NMR (ppm,d6-DMSO) d: 1.22e1.70 (m, 8H), 3.39 (m, 2H), 3.99 (t, J¼ 6.4 Hz, 2H), 4.35 (t, J ¼ 5.1 Hz, 1H), 7.00 (d, J ¼ 9.0 Hz, 2H), 7.14 (dd,J¼ 3.9 Hz, 1H), 7.53 (d, J ¼ 4.8 Hz, 1H), 7.55 (d,J¼ 3.6 Hz, 1H), 7.64 (m, 4H), 7.70 (d, J ¼ 8.7 Hz, 2H). B(OH)2 C8H17O Br C8H17O Br HO C8H17Br, K2CO3 Butan-2-one + n-BuLi, B(OMe)3, -78 °C THF Pd(PPh3)4 THF:2M Na2CO3 (aq) (3:2) S Br _C 8H17O S BBr3 Cl O CH2Cl2 N(Et)3, THF HO S OC6H12O S O HOC6H12O S BrC6H12OH, K2CO3 methyl ethyl ketone

B(OH)2 C8H17O Br C8H17O HO HOC6H12O O OC6H12O (1) (2) ₍₃₎ (3) (4) (5) (6) (7) (8) (9) (10) (11) M1 M2 Br 59 + CuBr, PMDETA Anisole SMi Mn=6196 Mw=6900 PDI=1.11 O O Br O O (12)

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2.3.8. 2-(40 -(6-Hydroxyhexyloxy)-biphenyl-4-yl)-9,9-diethyl-9H-fluorene (11)

Compound 11 was synthesized via analogous procedures of compound 10. Yield: 80%. 1H NMR (ppm, CDCl3) d: 0.35 (t, J¼ 7.2 Hz, 6H), 1.41e1.83 (m, 8H), 2.10 (m, 4H), 3.65 (m, 2H), 4.00 (t, J¼ 6.4 Hz, 2H), 6.97 (d, J ¼ 9.0 Hz, 2H), 7.30e7.35 (m, 3H), 7.54e7.76 (m, 10H). 2.3.9. M1 2-(40-(6-Hydroxyhexyloxy)-biphenyl-4-yl)-thiophene (10) (2.0 g, 5.7 mmol), triethylamine (5.7 g, 57 mmol), and 2,6-di-tertbutyl-4-methylphenol (100 mg, as a thermal inhibitor) were dissolved in 150 mL of anhydrous THF under a nitrogen atmosphere and then methacryloyl chloride (1.8 g, 17.1 mmol) was added dropwise. Afterward, the reaction mixture was heated under reflux overnight and then cooled to pour into 200 mL of aqueous NH4Cl solution (10%). The crude product

was extracted with CH2Cl2. The resulting organic layer was

washed with a saturated solution of NaCl and water, and the organic layer was dried over magnesium sulfate. After remov-ing the solvent by rotavapor, the resultremov-ing solid was purified by column chromatography using dichloromethane as an eluant to yield a colorless solid (1.9 g, 81%).1H NMR (ppm, CDCl3) d:

1.45e1.50 (m, 4H), 1.67e1.84 (m, 4H), 1.93 (s, 3H), 3.99 (t, J¼ 6.5 Hz, 2H), 4.15 (t, J ¼ 6.6 Hz, 2H), 5.53 (s, 1H), 6.09 (s, 1H), 6.96 (d,J¼ 9.0 Hz, 2H), 7.07 (dd, J ¼ 3.6 Hz, 1H), 7.26 (d, J¼ 5.1 Hz, 1H), 7.32 (d, J ¼ 3.6 Hz, 1H), 7.54 (m, 4H), 7.64 (d, J¼ 8.7 Hz, 2H). Elemental analysis for C26H28O3S:

Calc. C, 74.25; H, 6.71; found C, 74.43; H, 6.76. HRMS (EI)m/z: Calc. 420.1759; found 420.1754.

2.3.10. M2

Monomer M2 was synthesized by analogous procedures of M1. Yield: 83%. 1H NMR (ppm, CDCl3) d: 0.36 (t, J¼

6.9 Hz, 6H), 1.45e1.56 (m, 4H), 1.67e1.83 (m, 4H), 1.94 (s, 3H), 2.06 (m, 4H), 4.01 (t, J¼ 6.3 Hz, 2H), 4.16 (t,

J¼ 6.6 Hz, 2H), 5.54 (s, 1H), 6.09 (s, 1H), 6.98 (d,

J¼ 8.7 Hz, 2H), 7.32e7.36 (m, 3H), 7.55e7.77 (m, 10H). El-emental analysis for C39H42O3: Calc. C, 83.83; H, 7.58; found

C, 83.52; H, 7.54. HRMS (EI) m/z: Calc. 558.3134; found 558.3126.

2.4. Polymerization 2.4.1. Macroinitiator (SMi)

In a Schlenk flask,N,N,N0,N0,N00 -pentamethyldiethylenetri-amine (PMDETA, 3.46 mg, 0.5 mmol), CuBr (14.3 mg, 0.25 mmol), and styrene (6.8 g, 65 mmol) were added and stirred for 30 min. Ethyl 2-bromo-2-methylpropanoate (195 mg, 1 mmol) was added, and the mixture was immediately frozen in liquid nitrogen under vacuum. After several freezeethaw cycles, the flask was sealed under vacuum and put in an oil bath at 100C to react for 20 h. After reaction, the content was dissolved in chloroform. After being concentrated, the chloroform solution was precipitated into methanol and the precipitation was repeated for three times. The final

prod-uct was dried at 50C under vacuum. Yield: 75%. The

number-average molecular weight measured by GPC was Mn¼ 6196 g mol1with PDI (Mw/Mn)¼ 1.11.

2.4.2. Preparation of homopolymers (P1 and P2) and block copolymers (P3 and P4)

According to analogous procedures as shown inScheme 2, homopolymers (P1 and P2) and block copolymers (P3 and P4) were synthesized by utilization of initiators 12 and SMi, respectively.

2.4.2.1. P1. Yield: 80 mg (36%). N,N,N0,N0,N00 -Pentamethyl-diethylenetriamine (PMDETA, 8.7 mg, 0.05 mmol), CuBr (3.6 mg, 0.025 mmol), and M1 (210.3 mg, 0.5 mmol) were added and stirred for 30 min. After that, ethyl 2-bromo-2-methylpropanoate (12) (1.95 mg, 0.01 mmol) was added, and the mixture was immediately frozen in liquid nitrogen under vacuum. The mixture was degassed three times using the freezeepumpethaw procedure and sealed under vacuum. Af-ter stirring for 30 min at room temperature, the mixture was reacted at 100C in a preheated oil bath for 24 h. The mixture was reprecipitated more than twice in methanol and then washed by acetone. A white product of polymer was collected by filtration and dried under vacuum. By analogous proce-dures,1H NMR data, and GPC results of the other homopoly-mer and copolyhomopoly-mers are listed as follows.

2.4.2.2. P2. Yield: 75 mg (27%). 1H NMR (ppm, CDCl3) d:

0.31 (broad), 0.93 (broad), 1.53e1.93 (broad), 2.02 (broad), 3.97 (broad), 6.91 (broad), 7.29 (broad), 7.54 (broad). The number-average molecular weight measured by GPC was Mn¼ 5103 g mol1with PDI (Mw/Mn)¼ 1.25.

2.4.2.3. P3 (monomer/initiator¼ 30/1). Yield: 110 mg (40%).

1_{H NMR (ppm, CDCl}

3) d: 0.96 (broad), 1.41e1.93 (broad),

3.99 (broad), 6.55 (broad), 7.06 (broad), 7.31 (broad), 7.59 (broad). The number-average molecular weight of the soluble part of the polymer measured by GPC wasMn¼ 8713 g mol1

with PDI (Mw/Mn)¼ 1.19.

2.4.2.4. P4 (monomer/initiator¼ 30/1). Yield: 100 mg (29%).

1_{H NMR (ppm, CDCl}

3) d: 0.30 (broad), 0.85 (broad), 1.40e

1.84 (broad), 2.03 (broad), 3.98 (broad), 6.55 (broad), 7.02 (broad), 730 (broad), 7.58 (broad). The number-average

mo-lecular weight measured by GPC was Mn¼ 7898 g mol1

with PDI (Mw/Mn)¼ 1.17.

3. Results and discussion

3.1. Synthesis and characterization

Atom transfer radical polymerization (ATRP) has been proven to be a very powerful polymerization technique for the preparation of block copolymers from various monomers [24e27]. In this work, the styrene macroinitiator (SMi) [13] was used to copolymerize with methacrylate monomers con-taining biphenyl-4-ylthiophene (M1) and biphenyl-4-ylfluor-ene (M2) pendants to produce diblock copolymers. The

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number-average molecular weights (Mn) as well as PDI values

of homopolymer (P2) and diblock copolymers (P3 and P4) containing biphenyl-4-ylthiophene and biphenyl-4-ylfluorene LC blocks determined by GPC (THF as an eluant) are shown inTable 1. It indicates that all of the macroinitiator (SMi), ho-mopolymer (P2), and diblock copolymers (P3 and P4) with extended molecular weights had small PDI values determined by ATRP. Due to the poor solubility of longer biphenyl-4-ylth-iophene units in P1, homopolymer P1 was unable to be char-acterized, including the molecular weight. According to the number-average molecular weight (Mn) of homopolymer P2,

i.e. 5103 g mol1with a PDI value of 1.25, homopolymer P2 contains only about 9 units of biphenyl-4-ylfluorene monomer

O O Br OC6H12O S O O OC6H12O + CuBr, PMDETA Anisole O O O C 6_H 12 O S O * O O O C 6_H 12 O O * 9 m or SM i O C 6_H 12 O S O * O C 6 H 12 O O * 3 6 59 O O 59 O O P1, P2, P3, and P4 P1 _P2 Mn=5103 Mw=6402 PDI=1.25 (12) M1 M2 P3 Mn=8713 Mw=10370 PDI=1.19 P4 Mn=7898 Mw=9238 PDI=1.17

Scheme 2. Synthetic routes of homopolymers (P1 and P2) and block copolymers (P3 and P4).

Table 1

Molecular weight and thermal properties of polymers P1eP4

Sample Mn(g mol1) Mw(g mol1) PDI (Mw/Mn) Tda(C) Tgb(C) P1c ec ec ec 315 187.2 P2 5103 6402 1.25 338 81.3 P3d 8713 10,370 1.19 349 91.3 P4d 7898 9238 1.17 333 90.6 a

Temperature of 5% weight loss measured by TGA under nitrogen. b

The glass transition temperatures (_{C) were determined by DSC (with a}

heating and cooling rate of 5C/min). c

The molecular weight was undetermined since polymer P1 is insoluble in most organic solvents.

d

The number-average molecular weight (Mn) of macroinitiator SMi deter-mined by GPC was 6196 g mol1with a PDI value of 1.11.

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(M2). In addition, the number-average molecular weight (Mn)

of macroinitiator (SMi) determined by GPC was 6196 g mol1 (containing about 59 styrene monomer units) with a PDI value of 1.11, so diblock copolymers P3 and P4 only contain about 6 and 3 units of methacrylate monomers consisting of biphenyl-4-ylthiophene (M1) and biphenyl-4-ylfluorene (M2) pendants, respectively, as shown in the chemical structures ofScheme 2. 3.2. Thermal and mesogenic properties

The thermal stabilities of polymers (P1eP4) under an at-mosphere of nitrogen were evaluated by thermogravimetric analysis (TGA), which indicates that the values ofTd(the

deg-radation temperature of 5% weight loss in nitrogen) 315_C

for all polymers (as shown in Table 1). Due to the unclear glass transition temperatures (Tgs) of these block copolymers,

Tgvalues were only detectable during the first heating scans of

DSC measurements (with a heating rate of 5C/min) by fol-lowing the quench of polymers (from 200C) in liquid nitro-gen. By this quenching process, theTgvalues of all polymers

in DSC measurements were demonstrated more clearly in the range of 81e187C as shown inTable 1andFig. 1. Compar-ing polymers P1 and P2, they exhibited glass transition tem-peratures (Tgs) at 187 and 81C, correspondingly. P1 was

more rigid than P2 due to no lateral flexible chains on the pendants of P1 and thus to have poorer solubility. Therefore, the Tgvalue of P1 was higher than that of P2. However, the

Tg values of block copolymers P3 and P4 were comparable

(around 91C), which were mostly contributed from the flexible blocks of polystyrene (PS)[28,29] due to the longer block, i.e. larger molecular weight, of PS originated from macroinitiator SMi.

The mesogenic properties of all polymers were character-ized by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The phase transition tempera-tures and enthalpies of polymers P1eP4 are summarized in Table 2 and their DSC thermograms are displayed inFig. 1. To avoid thermal decomposition, these polymers were only

heated up to about 300C (with a heating rate of 5C/min). Since both monomers, i.e. biphenyl-4-ylthiophene monomer (M1) and biphenyl-4-ylfluorene monomer (M2), possess the smectic A and nematic phases, respectively, polymers P1e P4 indeed inherit the LC properties from their constituents (M1 and M2). Regarding their mesomorphism, homopolymer P1and block copolymer P3 which contain biphenyl-4-ylthio-phene units possessed the smectic A phase. P2 and P4 were amorphous polymers with Tg¼ 81 and 91C, respectively.

Homopolymer P2 showed a stable glass-forming nematic phase, but P4 did not possess any mesogenic phase due to a less monomer content (M2) of biphenyl-4-ylfluorene units in the copolymer composition of P4. Fig. 2shows a fan-shaped focal conic texture of homopolymer P1 observed by POM (at 255C cooling), which is a characteristic texture of the smectic A phase. In addition, their smectic phases had been further confirmed by X-ray diffraction (XRD) measurements (will be described later). Because of no lateral flexible chains on the thiophene pendants of P1 and P3, rigid rods of LC blocks generate stronger transverse interaction and thus induce the smectic phases in polymers P1 and P3. Interestingly, even though block copolymer P3 contains only about 6 biphenyl-4-ylthiophene monomer units (M1) and 59 styrene monomer

50 100 150 200 250 300 Endotherm (up) Temperature ( °C) P1 P2 P3 P4

Fig. 1. DSC thermograms of polymers P1eP4 during the first heating scan at 5C/min.

Table 2

Phase behavior of polymers P1eP4aec

Sample Transition temperature (_{C) and} enthalpy (in parentheses, kJ/g)

Tid(C) P1 K235.4 (15.8) SA w270 P2 G81.3 N w140 P3 K218.9 (4.1) SA w275 P4 G90.6 w145 a

Transition temperatures (_{C) and enthalpies (in parentheses, kJ/mol) were}

determined by DSC (a heating rate of 5C/min). b

G [ glass temperature; K ¼ crystalline; N ¼ nematic; SA¼ smectic A. c

Transition temperatures of M1 and M2 are as follows: M1: K 185.3C (52.2 kJ/g) SAwithTi¼ 245C; M2: K 60.4C (3.4 kJ/g) N withTi¼ 98C.

d

Ti: all isotropization temperatures, including monomers M1 and M2, were characterized by polarizing optical microscopy (POM).

Fig. 2. POM texture of the mesophase (SA) of polymer P1 observed at 255C (cooling).

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units, P3 still can sustain its mesogenic property ascribed to the rigid thiophene pendants of the short block (6 monomer units). Regarding homopolymer P2, the lateral diethyl groups on 9-position of fluorene pendants, which separate the rigid cores of the side-chains, cause the reduction of transverse in-teraction among rigid rods in P2 and thus favor the nematic phase. However, in contrast to polymers P1eP3, block copol-ymer P4 contains only about 3 biphenyl-4-ylfluorene mono-mer units (M2) but with 59 styrene monomono-mer units, so it led to no mesogenic phase in P4 due to the less rigidity and shorter block of biphenyl-4-ylfluorene LC units. Moreover, polymers P1 and P3 with thiophene pendant groups revealed higher isotropization temperatures (Ti around 270C) than

polymers P2 and P4 with fluorene pendant groups (Tiaround

140C). Therefore, the comparison of isotropization tempera-tures in monomers M1eM2 and polymers P1eP4 addition-ally suggests that the rigidity of biphenyl-4-ylthiophene units is higher than that of biphenyl-4-ylfluorene units.

3.3. X-ray measurements

In order to elucidate the structures of the mesophases, X-ray diffraction (XRD) measurements were carried out in the temperature ranges of mesophases for polymers P1eP4, and the XRD data are summarized inTable 3. Owing to no smectic phases in P2 (only the nematic phase) and in P4 (no meso-phase), there are no any peaks in the XRD patterns of poly-mers P2 and P4. As shown in Fig. 3, the XRD patterns of polymers P1 and P3 are almost identical and their layer d-spacing values are around 29 A˚ , which were attributed to the

layerd-spacing in the rigid blocks of biphenyl-4-ylthiophene units. In addition, the layer d-spacing values in the ratio of 1:1/2 indicate lamellar orders exist in the mesophases of P1 and P3. According to the molecular modeling calculation, the layerd-spacing value of coplanar structure in M1 is about 23.7 A˚ . Therefore, the layer d-spacing value of 29 A˚ by XRD measurements in polymers P1 and P3 was suggested to be in-terdigitated packing of biphenyl-4-ylthiophene units in rigid blocks. By this evidence, the layered structures of polymers P1and P3 have little relationship with respect to the flexible blocks of polystyrene (PS).

3.4. Photophysical properties

The photophysical properties, including the UVevisible absorption and photoluminescence (PL) spectral data, of all polymers in THF solutions are summarized in Table 4. Due to the identical rigid cores of luminescent biphenyl-4-ylthio-phene units in monomer M1 and polymers P1 and P3, they have almost the same maximum absorption wavelength around 312 nm and emit blue light at approximately lmax,PL¼ 377 nm

in solutions. Comparably, monomer M2 and polymers P2 and P4have the identical rigid cores of luminescent biphenyl-4-yl-fluorene units, therefore, they have almost the same values

Table 3

XRD diffraction data of polymers P1 and P3

Sample Mesophase Temperature (_C) _d-Spacinga_(A_{˚ )}

P1 SA 255 d001¼ 26.98

d002¼ 13.68

P3 SA 230 d001¼ 26.86

a

The theoreticald-spacing value is 23.7 A˚ for polymers P1 and P3.

5 10 15 20 25 30 35

2 theta ( °C)

Intensity (a. u.)

P1 P3

Fig. 3. X-ray diffraction patterns of polymers P1 and P3.

Table 4

UVevisible absorption and photoluminescence spectral data of monomers (M1 and M2) and polymers (P1eP4)

Sample lmax,Absa(nm) lmax,PLa(nm) Fb(%)

M1 312 375 17.5 M2 322 385 23.2 P1 313 377 15.3 P2 321 386 19.0 P3 312 378 15.8 P4 321 387 18.9 a

Absorption and PL spectra were recorded in dilute THF solutions at room temperature.

b

PL quantum yield in THF and 9,10-diphenylanthrance was the reference of the quantum yield.

300 400 500

Wavelength (nm)

PL Intensity (a. u.)

Absorption (a. u.)

UV-Vis. (M1) UV-Vis. (M2) PL. (M1) PL. (M2)

Fig. 4. UVevisible absorption (solid lines) and PL (dashed lines) spectra of monomers M1 and M2 in solutions (THF as solvent).

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of the maximum absorption wavelength around 322 nm and

the maximum PL wavelength (lmax,PL) around 386 nm in

solutions.

Fig. 4shows the UVevisible absorption and PL spectra of monomers M1 and M2 in solutions. Compared with M1, monomer M2 consisting of fluorene units results in longer maximum absorption wavelengths and PL wavelengths due to longer conjugation lengths in rigid cores. By the same rea-son, the maximum absorption and PL wavelengths of poly-mers P2 and P4 are more red-shifted than those of polypoly-mers P1 and P3. Because of the less aggregated form originated from the lateral diethyl groups on fluorene pendants in M2, the quantum yield of biphenyl-4-ylfluorene monomer M2 is higher than that of biphenyl-4-ylthiophene monomer M1. Ac-cordingly, the quantum yields of polymers P2 and P4 are larger than those of polymers P1 and P3.

Due to the poor solubility of polymers P1 and P3 and lower molecular weights of monomers M1 and M2, the photo-physical properties of these compounds in solid films were not

obtained. Compared with solutions, solid films of polymers P2 and P4 in Fig. 5 exhibit red-shifted PL emissions around 396 nm owing to the pep* aggregation of the rigid cores. In addition, the differences of photophysical properties be-tween polymers P2 and P4 (both in solutions and solid films) are trivial, so the flexible PS blocks did not reflect important contribution to the photophysical properties. Fig. 6 displays the PL spectra of P2 solid films by spin-coating and quenching (by liquid N2) from 130C (the nematic phase). Due to the

frozen nematic structure by quenching, P2 was more orderly aligned and it led to aggregation of rigid cores in Fig. 6. Hence, the shorter wavelength peak at 376 nm in PL spectrum of P2 solid film became a shoulder by quenching, which is similar to our previous report [7]. In order to evaluate the ef-fect of mesogenic structure on photoluminescence properties, polarized PL spectra (as shown inFig. 7) were measured from aligned P2 solid film by quenching from 130C on a rubbing PI substrate. Polarization ratio (PLk/PLt) was about 1.43,

where PLkis the maximum PL emission intensity as the

polar-izer is parallel to the rubbing direction, and PLt is the

max-imum PL emission intensity as the polarizer is perpendicular to the rubbing direction. This result shows the effect of meso-genic alignment of P2 on rubbing PI substrate can induce a polarized PL emission with a polarization ratio of 1.43.

4. Conclusions

Atom transfer radical polymerization (ATRP) was em-ployed to fabricate block copolymers consisting of PS macroinitiators and liquid crystalline polymethacrylate blocks containing biphenyl-4-ylthiophene (M1) and biphe-nyl-4-ylfluorene (M2) units.

Thermal and XRD investigations indicate that polymers P1 and P3 exhibited the interdigitated smectic A phase which has little relationship with respect to their flexible PS blocks.

350 400 450 500

Wavelength (nm)

P2-solution P4-solution P2-solid film P4-solid film

Fig. 5. PL spectra of polymers P2 and P4 in solid films and solutions (THF as solvent).

350 400 450 500 550

Wavelength (nm)

solid film by spin-coating

solid film by quenching from LC phase

Fig. 6. PL spectra of P2 solid films by spin-coating and quenching (by liquid N2) from 130C (the nematic phase).

350 400 450 500 550

Wavelength (nm)

parallel to polarizer perpendicular to polarizer

Fig. 7. Polarized PL spectra of aligned P2 solid film by quenching from 130_{C on a rubbing PI substrate, where PL}

k is the parallel PL intensity as the polarizer is parallel to the rubbing direction, and PLtis the perpendicular PL intensity as the polarizer is perpendicular to the rubbing direction.

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In terms of PL wavelengths of all block copolymers in di-lute solutions, P2 and P4 are more red-shifted than P1 and P3, which might be due to longer conjugation lengths of the lateral biphenyl-4-ylfluorene blocks in polymers P2 and P4.

The shorter wavelength peak at 376 nm in PL spectrum of

P2 solid film became a shoulder by quenching from

130C (the nematic phase). The effect of mesogenic alignment of P2 on rubbing PI substrate can induce a polar-ized PL emission with a polarization ratio of 1.43.

Acknowledgements

We are grateful for the financial support provided by the National Science Council of Taiwan (ROC) through NSC 94-2113-M-009-005. The powder XRD measurements were supplied by beamline BL17A (charged by Dr. Jey-Jau Lee) of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan.

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