Chapter 2. Synthesis and Characterization of Rod-Coil Polymers Based on Poly-
2.4 Conclusion
A novel conjugated aromatic core containing direct-coupled fluorene, thiophene, and biphenyl groups via Suzuki coupling reaction was synthesized in this study. The rigid hydrophobic core was combined with two different lengths of poly(ethylene oxide)s as hydrophilic flexible chains. It is interesting that the increasing flexible chains lead to different mesophases (and molecular arrangements) and decrease the phase transition temperatures of Tm and Ti. The XRD patterns and optical textures by POM have proved their mesophasic structures and molecular arrangements. The melting points of the commercial poly(ethylene oxide)s with Mn=750 and 2000 are 30℃ and 52℃, respectively. Hence, LC segregation might be influenced with poly(ethylene oxide)s. Besides mesophasic properties, the PL and EL properties of all rod-coil polymers and analogous derivatives are also investigated.
Chapter 3
Synthesis and Characterization of Liquid Crystalline Block Copolymers with Cyanoterphenyl Moieties by ATRP
3.1 Introduction
In recent years, many research groups have concentrated on the synthesis of liquid crystalline (LC) block copolymers and characterization of their phase behavior and morphology.29-34 These kinds of liquid crystalline (LC) block copolymers were synthesized via different tpyes of living free-radical polymerization.35-38 The interest in this type of materials resides in the combined properties of two (or more than two) completely different polymers which are chemically bonded to each other. The macrophase separation takes place due to the segregation of different polymer chains.
Regarding LC properties, the combination of rigid cores and flexible chains is required for the LC molecular design. In general, there are two kinds of LC block copolymers, main-chain and side-chain LC block copolymers. Mesogenic groups are connected along the polymer backbones as main-chain copolymers, and pendent mesogenic groups are attached to the polymer backbones via flexible spacers as side-chain copolymers.
Side-chain LC polymers are often used in electro-optical applications, where different kinds of rigid cores, including azobenzene 4, 39- 40 and biphenyl units 41-46, are used in mesogenic monomers. In previous studies, the aromatic cores in conjugation with a terminal cyano group have high values of birefringence and reasonable viscosities. Moreover, they are chemically and photochemically stable.
Cyanoterphenyl derivatives have been used in a wide range of nematic mixtures possessing high thermal, chemical and photochemical stabilities.47 Therefore, a series
of novel side-chain liquid crystalline block copolymers consisting of different flexible macroinitiators, including poly(ethylene oxide) (PEO), polystyrene (PS), and poly(ethylene oxide)-b-polystyrene, and polymethacrylate with a pendent cyanoterphenyl group, were synthesized through atom transfer radical polymerization (ATRP). Furthermore, thermal, mesomorphic, and PL properties of all polymers were also investigated in this study.
3.2 Experimental Section 3.2.1 Measurements
1H NMR spectra were recorded on a Varian unity 300 MHz spectrometer using CDCl3 and d6-DMSO solvents. Elemental analyses were performed on a HERAEUS CHN-OS RAPID elemental analyzer. Transition temperatures were determined by differential scanning calorimetry (DSC) (Perkin–Elmer, model: Diamond) with a heating and cooling rate of 5 °C/min. The mesophases were studied using a polarizing optical microscope (POM) (Leica, model: DMLP) equipped with a hot stage.
Thermogravimetric analysis (TGA) was conducted on a Du Pont Thermal Analyst 2100 system with a TGA 2950 thermogravimetric analyzer with a heating rate of 10°C/min under nitrogen. Gel permeation chromatography (GPC) analysis was conducted on a Waters 1515 separation module with chloroform as the eluant against a polystyrene calibration curve. UV-visible absorption spectra were recorded in dilute chloroform solutions (10-6 M) on a HP G1103A spectrophotometer.
Photoluminescence (PL) spectra were obtained on a Hitachi F-4500 spectrophotometer. Polymer solid films were spin-coated on quartz substrates from chloroform solutions with a concentration of 1 mg/mL.
3.2.2 Materials
Chemicals and solvents were reagent grade and purchased from Aldrich, ACROS, TCI, and Lancaster Chemical Co. Dichloromathane and THF were distilled to kept anhydrous before use. Pyridine was dried by refluxing over calcium hydride. The other chemicals were used without further purification.
3.2.3 Synthesis
Scheme 3.1 summarizes the steps involved in the synthesis, with details of each step given below.
4-Bromo-4’-octoxybiphenyl (2). 1-Bromooctane (11.6 g, 60 mmol), 4-bromo-4’
-hydroxybiphenyl (10 g, 40 mmol), and potassium carbonate (16.6 g, 120 mmol) were dissolved in butan-2-one (100 mL) and reacted 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 petroleum ether (bp: 35-60
°C) to yield a white solid (13.5 g, 93%). 1H NMR (ppm, CDCl3), δ: 0.89 (t, J = 6.9Hz, 3H), 1.29-1.47 (m, 10 H), 1.80 (quintet, J = 6.6 Hz, 2H), 3.98 (t, J = 8.6 Hz, 2H), 6.99 (d, J = 8.8 Hz, 2H), 7.40-7.54 (m, 6H).48
4’-Octoxybiphenyl-4-ylboronic Acid (3). 4-Bromo-4’-octoxybiphenyl (2) (5 g, 13.8 mmol) was dissolved in THF (200 mL) and then n-Butyllithium (8.9 mL, 2.5 M, 22.1 mmol) was added dropwise at -78 °C to react. The reaction mixture was maintained under these conditions for one more 1 h. Furthermore, it was added dropwise to trimethyl borate solution (3.5 g, 33.2 mmol) at -78 °C. The solution was allowed to cool to room temperature overnight. The final solution was acidified with 10 % HCl solution (100 mL) and stirred for 45 mins at room temperature. The solution was washed with saturated sodium carbonate solution and water, and the 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), δ: 0.85 (t, J = 7.2 Hz, 3H), 1.24-1.41 (m, 10H), 1.71 (quintet, J = 6.6 Hz, 2H), 3.98 (t, J = 6.6 Hz, 2H), 6.99 (d, J
= 8.8 Hz, 2H), 7.56-7.62 (m, 4H), 7.83 (d, J = 8.8 Hz, 2H), 8.03 (s, 2H).48
4-Octoxy-4”-cyanoterphenyl (5). Compound 4 (2.3 g, 12.8 mmol), compound 3 (5.0 g, 15.3 mmol), and tetrakis(triphenylphosphine)palladium(0) (740 mg, 0.64 mmol) were reacted in THF (100 mL) for 10 mins, and then 100mL of 2 M aqueous Na2CO3 solution was added. The mixture was reacted and refluxed for 48 h. After reaction, the cooled solution was washed with dilute hydrochloric acid (10%) and water, and dried over magnesium sulfate. The final solution was purified by column chromatography (silica gel, CH2Cl2/hexane 1:1) to yield a white solid (4.7 g, 83%).
1H NMR (ppm, CDCl3), δ: 0.87 (t, J = 6.6 Hz, 3H), 1.26-1.46 (m, 10 H), 1.80 (quintet, J = 6.9 Hz, 2H), 3.99 (t, J = 6.6 Hz, 2H), 6.98 (d, J = 9.0 Hz, 2H), 7.54 (d, J = 9.0 Hz , 2H), 7.62 (m, 4H), 7.71 (m, 4H).
4-Hydroxyl-4”-Cyanoterphenyl (6). 4-Octoxy-4”-cyanoterphenyl (5) (3.7 g, 9.5 mmol) was dissolved in dry chloroform (150 mL) under nitrogen and then boron tribromide (4.8 g, 19.1 mmol) was added dropwise and reacted 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). Then, the solution 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 (2.37 g, 92%).
1H NMR (ppm, d6-DMSO), δ: 6.86 (d, J = 8.4 Hz, 2H), 7.55 (d, J = 8.7 Hz , 2H), 7.71 (d, J = 8.4 Hz , 2H), 7.79 (d, J = 8.7 Hz , 2H), 7.91 (m, 4H), 9.63 (s, 1H).
4-(6-Hydroxyhexyloxy)-4”-cyanoterphenyl (7). 4-Hydroxyl-4”-Cyanoterphenyl (6) (2.4 g, 8.7 mmol), 6-chloro-1-hexanol (2.1 g, 11.3 mmol), K2CO3 (3.6 g, 26.1 mmol) and less KI (20 mg) were dissolved in 200 mL of DMF and refluxed overnight.
The reaction mixture was then cooled and poured into 200 mL of water and stirred for 2 h. The crude product was extracted with ethyl acetate and the organic layers were washed with a saturated 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 absolute ethanol to give a colorless solid (2.6 g, 80%).
1H NMR (ppm, d6-DMSO), δ: 1.28-1.70 (m, 8H), 3.44 (m, 2H), 3.99 (t, J = 6.6 Hz, 2H), 4.32 (t, J = 5.0 Hz, 1H), 7.00 (d, J = 8.6 Hz, 2H), 7.62 (d, J = 8.6 Hz, 2H), 7.71(d, J = 8.4 Hz, 2H), 7.80 (d, J = 8.4 Hz, 2H), 7.90 (m, 4H).
4-(6-Methacryloyloxyhexyloxy)-4”-cyanoterphenyl (8). 4-(6-Hydroxyhexyloxy)- 4”-cyanoterphenyl (7) (2.6 g, 7.0 mmol), triethylamine (2.1 g, 21 mmol), and 2,6-di-tertbutyl-4-methylphenol (200 mg, as a thermal inhibitor) were dissolved in 150 mL of anhydrous THF under a nitrogen atmosphere and then methacryloyl chloride (2.2 g, 21 mmol) was added dropwise. The reaction mixture was heated under reflux overnight and then cooled and poured into 200 mL of an aqueous solution of NH4Cl (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 removing the solvent by rotavapor, the resulting solid was purified by column chromatography using hexane/ethyl acetate (7:3) as an eluant to yield a colorless solid (2.3 g, 75%). 1H NMR (ppm, CDCl3), δ: 1.46 -1.53 (m, 4H), 1.62-1.74 (m, 2H), 1.82 (m, 2H), 1.93 (s, 3H), 4.00 (t, J = 6.3 Hz, 2H), 4.15 (t, J = 6.6 Hz, 2H), 5.53 (m, 1H), 6.09 (m, 1H), 6.97 (d, J = 8.7 Hz, 2H), 7.55 (d, J = 8.7 Hz, 2H), 7.64 (m, 4H), 7.71 (m, 4H). Element analysis for C29H29NO3: Calc. C, 79.24; H, 6.65; N, 3.19; Found C, 79.30; H, 6.71; N, 3.05.
Macroinitiator (I1). A solution of 1.8 g (7.7 mmol) of 2-bromo-2-methylpropionyl chloride in 10 mL of dry THF was added to a mixture of 1.1 g (10 mmol) of
triethylamine and 10 g (5 mmol) of PEG methyl ether with an Mn of 2000 gmol-1 in 30 mL of THF at 0 °C, and then the mixture was stirred for 18 h. After the mixture was filtered, half of the solvent was evaporated, and the PEG macroinitiator was precipitated into cold ether. After dissolution in ethanol, the solution was stored in refrigerator to recrystallize to yield a white solid. Yield: 55%. 1H NMR (ppm, CDCl3), δ: 1.94 (s, 6H), 3.38 (s, 3H), 3.54-3.76 (m, 174H), 4.33 (dd, 2H).
Preparation of Macroinitiators I2, I3 and I4
Polymerzation of macroinitiator I3. In a Schlenk flask, 3.46 mg of N,N,N’,N’,N”-pentamethyldiethylenetriamine (PMDETA, 0.02 mmol), 14.3 mg of CuBr (0.1 mmol), and 5.5 g of styrene (52.8 mmol) were added and stirred for 30 min.
74 mg of 1-(1-bromoethyl)benzene (0.4 mmol) was added, and the mixture was immediately frozen in liquid nitrogen under vacuum. After several freeze-thaw cycles, the flask was sealed under vacuum and put in an oil bath at 100 °C for 20 h. After the reaction, the content was dissolved in chloroform. After being concentrated, the chloroform solution was precipitated into methanol. The precipitation was repeated three times. The final product was dried at 50 °C under vacuum. Mn = 10337 gmol-1 and PDI (Mw/ Mn) = 1.28 (by GPC).
The macroinitiators (I2 and I4) were synthesized by using analogous procedures via ATRP and the information of Mn and PDI for I2 and I4 are listed below.
Macroinitiator, I2: Mn = 1016 gmol-1 and PDI (Mw/ Mn) = 1.11 (by GPC).
Macroinitiator, I4: Mn = 27474 gmol-1 and PDI (Mw/ Mn) = 1.35 (by GPC).
Preparation of Homopolymer and Block Copolymers
Block copolymers (P1-P4) and homopolymer (P5) were synthesized by using the analogous procedure except for the utilization of different initiators (see Scheme 3.2).
Preparation of polymer P1 (polymerization of monomer 8 with macroinitiator I1). 4 mg (0.04 mmol) of CuCl, 20 mg (0.01 mmol) of I1, and 440 mg (1 mmol) of
monomer (8) were mixed under nitrogen. 11 μL (23 mg, 0.1 mmol) of 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) in 6 mL of anisole was added through a syringe. The mixture was degassed three times using the freeze-pump-thaw procedure and sealed under vacuum. After stirring for 30 min at room temperature, the mixture was reacted in a preheated 80 °C oil bath for 12 h. The solution was passed through a neutral Al2O3 column with THF as an eluant to remove the catalyst. The white filtrate was concentrated under reduced pressure and reprecipitated twice into methanol. The white product of polymer was collected by filtration and dried under vacuum. Yield: 150 mg (34 %). Mn = 10258 gmol-1 and PDI (Mw/ Mn) = 1.17 (by GPC).
P2: Yield: 158 mg (33 %). Mn = 9772 gmol-1 and PDI (Mw/ Mn) = 1.28 (by GPC, the soluble part of the polymer).
P3: Yield: 206 mg (32 %). Mn = 15632 gmol-1 and PDI (Mw/ Mn) = 1.34 (by GPC, the soluble part of the polymer).
P4 and P5: No data obtained due to poor solubilities of longer cyanoterphenyl blocks.
Scheme 3.1 Synthetic routes of monomers and macroinitiators
Scheme 3.2 Synthetic routes of polymers
I1, I2, I3, and I4 + OC6H12O CN
O
(8), monomer
CuCl, HMTETA
Anisole P1, P2, P3, and P4
C2H5OOC(CH3)2Br + OC6H12O CN
Atom transfer radical polymerization (ATRP) has proven to be a very powerful polymerization technique for the preparation of block copolymers from a wide variety of monomers.49, 50 In this work, macroinitiators, including poly(ethylene oxide) (I1), 4 polystyrenes (I2 and I3), 34 and poly(ethylene oxide)-b-polystyrene (I4), were used to copolymerize cyanoterphenyl mathacrylate monomers to produce LC cyanoterphenyl block copolymers. GPC measurements indicated that all macroinitiators (I1-I4) and
the diblock copolymers (P1-P3) with extended molecular weights had narrow polydispersities. Due to poor solubilities of longer cyanoterphenyl blocks in P4 and P5, no GPC data were obtained for these polymers. The number-average molecular
weights (Mn) of macroinitiators (I2, I3, and I4) determined by GPC are 1016, 10337, and 27474 gmol-1 with polydispersities (PDI) = 1.11, 1.28, and 1.35, respectively. The precursor of I1 was purchased from the commercially available poly(ethylene oxide) (Mn=2000 gmol-1) with PDI = 1.04. The cyanoterphenyl homopolymer (P5) exhibited poor solubility in conventional organic solvents so as not to characterize and process into films.5 Figure 3.1 shows the NMR spectra of block copolymers P1, P3 and P4, where the NMR spectrum of P2 is omitted due to its similarity with that of P3. In Table 3.1, the number-average molecular weights (Mn) of the diblock copolymers containing LC cyanoterphenyl blocks (P1-P3) were determined by GPC, in which chloroform was used as an eluant. The triblock copolymer P4 also exhibited poor solubility in conventional organic solvents. Though the flexible chains of PEO and PS blocks was the longest one in P4, it suggested that the molecular weight of LC cyanoterphenyl block might be polymerized to such a high degree of polymerization to induce poor solubility.
Figure 3.1 1H NMR spectra of block copolymers P1, P3, and P4.
Table 3.1 Molecular Weights and Thermal Properties of Block Copolymers P1-P4
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 5 °C /min).
3.3.2 Thermal Properties and X-ray Investigation
The average molecular weights and polydispersity indices of these macroinitiators (I1-I4) and block copolymers (P1-P3) were obtained by GPC. The thermal stability of polymers (P1-P4) under an atmosphere of nitrogen was evaluated by thermogravimetric analysis (TGA), which indicates that Td (the degradation temperature of 5% weight loss in nitrogen) ≥ 325 °C for all polymers (shown in Table 3.1). The mesomorphism was characterized by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The phase transition temperatures and enthalpies of all polymers are summarized in Table 2. Regarding these results, all block copolymers (P1-P4) possessed the smectic A phase, which also existed in the cyanoterphenyl homopolymer (the same structure as P5) in a previous study.5 The DSC thermograms are displayed in Figure 3.2. To avoid thermal decomposition, these polymers were heated up to about 250 °C (with a heating rate of 5 °C/min) and their melting temperatures were not observed even over 250 °C. All block copolymers revealed clearing temperatures (Tc) around 275~300 °C where thermal decomposition occurred. Figure 3.3 showed that a fan-shaped texture of the corresponding smectic A phase of P1 observed by POM at 270 °C (cooling).
Table 3.2 Phase Behavior of Block Copolymers P1-P4 a,b
a Transition temperatures (°C) and enthalpies (in parentheses, kJ/mol) were determined by DSC (a heating rate of 5 °C /min).
b K = crystalline; SA = smectic A
c Tc : the clearing (isotropization) temperature which was observed by polarizing optical microscopy (POM).
Figure 3.2 DSC thermograms of block copolymers P1-P4 during the first heating
scan.
Figure 3.3 The optical texture of the mesophase (SA) of P1 observed by POM at 270
°C (cooling).
Because it is not easy to observe the glass transition temperatures (Tg) of these bock copolymers, Tg values were detectable by liquid nitrogen quenching of polymers in the first heating scans of DSC measurements (with a heating rate of 5
°C/min). At this rate, the DSC results can indicate Tg values of all block copolymers more clearly and their Tg values are in the range of 82 to 163 °C referred to Table 3.1.
For all block copolymers in Figure 3.2, P3 and P4 can easily reveal the glass transition temperatures (Tgs), but Tgs of P1 and P2 could not be observed due to the low Tg of PEO block in P1 and the short block of polystyrene in P2. The glass transition temperature (Tg) of P3 (at 98 °C) was mostly contributed from the polystyrene block with PS repeating units of 98.51, 52 However, two glass transition temperatures (at 83 and 154 °C) were present in P4, which are attributed to the immiscibility between the more extended polystyrene block (with PS repeating units of 244 and Tg = 83 °C) and the LC cyanoterphenyl block (with Tg = 154 °C). The lower Tg of the more extended polystyrene block (with PS repeating units of 244 and Tg = 83 °C) in P4 in comparison with the higher Tg of the shorter polystyrene block (with PS repeating units of 98 and Tg = 98 °C) in P3 is due to the plasticizer effect of PEO block in the triblock copolymer P4. Hence, this situation may serve as evidence
for the microphase separation morphology of the triblock copolymer P4.53
In order to elucidate the structures of the mesophases, X-ray diffraction (XRD) measurements were carried out at the temperature ranges of mesophases for polymers P1-P5. As shown in Figure 3.4, the XRD patterns of polymers P1-P5 are almost
identical and their layer d-spacing values are around 37 Å. In addition, the layer d-spacing values in the ratio of 1:1/2 indicate a lamellar order exits in the mesophases, and the XRD data are summarized in Table 3.3. Furthermore, a fan-shaped texture is clearly observed by POM as shown in Figure 3.3, which is a characteristic texture of the smectic A phase. According to the molecular modeling calculation, the layer d-spacing value of coplanar structure in monomer (8) is around 35.6 Å (the layer d-spacing value around 37 Å by XRD patterns). Therefore, a possible layer structures of block copolymers P1-P5 is suggested to be interdigitated packing of rods. From this evidence, the layer structures of all polymers, P1-P5, have little relationship with respect to the flexible blocks, such as PS and PEO blocks.
10 20 30 40
Intensity (a. u.)
P5
P4
P3
P2
2 theta ( o )
P1
Figure 3.4 X-ray diagrams of all polymers P1-P5.
Table 3.3 XRD Diffraction Data of All Polymers P1-P5 at 190 °C
a The theoretical d-spacing value is 35.6 Å for all polymers P1-P5.
3.3.3 Optical Properties
The photophysical properties of all block polymers P1-P4 containing luminescent cyanoterphenyl blocks were studied by photoluminescence (PL) and UV-visible absorption spectra in dilute chloroform solutions and solid films. The optical properties of all polymers are summarized in Table 3.4. Due to the identical rigid cores of luminescent cyanoterphenyl blocks, all synthesized polymers in solutions have almost the same maximum absorption wavelength around 310 nm and emit blue light at approximately λmax,PL = 435 nm in solid films.
Figure 5 shows an example of UV-visible and PL spectra of diblock copolymer P1.
Compared with the maximum PL wavelength in solutions, the materials in solid films exhibit red-shifted PL emission owing to the π- π* aggregation of the rigid cores (luminescent cyanoterphenyl blocks). In terms of PL wavelengths of all block copolymers in dilute solutions, Figure 3.6 indicates that P2 and P4 are more red-shifted than P1 and P3. The red-shifted PL emission in P4 might result from a large molecular weight in LC cyanoterphenyl block with higher aggregation of
emitting cyanoterphenyl moieties. In contrast to P1and P3, P2 has shorter flexible polystyrene chains resulting in a stronger π- π* aggregation effect of the cyanoterphenyl blocks.
Table 3.4 Absorption and Photoluminescence Spectral Data of Block Copolymers P1-P4.
λmax,Abs (nm) λmax,PL (nm)
Sample
solutiona film solutiona film
P1 310 309 403 427
P2 312 314 416 435
P3 310 308 404 434
P4 312 310 431 438
a Absorption and PL emission spectra were recorded in dilute CHCl3 solutions at room temperature.
250 300 350 400 450 500 550 600 650
PL Intensity (a.u.)
Absorption (a. u.)
Wavelength (nm)
UV-Vis. (solutions) UV-Vis. (solid films) PL (solutions) PL (solid films)
Figure 3.5 Absorption (solid lines) and PL (dash lines) spectra of P1 in solutions (CHCl3 as solvent) and solid films.
350 400 450 500 550 600
PL Intensity (a.u.)
Wavelength (nm)
P1 P2 P3 P4
Figure 3.6 PL spectra of block copolymers P1-P4 in solution (CHCl3 as solvent).
3.4 Conclusion
Atom transfer radical polymerization (ATRP) was employed to fabricate block copolymers composed of different macroinitiators and liquid crystalline cyanoterphenyl-based polymethacrylate blocks. Thermal and XRD investigations indicate that all polymers exhibit the interdigitated packing smectic A phase which have little relationship with respect to the flexible PS and PEO blocks. In terms of PL wavelengths of all block copolymers in dilute solutions, P2 and P4 are more red-shifted than P1 and P3, which might be due to the π- π* aggregation effect of the cyanoterphenyl blocks in block copolymers.
Chapter 4
Synthesis and characterization of side-chain liquid crystalline homopolymers and block copolymers containing biphenyl-4-ylthiophene and biphenyl-4-ylfluorene pendants
4.1 Introduction
A wide variety of liquid crystalline (LC) copolymers 31, 33, 54-58 with optimized structures have been developed in recent years. In addition, numerous liquid crystalline (LC) block copolymers consisting of mesogenic blocks and isotropic blocks were synthesized via different types of living free-radical polymerization 35-36,
38, 59 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 copolymers, where mesogenic groups connected along the backbones are main-chain copolymers (such as copolyesters composed of p-hydroxybenzoic acid and poly(ethylene terephthalate), etc.)60-62, and pendent mesogenic groups attached to the backbones via flexible spacers are side-chain copolymers.
Among these LC copolymers, side-chain LC copolymers have attracted significant interests because of their liquid crystalline behavior as low molecular mass pendent mesogens and their easy processing characteristic as polymers. Furthermore, side-chain LC polymers are often used in electro-optical applications due to their lower viscosities and easier alignment tendencies than those of main-chain LC polymers. Several kinds of rigid cores, including azobenzene 4, 36-37 and biphenyl units
41-46, were applied to mesogenic groups of side-chain LC polymers. In previous researches 63, 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 composed of methacrylates containing pendent biphenyl-4- ylthiophene (M1) and biphenyl-4-ylfluorene (M2) groups were synthesized by atom transfer radical polymerization (ATRP), where block copolymers P3 and P4 were produced from styrene-macroinitiator (SMi). Furthermore, the thermal, mesogenic, and photoluminscent (PL) properties of all polymers were also investigated in this study.
4.2 Experimental Section 4.2.1 Measurements
1H NMR spectra were recorded on a Varian unity 300 MHz spectrometer using CDCl3 and d6-DMSO solvents. Elemental analyses were performed on a HERAEUS CHN-OS RAPID elemental analyzer. Transition temperatures were determined by differential scanning calorimetry (DSC) (Perkin–Elmer, model: Diamond) with heating and cooling rates of 5 °C/min. The mesomorphic properties were studied using a polarizing optical microscope (POM) (Leica, model: DMLP) equipped with a hot stage. Thermogravimetric analysis (TGA) was conducted on a Du Pont Thermal Analyst 2100 system with a TGA 2950 thermogravimetric analyzer at a heating rate of 10 °C/min under nitrogen. Gel permeation chromatography (GPC) analysis was conducted on a Waters 1515 separation module with chloroform as the eluant against a polystyrene calibration curve. UV-visible absorption spectra were recorded in dilute THF solutions (10-6 M) on a HP G1103A spectrophotometer. Photoluminescence (PL) spectra were obtained on a Hitachi F-4500 spectrophotometer.
4.2.2 Materials
Unless otherwise specified, chemicals and solvents were reagent grade and
purchased from Aldrich, ACROS, TCI, and Lancaster Chemical Co. Dichloromathane and THF were distilled to keep anhydrous before use. Pyridine was dried by refluxing over calcium hydride. The other chemicals were used without further purification.
purchased from Aldrich, ACROS, TCI, and Lancaster Chemical Co. Dichloromathane and THF were distilled to keep anhydrous before use. Pyridine was dried by refluxing over calcium hydride. The other chemicals were used without further purification.