Chapter 4. Synthesis and Characterization of Side-Chain Liquid Crystalline
4.3.4 Photophysical Properties
The photophysical properties, including the UV-visible absorption and photoluminescence (PL) spectral data, of all polymers in THF solutions are summarized in Table 4.4. By reason of the identical rigid cores of luminescent biphenyl-4-ylthiophene 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 λmax,PL = 377 nm in solutions. Comparably, monomer M2 and polymers P2 and P4 have the identical rigid cores of luminescent biphenyl-4-ylfluorene units, therefore, they have almost the same values of the maximum absorption wavelength around 322 nm and the maximum PL wavelength (λmax,PL) around 386 nm in solutions.
Table 4.4 UV-visible absorption and photoluminescence spectral data of monomers (M1 and M2) and polymers (P1-P4)
Sample λmax,Abs (nm)a λmax,PL (nm)a Φ (%)b
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.
Figure 4.4 shows the UV-visible absorption and PL spectra of monomers M1 and M2 in solutions. Compared with M1, monomer M2 consisting of fluorene units result in longer maximum absorption wavelengths and PL wavelengths due to longer conjugation lengths in rigid cores. By the same reason, the maximum absorption and PL wavelengths of polymers P2 and P4 are more red-shifted than those of polymers 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.
Accordingly, the quantum yields of polymers P2 and P4 are larger than those of polymers P1 and P3.
300 400 500 Wavelength (nm)
PL Intensity (a. u.)
Absorption (a. u.)
UV-Vis. (M1) UV-Vis. (M2) PL. (M1) PL. (M2)
Figure 4.4 UV-visible absorption (solid lines) and PL (dash lines) spectra of monomers M1 and M2 in solutions (THF as solvent).
Due to the poor solubility of polymers P1 and P3 and lower molecular weights of monomers M1 and M2, the photophysical properties of these compounds in solid films were not obtained. Compared with solutions, solid films of polymers P2 and P4 in Figure 4.5 exhibit red-shifted PL emissions around 396 nm owing to the π-π*
aggregation of the rigid cores. In addition, the differences of photophysical properties between 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. Figure 4.6 displays the PL spectra of P2 solid films by spin-coating and quenching (by liquid N2) from 130 °C (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 Figure 4.6. 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 report56. In order to evaluate the effect of mesogenic structure on photoluminescence properties, polarized PL spectra (as shown in Figure 4.7) were measured from aligned P2 solid film by quenching from 130 °C on a rubbing PI substrate. Polarization ratio (PL⎜⎜/ PL⊥) was about 1.43, where PL⎜⎜is the maximum PL emission intensity as the polarizer is parallel to the rubbing direction, and PL⊥is the maximum PL emission intensity as the polarizer is perpendicular to the rubbing direction. This result shows the effect of mesogenic alignment of P2 on rubbing PI substrate can induce a polarized PL emission with a polarization ratio 1.43.
350 400 450 500
Wavelength (nm)
PL Intensity (a. u.)
P2-solution P4-solution P2-solid film P4-solid film
Figure 4.5 PL spectra of polymers P2 and P4 in solid films and solutions (THF as solvent).
350 400 450 500 550
PL Intensity (a. u.)
Wavelength (nm)
solid film at room temperature solid film quenching from LC phase
Figure 4.6 PL spectra of polymer P2 in the solid state by spin-coating and quenching (by liquid N2) from 130 °C (the nematic phase).
350 400 450 500 550
Wavelength (nm)
PL Intensity (a. u.)
parallel to polarizer perpendicular to polarizer
Figure 4.7 Polarized PL spectra of aligned P2 solid film by quenching from 130 °C on a rubbing PI substrate, where PL⎜⎜ is the parallel PL intensity as the polarizer is parallel to the rubbing direction, and PL⊥ is the perpendicular PL intensity as the polarizer perpendicular to the rubbing direction.
4.4 Conclusion
Atom transfer radical polymerization (ATRP) was employed to fabricate block copolymers consisting of PS macroinitiators and liquid crystalline polymethacrylate blocks containg biphenyl-4-ylthiophene (M1) and biphenyl-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. 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 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 130 °C (the nematic phase). The effect of mesogenic alignment of P2 on rubbing PI substrate can induce a polarized PL emission with a polarization ratio 1.43.
Chapter 5
Synthesis and Characterization of Liquid Crystalline Side-Chain Block Copolymers Containing Luminescent
4,4’-Bis(biphenyl)fluorene Pendants
5.1 Introduction
During the past decade, there are several types of liquid crystalline (LC) block copolymers produced by living free-radical polymerizations.35-38, 64 Matyjaszewski’s group 65 and Sawamoto and co-workers 66 independently developed transition metal catalyzed living free radical polymerization known as atom transfer radical polymerization (ATRP), which is typically initiated by an alkyl halide (R-X) and catalyzed by a transition metal complex, such as CuX/bpy. ATRP has been demonstrated to provide controlled polymerizations of styrenes,methacrylates, and acrylonitriles with variations of compositions and architectures.
Recently, ATRP were also used in the synthesis of side-chain LC block copolymers, 34, 67-70 in which mesogenic pendent groups were grafted onto polymer backbones to form side-chain LC blocks. The interest in this type of materials resides in the cohesive 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. In order to introduce luminescent properties into LC polymers, the combination of long conjugation rigid cores with flexible chains is required for the molecular design of luminescent LCs.71-73 However, the microphase structures of side-chain LC polymers were influenced by the mesogenic groups, such as azobenzene 4,39-40 and biphenyl 41-46 units, which were
frequently used in LC monomers.
In our previous studies 58,63, main-chain block copolymers containing conjugated fluorene, thiophene, and biphenyl backbones as well as side-chain block copolymers containing biphenyl-4-ylthiophene or cyanoterphenyl pendants possess the smectic A and columnar (Colh and Colr) phases, but side-chain block copolymers consisting of biphenyl-4-ylfluorene pendants possess the nematic phase. Herein, a series of a new mesogenic homopolymer and block copolymer (P1 and P2), which were composed of styrene-macroinitiators (SMi) and methacrylates with pendent 4,4′-bis(biphenyl)fluorene (M1) groups, were synthesized through atom transfer radical polymerization(ATRP). In addition, random copolymer P3 and block copolymer P4 consisting of pendent 4,4′-bis(biphenyl)fluorene units (M1) and biphenyl-4-ylfluorene units (M2) were also prepared by ATRP. Furthermore, thermal, mesogenic, and PL properties of all polymers were also investigated in this study.
5.2 Experimental Sections 5.2.1 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 mesogenic 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. High-resolution electron impact mass data were obtained on a Finnigan-MAT-95XL. 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.
5.2.2 Materials
Chemicals and solvents were reagent grade and purchased from Aldrich, ACROS, TCI, and Lancaster Chemical Co. Dichloromathane and THF were distilled to keep anhydrous before use. Pyridine was dried by refluxing over calcium hydride. The other chemicals were used without further purification.
5.2.3 Synthesis
Atom transfer radical polymerization (ATRP) is very versatile and is tolerant for a wide range of functional groups present either in the monomers, solvents, or
initiators.49,50, 63 Schemes 5.1 and 5.2 summarize the steps involved in the syntheses of the macroinitiator (SMi), monomers (M1 and M2), and polymers (P1-P4), and the details of each step are 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 = 6.9 Hz, 3H), 1.29-1.47 (m, 10H), 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).63
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 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 allowed to cool to room temperature overnight. The final solution was acidified with 100 mL of 10 % HCl solution and stirred for 45 min at room temperature. The solution was washed with saturated sodium carbonate solution 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), 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).63
Compound 5. Compound 4 (2.0 g, 5.3 mmol), compound 3 (4.0 g, 12.3 mmol), and tetrakis(triphenylphosphine)palladium(0) (307 mg, 0.27 mmol) were reacted in THF (120 mL) for 10 min, and then 80 mL 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 (3.3 g, 87%). 1H NMR (ppm, CDCl3), δ:
0.42 (t, J = 6.9 Hz, 6H), 0.89 (t, J = 6.9 Hz, 6H), 1.31-1.61 (m, 20H), 1.84 (m, 4H), 2.14 (q, J = 7.3 Hz, 4H), 4.01(t, J = 6.0 Hz,4H), 6.98 (d, J = 7.8 Hz, 4H), 7.57-7.80 (m, 18H).
Compound 6.Compound 5 (3.3 g, 4.5 mmol) was dissolved in dry chloroform
(150 mL) under nitrogen and then boron tribromide (3.4 g, 13.6 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 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 (2.4 g, 94%). 1H NMR (ppm, d6-DMSO), δ: 0.29 (t, J = 7.2 Hz, 6H), 2.16 (q, J = 7.2 Hz, 4H), 6.87 (d, J = 8.4 Hz, 4H), 7.5 (d, J = 8.7 Hz, 4H), 7.70 (m, 6H), 7.79 (m, 6H), 7.91 (d, J = 7.8 Hz, 2H), 9.63 (s, 2H).
Compound 7.Compound 6 (2.2 g, 3.94 mmol) and potassium carbonate (1.09 mg, 7.88 mmol) were dissolved in 50 mL of DMF and then 1-bromooctane (760 mg, 3.94 mmol) was added in solution to react for 24 h by reflux. After cooling to room temperature, the solution was extracted with dichloromethane and water, and the organic layer was dried over magnesium sulfate. The final solution was purified by column chromatography (silica gel, CH2Cl2) to yield a white solid (1.3 g, 54%).1H NMR (ppm, CDCl3), δ: 0.43 (t, J = 7.2 Hz, 6H), 0.90 (t, J = 6.6 Hz, 3H), 1.25-1.48 (m, 10H), 1.82 (m, 2H), 2.14 (q, J = 6.9 Hz, 4H), 4.01 (t, J = 6.6 Hz, 2H), 4.90 (s, 1 H), 6.92-7.01 (m, 4H), 7.53-7.67 (m, 12H), 7.73-7.80 (m, 6H).
Compound 8.Compound 7 (1.2 g, 1.80 mmol), 6-bromo-1-hexanol (421 mg, 2.33 mmol), K2CO3 (740 mg, 5.4 mmol), and KI (20 mg) were dissolved in 80 mL of DMF to reflux 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 layer as 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 (1.1 g, 80%).1H NMR (ppm, CDCl3), δ: 0.43 (t, J = 7.2 Hz, 6H), 0.88 (t, J = 6.0 Hz, 3H), 1.31-1.86 (m, 20H), 2.15 (m, 4H), 3.67 (m, 2H), 4.01 (m, 4H),
7.00 (m, 4H), 7.57-7.80 (m, 18H).
M1. Compound 8 (1.1 g, 1.4 mmol), triethylamine (1.4 g, 14.3 mmol), and 2,6-di-tertbutyl-4-methylphenol (30 mg, as a thermal inhibitor) were dissolved in 100 mL of anhydrous THF under a nitrogen atmosphere and then methacryloyl chloride (448 mg, 4.3 mmol) was added dropwise. 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 removing the solvent by rotavapor, the resulting solid was purified by column chromatography using dichloromethane as an eluant to yield a colorless solid (1.1 g, 92%). 1H NMR (ppm, CDCl3), δ: 0.41 (t, J = 7.2 Hz, 6H), 0.88 (t, J = 6.9 Hz, 3H), 1.31-1.82 (m, 20H), 1.94 (s, 3H), 2.12 (m, 4H), 3.98(m, 2H), 4.15 (m, 4H), 5.54 (s, 1H), 6.09 (s, 1H), 6.98 (m, 4H), 7.55-7.79 (m, 18H). Elemental analysis for C59H66O4: Calc. C, 84.45; H, 7.93; Found C, 84.12; H, 7.74. LRMS (EI) m/z: Calc. 838.4; Found 838.4.
M2. The synthetic route has been reported in our previous study.74 Yield: 83%.
1H NMR (ppm, CDCl3), δ: 0.36 (t, J = 6.9 Hz, 6H), 1.45 -1.56 (m, 4H), 1.67-1.83 (m, 4H), 1.94 (s, 3H), 2.06 (m, 4H), 4.01 (t, J = 6.3 Hz, 2H), 4.16 (t, J = 6.6 Hz, 2H), 5.54 (s, 1H), 6.09 (s, 1H), 6.98 (d, J = 8.7 Hz, 2H), 7.32-7.36 (m, 3H), 7.55-7.77 (m, 10H).
Elemental 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.
Polymerzation of macroinitiator SMi. In a Schlenk flask, N,N,N’,N’,N”-pentamethyldiethylenetriamine (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
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 and the precipitation was repeated for three times. The final product was dried at 50 °C under vacuum. Yield: 75%. The number-average molecular weight measured by GPC is Mn
= 6196 gmol-1 with PDI (Mw/ Mn) = 1.11.
General synthetic procedures of all polymers
According to analogous procedures as shown in scheme 5.2, P1 -P4 were synthesized by utilization of different initiators.
An example of polymerization for polymer P2
4 mg (0.04 mmol) of CuCl, 63 mg (0.01 mmol) of SMi, and 251 mg (0.3 mmol) of M1 were mixed under nitrogen. 27 μL (22.9 mg, 0.1 mmol) of 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) in 3 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 at 100 °C in a preheated 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: 125 mg (40%). 1H NMR (ppm, CDCl3), δ:
0.35 (broad), 0.88 (broad), 1.28-1.78 (broad), 2.01 (broad), 3.94 (broad), 6.56 (broad), 6.98 (broad), 7.55(broad). The number-average molecular weight measured by GPC is Mn = 14852 gmol-1 with PDI (Mw/ Mn) = 1.15.
P1: Yield: 85 mg (34%). 1H NMR (ppm, CDCl3), δ: 0.36 (broad), 0.88 (broad), 1.28-1.94 (broad), 2.03 (broad), 3.97 (broad), 6.94 (broad), 7.57 (broad). The
number-average molecular weight measured by GPC is Mn = 10007 gmol-1 with PDI (Mw/ Mn) = 1.21.
P3: (M1/M2/initiator =15/15/1). Yield: 75 mg (30%). 1H NMR (ppm, CDCl3), δ:
0.36 (broad), 0.86 (broad), 1.24-1.94 (broad), 2.04 (broad), 3.99 (broad), 6.96 (broad), 7.30 (broad), 7.62 (broad). The number-average molecular weight measured by GPC is Mn = 6275 gmol-1 with PDI (Mw/ Mn) = 1.31.
P4: (M1/M2/initiator =15/15/1). Yield: 121 mg (39%).1H NMR (ppm, CDCl3), δ:
0.35 (broad), 0.87 (broad), 1.28-1.79 (broad), 2.03(broad), 3.96 (broad), 6.55 (broad), 6.94(broad), 7.29 (broad), 7.55 (broad). The number-average molecular weight measured by GPC is Mn = 10643 gmol-1 with PDI (Mw/ Mn) = 1.22.
Scheme 5.1 Synthetic routes of the monomer (M1) and the macroinitiator (SMi).
Scheme 5.2 Synthetic routes of polymers (P1-P4).
5.3 RESULTS AND DISCUSSION
5.3.1 Synthesis and Characterization
In this study, the styrene-macroinitiator (SMi),65 was used to copolymerize with mathacrylate monomers containing luminescent 4,4′-bis (biphenyl)fluorene units (M1) to produce diblock copolymer P2 by ATRP. In addition, random copolymer P3 and block copolymer P4 containing pendent 4,4′-bis(biphenyl)fluorine units (M1) and biphenyl-4-ylfluorene units (M2) were synthesized by similar (ATRP) procedures.
The number-average molecular weights (Mn) and PDI values of all polymers (P1-P4) containing mesogenic 4,4′-bis(biphenyl)- fluorene units and biphenyl-4-ylfluorene units are shown in Table 5.1. The number-average molecular weight (Mn) of macroinitiators (SMi) is 6196 gmol-1 with a PDI value of 1.11. Comparing NMR sprctra of random copolymer P3 and block copolymer P4, a broad peak of P4 at 6.56 ppm belonged to the chemical shift of polystyrene block and a broad peak at 6.98 ppm incorporated chemical shifts of M1 and M2 with that of polystyrene block, so P4 could be recognized as a diblock copolymer. However, due to the overlapped proton peaks around phenyl and alkyl groups of M1 and M2 in NMR spectra of polymers P3 and P4, the molar ratio of M1 and M2 in random copolymer P3 and block copolymer P4 could not be distinguished, so the values of x and y in the chemical structures of random copolymer P3 and block copolymer P4 in Scheme 6.2 are undetermined.
Table 5.1 Molecular weights and polydispersity indexes of polymers P1-P4
5.3.2 Thermal and Mesogenic Properties
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) ≥ 350 °C for all polymers (shown in Table 5.2). The mesogenic properties were characterized by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The phase transition temperatures and enthalpies of all polymers are summarized in Table 5.2.
Regarding their mesomorphism, all polymers exhibit the nematic phase with mesophasic ranges wider than 200 °C. In our previous studies 63,74, block copolymers containing conjugated main-chain fluorene, thiophene, and biphenyl blocks as well as block copolymers containing side-chain biphenyl-4-ylthiophene or cyanoterphenyl blocks possess the smectic A and columnar (Colh and Colr) phases, but block copolymers consisting of side-chain biphenyl-4-ylfluorene blocks possess the nematic phase. Compared with the smectic A phase of the polymers in our previous studies, the side-chains of the rigid cores in polymers P1-P4 composed of pendent 4, 4′-bis(biphenyl)fluorene and biphenyl-4-ylfluorene units were separated by the diethyl groups on 9 position of fluorene units which cause the reduction of lateral interaction among rigid rods and thus to favor the nematic phase. To avoid thermal decomposition, these polymers were only heated up to about 320 °C (with a heating rate of 5 °C/min) and all polymers (P1-P4) revealed isotropization temperatures (Ti) around 299 ~ 316 °C. Figure 5.1 showed that a schlieren texture of the corresponding nematic phase of P1 observed by POM at 280 °C (cooling).
Table 5.2 Thermal properties of polymers P1-P4 a,b,c
a Temperature of 5% weight loss measured by TGA under nitrogen.
b Transition temperatures (°C) and enthalpies (in parentheses, kJ/g)were determined by DSC (with heating rates of 5 °C /min).
c N = nematic phase; I = isotropization temperature.
Transition temperatures (measured by DSC) of M1 and M2:
M1 : K 104.4 °C (31.5 kJ/g) N 266.0 °C (1.86 kJ/g) Ti
M2 : K 60.4 °C (3.4 kJ/g) N 98.0 °C Ti (Ti was characterized by POM)
Figure 5.1 The schlieren texture (the nematic phase) of P1 observed by POM at 280°C (cooling).
Because of the unclear glass transition temperatures (Tg) of these block copolymers, Tg values were detectable in the first heating scans of DSC measurements (with a heating rate of 5°C/min) by quenching of polymers (from 200 °C) in liquid nitrogen.
By this quenching process, the DSC results can reveal Tg values of all polymers more clearly (in the range of 56 to 90 °C as shown in Table 5.2). In Table 5.2, polymers P1 and P3 exhibit the glass transition temperatures (Tgs) at 60 and 56 °C, respectively.
Tgs of P2 and P4 (around 90 °C) were mostly contributed from the polystyrene block.51, 52
In order to elucidate the structures of the mesophases, X-ray diffraction (XRD) measurements were carried out at the temperature ranges of mesophases, which were determined by DSC and POM, for polymers P1–P4. The nematic phase of all polymers characterized by the schlieren texture of POM is further confirmed by the results of no refraction peaks observed in the XRD patterns.
5.3.3 Photophysical Properties
The photophysical properties of polymers P1-P4 including UV-visible absorption spectra and photoluminescence (PL) spectra in THF solutions are summarized in Table 5.3. Due to identical rigid cores of luminescent 4,4′-bis(biphenyl)fluorene pendants (M1), the synthesized polymers P1 and P2 have almost the same maximum absorption wavelength around 343 nm in solutions and solid films. In addition, the maximum photoluminescence wavelengths (λmax,PL ) were around 396 nm in solutions and 425 nm in films, respectively. Polymers P3 and P4 both possess the biphenyl-4-ylfluorene pendants (M2), therefore, they had the same maximum absorption wavelength around 336 nm in solutions and films. Figure 5.2 shows an example of PL spectra of the monomers (M1 and M2) and polymers (P1 and P3) in solutions. Compared with M2 (which was reported in our previous study) 58, M1 consisting of an additional biphenyl unit results in the longer conjugation lengths than M1. Hence, the maximum absorption wavelengths and PL wavelengths of M1 are
more red-shifted than M2. In Figure 5.3, due to the biphenyl-4-ylfluorene pendants of P3, the maximum absorption wavelength of P3 was more blue-shifted than that of P1 (around 343nm) in films and solutions. However, the λmax,PL values of P1 and P3 were almost the same (396 nm in solutions and 423 nm in films). This phenomenon may be originated from the energy transfer of biphenyl-4-ylfluorene pendants to 4,4′-bis(biphenyl)fluorene pendants which can be confirmed by the overlapped UV-visible spectra of M2 and PL spectra of M1. However, Intermolecular aggregation of luminescent pendants could cause shoulders of P1 and P3 around 440 nm in Figure 3. In addition, the quantum yields of all polymers consist of 4,4′-bis(biphenyl)fluorene groups (70%) were better than other analogous polymers (around 20%) in our previous studies.63,74
Table 5.3 UV-visible Absorption and Photoluminescence Spectral Data of Monomers M1-M2 and Polymers P1-P4
λmax,Abs (nm) λmax,PL (nm)
Sample
solutiona film solutiona film Φ (%)b
M1 344 - 392 - 74
M2 c 322 - 385 - 23
P1 344 343 396 425 73
P2 344 340 397 424 70
P3 338 336 395 422 70
P4 337 336 394 423 72
a Absorption and PL emission spectra were recorded in dilute THF solutions at room temperature.
b 9,10-Diphenylanthrance in THF is used as the reference of the quantum yield.
c UV-visible absorption and PL data of M2 in solutions was reported in our previous study.22
350 400 450 500
PL Intensity (a. u.)
Wavelength (nm)
M1-solution M2-solution P1-solution P3-solution
Figure 5.2 PL spectra of monomers (M1 and M2) and polymers (P1 and P3) in solutions.
300 350 400 450 500 550
wavelength (nm)
Absorption (a. u.) PL intensity (a. u.)
P1 (UV-solid film) P3 (UV-solid film) P1 (PL-solid film) P3 (PL-solid film)
Figure 5.3 UV-visible absorption and PL spectra of P1 and P3 in solid films.
5.4 CONCLUSION
Polymers (P1 and P2) containing 4,4′-bis(biphenyl)fluorene pendants (M1) and random copolymers (P3 and P4) composed of 4,4’-bis(biphenyl)fluorene pendants (M1) and biphenyl-4-ylfluorene pendants (M2) are polymerized by atom transfer radical polymerization (ATRP). Thermal and XRD investigations indicate that polymers exhibit the nematic phases which have little relationship with respect to the flexible polystyrene block, but the glass transition temperatures (Tgs) of block copolymers P2 and P4 containing polystyrene blocks are higher than those of
Polymers (P1 and P2) containing 4,4′-bis(biphenyl)fluorene pendants (M1) and random copolymers (P3 and P4) composed of 4,4’-bis(biphenyl)fluorene pendants (M1) and biphenyl-4-ylfluorene pendants (M2) are polymerized by atom transfer radical polymerization (ATRP). Thermal and XRD investigations indicate that polymers exhibit the nematic phases which have little relationship with respect to the flexible polystyrene block, but the glass transition temperatures (Tgs) of block copolymers P2 and P4 containing polystyrene blocks are higher than those of