Jin-Jin Lia,
Jian-Jian Wangb,
Yin-Ning Zhoua and
Zheng-Hong Luo*ab
aDepartment of Chemical Engineering, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. E-mail: luozh@sjtu.edu.cn; Fax: +86-21-54745602; Tel: +86-21-54745602
bDepartment of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P.R. China
First published on 18th April 2014
The origin of the low-energy emission of fluorene-based rod-coil block copolymers still remains controversial. In this work, a series of polyfluorene-based rod-coil block copolymers having different coil segments, i.e., poly[2,7-(9,9-dihexylfluorene)]-block-poly(2,2,3,3,4,4,4-heptafluorobutyl methacrylate, (PF-b-PHFBMA), PF-b-poly(butylmethacrylate) (PF-b-PBMA), PF-b-poly(2-hydroxyethyl methacrylate) (PF-b-PHEMA) and PF-b-poly(acrylic acid) (PF-b-PAA), were synthesized using the ATRP technique. The optical and surface properties and thermal behaviors of these copolymers were systematically investigated. In particular, different thermal treatment conditions, including annealing temperature, annealing time and annealing atmosphere were introduced to study the effect of coil segment on the copolymer spectral stability. The incorporation of PBMA, PHEMA and PAA segments to PF could indeed improve the copolymer spectral stability, while the PHFBMA block brought undesirable low-energy emission. In addition, water contact angle (WCA) measurements of the copolymer films before and after annealing further demonstrated that the low-energy emission of PF-based rod-coil block copolymers was attributed to the molecular aggregation rather than the formation of fluorenone defects.
In recent studies, coil-like blocks nonconjugated and hole/electron transporting molecules were incorporated into polyfluorene backbones to suppress the aforementioned low-energy emission.21–24 So far, many coil segments have been used to form PF-based rod-coil block copolymers, such as poly(methyl methacrylate) (PMMA), polystyrene (PS), poly(2-hydroxyethyl methacrylate) (PHEMA), poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(ethylene oxide) (PEO).25–30 The aggregation and microphase separation behavior of these block copolymers caused to the incompatibility of rod and coil segments can lead to the formation of unusual nanoscale morphologies and photophysical properties in both solution and solid state.14,31 These functions, which can be adjusted flexibly through introducing different coil segments, have attracted great attention for optoelectronic or sensory applications.32,33 However, the influence of coil segments related to the microphase separation behavior on the photophysical properties of PF-based rod-coil block copolymers has not been fully explored.
Recently, we have synthesized a series of PF-based rod-coil block copolymers with PHFBMA as the coil segment by ATRP and reported that the micelle structure formed by different selective solvents has a dramatic influence on their optical properties.34 The previous study was achieved in solution, however the influence of separation behavior on the optical properties of these block copolymers in solid state has not ever been studied. Based on this idea, four coil segments having different polarity and solubility are selected to form PF-based rod-coil block copolymers PF-b-PHFBMA, PF-b-PBMA, PF-b-PHEMA and PF-b-PAA in this study. These copolymers with a similar polymerization degree are well synthesized through ATRP technique. Scheme 1 shows the synthetic routes and corresponding synthetic conditions. Particular attention is focused on the effect of coil segment on the copolymer spectral stability under different thermal treatment conditions. In addition, water contact angle (WCA) measurements about the surface properties and thermal stability of resulting polymers were also carried out.
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Scheme 1 Synthesis route of polyfluorene-based rod-coil block copolymers PF-b-PHFBMA, PF-b-PBMA, PF-b-PHEMA, PF-b-PAA. |
Fourier-transform infrared (FTIR) spectra were obtained on an Avatar 360 FTIR spectrophotometer by dispersing samples in KBr disks.
The molecular weight (Mn) and molecular weight distribution (Mw/Mn, PDI) of the polymers were determined at 40 °C by GPC equipped with a waters 1515 isocratic HPLC pump, three styragel columns (Waters HT4, HT5E, and HT6) and a waters 2414 refractive index detector (set at 30 °C), using THF as the eluent at the flow rate of 1.0 mL min−1. A series of polymethyl methacrylate narrow standards were used to generate a conventional calibration curve.
Thermogravimetric analysis (TGA) was conducted on a SDT Q600 under a heating rate of 10 °C min−1 from room temperature to 800 °C in nitrogen gas atmosphere.
The fluorescence measurement was carried out on a Hitachi 7000 spectrofluorometer with a xenon lamp as a light source.
Water contact angle (WCA) measurements were carried out on a Contact Angle Measuring Instrument (KRUSS, DSA30). The wetting liquid used was water. For each angle reported, at least five sample readings from different surface locations were averaged.
The films used for fluorescence and WCA measurements were prepared as follows. Glass slides were first cleaned successive using acetone, ethyl alcohol and deionized water. The polymer solution (10 mg mL−1 in THF) was then spin-casted onto clean glass slide at 2000 rpm for 30 s, and dried naturally for 24 h.
All polyfluorene-based rod-coil block copolymers were synthesized via ATRP method with the bromo-ended polyfluorene as macroinitiator (shown in Scheme 1). And their chemical structures were confirmed by 1H NMR and FTIR. Fig. 1A-a and A-b show the 1H NMR spectra of PF-b-PHFBMA and PF-b-PBMA respectively. The appearance of peak at 4.42 ppm in Fig. 1A-a and peak at 3.95 ppm in Fig. 1A-b, corresponding to the protons of the –O–CH2–CF2– group of HFBMA and the –O–CH2– group of BMA, respectively, indicate the successful synthesis of PF-b-PHFBMA and PF-b-PBMA. The FTIR spectra of PF-b-PHFBMA (Fig. 2A-a) and PF-b-PBMA (Fig. 2A-b) both exhibit more stronger absorption peak at 1730 cm−1 which is assigned to the stretching vibrations of CO group of PHFBMA and PBMA. These results also confirm the successful synthesis of PF-b-PHFBMA and PF-b-PBMA (the original 1H NMR and FTIR spectra of PF-Br for comparison are respectively shown in Fig. S1-b and S2-b of the ESI†).
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Fig. 2 FTIR spectra of polyfluorene-based rod-coil block copolymers (A-a) PF-b-PHFBMA and (A-b) PF-b-PBMA; (B-a) PF-b-PtBA and (B-b) PF-b-PAA; (C-a) PF-b-P(HEMA-TMS) and (C-b) PF-b-PHEMA. |
Fig. 1B-a and 2B-a respectively show the 1H NMR and FTIR spectra of PF-b-PtBA. The appearance of peak at 1.44 ppm corresponding to the protons of tert-butyl group (Fig. 1B-a) and the more stronger absorption peak at 1730 cm−1 which is assigned to the stretching vibrations of CO group of PtBA (Fig. 2B-a) both indicate the successful synthesis of PF-b-PtBA. After hydrolysis, the 1H NMR spectrum of PF-b-PAA in DMSO (Fig. 1B-b) show the obviously decreases of the tert-butyl resonance at 1.44 ppm and the appearance of the peak at 12.25 ppm assigned to the protons of carboxyl groups. In the FTIR spectrum of PF-b-PAA (Fig. 2B-b), a new adsorption peak characteristic of carboxylic acid group appears at 3436 cm−1 and the signal at 1370 cm−1 attributed to the symmetric bending vibration of the tert-butyl group disappears. These results reveal the successful preparation of PF-b-PAA (the original 1H NMR and FTIR spectra of PF-Br for comparison are respectively shown in Fig. S1-b and S2-b of the ESI†).
The 1H NMR spectra of PF-b-PHEMA-TMS and PF-b-PHEMA are shown in Fig. 1C. In Fig. 1C-a, the appearance of peak at 3.75 ppm corresponding to the protons of the –CH2– group of –CH2–O–Si(CH3)3, and the appearance of peak at 0.16 ppm corresponding to the protons of –CH3 groups of –Si(CH3)3, both indicate the successful incorporation of HEMA-TMS to PF segment. After the deprotection reaction using KF/TBAF as the catalyst, the disappearance of peak at 0.16 ppm (Fig. 1C-b) indicate that the –TMS groups on PHEMA are successful removed. The successful synthesis of PF-b-PHEMA also can be confirmed by the appearance of the adsorption peak at 3436 cm−1 in Fig. 2C-b, which is assigned to the stretching vibrations of the –OH group of PHEMA (the original 1H NMR and FTIR spectra of PF-Br for comparison are respectively shown in Fig. S1-b and S2-b of the ESI†).
Fig. 3 shows the GPC traces for the four as-prepared copolymers. The relatively narrow polydispersity indicates the good controllability of polymerization. However, the result of PF-b-PHFBMA seems to be unusual, which might be due to the collapsed coil structure of PHFBMA block in THF.34 Additionally, the molecular weights of PF-b-PHEMA (Mn = 13137 g mol−1) and PF-b-PAA (Mn = 10
502 g mol−1) can be calculated by the results of PF-b-PHEMA-TMS and PF-b-PtBA, since PF-b-PHEMA-TMS and PF-b-PtBA exhibit better solubility in THF eluent.
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Fig. 3 GPC traces of polyfluorene-based rod-coil block copolymers PF-b-PHFBMA, PF-b-PBMA, PF-b-P(HEMA-TMS), PF-b-PtBA. |
After excluding the impact of fluorenone defects, the effect of incorporating different coil segments on the spectral stability of polyfluorene-based copolymers have been investigated under different thermal treatment conditions, including annealing temperature, annealing time and annealing atmosphere. Green index φ, defined as intensity ratio of green emission (∼530 nm) to the strongest peak of the blue emission (Igreen/Iblue),37,38 was introduced to depict the low-energy emission. The larger the value of φ is, the stronger the low-energy emission is, which indicates the poorer the spectral stability.
First, the spectral stability of polyfluorene-based polymers under different annealing temperature for 1 h was investigated. From the results shown in Fig. 5(A)–(D), no new emission band appears in the spectra of PF-Br, PF-b-PHFBMA, PF-b-PBMA, PF-b-PHEMA and PF-b-PAA when the annealing temperature is 50 °C or 100 °C. As the temperature increases to 150 °C, emission band at 525 nm only appears in the spectrum of PF-b-PHFBMA, the green index φ = 0.25. When the temperature continues increasing to 200 °C, this emission band is also observed in the case of PF-Br, the green index φ = 0.23, simultaneously, the intensity of this emission band in the spectrum of PF-b-PHFBMA increases significantly, the green index φ = 0.34. As a whole, the annealing temperature has influence on the spectral stability to some extent.
Fig. 6(A)–(D) show the spectra of polyfluorene-based polymers PF-Br, PF-b-PHFBMA, PF-b-PBMA, PF-b-PHEMA and PF-b-PAA under different annealing time at 150 °C in air. When the annealing time is 1 h, the emission band at 525 nm only appears in the spectrum of PF-b-PHFBMA, the green index φ = 0.25 (Fig. 6(B)). As the annealing time go on, the spectra of PF-b-PBMA, PF-b-PHEMA and PF-b-PAA still maintain steady while the emission band at 525 nm is observed in the spectrum of PF-Br at 6 h, the green index φ = 0.22 (Fig. 6(D)). The intensity of the emission band at 525 nm appeared in the spectrum of PF-b-PHFBMA increases significantly (φ = 0.69) as the annealing time increase to 6 h (Fig. 6(D)). The above results suggest that polyfluorene can maintain the spectral stability in a short period annealing time. However, when the annealing time exceeds a certain value, low-energy emission begin to appear. In a word, introducing the coil blocks PBMA, PHEMA and PAA can improve the spectral stability of polyfluorene while incorporating the coil block PHFBMA has the opposite effect.
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Fig. 6 Fluorescence spectra of PF-Br, PF-b-PHFBMA, PF-b-PBMA, PF-b-PHEMA and PF-b-PAA fims (A) pristine spin-cast, (B) annealed 1 h, (C) annealed 3 h and (D) annealed 6 h at 150 °C in air. |
To confirm whether the fluorenone defect structure was formed due to annealing in air, the spectral stability of polyfluorene-based polymers annealing in different atmosphere at 150 °C for 1 h was investigated. The corresponding emission spectra are depicted in Fig. 7. When compared to the pristine spin-casted film, no low-energy emission is present in the spectra of PF-Br, PF-b-PBMA, PF-b-PHEMA and PF-b-PAA either in vacuum or in air. In contrast, PF-b-PHFBMA exhibits the 525 nm emission band in air and in vacuum. From the results above, we suppose that the fluorenone defect structure might not form in air due to the limited oxidation in solid state, and the low-energy emission of PF-b-PHFBMA film might be resulted from molecular aggregation.
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Fig. 7 Fluorescence spectra of PF-Br, PF-b-PHFBMA, PF-b-PBMA, PF-b-PHEMA and PF-b-PAA films (A) pristine spin-cast, (B) annealed 1 h at 150 °C in air, (C) films annealed 1 h at 150 °C in vacuum. |
In combination with the results discussed above, both PF-Br and PF-b-PHFBMA exhibit the 525 nm low-energy emission band after annealing in air at 200 °C for 1 h (Fig. 5(D)) or at 150 °C for 6 h (Fig. 6(D)). The 13C NMR spectrum of PF-OH (Fig. 4) and the spectra of rod-coil copolymer films annealed in vacuum (Fig. 7(C)) excluding the impact of fluorenone defects on the spectral stability of polyfluorene-based polymers. Therefore, we tentatively suggest that introducing the coil blocks PBMA, PHEMA and PAA can inhibit the low-energy emission of polyfluorene while incorporating the coil block PHFBMA has the opposite effect, and aggregation behavior between fluorene chain is the cause of the low-energy emission band.
To PF-b-PHFBMA, the unfavorable enthalpic interactions between the fluorinated segment and the PF block can drive the segregation of copolymer. During the segregation process, PHFBMA is oriented to gather on the air–polymer surface due to its low surface free energy and self-aggregated property. If the annealing temperature is constant, the amount of fluorinated segments on the surface will increase gradually with the increase of annealing time, and finally reached a maximal value.39,40 The accompanied aggregation of PF blocks on the other side causes the low-energy emission band, and the intensity of the emission gradually increase as the annealing time go on (Fig. 6). The increase of the annealing temperature can promote the moving of PHFBMA segments and the aforementioned maximal value can be reached in a shorter period time when the temperature is higher.39,40 It is a reasonable explanation about the appearance of low-energy emission band and the enhanced intensity of the emission as the increase of the annealing temperature (Fig. 5). Compared with PHFBMA block, PBMA, PHEMA and PAA blocks have a better compatibility with PF, the separation trend of copolymers PF-b-PBMA, PF-b-PHEMA and PF-b-PAA are relatively weaker. Aggregation of PF blocks is less likely to occur even at a relative high temperature. Therefore, increasing annealing temperature or extending annealing time, a low-energy emission band never appear in the spectra of the polyfluorene-based copolymers with PBMA, PHEMA and PAA as coil blocks (Fig. 5 and 6).
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Fig. 8 The static WCA images of films (A) before annealing and (B) after annealing at 150 °C in air for 1 h. |
The thermal behaviors of all polymers investigated by TGA indicate that incorporation of coil blocks with polyfluorene does not much affect the thermal properties of polymers, which can be used to prepare the electronic device with inhibited low-energy emission.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra01616e |
This journal is © The Royal Society of Chemistry 2014 |