Chiang Mai Journal of Science

Synthesis and Characterization of Fluorescence Properties of PESNA-b-PBIT White Fluorescent Copolyester

1. INTRODUCTION

     With the rapid development of optical display materials, white-emitting polymer materials have garnered significant attention due to their crucial role in large-area planar lighting, flexible displays, and optoelectronics [1-3]. Since 1931, the International Commission on Illumination (CIE) has stipulated that all visible spectral colors can be represented by two coordinates, with the color coordinates for white light being (0.33, 0.33) [4,5]. Based on optical principles, white-emitting organic luminescent materials are primarily achieved through two systems: single-component systems, where the proportion of different fluorescent groups within a single fluorescent molecule is adjusted to produce white light; and by blending primary colors (red, green, blue) or complementary colors (yellow, blue) [6-9]. Similarly, white-emitting fluorescent polymers can also be synthesized via two primary strategies: blending or copolymerization [10,11].
     Blending small molecules with polymers or polymers with polymers is a commonly used method to obtain white-emitting fluorescent polymers. In such systems, the white light emission relies on host-guest interactions [12,13]. This is currently the primary approach to achieving white light emission in materials. For instance, Kim [14], Shu [15], Kang [16], Liu [17], and others have successfully blended two-color fluorescent substances to obtain effective white-emitting fluorescent materials. Han [18] has also achieved white light emission through blending using red, blue, and green fluorescent substances, employing electrostatic spinning technology. However, materials prepared by using this method often exhibit unavoidable phase separation issues, which may affect their stability and reproducibility during use. Subsequently, scholars proposed a scheme where dyes are chemically attached to polymer chains, known as copolymerization [19,20]. This approach not only allows for the combination of multiple fluorophores to control their ratios and achieve different colored emissions, but it also ensures uniform dispersion of the fluorophores at the molecular level, which contributes to improved energy transfer as reflected by fluorescence spectra [20]. Liu et al [21]. achieved white light emission by covalently attaching an orange dopant and an alkyl spacer to the side chains of a blue polymer host (polyfluorene), adjusting the dopant content to emit white fluorescence. Yang [22] obtained organic molecules with high quantum efficiency white light emission by modifying the terminal groups of polymethylene chains with bifunctional molecules. Choi [23] utilized direct C-H amidation polymerization (DCAP) to generate polysulfonamides without defects, achieving white light emission through excited-state intramolecular proton transfer (ESIPT). Zhang [24] introduced chiral dibenzoyloxyethyl units into three polymer backbones, enabling the polymers S-/R-WP2 to emit standard white light with CIE coordinates of (0.33, 0.34) and a high color rendering index of 95. Chen [25] investigated the effects of different polymerization methods and solvents on the optical properties of poly (maleic anhydride-alt-vinyl acetate) (PMV), finding that solution polymerization exhibited higher quantum yields compared to self-stable precipitation polymerization. Given the advantages of copolymerization over blending, this study aims to select suitable fluorescent dyes for attachment to polymer chains [26,27]. Herein, 1,8-naphthalic anhydride derivatives are chosen due to their favorable photophysical properties (e.g., tunable fluorescence intensity and good photostability) [28] and wide application in dyes, sensing materials, and liquid crystals [29]. Electron-donating groups can enhance the fluorescence intensity of naphthalic anhydride, while electron-withdrawing groups can decrease it [30,31]. Therefore, the fluorescence intensity of 1,8-naphthalic anhydride derivatives can be modulated by introducing different functional groups, which in turn enables the adjustment of luminescence intensity to achieve white light emission during copolymerization. Hence, this study selects 4-chloro-1,8-naphthalic anhydride as the blue fluorescent dye.
     Compared with polyester, copolyesters have additional monomer molecules incorporated for modification, by which certain performance aspects can be enhanced while the original characteristics are maintained [27,30,43]. In copolyesters, fluorescence emission is generated by the aggregation of carbonyl groups within each polyester unit. Considering that the luminescence color is affected by the rigidity and flexibility of the molecular chain [36], the fluorescence properties of the polyester have the potential to be altered when it is modified with other monomers or fluorescent dyes [44].
     In our prior research, we developed and validated the "blue-orange complementary" block copolymerization strategy for white fluorescent copolyesters, laying a solid foundation for the current work: We previously synthesized blue-emitting poly(ethylene succinate-co-cyclohexanedimethanol) (PESC) and orange-emitting 1,6,7,12-tetrachloroperylene tetracarboxylic dianhydride (TTAD)-modified poly(butylene terephthalate-co-propylene terephthalate) (PBTT) via melt polycondensation, then linked them by chain extension to form PESC-b-PBTT with white light emission (CIE: (0.27, 0.33) at 85 wt% PESC) [32]; After that, we extended this paradigm by preparing blue-emitting poly(ethylene succinate-co-2,5-furandicarboxylate) (PESF, with 2,5-furandicarboxylic acid to adjust blue fluorescence) and orange-emitting TTAD-grafted poly(butylene isophthalate-co-butylene terephthalate) (PBIT) via the same polycondensation method, followed by chain extension to obtain PESF-b-PBIT with tunable white light (typical CIE: (0.31, 0.35); fine-tuned to (0.32, 0.35) at 96.7 mol% PESF) [33]. These studies confirmed aliphatic polyesters (PESC, PESF) as blue-emitting units, TTAD-functionalized aromatic polyesters (PBTT, PBIT) as orange-emitting units， both validated by previous white light emission results [32,33].
     Building on the previously established "blue-orange complementary" block copolymerization paradigm for white fluorescent copolyesters, this study still adopts the copolymerization strategy, wherein two fluorescent dyes (blue and orange) are respectively grafted onto polyester molecular chains via melt polycondensation to construct the target fluorescent system. For the blue-emitting polymer, this study will utilize 4-chloro-1,8-naphthalic anhydride as the blue fluorescent dye. Poly (butylene succinate) (PES) is chosen as the carrier for the blue fluorescent dye due to its good flexibility, thermal stability, mechanical properties, and processability [34-37]. Additionally, the diluting effect of the PES molecular chain on the blue dye is expected to suppress dye aggregation and fluorescence quenching (ACQ) effects. For the orange-emitting polymer, 1,6,7,12-tetrachloro-3,4,9,10-perylenetetracarboxylic dianhydride is incorporated into the molecular chain of poly(butylene isophthalate-co-butylene terephthalate) (PBIT) via grafting to function as the orange fluorescent unit. The orange light emitted by this perylene-based dye exhibits spectral complementarity with the blue light from the 4-chloro-1,8-naphthalic anhydride- modified poly(butylene succinate) (PESNA), thereby laying an important foundation for the realization of stable white light emission by achieving spectral complementarity between blue and orange emission in the final copolymer system.

2. MATERIALS AND METHODS

2.1 Raw Materials
    Dimethyl terephthalate (DMT), 99%, Macklin; Dimethyl isophthalate (DMI), 99%, Macklin; 1,4-Butanediol (BDO), 99%, Macklin; 1,6,7,12-Tetrachloro-3,4,9,10-peryltetracarboxylic dianhydride (TTAD), 96%, Macklin; Butyl titanate (TBT), AR, Tianjin Kemiou Chemical Reagent Co. Ltd (China); 1,4-Succinic acid (SA), AR, Macklin; Ethylene glycol (EG), AR, Macklin; 4-Chloro-1,8-naphthalenedicarboxylic anhydride (94%, Aladdin); Butyl titanate (TBT), AR, Tianjin Kemiou Chemical Reagent Co. Ltd (China); Triphenyl phosphate, 98%, Macklin; dichloromethane, AR, Damao Chemical Reagent Factory (China); methanol, AR, Tianjin Kemiou Chemical Reagent Co. Ltd (China); N,N-Dimethylformamide (DMF), GR, Tianjin Kemiou Chemical Reagent Co. Ltd (China); dicyclohexylmethane-4,4'-diisocyanate (HMDI), 90%, Macklin.

2.2 Synthesis of Copolyesters
     The orange luminescent copolyester, poly(butylene glycol isophthalate-co-butylene glycol terephthalate)(PBIT), was prepared by melt polycondensation method. Dimethyl terephthalate (DMT) and dimethyl isophthalate (DMI) were added to a three-necked flask at a molar ratio of 1:1, followed by the addition of 1,4-butanediol (BDO) at an acid-to-alcohol molar ratio of 1:1.8. Subsequently, TBT, triphenyl phosphate, and TTAD were added, with each of them accounting for 0.1wt% of the total dicarboxylic acid. The mixture was heated to 180 °C under a nitrogen atmosphere for an esterification reaction lasting 4 hours, which terminated when methanol ceased to be produced. The temperature was then raised to 230 °C, which is significantly lower than the critical threshold required for the carbonization of PESNA/PBIT precursors, as their limited aromatic moiety content is insufficient to drive the formation of polycyclic carbon domains [42]. Then，the vacuum device was connected to initiate the polycondensation reaction under vacuum. After approximately 1 hour of reaction, the product was obtained. The synthetic method is shown in Figure 1a.
     Blue fluorescent copolyester was prepared via melt polycondensation. Succinic acid (SA) and ethylene glycol (EG) were added to a three-neck flask in a molar ratio of 1:1.2. Then, 4-Chloro-1,8-naphthalic anhydride, triphenyl phosphate, and butyl titanate were introduced as the fluorescent molecule, thermal stabilizer, and catalyst, respectively, each accounting for 0.1wt% of SA. Under a nitrogen atmosphere, the temperature was raised to 180 °C for the esterification reaction. The esterification process was considered complete after 4 h when no further water was produced. Subsequently, the temperature was increased to 230 °C, and a vacuum apparatus was connected to perform polycondensation under reduced pressure. The reaction system's pressure was lowered to 100 Pa, and the reaction continued for approximately 2 h to obtain the product. The product was then dissolved in dichloromethane and precipitated using an excess of methanol. After filtration, the product was placed in a vacuum oven and dried for 12 h, named PESNA. The synthesis pathway is depicted in Figure 1b.
    White fluorescent copolyesters were prepared by melt chain extension using pre-dried PESNA and PBIT. With a fixed total mass of 5 g, PESNA and PBIT were mixed in different mass ratios of 80:20, 75:25, 70:30, and 65:35, followed by melt chain extension. Once the reaction was complete, the samples were collected and labeled as B20, B25, B30, and B35, respectively. The pre-weighed PESNA and PBIT samples (with a total mass of 5 g) were added to a 100 mL three-necked flask and heated to 180 °C under a nitrogen atmosphere. Once both components were completely melted, stirring was started for 10 min to ensure thorough mixing. Following this, 8 drops of 4,4-diisocyanatodicyclohexylmethane (HMDI) were added dropwise, with one drop added every 10 min.

2.3 Characterization
     Nuclear Magnetic Resonance (NMR) spectra were recorded with chloroform as solvent, TMS as an internal standard. The measurements were conducted at a frequency of 600 MHz.
     The attenuated total reflectance (ATR) method was employed for Fourier Transform Infrared (FTIR) spectroscopy, using a zinc selenide internal reflection crystal. Measurements were conducted over the wavenumber range of 4000-500 cm^-1, with a resolution of 1cm⁻¹ and averaged over 32 scans.
     The samples were dissolved in N,N-dimethylformamide, and Gel Permeation Chromatography (GPC) was employed to determine their molecular weights and polydispersity indices and polydispersity index, using polystyrene as the standard sample.
     Crystallization and melting behavior of copolyesters were performed on a differential scanning calorimeter (DSC，DSC8000, Perkin-Elmer, USA). All of the samples were heated from 50 °C to 170 °C at a heating rate of 60 °C/min under nitrogen atmosphere, held for 3min; then cooled to -40 °C at a rate of 60 °C/min, held for 3min; then heated again to 170 °C at a rate of 10 °C/min, held for 3 min; and then cooled to -40 °C at a rate of 2 °C/min, held for 3min; finally, heated to 170 °C at a rate of 10 °C/min.
     In accordance with the GB/T 1040-2006 standard, the tensile performance test of the sample was conducted using a universal material testing machine at a rate of 50 mm/min. The sample was in the shape of a dumbbell.
     The fluorescent properties of the product were characterized using a fluorospectrophotometer with an excitation wavelength of 365 nm. Subsequently, various excitation wavelengths were employed to investigate the color change of the sample as a function of the excitation wavelength. The sample dimensions were 20 mm in diameter and 1 mm in thickness.

3. RESULTS AND DISCUSSIONS

3.1 Chemical Structure of Copolyesters
     The ¹H NMR of the PESNA is shown in Figure 1c. In Figure 1c, it can be observed that the proton signal peak at position 1 has a chemical shift value (δ) of 4.29 ppm, corresponding to the H-atoms of the EG (ethylene glycol) methylene group. The proton signal peak at position 2, with a δ value of 2.66 ppm, belongs to the H-atoms of the methylene group on SA (presumably referring to a specific monomer or segment in the copolyester structure).
     The ¹H NMR spectrum of the chain-extended B series copolyester was presented in Figure 1d. The signal peaks at δ values of 4.29ppm and 2.66 ppm, labeled as 1 and 2 respectively, represent the H-atoms on the methylene groups of EG and SA in the PESNA block. The signal peak at δ value of 4.42 ppm, labeled as a, is attributed to the H-atoms on the methylene group adjacent to the ester group in the butanediol segment of the PBIT block. The signal peak at δ value of 1.96 ppm, labeled as b, corresponds to the H-atoms on the inner methylene group of butanediol. The signal peak at δ value of 8.08 ppm, labeled as c, represents the hydrogen atoms on the benzene ring of dimethyl terephthalate. The signal peak at δ value of 8.66 ppm, labeled as f, corresponds to the H-atoms located between the two ester groups on the benzene ring of DMI (presumably a specific monomer or segment). The signal peak at δ value of 7.51 ppm, labeled as d, belongs to the H-atoms in the para-position on the benzene ring, while the signal peak at δ value of 8.21 ppm, labeled as e, represents the other two H-atoms on the benzene ring.

$F_{\mathrm{PBIT}}=I_{\text{c+d+e+f}}÷I_{\text{c+d+e+f+2}}$                                   (1)

F_PBIT: molar ratio of PBIT in copolyester, %; I: refers to the integrated area indicated by the subscript.

3.4 Thermodynamic Performance Analysis and Mechanical Properties
     Thermal analysis of PESNA, PBIT, and the entire B series of copolyesters were conducted by DSC. Figure 3a presents the DSC heating curves obtained at a heating rate of 10 °C/min after melt quenching. The fundamental thermodynamic parameters derived from these curves are summarized in Table 3. The glass transition temperature (T_g) of PESNA is -11.2 °C, whereas the T_g of PBIT is 20.9 °C. This difference can be attributed to the presence of numerous rigid benzene ring units in PBIT. In contrast, PESNA is an aliphatic macromolecule with high chain flexibility, resulting in a lower T_g compared to PBIT. Interestingly, all copolyesters in the B series exhibit two distinct glass transition temperatures: the lower one (T_g1) corresponds to the PESNA segment, and the higher one (T_g2) corresponds to the PBIT segment. According to examination of Figure 3a and Table 3, it becomes apparent that the incorporation of the PBIT segment into the PESNA backbone leads to an increase in the T_g1 of the PESNA segment from -11.2 °C in the pure form to -8.1 °C in B30, increasing by approximately 3 °C. This increase is due to the introduction of rigid benzene structural units from the PBIT segment, which enhances the overall rigidity of the polymer chain, reduces its flexibility, and hinders chain mobility [38]. Consequently, the copolyester requires more energy to facilitate segment motion at higher temperatures. Notably, the T_g2 associated with the PBIT segment also exhibits an increase, but still does not approach T_g1. This observation suggests significant structural differences between the PBIT and PESNA segments, indicating their incompatibility. This incompatibility is beneficial for the subsequent preparation of white fluorescent copolyesters, as it avoids excessive energy transfer between the two segments.

3.5 Analysis of Fluorescence Characteristics
     Figure 4a presented the fluorescence emission spectra and CIE coordinates of PESNA and PBIT under an excitation wavelength of 365 nm. As seen from the figure, the maximum emission wavelength of PESNA is around 424 nm, and the calculated CIE coordinates are (0.16, 0.07), which fall within the blue wavelength range in the right panel. On the other hand, the maximum emission wavelength of PBIT is at 563 nm, and the CIE coordinates are (0.47, 0.52), which are located in the orange region in the right figure. Furthermore, plotting the coordinates of both blue-emitting and orange-emitting polyesters in the CIE chromaticity diagram reveals that the line connecting them passes through the white light region. By combining these two components according to the donor-acceptor strategy, this study meets the requirement for white light emission, as evidenced by the CIE chromaticity coordinates of the resulting copolymer.
     Figure 4b presents the fluorescence emission spectra and CIE coordinate diagrams of the B-series copolyesters excited at the wavelength of 365 nm. As can be observed from the figure, the emission spectra of the B-series copolyesters exhibit two distinct emission peaks, located near 417 nm and 555 nm, respectively. The emission peak at 417 nm falls within the blue wavelength range, attributed to the fluorescence emission zone of PESNA (with CIE chromaticity coordinates of (0.16, 0.07)), while the emission peak at 555 nm is situated in the orange wavelength range, attributed to the fluorescence emission zone of PBIT (with CIE chromaticity coordinates of (0.47, 0.52)). According to the donor-acceptor strategy, the blue-emitting donor unit serves as the component that absorbs the excited light energy. Upon excitation by ultraviolet light, it needs to transfer a portion of the energy to the orange-emitting acceptor unit, causing the acceptor to emit orange light. This process enables the overall white light emission. Therefore, even if the fluorescence intensity of the blue-emitting donor and the orange-emitting acceptor is comparable, the blue-emitting unit contributes significantly to white light emission.
     As observed in Figure 4(c-f), with the increase in PBIT content, the intensity of the blue emission peak gradually decreases, while the intensity of the orange emission peak at 555 nm gradually increases for the B-series copolyester samples. This trend is consistent with the changing ratio of feedstocks during the preparation of the two-block copolymer. Furthermore, by examining the CIE coordinates of the four copolyesters, it is evident that the emission peak intensity at 414 nm is higher than that at 555 nm for B₂₀ and B₂₅, resulting in a fluorescence color that is biased towards blue light. The CIE coordinates for B₂₀ and B₂₅ are (0.28, 0.27) and (0.30, 0.30), respectively. Conversely, for B₃₅, the emission peak intensity at 414 nm is lower than that at 555 nm, leading to a fluorescence color biased towards orange light with CIE coordinates of (0.34, 0.35). B₃₀ exhibits nearly equal emission peak intensities at 414 nm and 555 nm, resulting in CIE chromaticity coordinates of (0.32, 0.33), which closely approximate the standard white-light coordinates of (0.33, 0.33) with a deviation of less than 0.01 in both x and y axes.
     Additionally, the influence of excitation wavelength on the fluorescence color of the copolyesters was investigated. Figure 4(c-f) presents the emission spectra of the B-series copolyesters under different excitation wavelengths. It is observed that the orange emission peak at 555 nm remains unchanged in position but varies in intensity with increasing excitation wavelength. Meanwhile, the blue emission peak experiences a slight shift towards longer wavelengths as the excitation wavelength increases. The intensity of the blue emission peak initially increases and then decreases, reaching a maximum at an excitation wavelength of 380 nm.
     By utilizing the variation patterns of the two emission peaks with excitation wavelength in the copolyester, it is feasible to achieve fine-tuning of the fluorescent color by altering the excitation wavelength. Specifically, when the intensity of the blue emission peak is stronger than that of the orange emission peak, increasing the excitation wavelength can enhance the intensity of the orange emission peak. Conversely, decreasing the excitation wavelength can weaken the orange emission intensity. For instance, when the excitation wavelength of the B₂₀ sample is increased from 365 nm to 380 nm, the CIE chromaticity coordinates shifts from (0.28, 0.27) to (0.30, 0.33). Similarly, the CIE color coordinate of the B₂₅ sample also changes from (0.30, 0.30) to (0.33, 0.36).
     Figure 5 shows the fluorescence decay curves of PESNA, PBIT, and B-series block copolyesters, and it can be observed that the fluorescence decay profiles of B-series copolyesters (B₂₀–B₃₅) fall between those of PESNA segment and PBIT segment. Although the fluorescence lifetime curves of the B-series samples are very close, it can also be observed that as the PESNA composition increases, the fluorescence lifetime curves of the samples shift towards the long time axis, which intuitively reflects the donor-to-acceptor energy transfer process in the copolymers. Notably, B₃₀ (the sample with CIE coordinates closest to standard white light) exhibits a moderate decay rate, matching the balanced energy transfer efficiency between the donor and acceptor, which further validates the feasibility of the donor-acceptor strategy for white light emission.

4. CONCLUSIONS

     In this work, PESNA, PBIT and a series of PESNA-b-PBIT block copolyesters were fabricated, and their chemical structures and physical properties were systematically characterized. Crystallization and melting tests indicated the incompatibility between the PBIT and PESNA segments, as well as an increase in the overall chain rigidity. Fluorescence spectroscopy results showed that white light emission was achieved under excitation at 365 nm, with CIE chromaticity coordinates of (0.32, 0.33). Compared with previous results, this result is even closer to the international standard (0.33, 0.33) [32,33], with a smaller deviation than the previously reported CIE coordinates of (0.27, 0.33) [32] and (0.31, 0.35) [33]. In summary, this study demonstrates that white fluorescent block copolymers with good thermal and optical stability can be successfully prepared using PESNA and PBIT. This study thus presents a strategy for designing and preparing white fluorescent copolyesters with tunable properties and maybe broad application prospects in flexible displays, planar lighting, and other optoelectronic devices.

ACKNOWLEDGEMENT

     The authors express their thanks to the financial assistance of the Nature Science Foundation of Hebei Province (the grant no. B2021201042).

AUTHOR CONTRIBUTIONS

Yunxiang Fu: Experiment, Data curation, Writing-Original draft preparation.
Jicai Xu, Wang Yu, Mohan Di: Assist in Data curation, text editing;
Ming-tao Run: Conceptualization, Methodology, Software, Supervision.

CONFLICT OF INTEREST STATEMENT

     The authors report no declarations of interest.

FUNDING

     This project is funded by the Nature Science Foundation of Hebei Province: Contract number B2021201042.

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