Synthesis and Optical Performances of a Waterborne Polyurethane-Based Polymeric Dye

A waterborne polyurethane-based polymeric dye (WPU-CFBB) was synthesized by anchoring 1, 4-bis(methylamino)anthraquinone (CFBB) to waterborne polyurethane chains.The number molecular weight, glass transition temperature, and average emulsion particle size for the polymeric dye were determined, respectively. This polymeric dye exhibited intriguing optical behaviors. The polymeric dye engendered two new absorption bands centered at about 520 nm and 760 nm if compared with CFBB in UV-vis spectra. The 760 nm peak showed hypsochromic shift with the decrease of average particle sizes. The polymeric dye dramatically demonstrated both hypsochromic and bathochromic effects with increasing temperature. The fluorescence intensity of the polymeric dye was much higher than that of CFBB. It was found that the fluorescence intensities would be enhanced from 20C to 40C and then decline from 40C to 90C. The fluorescence of the polymeric dye emulsion was very stable and was not sensitive to quenchers.


Introduction
Polymeric dyes have attracted a great deal of attention in recent years, because of low toxicity, good colorfastness, abrasion resistance, resistance to migration, better processability, and so forth [1][2][3][4][5].The polymeric dyes hold great promise in a wide range of applications.The growing interest in polymeric dyes is currently focused on anchoring monomeric dyes onto polymer matrixes to afford polymeric dyes [6][7][8].
Polyurethane (PU) is the most versatile polymer material to meet the highly diversified demands of modern technologies such as coatings, adhesives, fiber, foams, and thermoplastic elastomers.The development of polyurethanebased polymeric dyes has aroused some interests [9][10][11].However, PU products are usually rather poor in their dyeing properties (dyeing ability and color fastness) as a result of lack of dyeing sites.A colored PU is mostly made of a mixture of pigment and polymer.However, the color is dull, with a lack of brightness and also poor abrasion resistance [12].Moreover, elastic PU fibers are usually dyed in an energy/time consuming way, that is, at a high temperature with a long time [13].
Efforts have been exerted to improve the dyeing properties of formed membrane objects including introducing dyeable functional groups into PU, directly grafting dyestuff molecules into PU, copolymerizing with dyestuff, or fixation treatment after the dyeing process.Wang et al. synthesized a PU-based dye and found that the dyeability and thermal migration value of PU-based dye were considerably improved [11,14].The formed film tended to show the glass transition point of the soft segment, lower strength, and better elongation.Chao et al. prepared a PU-based dye ionomer and found that the particle size became larger when the dye concentration was increased, and the formed film became stronger with decreased elongation [14][15][16].Buruiana et al. synthesized waterborne polyurethane with various covalently bonded chromophores [17][18][19].They found that tensile and thermal properties were similar to those of other elastomeric polyurethanes, but optical properties of the polymer differed from those of the monomers.
Scheme 1: Synthesis and structure formula of WPU-CFBB or WPU.
From the above-mentioned works, study of polyurethane-based dye focused on synthesizing and modifying dyes via reaction with isocyanates to improve the dyeing properties.Other examples found in the literature usually pay attention to the performances of polymer; however, optical properties of polyurethane-based dye such as absorbance and fluorescence change with external circumstances are rarely described.
In this paper, a waterborne polyurethane-based polymeric dye (WPU-CFBB) was synthesized by anchoring 1, 4bis(methylamino)anthraquinone (CFBB, C.I.61500) to polyurethane chains.Special absorbance and fluorescence performances of WPU-CFBB were discussed in detail.WPU-CFBB can be used as an optical intelligent material with response to temperature.Because CFBB was anchored into polyurethane chains, the dye migration could be avoided, and color could be preserved permanently with brightness, solvent resistance, and good abrasion resistance as well.Additionally, WPU-CFBB is an environment friendly material to meet the diversified demands of environmental protection.Moreover, WPU-CFBB possessed the peculiar properties of optical properties, which might significantly expand the range of application in chemosensor, organic LEDs, laser active media, and so forth.

Synthesis of WPU-CFBB.
The synthesis of WPU-CFBB was presented in Scheme 1. TDI, PPG, DMPA, and DMF of 5 mL were charged into a dry four-necked flask equipped with a stirrer, thermometer, and condenser according to the molar ratio of TDI : PPG : DMPA = 1 : 0.35 : 0.28.DBTDL was added to the flask under stirring (the molar ratio of TDI : DBTDL = 1 : 0.02).The reaction was then carried out with vigorous agitation at 80 ∘ C under N 2 atmosphere until the NCO content reached the theoretical value to produce NCO-terminated prepolymer I.During the reaction, the content of -NCO groups was monitored via a standard dibutylamine titration method [20].In order to dilute prepolymer I, suitable amount of acetone was also added into the system to decrease the viscosity of the reaction system.Then, CFBB (the molar ratio of TDI : CFBB = 1 : 0.3) dissolved in acetone was added and reacted with prepolymer I to form prepolymer II at 80 ∘ C for 6 h.The prepolymer mixture was slowly cooled to 30 ∘ C. TEA is just a neutralizing agent not part of the polymer backbone (the molar ratio TEA : DMPA = 1.2 : 1).It was added into the reaction vessel and reacted with the carboxylic group of the side chain of prepolymer II for 30 min to obtain quaternized prepolymer.Deionized water was added into the reaction mixture under vigorous agitation for 30 min.The uniform emulsion of WPU-CFBB was obtained after acetone was removed at 50 ∘ C from the emulsion using a rotary evaporator under reduced pressure.The solid content of obtained WPU-CFBB was about 30 wt%.In order to compare with WPU-CFBB, waterborne polyurethane (WPU) was synthesized by means of substituting 1, 4-butanediol (BDO) for CFBB in the same conditions.

Purification of WPU-CFBB.
The purification of WPU-CFBB was carried out in three steps: (i) the thin film of WPU-CFBB was prepared by casting the emulsion on a Teflon plate and drying at room temperature for 7 days and then in a vacuum system at 50 ∘ C for 2 days; (ii) the thin film of WPU-CFBB was dissolved in acetone; (iii) precipitation was carried out with hexane; the precipitated WPU-CFBB fraction was obtained after the solvent evaporation.The precipitated WPU-CFBB fraction was dissolved, precipitated, and evaporated again; the final purified film of WPU-CFBB was obtained.

Measurements.
FTIR spectra (a Perkin-Elmer FTIR spectrometer) were recorded from the samples of the final purified films. 1 H-NMR spectra were recorded on a Bruker AC 300 spectrometer operating at 300 MHz.The UV-vis spectra were recorded in a Shimadzu spectrophotometer UV-2501PC.The temperature was measured with a thermocouple connected to the cell holder.Average particle size of the emulsion was determined by using a Shimadzu SALD-7101 laser particle size analyzer.Differential scanning calorimetric studies were conducted with Perkin-Elmer Pyris-1 (DSC) at a heating rate of 10 ∘ C/min under nitrogen atmosphere.Chromatography (GPC) using a series of two linear Styragel columns HT3, HT4 and a column temperature of 35 ∘ C was performed on a Waters 1515 pump and Waters 2414 differential refractive index detector (set at 30 ∘ C).The eluent was DMF at a flow rate of 1.0 mL/min.Fluorescence spectrometer was recorded on a Shimadzu RF-5301PC luminescence spectrometer.The temperature was measured with a thermocouple connected to the cell holder.The slit widths of monochromators were both 5 nm.The characteristic peak could also be observed in the spectrum of WPU-CFBB and could not be observed in WPU, indicating that CFBB has been successfully embedded into the polyurethane chains.The typical NCO peak at about 2270 cm −1 disappeared, showing that the NCO group had been completely reacted during the reaction between CFBB and prepolymer II.

Results and Discussion
The Based on comparison of the extinction coefficient of WPU-CFBB with that of CFBB, the weight percentage of 6.8% for CFBB segments in WPU-CFBB was determined by UV-vis absorption of WPU-CFBB.The number-averaged molecular weight (  ) of WPU-CFBB measured by GPC was about 3.3 × 10 4 and the molecular weight distribution index (  /  ) was 1.66.Glass transition temperature (  ) determined by DSC was 55 ∘ C.

UV-Vis Spectra of WPU-CFBB. WPU-CFBB formed
stable emulsion in water.WPU-CFBB emulsion showed a group of absorption bands peaked at 520, 555, 600, 650, and 760 nm.WPU itself did not show any significant absorption in this range.The UV-vis spectra of WPU-CFBB emulsion and its film dissolved in chloroform were shown in Figure 3.It was found that WPU-CFBB emulsion had a new absorption band centered at 760 nm if compared with its film dissolved in chloroform.Intramolecular dipoles, ionization, and hydrogen bonds in the polyurethane chains were strengthened due to WPU-CFBB emulsion with electrical double surface layers through the ionization of acidic groups on the polyurethane chains, resulting in engendering new absorption in long wavelength areas corresponding to the - * transition of anthraquinone in CFBB [21].
The UV-vis spectra of CFBB in chloroform and WPU-CFBB film dissolved in chloroform were shown in Figure 4. WPU-CFBB showed three typical absorption bands centered at 555, 600, and 650 nm associated with the anthraquinone unit of CFBB, which further confirmed that CFBB was attached to the polyurethane chains.WPU-CFBB film dissolved in chloroform showed a new absorption band centered at 520 nm compared with CFBB, corresponding to the - * transition resulting from decreasing  energy grade and enhancing  * energy grade of anthraquinone unit through inductive effect [21].
In order to demonstrate the relations of two new absorption maxima to the sizes of emulsions, a series of WPU-CFBB emulsions with different particle sizes were prepared by changing isocyanate index according to 6 : 1, 5 : 1, 4 : 1, and 3 : 1 molar ratio of TDI to PPG during the synthesis.Generally, PPG was added to react with TDI to form prepolymer.The isocyanate index could determine directly average particle size of the emulsions [22,23].The correlations between the isocyanate index, the average particle sizes, and the two new absorption maxima for WPU-CFBB emulsion were Figure 5.
With the decrease of the isocyanate index, the average particle sizes of emulsion were significantly enlarged from 75 nm to 126 nm.The new absorption maxima of 520 nm did not change with enlarged average particle sizes.However, the new absorption maxima of 760 nm were obviously hypsochromic shift with decreased average particle sizes from 760 nm to 740 nm.The phenomenon could be attributed to weakened microzones of CFBB segments in the decreased average particle sizes of WPU-CFBB emulsions with the isocyanate index increased.Normally, absorption spectrum displayed bathochromic effect along with increasing temperature.However, WPU-CFBB latex dramatically demonstrated both hypsochromic International Journal of Polymer Science  and bathochromic effects with increasing temperature.As could be seen from Figure 6, three strong absorption bands centered at 555, 600, and 650 nm associated with the anthraquinone unit showed bathochromic effects along with temperature increased from 30 to 90 ∘ C, corresponding to the - * transition, whereas bathochromic effects are much stronger from 80 to 90 ∘ C. The two new absorbance bands of WPU-CFBB latex centered at 520 and 760 nm unusually showed hypsochromic effect, which were attributed to reducing aggregation tendency of WPU-CFBB and decreasing the hydrogen bonding in WPU-CFBB with increasing temperature [24,25].

Fluorescence of WPU-CFBB.
The fluorescence spectra of WPU-CFBB emulsion and CFBB in acetone were shown in Figure 7.The fluorescence intensity of WPU-CFBB was much higher than that of CFBB at the same molar concentrations of chromophores.In fact, CFBB displayed very weak fluorescence emission.The fluorescence quantum yield (Φ  ) of WPU-CFBB and CFBB was determined using Rhodamine B (Φ 0 = 0.89) as the reference.The calculated result showed that Φ  of WPU-CFBB was 0.18 while that of CFBB in acetone is 0.01.The Φ  of WPU-CFBB emulsion was 18 times higher than that of CFBB in acetone.
The weak fluorescent emission of CFBB was not due to the so-called "concentrational self-quenching effect" but because of "structural self-quenching effect" [26,27].This phenomenon was attributed to the intermolecular excimers formation of CFBB between electron-donating imino group and electron-accepting carbon-oxygen double bond due to the molecular structure of CFBB containing both electronwithdrawing and electron-donating groups, as illustrated in Scheme 2. The formation of abundant excimers caused fluorescence quenching and engendered a weak fluorescence emission.
In order to prove that such a low fluorescence intensity of CFBB was not due to the "concentrational self-quenching effect, " the fluorescence intensity of WPU-CFBB and CFBB was measured in a wide concentration range of 10 −7 to 10 −3 M. Figure 8 showed the relationships between the fluorescence intensity and the solution concentration.The fluorescence intensity displayed a maximum at the concentration of about 10 −5 M for WPU-CFBB.It was clear that the fluorescence intensity of WPU-CFBB was always much higher than that International Journal of Polymer Science of CFBB at all concentrations.The concentration changing could not enlarge visibly the fluorescence intensity of CFBB.
The fluorescence intensity of WPU-CFBB was greatly enhanced comparing with that of CFBB, which was mainly attributed to the following two factors.First, CFBB was anchored to the polyurethane chains, which hindered the formation of excimers among CFBB.Furthermore, the intramolecular rotation and vibration of CFBB were restricted, resulting in a decrease in nonradiative transition and thus an increase in radiative transition process [28,29].Secondly, WPU-CFBB formed emulsion with electrical double surface layers due to the ionization of acidic groups, leading to augmented light absorption area [30].In addition, the CFBB segments of WPU-CFBB were inside electrical double layer; thus the mobility of CFBB was decreased and CFBB was shielded from possible quenchers [31,32].
Generally speaking, fluorescent intensity would gradually decline in solution along with increasing temperature.However, WPU-CFBB latex exhibited intriguing optical behavior.As shown in Figure 9, the fluorescence intensity of WPU-CFBB was enhanced as the temperature increased from 20 to 40 ∘ C and then declined from 40 to 90 ∘ C. The enhanced fluorescence intensity from 20 to 40 ∘ C could be attributed mainly to a lessened energy difference between  1 and  1 by thermal energy and decreased hydrogen bonding in WPU-CFBB with increasing temperature [33].WPU-CFBB can be used as an optical intelligent material with response to temperature.
The fluorescence of WPU-CFBB was very stable during storage.Hardly any difference of fluorescence intensity was observed after 30 days.Furthermore, fluorescence of WPU-CFBB decreased only a little after some quenchers (e.g., hydroquinone) were added into WPU-CFBB.
The fluorescence of WPU-CFBB was regularly quenched by adding the hydroquinone (Figure 10(a)).Fluorescence quenching efficiency of quencher hydroquinone can be analyzed according to the Stern-Volmer equation: where  0 is fluorescence intensity of WPU-CFBB emulsion without quencher while   is the fluorescence intensity containing quencher; [] is the concentration of quencher, and  SV is dynamic quenching constant. SV value indicates the fluorescence quenching efficiency.

Conclusions
A waterborne polyurethane-based dye WPU-CFBB was prepared by chemically anchoring CFBB into waterborne polyurethane chains.The number average molecular weight and its distribution index and glass transition temperature for WPU-CFBB were determined to be 3.3 × 10 4 , 1.66, and 55 ∘ C, respectively.Compared with UV-vis spectrum of CFBB, WPU-CFBB emulsion demonstrated two new absorption bands centered at about 520 nm and 760 nm, respectively.The 520 nm peak was no change with enlarged average particle sizes.However, the 760 nm peak showed hypsochromic shift with the decrease of average particle sizes.WPU-CFBB demonstrated both hypsochromic and bathochromic effects with increasing temperature.The fluorescence intensity of WPU-CFBB was prominently enhanced in comparison with CFBB, which was mainly attributed to the hindered formation of excimers among CFBB and the augmented light absorption area.The fluorescence intensity of WPU-CFBB was enhanced from 20 to 40 ∘ C and then declined from 40 to 90 ∘ C. The enhanced fluorescence intensity from 20 to 40 ∘ C could be mainly attributed to a lessened energy difference between  1 and  1 and decreased hydrogen bonding in WPU-CFBB.WPU-CFBB can be used as an optical intelligent material with response to temperature.In addition, the fluorescence of WPU-CFBB emulsions was very stable and was not sensitive to quenchers according to a fluorescence quenching analysis.

Figure 4 :Figure 5 :
Figure 4: The UV-vis spectra of CFBB in chloroform and WPU-CFBB film dissolved in chloroform.

Figure 9 :
Figure 9: The fluorescence spectra of WPU-CFBB under different temperature under 325 nm excitation.

Figure 10 :
Figure 10: (a) Fluorescent spectra of WPU-CFBB emulsion in increasing quencher concentration of hydroquinone under 325 nm excitation; (b) the plot of  0 /  versus the concentration of hydroquinone [].
Figure 10(b)  demonstrated a linear relationship between the  0 /  and [].The very small dynamic quenching constant  SV of 14.8 M −1 could be determined from the slope of the line, indicating clearly the fluorescence stability of WPU-CFBB emulsions.The CFBB segments of WPU-CFBB were inside electrical double layer due to the ionization of acidic groups on the polyurethane chains; thus CFBB segments were shielded from quenchers.Generally speaking, the fluorescence of WPU-CFBB was not sensitive to quenchers.