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 20°C to 40°C and then decline from 40°C to 90°C. The fluorescence of the polymeric dye emulsion was very stable and was not sensitive to quenchers.
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 [
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 polyurethane-based polymeric dyes has aroused some interests [
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 [
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, 4-bis(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.
2, 4-Tolylene diisocyanate (TDI, Shanghai Chemical Reagent Co., Ltd., China) was distilled under reduced pressure of 10 mmHg at 120°C. Poly(propylene glycol) (PPG, Mn = 1000 g/mol, BASF Co., Germany) was dried under the pressure of 10 mmHg at 110°C for 12 h to remove residual water. Dimethylol propionic acid (DMPA, Aldrich Co., USA) was dried in oven reaching 120°C for 48 h.
The synthesis of WPU-CFBB was presented in Scheme
Synthesis and structure formula of WPU-CFBB or WPU.
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.
FTIR spectra (a Perkin-Elmer FTIR spectrometer) were recorded from the samples of the final purified films. 1H-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 expected structure of WPU-CFBB was confirmed by FTIR spectroscopy. FTIR spectra of WPU, WPU-CFBB, and CFBB were shown in Figure
FTIR spectra of WPU, WPU-CFBB, and CFBB.
The 1H-NMR (DMSO-d6,
The 1H-NMR (DMSO-d6) spectrum of WPU-CFBB.
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 (
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
The UV-vis spectra of WPU-CFBB emulsion and WPU-CFBB film dissolved in chloroform.
The UV-vis spectra of CFBB in chloroform and WPU-CFBB film dissolved in chloroform were shown in Figure
The UV-vis spectra of CFBB in chloroform and WPU-CFBB film dissolved in chloroform.
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 [
Relationships between the isocyanate index, the average particle size, and the two new absorption maxima for WPU-CFBB emulsion.
Normally, absorption spectrum displayed bathochromic effect along with increasing temperature. However, WPU-CFBB latex dramatically demonstrated both hypsochromic and bathochromic effects with increasing temperature. As could be seen from Figure
The UV-vis spectra of WPU-CFBB emulsion varying with temperature.
The fluorescence spectra of WPU-CFBB emulsion and CFBB in acetone were shown in Figure
Fluorescence spectra of WPU-CFBB and CFBB in acetone under 325 nm excitation.
The weak fluorescent emission of CFBB was not due to the so-called “concentrational self-quenching effect” but because of “structural self-quenching effect” [
Intermolecular excimer of CFBB.
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
Fluorescence intensity of WPU-CFBB and CFBB at different concentrations under 325 nm excitation.
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 [
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
The fluorescence spectra of WPU-CFBB under different temperature under 325 nm excitation.
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
(a) Fluorescent spectra of WPU-CFBB emulsion in increasing quencher concentration of hydroquinone under 325 nm excitation; (b) the plot of
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 × 104, 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
The authors declare that there is no conflict of interests regarding the publication of this paper.
Financial support from the National High Technology Research and Development Program of China (no. 2014AAQ00294), the National Natural Science Foundation of China (no. 51073144), and the Natural Science Foundation of Anhui Education Department (no. KJ2014A045) is acknowledged.