Preparation and properties of a novel polymerizable amphiphilic anthraquinone derivative and its cationic colored copolymer latexes

Abstract

A novel polymerizable amphiphilic purple dye, 4-((4-(p-toluidino)anthraquinone-1-yl)oxy)-N-(2-(methacryloyloxy)ethyl)-N,N-dimethylbutan-1-aminium bromide (TAQMAB), was first designed and synthesized, and then used to prepare cationic covalently purple copolymer latexes through semi-continuous emulsion polymerization using water-soluble 2,2′-azobis(2-methylpropionamide) dihydrochloride (AIBA) as initiator. The chemical structure and properties of the dye were confirmed and characterized. Results showed that the critical micelle concentration and the hydrophilic lipophilic balance of TAQMAB were 975 mg L⁻¹ and 32.8, respectively. The amphiphilic structure of TAQMAB gave the dye considerable water solubility and an ability to migrate from the monomer droplets into the latex particles during emulsion polymerization, which overcame the limitation of solubility, dye conversion, color depth of the latexes, and made the polymerization proceed smoothly. The total monomer conversion decreased gradually with dye amount increasing, while the dye still kept a relatively high conversion when an adequate amount of AIBA was used. Both the total monomer conversion and the dye conversion decreased with AIBA amount decreasing. The light fastness of the covalently colored film was significantly enhanced compared with the noncovalently colored one, which was ascribed to electrons flowing through the polymer chains.

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Introduction

As basic materials, organic dyes are widely used in the applications of coatings, paints, printing, textiles, energy, and biosensors.^1–10 Polymer latexes are environmentally friendly systems with water as continuous phase, and have quite low cost in quantity production.^11,12 Normally, there are two methods to introduce colors into a latex. On the one hand, oil-soluble dyes can be dispersed by surfactants to obtain a water-based color paste, which is a kind of concentrated color dispersion and utilized as colorful additive in waterborne polymer systems.¹³ On the other hand, pigments can be encapsulated during the latex preparation to form complex structured polymeric particles.^14–17 In the above two processes, dyes are just physically blended with the polymer matrix and can be easily destroyed by intense light irradiation inducing fading and discoloration. Moreover, due to the poor compatibility between dye and polymer, phase separation of colors from the polymer matrix always occurs during long-term storage. So macromolecule dyes, which possess covalently bonded chromophores on polymer chains, have attracted more and more attention in recent years.^18–22

Amphiphilic quaternary ammonium salt is a cationic surfactant, which generally possesses both a hydrophilic positively charged ammonium group and a hydrophobic group such as a long alkyl chain.^23,24 Since these kinds of surfactants not only can be easily absorbed onto an oily surface and stabilize emulsion polymerization, but also show significant resistance to pathogens like bacteria, fungi and other microorganisms by killing or inhibiting microbial growth,^25–27 they have been widely used as surfactants in emulsion polymerization to prepare cationic polymer latexes.^28–31

In our previous work, a series of polymerizable anthraquinone dyes were synthesized and the corresponding anionic colored polymer latexes were prepared by emulsion polymerization. We found that the latex stability and the light fastness of the latex films were improved significantly by covalently bonding chromophores on the polymer chains.^32–34 However, since the synthesized dyes had poor solubility both in water and in monomers such as styrene and acrylates, the migration of dye molecules from the monomer droplets to the latex particles was hindered during emulsion polymerization, which induced unstable polymerization, low dye conversion and insufficient color depth of the latexes.^35,36

In the present work, 4-((4-(p-toluidino)anthraquinone-1-yl)oxy)-N-(2-(methacryloyloxy)ethyl)-N,N-dimethylbutan-1-aminium bromide (TAQMAB), a polymerizable amphiphilic anthraquinone derivative with the functionalities of both dye and surfactant, was first designed and synthesized, and then using 2,2′-azobis(2-methylpropionamide) dihydrochloride (AIBA) as initiator, the semi-continuous emulsion copolymerization of styrene (St), butyl acrylate (BA) and TAQMAB was carried out to fabricate covalently colored P(St–BA–TAQMAB) latexes. The chemical structure, the critical micelle concentration (CMC) and the hydrophilic lipophilic balance (HLB) of TAQMAB were characterized. Effects of TAQMAB and AIBA amounts on the emulsion polymerization and the latex properties were investigated, and the light fastness of P(St–BA–TAQMAB) latex film was evaluated by comparing with a noncovalently colored latex film.

Experimental

Materials

St, BA (First Chemical Reagent Factory, Tianjin, China) and N,N′-dimethylaminoethyl methacrylate (DMAEMA) (Beijing Eastern Acrylic Chemical Technology Co. Ltd, China) were purified by vacuum distillation. Hexadecyltrimethylammonium bromide (CTAB), 1,4-dibromobutane (Sinopharm Chemical Reagent Co. Ltd, Shanghai, China), 2,6-ditertbutyl-4-methylphenol (BHT) (Fuchen Chemical Works, Tianjin, China), 1-hydroxy-4-(p-toluidino)anthraquinone (Aladdin Industrial Co. Ltd, Shanghai, China), octylphenol polyoxyethylene (10) ether (OP-10) (Beijing Chemical. Co. Ltd, Beijing, China) and AIBA (Shanghai Yuanye Co. Ltd, Shanghai, China) were used as received. Dichloromethane, ethyl acetate, ethyl ether, petroleum ether, acetone and triethylamine (Beijing Chemical Works, Beijing, China) were dried with anhydrous MgSO₄ and stored with molecular sieve.

Synthesis of 1-(4-bromobutoxy)-4-(p-toluidino)anthraquinone (2)

In a typical experiment, 2 g 1-hydroxy-4-(p-toluidino)anthraquinone, 2 g potassium carbonate, 3 g potassium iodide, 0.5 g CTAB, and 9 g 1,4-dibromobutane were all dissolved in 150 mL acetone and added into a 250 mL round-bottom flask. The reaction was carried out with stirring at 60 °C for 12 h, and the resultant solution was cooled down to room temperature. After removing acetone by evaporation at reduced pressure, the solid was washed and filtered with petroleum ether and potassium hydroxide solution three times, successively. Finally, the obtained purple solid was purified by silica column chromatography using a dichloromethane and ethyl acetate mixture (3 : 1 by volume) as eluent to obtain the product with a yield of 96%.

¹H-NMR (600 MHz, DMSO-d₆, TMS, d/ppm): 8.18 (d × d, 2H, AQ 6,7-H), 7.85 (d × d, 2H, AQ 5,8-H), 7.55 (d × d, 2H, AQ 2,3-H), 7.25 (s, 4H, phenyl 1,2,3,4-H), 4.13 (t, 2H, 2-alkoxy H), 3.70 (t, 2H, 5-alkoxy H), 2.33 (s, 3H, phenyl CH₃), 2.15 (m, 2H, 3-alkoxy H), 1.90 (m, 2H, 4-alkoxy H).

Synthesis of 4-((4-(p-toluidino)anthraquinone-1-yl)oxy)-N-(2-(methacryloyloxy)ethyl)-N,N-dimethylbutan-1-aminium bromide (TAQMAB) (3)

In a typical experiment, 1.5 g 1-(4-bromobutoxy)-4-(p-toluidino)anthraquinone (2), 6 g DMAEMA and 0.5 g BHT were dissolved into 150 mL acetone, and added into a 250 mL round-bottom flask. The reaction was carried out with stirring at 40 °C for 48 h, and the resultant solution was cooled down to room temperature. After removing acetone by evaporation at reduced pressure, the solid was washed and filtered with ethyl ether three times, and the resultant TAQMAB (3) was obtained after vacuum drying with a yield of 81%.

¹H-NMR (600 MHz, DMSO-d₆, TMS, d/ppm): 8.18 (d × d, 2H, AQ 6,7-H), 7.85 (d × d, 2H, AQ 5,8-H), 7.55 (d × d, 2H, AQ 2,3-H), 7.25 (s, 4H, phenyl 1,2,3,4-H), 6.06 (d, 1H, 3-acryloyl cis H), 5.70 (d, 1H, 3-acryloyl trans H), 4.57 (t, 2H, NCH₂CH₂OCO), 4.16 (t, 2H, 2-alkoxy H), 3.78 (t, 2H, NCH₂CH₂OCO), 3.65 (t, 2H, 5-alkoxy H), 3.18 (s, 6H, NCH₃), 2.33 (s, 3H, phenyl CH₃), 2.05 (m, 2H, 3-alkoxy H), 1.88 (m, 5H, 4-alkoxy H and CH₃C=CH₂).

Synthesis of 4-((4-(p-toluidino)anthraquinone-1-yl)oxy)-N,N,N-triethylbutan-1-aminium bromide (TAQTAB) (4)

In a typical experiment, 1.5 g 1-(4-bromobutoxy)-4-(p-toluidino)anthraquinone (2) and 4 g triethylamine were dissolved into 150 mL acetone, and added into a 250 mL round-bottom flask. The reaction was carried out with stirring at 40 °C for 48 h, and the resultant solution was cooled down to room temperature. After removing acetone by evaporation at reduced pressure, the solid was washed and filtered with ethyl ether three times, and the resultant TAQTAB (4) was obtained after vacuum drying with a yield of 85%.

¹H-NMR (600 MHz, chloroform-d₃, TMS, d/ppm): 8.18 (d × d, 2H, AQ 6,7-H), 7.85 (d × d, 2H, AQ 5,8-H), 7.55 (d × d, 2H, AQ 2,3-H), 7.25 (s, 4H, phenyl 1,2,3,4-H), 4.24 (t, 2H, 2-alkoxy H), 3.86 (t, 2H, 5-alkoxy H), 3.58 (q, 6H, NCH₂CH₃), 2.33 (s, 3H, phenyl CH₃), 2.25 (m, 2H, 3-alkoxy H), 2.04 (m, 2H, 4-alkoxy H), 1.42 (t, 9H, NCH₂CH₃).

Preparation of cationic colored latex

The colored latexes were prepared via semi-continuous emulsion polymerization, and typical recipes are listed in Table 1. CTAB, OP-10, AIBA, and polymerizable dye were all dissolved in 90 g deionized water, and then the solution was added into a round-bottom flask, in which the monomer mixture of St and BA was charged beforehand. After emulsification by mechanical stirring at 300 rpm at room temperature for 0.5 h, half of the monomer emulsion was charged into a 250 mL four-necked round-bottom flask and heated in a bath at 65 °C under a nitrogen atmosphere with an agitation of 250 rpm. After 15 min polymerization, the other half of the monomer emulsion was added dropwise into the flask in 3 h. After that, the system was maintained at 65 °C for an additional 3 h, and cooled down to room temperature to obtain the cationic covalently colored P(St–BA–TAQMAB) latexes.

Table 1 Compositions of mixtures used for the preparation of P(St–BA–TAQMAB) latexes

Ingredients	Amount (g)
a 0–0.5 wt% based on the total monomers.b 2.5–3.0 wt% based on the total monomers.
St	5.0
BA	5.0
TAQMAB	Variable^a
AIBA	Variable^b
CTAB	0.40
OP-10	0.20
H2O	90

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As a control experiment, cationic noncovalently colored P(St–BA)/TAQTAB latex was prepared by the same procedure using the same amount of TAQTAB to replace TAQMAB.

Characterization

¹H-NMR spectra were obtained with a JNM-ECA600 spectrometer (JEOL Ltd, Japan). UV-visible absorption was investigated using a UV-visible spectrometer (T6, Persee Ltd Co., Beijing, China). The hydrodynamic diameter (D^DLS_p), polydispersity index (PI_w) and zeta potential (ζ) were measured with a Zetasizer 3000HS instrument (Malvern, UK) at 25 °C. The morphology of the dried latex particles was observed by transmission electron microscopy (TEM, Hitachi 800, Japan) at a voltage of 80 kV. The surface tension of TAQMAB aqueous solution was obtained from a surface tension meter (DCAT 21, Data Physics Co., Germany).

Monomer conversion (Conv.) was measured by gravimetric analysis, which was calculated as follows:

Conv. = S/S_t × 100% (1)

where S is solid content and S_t is the theoretical solid content assuming all of the monomers were consumed in polymerization.

Dye conversion was measured as follows. The colored latexes were coated on a glass plate and dried in an oven at 40 °C, and the resulting colored latex film was then extracted in a Soxhlet extractor by methanol for 5 h. The extracted dye in methanol solution was quantified by the UV-visible absorption method, and the dye conversion (Conv._dye) was calculated as follows:

Conv._dye = (1 − M/M_t) × 100% (2)

where M is the mass of the extracted dye and M_t is the mass of the dye used in the recipe.

Light fastness of the latex film was measured as follows. The colored latex film was irradiated in a xenon lamp aging chamber (LG-XD25U, HanZhan Instruments Co. Ltd, Chongqing, China) at 40 °C for 6 days. The irradiation intensity was 180 W m⁻², and the relative humidity was 50%. The color difference (ΔE) of the film before and after irradiation was measured on a white printing paper by a precise colorimeter (HP-200, Chinaspec Co., Shenzhen, China).

Results and discussion

Design and synthesis of the polymerizable dye TAQMAB and the unreactive dye TAQTAB

The polymerizable amphiphilic anthraquinone quaternary ammonium dye TAQMAB was synthesized by a two-step process as shown in Scheme 1. 1-Hydroxy-4-(p-toluidino)anthraquinone (1), a dark blue solid with maximum absorption wavelength (λ_max) of 579.8 nm, was used as reactant. A butyl chain was introduced between the anthraquinone and C=C bond by 1,4-dibromobutane, which could act as a flexible spacer to decrease the steric hindrance during polymerization. Since the alkyl chain was an electron-donating substituent of anthraquinone group and caused a hypsochromic effect, λ_max of 1-(4-bromobutoxy)-4-(p-toluidino)anthraquinone (2) was 547.2 nm, the compound being a dark purple solid. To increase the solubility of the dye and to introduce the polymerizable group, DMAEMA was used to react with the bromine atom of compound (2) to obtain TAQMAB (3). Such a reaction could be carried out at a relatively low temperature to keep the double bond stable with a yield higher than 80%.

Scheme 1 Synthesis of the polymerizable dye TAQMAB (3) and the unreactive dye TAQTAB (4).

As a control molecule, unreactive dye TAQTAB (4) was also synthesized to investigate the difference in properties as compared to TAQMAB. From Table 2, it can be seen that λ_max of compounds (2), (3), (4) and the colored latex polymer was almost the same, indicating that the color of the dyes did not change both in the synthesis step and in the emulsion copolymerization with other monomers.

Table 2 UV-visible maximum absorption wavelength of the dyes and the P(St–BA–TAQMAB) latex film (N-methylpyrrolidone was used as solvent)

Dye	λmax/nm
Compound (1)	579.8
Compound (2)	547.2
TAQMAB (3)	547.8
TAQTAB (4)	547.4
P(St–BA–TAQMAB)	547.8

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Properties of dyes TAQMAB and TAQTAB as surfactants

Both TAQMAB and TAQTAB had an amphiphilic structure and could act as cationic surfactant, in which the anthraquinone group was hydrophobic and the quaternary ammonium group was hydrophilic. By measuring the surface tension of aqueous solutions of the dyes with various concentrations, the CMC values of TAQMAB and TAQTAB were obtained at the inflection point of the surface tension–concentration curve (Fig. 1), the values being 975 mg L⁻¹ (1.57 mM) and 800 mg L⁻¹ (1.42 mM), respectively. The higher CMC of TAQMAB is due to its a larger hydrophilic group compared with TAQTAB.³⁷

Fig. 1 Surface tension of the dye aqueous solution versus the dye concentration: (a) TAQMAB; (b) TAQTAB.

Based on Davies' group contribution method, HLB values were calculated by the following equation:³⁸

HLB = 7 + Σ(hydrophilic group number) + Σ(lipophilic group number) (3)

When all the group numbers were substituted into eqn (3),^38–41 HLB of TAQMAB and TAQTAB were calculated to be 32.8 and 30.9, respectively, which was consistent with the fact that the more hydrophilic surfactant has a larger HLB value.

Discoloration of polymerizable dye TAQMAB

Because the nitrogen atom linked to anthraquinone of TAQMAB was a comparatively electron-rich center in the excited state, when exposed to light, electrophilic attack occurred easily at the nitrogen atom of phenylamino-substituted anthraquinone inducing discoloration.⁴² The discoloration of TAQMAB was believed to be an oxidation process.⁴³ When TAQMAB aqueous solution was heated to 80 °C and maintained at this temperature, the solution color faded from purple to orange gradually during 4 h. Fig. 2 shows that the absorption intensity of TAQMAB solution at λ_max was weakened gradually and a new peak near 440 nm became larger and larger. If we added some oxidizing agent such as ammonium persulfate or introduced more air into the solution, the discoloration process was accelerated. From the theory of Freeman et al.,⁴³ the phenylamino-substituted anthraquinone can be easily oxidized into hydroxyl-substituted or amide-substituted anthraquinone. Due to the weak electron-donating ability of hydroxyl group and electron-attracting ability of amide group, the UV-visible absorption peak of the dye had a hypsochromic shift, and as a result, the solution color turned from purple to orange.

Fig. 2 UV-visible absorption spectra of TAQMAB solution (0.1 mg L⁻¹ in water) versus incubating time at 80 °C.

Semi-continuous emulsion polymerization

As the cationic amphiphilic TAQMAB was easily oxidized, AIBA, a water-soluble non-oxidizing initiator with positive charge, was used in the semi-continuous emulsion copolymerization of St, BA and TAQMAB under N₂ atmosphere. To maintain the latex stability, surfactant CTAB and OP-10 were also employed. A series of the covalently colored polymer latexes were prepared according to the recipes listed in Table 1, and the properties of the latexes, such as particle size and size distribution, zeta potential, total monomer conversion, and dye conversion, were investigated to evaluate the effects of TAQMAB and AIBA amounts on the emulsion polymerization.

The morphologies of the covalently (sample A3) and noncovalently (sample C3) colored nanoparticles were investigated by TEM imaging. As shown in Fig. 3, the diameters of the dried particles were in the range of 40–50 nm with a narrow size distribution. It was noted that the particles aggregated tightly in the images, and the reason was that the small size of the particles created considerable surface energy, which made the system unstable and could be significantly decreased by aggregation. On the other hand, the particles were quite soft, which was determined by the formulation. In the drying process of TEM sample preparation, the particles tended to coalesce and, as a result, significant deformation of the particles is observed in Fig. 3.

Fig. 3 TEM images of the cationic colored latex nanoparticles: (a) P(St–BA–TAQMAB) (sample A3), and (b) P(St–BA)/TAQTAB (sample C3).

Influence of dye amount

The cationic amphiphilic TAQMAB was easy to dissolve in water, which overcame the limitation of the dye content in emulsion polymerization. Moreover, with the introduction of phenylamino group that was an electron-donating substituent on anthraquinone, the dye would have improved color strength.³⁴ A series of P(St–BA–TAQMAB) latexes with TAQMAB amounts from 0 wt% to 1.0 wt% were prepared, and results are listed in Table 3. The D^DLS_p of all the samples was less than 60 nm with a quite narrow size distribution. It is worth pointing out that the particle size deceased regularly as the dye amount increased, which resulted from the emulsifying property of the dye molecules. When the dye amount was less than 1.0 wt%, the emulsion polymerization proceeded smoothly, and the resulting latexes exhibited excellent colloidal stability of several months. If the dye amount reached 1.0 wt% or higher, the emulsion system became unstable and the size distribution was broadened.

Table 3 Influence of TAQMAB amount on the emulsion polymerization and the latex properties^a

Sample	TAQMAB/wt%	Conv./wt%	Conv.dye/wt%	D^DLSp/nm	PIw	ζ/mV
a 3.0 wt% AIBA to the total monomers was used.b The dye in sample C3 was TAQTAB. Conversion of TAQTAB referred to the adsorption ratio of dye in latexes.
A1	0	95.6	—	57.3	0.074	66.9
A2	0.1	95.1	95.7	53.5	0.070	73.4
A3	0.2	94.5	94.5	52.7	0.061	67.2
A4	0.5	93.3	95.1	47.7	0.075	67.5
A5	1.0	90.0	93.7	38.5	0.209	69.5
C3^b	0.2	95.5	20.7	57.2	0.087	65.9

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The total monomer conversion was also significantly influenced by the dye amount. Accounting for the inhibition of the anthraquinone group in emulsion polymerization,^44,45 the total monomer conversion decreased gradually with dye amount increasing. However, the dye still kept a relatively high conversion in all samples. A possible reason was that the dye molecules could act as surfactant and tended to migrate into the latex particles during emulsion polymerization. Due to the favorable solubility and migration, the polymerizable dye TAQTAB had a satisfactory conversion when an adequate amount of AIBA was used. Interestingly, if part of BA (10 wt% to the total monomers) in sample A3 was replaced by DMAEMA, which was also a kind of amphiphilic water-soluble monomer and competed with the dye molecules during the emulsion polymerization, the dye conversion decreased evidently to 74.0%. Such a phenomenon also confirmed the explanation proposed above.

The P(St–BA)/TAQTAB latex with unreactive dye TAQTAB was also prepared as a control experiment (sample C3). Because there was no chemical bond between TAQTAB and polymer chain, the dye could be easily extracted and only about 20 wt% remained in the polymer matrix by physical adsorption after 5 h extraction.

The P(St–BA–TAQMAB) latex films were dissolved in N-methylpyrrolidone, and their UV-visible absorption spectra were measured and are shown in Fig. 4. The λ_max remained at 547.8 nm, and the absorbance intensity increased significantly with an increase of the dye amount, demonstrating that the dye molecules were evenly copolymerized with other monomers and the color strength could be adjusted by altering the dye amount.

Fig. 4 UV-visible absorption spectra of P(St–BA–TAQMAB) prepared with different TAQMAB amounts (N-methylpyrrolidone was used as solvent).

Influence of initiator amount

To investigate how the initiator affects the emulsion polymerization and the resulting latex properties, a series of P(St–BA–TAQMAB) latexes were prepared using different amounts of AIBA from 1.0 wt% to 3.0 wt% to the total monomers, and the results are summarized in Table 4. The total monomer conversion and the dye conversion both increased with the AIBA amount increasing. When 1.5 wt% of AIBA was used, the total monomer conversion and the dye conversion were only 82.5% and 76.1%, respectively. Meanwhile, the particle size was relatively small and the size distribution was broadened, indicating 1.5 wt% of AIBA was insufficient. So initiator amount was an essential factor in the preparation of a stable and uniform P(St–BA–TAQMAB) latex.

Table 4 Influence of AIBA amount on the emulsion polymerization and the P(St–BA–TAQMAB) latex properties^a

Sample	AIBA/wt%	Conv./wt%	Conv.dye/wt%	D^DLSp/nm	PIw	ζ/mV
a 0.2 wt% TAQMAB to the total monomers was used.
B1	1.5	82.5	76.1	47.2	0.219	77.8
B2	2.0	92.2	85.1	52.5	0.109	70.1
B3	2.5	93.5	88.2	52.8	0.079	70.6
B4/A3	3.0	94.5	94.5	52.7	0.064	67.2

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Light fastness of the latex films

The light fastness of covalently colored P(St–BA–TAQMAB) latex film (sample A3) and noncovalently colored P(St–BA)/TAQTAB latex film (sample C3) was investigated by an irradiation test, and color difference (ΔE) was used to evaluate the light fastness. As illustrated in Fig. 5, ΔE of P(St–BA–TAQMAB) film was much lower than that of P(St–BA)/TAQTAB film, indicating that the former film had better photostability than the latter. It is known that the color difference mainly results from the dye structural change or destruction under intense xenon lamp irradiation. The enhancement of light fastness could be ascribed to electron transfer from the covalently bonded chromophore to the main polymer chain. The polymer chains transported the extra energy from irradiation and kept the anthraquinone group in the ground state rather than transiting to an excited state.⁴⁶ In contrast, the unreactive dye TAQTAB in the P(St–BA)/TAQTAB latex film could not dissipate the intrusive energy and was easily destroyed under irradiation, which resulted in a higher ΔE.

Fig. 5 ΔE of purple films versus irradiation time: (■) P(St–BA)/TAQTAB latex film (sample C3), and (●) P(St–BA–TAQMAB) latex film (sample A3).

ΔE was also related to the chemical structure of the dyes. Compared with the previous work of our lab (Scheme 2),^33,34 the colored latex films prepared with dye AHAQ (red) and AAQCB (yellow) have smaller ΔE, while the film prepared with AHMAQ (blue) has as large a ΔE as that of TAQMAB (purple) film. As discussed in the section on dye discoloration, the nitrogen atom bonded to the anthraquinone group is an electron-rich center and is easily attacked by electrophilic reagent. That is, a larger electron density of the nitrogen atom will induce less stability of the dye and higher ΔE of the colored films.

Scheme 2 Chemical structures of the four synthesized polymerizable dyes.

The dye TAQMAB has two substituents on anthraquinone group, which are phenylamine and alkoxy. The alkoxy substituent is electron donating and leads to the nitrogen possessing larger electron density. AHMAQ has two alkylamino substituents, which also induce larger electron density on the nitrogen atom. As a result, AHMAQ and TAQMAB are both easy to oxidize and their colored latex films have relatively high ΔE. On the contrary, AHAQ and AAQCB just have one substituent without any other electron-donating group on anthraquinone. Moreover, the carbonyl group, which is an electron-withdrawing group, can further reduce the electron density of the nitrogen atom of AAQCB. Therefore, AAQCB and AHAQ are relatively stable, and their colored latex films have small ΔE.

Conclusions

Polymerizable amphiphilic anthraquinone dye TAQMAB was synthesized and its chemical structure was confirmed by ¹H-NMR, and the values of its CMC and HLB were measured and calculated to be 975 mg L⁻¹ and 32.8, respectively. Then, semi-continuous emulsion copolymerization initiated by AIBA was carried out to prepare cationic P(St–BA–TAQMAB) latexes. The amphiphilic structure of TAQMAB gave the dye considerable water solubility and the ability to act as a surfactant during the emulsion polymerization, which helped the dye conversion to achieve a relatively high level. Both total monomer conversion and dye conversion increased with AIBA amount increasing. The light fastness of the covalently colored film was significantly enhanced compared with the noncovalently colored film, which was ascribed to electrons flowing through the polymer chains. Polymerizable amphiphilic dyes, which overcome the limitation of solubility, dye conversion in emulsion polymerization and color depth of the resulting latexes, should have great potential in future applications.

Acknowledgements

This work is supported by grants from the National Basic Research Program of China (no. 2014CB932202).

Notes and references

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