Systemic research of fluorescent emulsion systems and their polymerization process with a fluorescent probe by an AIE mechanism

Abstract

In this paper, an AIE luminogen, which was used as a fluorescent probe, was synthesized and copolymerized with acrylate monomers to study the process of emulsion polymerization and properties of a fluorescent emulsion. At first, according to the changes in the fluorescence spectra, the emulsion polymerization process can be followed with real-time monitoring. Then, by varying the relative content of the AIE luminogen, the glass transition temperature of the synthesized emulsion, the size of the emulsion particle, the contents of the emulsion, and the detection temperature, etc., the relationship between the fluorescence properties and intrinsic properties of the emulsion was studied systematically. It should be pointed out that the microscopic motion of a segment of polymer can be studied by fluorescence spectra with the help of a fluorescent probe. Traditionally, AIE luminogens are applied in optoelectronics and biological domains as small organic molecules. When an AIE luminogen is connected with polymer chains by a chemical bond, a lot of interesting phenomena can be observed. The research results not only provide a new method to study the emulsion polymerization process and properties of emulsion, but also, the synthesized emulsion with properties of fluorescence may broaden the application of the AIE mechanism.

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Introduction

Polymer particles have attracted a great deal of interest and have been utilized in a wide range of fields.¹ Waterborne polymer colloids, often referred to as ‘emulsion dispersions’, are well-established for diverse applications in many commercial products from different industrial segments, including cosmetics, pharmaceuticals, adhesives, coatings and paints^2–5. A key advantage of the colloidal approach is that it enables direct control of the internal structure organization of the particles at nanoscale.⁶ Nanostructured lattices have been commonly prepared by emulsion polymerization processes in the presence of emulsifying agents to ensure the physical stability and nanometric size of the resultant particles. Examples of miniemulsion, suspension or dispersion polymerization processes can be found in the literature, but emulsion polymerization is by far the most frequently used technique.⁷ Traditional AE methods to study emulsions and emulsion polymerization processes include FTIR, ¹H NMR or ¹³C NMR, DSC, TGA, TEM, SEM, and laser diffraction particle size analysis, which study chemical substance, structure, morphologies, and thermodynamic performance.^1,7 However, as a multiphase system, a lot of research conclusions about it are still obscure, and further investigation is still needed to research the properties of emulsions systematically.

Besides studying the intrinsic properties of emulsions, functionalization of the emulsion particles is also important research work. When fluorescent molecules were incorporated into emulsion particles, they attracted increasing interest owing to their potential applications ranging from optoelectronics to biological imaging and disease therapy.⁸ The incorporation of polymerizable organic dyes, including uoresceins, rhodamines, 1,8-naphthalimides, diverse aromatic dyes, coumarins, azo-dyes and oxadiazoles, into polymers using polymerization methods including free radical polymerization, reversible addition–fragmentation transfer polymerization, atom transfer radical polymerization, nitroxide mediated polymerization and emulsion polymerization etc. has been reported.⁸ However, these fluorescent organic particles based on conventional organic dyes often lead to a significant decrease in their fluorescence intensity after their self-assembly into nanoparticles because of the notorious aggregation-caused quenching effect.^9,10

In 2001, the novel phenomenon of aggregation-induced emission (AIE) was first found by Tang’s group from propeller-like siloles, whose emission was very weak in solution but became intense as aggregates formed.¹¹ Such abnormal emission behavior has drawn great research interest, for it is exactly opposite to the common belief that the emission of chromophores decreases in an aggregated state.^12–14 This intriguing finding paves a new avenue to tackle the notorious ACQ of conventional chromophores. Bin Liu and Ben Zhong Tang et al. synthesized TPE-TPA-DCM, fabricated its BSA nanoparticles, and explored their in vivo and in vitro bioimaging applications. The TPE-TPA-DCM possesses both TICT and AIE features, and the BSA-formulated NPs show efficient FR/NIR fluorescence with low cytotoxicity, uniform size and spherical morphology.¹⁵ Xiaoyong Zhang, Yen Wei et al. obtained polymer nanoparticles (named as PhE–Pst NPs) which exhibited strong fluorescence and high water dispersibility owing to the partial aggregation of PhE and surface coverage with a hydrophilic shell. More importantly, these FONs showed spherical morphology, uniform size (about 200 nm) and excellent biocompatibility, making them promising for bioimaging applications.¹⁶ Tang et al. reported highly emissive AIE-based inorganic–organic nanoparticles with core–shell structures which were fabricated by a one-pot, surfactant-free hybridization process. The resultant FSNPs were monodispersed with smooth surfaces. They possessed high surface charges, AIE features and excellent colloidal stability. Furthermore, the particle diameters and emission efficiencies of the FNPs could be manipulated by changing the reaction conditions and luminogen loadings.^17,18

Although fluorescent organic particles had been studied for a long time and showed great prospects in applications, the relationship between their fluorescence properties and other intrinsic properties still needs to be researched carefully. In this paper, tetraphenylethene-containing tetra-acrylates were synthesized and used as a fluorescent probe to carry out systematic research work. Tetraphenylethene (TPE), as one of the typical fluorescent molecules with AIE character, is found to be non-emissive in dilute solutions but becomes highly luminescent when its molecules are aggregated in concentrated solutions or cast in solid films.¹⁹ In detail, in a dilute solution, TPE undergoes dynamic intramolecular rotations against its double bound and renders its molecule non-luminescent. On the other hand, in the aggregate state, the molecules cannot pack through a π–π stacking process due to their propeller shape, while the intramolecular rotation of their aryl rotors is greatly restricted owing to physical constraint. This restriction blocks the non-radiative pathway and opens up a radiative channel.^20,21 Taking advantage of its unique properties, the fluorescence intensity of a polymer modified with TPE-containing tetra-acrylate can be efficiently enhanced by increasing its concentration and loading without the need to avoid the aggregation of the probes. It is possible to study the emulsion polymerization process and the properties of emulsion with fluorescence spectra by taking advantage of the AIE mechanism.

Results and discussion

Tetraphenylethene-containing tetra-acrylates were synthesized according to the previous document.²² Briefly, a McMurry coupling reaction of 4,4′-dihydroxybenzophenone catalyzed by TiCl₄–Zn in THF generated the intermediate tetrahydroxy tetraphenylethene. Since it readily underwent an oxidation reaction in air, it was treated with acryloyl once isolated to furnish the product, TPE-containing tetra-acrylate. White solid; overall yield 38% (Scheme 1). Detailed structural analyses are provided in Fig. 1S.†

Scheme 1 Synthesis of TPE-containing tetra-acrylate.

The standard emulsion polymerization process was carried out as follows. Synthesized TPE-containing tetra-acrylate was blended with acrylate monomers and fluorescent acrylate emulsions prepared via emulsion polymerization with sodium dodecyl sulfate (SDS) as the emulsifying agent and potassium persulfate (KPS) as the thermal initiator. In brief, acrylate monomers mixed with the AIE luminogen were dispersed in water, which contained a minute amount of the emulsifying agent and thermal initiator. After the blending mixtures were emulsified with intense mixing, polymerization was carried out at 80 °C for 10 h with continuous stirring. Different conditions were set to study the relationship between the fluorescence properties and other intrinsic properties of the emulsion, such as the relative content of the AIE luminogen, the size and content of the fluorescent acrylate emulsion, the glass transition temperature of the polymers, and the detection temperature. To identify the samples accurately, the different prepared fluorescent acrylate emulsions were named from AE-TPE-A to AE-TPE-R. Details of the preparation process are presented in the ESI.†

At first, the sample AE-TPE-D was selected as the example to study the emulsion polymerization process, which is presented in Fig. 1. During the process of polymerization, the AIE luminogen acted as a probe for tracking the whole reaction process according to the fluorescence spectra. Owing to the electronic transition of the TPE unit, the absorption spectra of TPE-containing tetra-acrylate and fluorescent acrylate emulsion exhibited an absorption peak at around 460 nm.

Fig. 1 (a) Fluorescence spectra of the ongoing reacting fluorescent acrylate emulsion, (b) wavelength and intensity of the PL spectra of the ongoing reacting fluorescent acrylate emulsion and (c) pictures of the ongoing reacting fluorescent acrylate emulsion taken by a camera with a varied reaction time (from 5 to 600 min).

At first, no obvious polymerization phenomenon happened before 50 min according to the pictures taken from the camera and the results of the fluorescence spectra. During this stage, the AIE luminogen was just physically dispersed into micelles with the methylmethacrylate monomer, and the thermal initiators started to decompose and generate free radicals in water. The fluorescence spectra of the AIE luminogen showed a distinct emission peak at around 485 nm because of the poor solvent system. However, when the reaction carried on, an autoacceleration effect²³ happened from 55 min due to the higher concentration of monomers and free radicals in the micelles. The peak position red shifted to 455 nm and the intensity enhanced about fourteen times. We have proposed that the restriction of intramolecular rotation (RIR) is the main cause for the AIE phenomenon.²⁴ When the AIE luminogen and monomers are knitted together by covalent bonds to form polymer chains, the RIR process will partially activate. This restriction blocks the non-radiative pathway and opens up the radiative channel,²⁵ and thus makes the synthesized acrylate emulsion somewhat emissive. From 55 min to 120 min, in the second process of polymerization, the intramolecular rotation of the AIE luminogen was further restricted due to the ongoing formation of the polymer chains. According to the AIE mechanism, the fluorescence intensity increased with the content of the reacted AIE luminogens. So in this stage, the fluorescence intensity of the reacting system increased with time. When the reaction continued, the fluorescence intensity of the reaction system maintained a relatively stable state from 120 min to 300 min. This means that the AIE luminogens had been used up during the third stage. However, due to the metastable nature of the acrylate emulsion, it might collide, aggregate and deposit at high temperature. So, when the reaction preceded, a part of the fluorescent acrylate emulsion formed a gel and precipitated, and the fluorescence intensity decreased with reacting time.

To verify the results of the fluorescence spectra, GPC and solid content of the emulsion measurements were carried out. As can be seen from the ESI,† the results confirm the results of fluorescence spectra. The typical process of the emulsion polymerization can be summarized as follows: in the beginning, the reactive monomers were wrapped in the micelles formed by the emulsifying agent and dispersed in water. At this stage, the content of free radicals was quite low, and the reaction proceeded gently. When the reaction carried on, the concentration of free radicals increased gradually, and some of them entered into the micelles. Because the concentration of the reaction monomers in the micelles is quite high, the auto-acceleration effect of free-radical polymerization happened during this stage, and most of the monomers (over 90%) reacted. The residual monomers were consumed gradually in the next two or three hours. Due to the acrylate emulsion being a metastable system, some of the emulsion particles might collide, aggregate and deposit in the last stage.

Fig. 2(a) presents the fluorescent acrylate emulsion (AE-TPE-D) and commercially purchased acrylate emulsion under UV light (365 nm). Owing to the AIE mechanism as mentioned above, the synthesized AE-TPE-D emitted a strong fluorescence. Fig. 2(b) represents the typical morphology of the synthesized fluorescent acrylate emulsion (AE-TPE-D) from TEM measurements. With the packing of the micelle formed by the emulsifying agent, acrylate monomers and AIE luminogens were dispersed and polymerized in the micelles via emulsion polymerization.²⁶ The fluorescent acrylate emulsion presented a typical core–shell structure, in which, due to the hydrophobic properties, methylmethacrylate and the AIE luminogens were wrapped into the core layer of the fluorescent acrylate emulsion, while the emulsifying agent with amphipathic properties formed the shell layer to allow them to be dispersed in water. With these characteristics, the composition of the fluorescent acrylate emulsion, preparation process and external environment can influence the fluorescence properties of the synthesized fluorescent acrylate emulsion. So in the next part, we will study the interrelationships between them systematically.

Fig. 2 (a) Fluorescent acrylate emulsion and ordinary acrylate emulsion irradiated under UV light and (b) fluorescent acrylate emulsion observed from TEM measurements.

At first, taking AE-TPE-D and TPE-containing tetra-acrylate as examples, the interaction between the AIE luminogen and polymer chains was studied, and the fluorescence spectra are presented in Fig. 3. As we know that the result of fluorescence spectroscopy is a relative value, the content of the AIE luminogen in the two samples was kept identical in order to study their fluorescence properties. Compared to pure TPE-containing tetra-acrylate in a THF–water (1 : 99) mixture, the emission peak of AE-TPE-D is at about 455 nm, which blue shifted about 30 nm. Also, it should be pointed out that the fluorescence intensity increases by about 7 times for AE-TPE-D with the content of the AIE luminogen. This is partly attributed to the wrapping and coiling of the rigid polymer chains.²⁷ As for the small AIE luminogen, it tends to aggregate in poor solvents and radiate fluorescent light when it is induced by UV light, however, the aggregation is activated just by physical stacking. When it is knitted to the rigid polymer chains with a chemical bond, its intramolecular rotation may be restricted even more. So a higher absorbed energy can be released by the fluorescent light, the emission peak tends to blue shift, and the fluorescence intensity also increases.

Fig. 3 Fluorescence spectra of TPE-containing tetra-acrylate in a THF–water (1 : 99) mixture and the fluorescent acrylate emulsion (AE-TPE-D).

The fluorescence properties of the fluorescent acrylate emulsion which are affected by the relative content of the AIE luminogen are presented in Fig. 4. It is obvious that the peak intensity increased linearly with the content of TPE-containing tetra-acrylate for samples from AE-TPE-A to AE-TPE-D. This is inaccessible to ACQ probes such as pyrene, where increasing the concentration of the probes leads to the formation of excimers or aggregates and consequent fluorescence quenching.^28–30 For AIE luminogens such as TPE-containing tetra-acrylate, aggregation favors emission, and thus allows for the enhancing of the fluorescence intensity by increasing the AIE luminogen loading.³¹ However, for the sample AE-TPE-E, the fluorescence intensity decreases when compared to AE-TPE-D. With the four functional groups of the AIE luminogen, the stability of the synthesized fluorescent acrylate emulsion may be affected due to cross linking polymerization. When the content of the AIE luminogen extends to a certain limit, some of the synthesized fluorescent acrylate emulsion tends to gather and precipitate due to excessive cross-linking. So it is understandable that the peak intensity of AE-TPE-E is lower than AE-TPE-D because some of the AIE luminogens have been precipitated. To confirm this conclusion, we also measured the fluorescence quantum yield (Φ_F) of the fluorescent acrylate emulsions with varied content of the AIE luminogen and the results are summarized in Fig. 5 and Table 1. The trend of Φ_F of the different fluorescent acrylate emulsions also indicates that the content of the AIE luminogens will determine the luminescence behavior of the fluorescent acrylate emulsions, and it increases with the content of the AIE luminogen according to the AIE mechanism.

Fig. 4 Fluorescence spectra of the fluorescent acrylate emulsion with a varied content of TPE-containing tetra-acrylate.

Fig. 5 Fluorescence quantum yield (Φ_F) of the fluorescent acrylate emulsions with a varied content of the AIE luminogen.

Table 1 Optical properties of the fluorescent acrylate emulsion with a varied content of TPE-containing tetra-acrylate^a

Compd	λab/nm	λem/nm	Φagree
a Abbreviations: λab = absorption maximum of different fluorescent acrylate emulsions, λem = emission maximum of different fluorescent acrylate emulsions, and Φagree = fluorescence quantum yields of different fluorescent acrylate emulsions.
AE-TPE-A	340	455	27.80%
AE-TPE-B	340	455	31.25%
AE-TPE-C	340	455	31.30%
AE-TPE-D	340	455	33.06%
AE-TPE-E	340	455	28.72%

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Fluorescent acrylate emulsions of different sizes with a varied content of the emulsifying agent were prepared and studied. Fig. 4S† presents their particle size and particle size distribution. From Fig. 6, the fluorescence intensities increase with the decreased size of the fluorescent acrylate emulsion particles. As is presented in Fig. 2(b), the fluorescent acrylate emulsion particles present a typical core–shell structure and the AIE luminogens with hydrophobic properties are wrapped in the inner core. With the size of the fluorescent acrylate emulsion decreasing, the space for segmental motion of the polymer chains in the fluorescent acrylate emulsion particles becomes smaller. The restricted movement of the polymer chains will strengthen the RIR effect of the AIE luminogens because they are knitted to the rigid polymer chains with a chemical bond.

Fig. 6 Fluorescence spectra of the fluorescent acrylate emulsion with varied particle size.

Sample AE-TPE-D was selected to study the relationship between fluorescence properties and the content of the fluorescent acrylate emulsion by the addition of water (Fig. 7). From the sample AE-TPE-D to sample AE-TPE-L, the fluorescence intensities of the samples increase with decreasing concentration. For the synthesized fluorescent acrylate emulsion, the component and particle size were fixed. According to the AIE mechanism, the fluorescence intensity should decrease with a decreasing concentration of the AIE luminogens. However, contrary to theoretical analysis, the fluorescence intensity increased in fact. We deem that the changed polarity and the squeezing effect of the hydrophilic layer may be the reasons for this phenomenon. As the AIE luminogens are knitted to the polymer chains by chemical bonds, their fluorescence properties will be affected by a polymer segmental motion. The polarity of the system was increased with the added water, which will block the segment motion of the hydrophobic polymer chains.³² In addition, the increased squeeze force of the hydrophilic layer by the added water may enhance this effect. All of these influences can reinforce the RIR effect of the AIE luminogens.

Fig. 7 Fluorescence spectra of the fluorescent acrylate emulsion (AE-TPE-D) with varied concentration.

The testing temperature was set to be at 25 °C, and different acrylate emulsions with a varied T_g value were studied by the fluorescence spectra. From Fig. 8(a), by increasing the glass transition temperature, the fluorescence intensity increases. In addition, the peak positions can be changed also. The peak positions of the fluorescent acrylate emulsions were at around 455 nm when their T_g values were higher than room temperature, while for those whose T_g values were lower than room temperature, their peak positions were at around 470 nm. A possible reason is that in the glassy state, the intramolecular motions of the phenyl rings of the AIE luminogens are restricted to some extent when they are knitted and embedded in the rigid polymer matrices. The energy of the excited state is annihilated through radiation decay, and thus the fluorescent acrylate emulsions emit efficiently. When the polymers are in the rubbery state, the intramolecular motions of the AIE luminogens are activated due to the movement of the polymer segments and the significantly increased free volume in the polymer matrices. The intramolecular motions consume the energy of the excited state. This leads to the weak and red shift of the fluorescence emission in the rubbery state of the polymers.^11,33

Fig. 8 (a) Fluorescence spectra of the fluorescent acrylate emulsions with varied T_g values, and (b) fluorescence wavelength & intensity of the fluorescent acrylate emulsions with varied T_g values.

SEM images of the coatings cured at room temperature of the different fluorescent acrylate emulsions with varied T_g values are presented in Fig. 5S.† As we know, the process of film formation for ordinary emulsions is carried out as follows: at first, the emulsion particles are aggregated by the evaporation of water; then they tend to be warped and destroyed due to the capillary forces between the gaps of the emulsion particles; finally, the damaged emulsion particles tend to intermix with each other and form a flat coating on the substrate surface. The T_g value of emulsions can determine the film forming ability of the emulsion particles. For those emulsion particles whose T_g value is lower than room temperature, soft polymer chains benefit the deformation of the emulsion particles and form a flat coating. While for those emulsions with a high T_g value (beyond room temperature, 25 °C), the emulsion particles cannot wrap due to the stiff polymer chains, and they still present as nano-sized particles after the evaporation of water. According to the AIE mechanism, a trace amount of the AIE luminogen can be used as the fluorescent probe to study the film forming ability of the emulsion particles.

Glass transition is an intrinsic property of amorphous polymers, and it is a macro-reflection of the transformation for the movement of polymer chains, which will affect the materials’ use, properties and processability.^34,35 According to the molecular structural theory, glass transition is the relaxation phenomenon for the amorphous area of polymer materials from the freeze state to unfrozen state. At room temperature under the T_g, only atoms or functional groups on the polymer chains can vibrate at the equilibrium position. In addition, according to the mechanical properties of the polymer materials, the whole glass transformation region of polymer materials can be divided into the “tough” glass state and “soft” glass state. The temperature that identifies these two states is the brittle temperature (T_b).³⁶ When the temperature is lower than the T_b, the polymer materials present a brittleness property; however, when the temperature is higher than the T_b, the polymer materials present extensibility.

As is presented in Fig. 9, AE-TPE-P, whose T_g is set as 40 °C, is used as the example to study the interrelation between the fluorescence properties of the fluorescent acrylate emulsions and the test temperature. As the segmental motion ability increases with increased temperature, according to the RIR mechanism, the intramolecular motions of TPE are activated due to the movement of the polymer segments and significantly increased free volume in the polymer matrices.³⁷ So the fluorescence intensity decreases with increased test temperature. It should be pointed out that there are two apparent lowering platforms for decreased PL intensity in the glass transition region. As is discussed above, there is the “tough” glass state and “soft” glass state of amorphous polymers. Although subtle changes with varied temperature of the polymer chains cannot be observed from ordinary DSC measurements (Fig. 10(a)), they might be observed indirectly from the fluorescence spectra because the fluorescent probe and polymer chains were knitted together at the molecular level with a covalent bond, and the polymer segmental motion can affect the luminescence behavior of the fluorescent acrylate emulsions directly. From Fig. 10(b), the fluorescence intensity of dried AE-TPE-P with a varied test temperature is presented. For the polymer matrix, the segmental motion in the solid state was not obvious compared to when it was dispersed in the emulsion particles due to the high packing density of the polymer chains in the solid state. From the trend of fluorescence intensity with varied temperature, the T_g value of the synthesized emulsion can be obtained, which is close to the theoretical calculation value.

Fig. 9 (a) Fluorescence spectra of the fluorescent acrylate emulsion (AE-TPE-P, T_g = 40 °C) with varied testing temperature, and (b) fluorescence intensity of AE-TPE-P with varied testing temperature.

Fig. 10 Study the properties of the dried AE-TPE-P powers by different testing methods. (a) DSC thermograms; (b) fluorescence spectra.

Conclusion

Due to the AIE luminogen tetraphenylethene-containing tetra-acrylates knitting with polymer chains by chemical bonds, and the synthesized fluorescent emulsion particles dispersing in water uniformly, the changes of the emulsion system and the segmental motion of the polymer matrix will affect the fluorescence properties of the AIE luminogen owing to the restriction of the intramolecular rotation (RIR) effect. With this characteristic, the process of emulsion polymerization can be monitored in real-time, and changes of its internal characteristics and external environments can interfere with the fluorescence spectrometry of the emulsion directly. This study thus opens up a new avenue for the research of the emulsion polymerization process and develops an AIE-active emulsion. In addition, the synthesized emulsion with properties of fluorescence may broaden the application of the AIE mechanism.

Materials

THF was distilled under normal pressure from sodium benzophenone ketyl under nitrogen immediately prior to use. 4,4′-Dihydroxybenzophenone, zinc powder, titanium(IV) chloride (TiCl₄), triethylamine, acryloyl chloride, methylmethacrylate (MMA, C.P.), ethylhexyl acrylate (2-EHA, C.P.), dodecyl sulfate (SDS, A.R.), and potassium persulfate (KPS, A.R.) were used as received without further purification.

Instruments

¹H NMR spectra were measured on Bruker ARX 400 NMR spectrometers using CDCl₃ as the deuterated solvent and tetramethylsilane (TMS; δ = 0 ppm) as the internal standard. FTIR spectra were recorded on a Perkin-Elmer 16 PC FT-IR spectrophotometer. Relative number (M_n) and weight-average (M_w) molecular weights and polydispersity indices (PDI or M_w/M_n) of the fluorescent emulsion particles were estimated by a Waters Associates Gel Permeation Chromatography (GPC) system equipped with RI and UV detectors. THF was used as the eluent at a flow rate of 1.0 mL min⁻¹. A set of monodispersed linear polystyrenes covering the molecular weight range of 10³ to 10⁷ were used as standards for the molecular weight calibration. DSC measurements were carried out using a DSC Q100 (TA Instruments) (TA Instruments, USA) over a temperature of −50 °C to 150 °C at a scan rate of 10 °C min⁻¹. The thermogram was base line corrected and calibrated using indium metal. The experimental specimens (8–10 mg) were dried at 60 °C under vacuum for 24 h, before being measured. All of the samples were firstly annealed at 120 °C for 3 min, and cooled to −50 °C by using liquid nitrogen and then scanned for the measurement. UV-vis absorption and light transmission spectra were measured on a Milton Roy Spectronic 3000 array spectrophotometer. Fluorescence spectra were recorded using a steady state spectrometer (Edinburgh Instrument Ltd, FLSP920) equipped with a temperature control system (Oxford instruments). To eliminate the interference of heating rates on the thermal properties of the fluorescent elastomer, the heating rate was fixed at 1 min⁻¹ for all specimens. Fluorescence spectra were scanned every 1 min.

The absolute fluorescence quantum yield (Φ_F) of the fluorescent acrylate emulsions was measured using an integrating sphere (FLS 980, Edinburgh). Images from the scanning electron microscopy (SEM) analysis were taken on a JSM-6700F electron microscope. Images from the transmission electron microscope (TEM) analysis were taken on a JEOL 2010 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. Particle sizes of the fluorescent emulsion particles were measured on a Brookhaven Instruments Corporation 90 Plus/B1-MAS Zetaplus Zeta Potential Analyzer.

Acknowledgements

The project was funded by the State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, NSFC (No. 21201087), and Postdoctoral Foundation of Jiangsu Province (No. 1501091c). The authors also thank the Jiangsu Overseas Research & Training Program for University Prominent Young & Middle-aged Teacher and Presidents, and the Innovative Programs for Undergraduate Students of and Innovative Programs for Graduate Students of Jiangsu University of Science and Technology. We thank Dr Hongkun Li from Soochow University for helping us carry out the fluorescence quantum yield measurement.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04076d

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