Dual responsive macroemulsion stabilized by Y-shaped amphiphilic AB₂ miktoarm star copolymers

Abstract

Dual responsive macroemulsions stabilized by Y-shaped amphiphilic AB₂ miktoarm star polymeric emulsifiers were presented in this study. First, a amphiphilic Y-shaped AB₂ miktoarm star polymer composed of poly(N,N-dimethylaminoethylmethacrylate) (PDMAEMA) and polystyrene (PS) arms was synthesized by sequential reversible addition–fragmentation chain transfer (RAFT) polymerization of styrene monomer and atom transfer radical polymerization (ATRP) of N,N-dimethylaminoethyl methacrylate (DMAEMA) monomer. The structure and the molecular weight as well as the molecular weight distribution were carefully characterized by ¹H NMR and GPC, respectively. The obtained PS–(PDMAEMA)₂ miktoarm star polymers were then applied as polymer emulsifiers for both o/w and w/o macroemulsions formation, and stabilized macroemulsions could be produced at a lower emulsifier content. Meanwhile, the emulsifying performance of PS–(PDMAEMA)₂ miktoarm star polymer and stimulus-response of the macroemulsion were also investigated. The PS–(PDMAEMA)₂ stabilized o/w macroemulsion showed pH-induced demulsification and temperature-induced phase inversion. However, the inversion of the PS–(PDMAEMA)₂ emulsifier at the oil–water interface could not be spontaneously accomplished. Furthermore, successful phase inversion was only smoothly realized for those emulsions with pH 7 water in the presence of a modulate stirring.

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1. Introduction

Emulsions are a class of disperse systems consisting of two immiscible liquids,¹ where disperse phase droplets are dispersed in a continuous phase medium in the presence of emulsifiers. Emulsions, not including Pickering emulsion,² can be divided into three types: a kinetically stable macroemulsion, thermodynamically stable microemulsion, and an “approaching thermodynamically stable” nanoemulsion³ according to the droplet size; the corresponding droplet size ranges are 0.1–5 μm, 5–50 nm and 20–100 nm, respectively.⁴ Because of their high stability, microemulsions and nanoemulsions have huge amounts of applications in food technology, personal care and cosmetics, and drug delivery as well as materials synthesis.^5–9 However, preparation of a microemulsion is not easy due to a lack in general theory for predicting the microemulsion structure in a particular system containing oil, water, and surfactant.¹⁰ From this point of view, preparation of kinetically stable macroemulsions is relatively easy because they can be obtained by simply dispersing oil and water phases in the presence of emulsifiers without a co-stabilizer requirement. Furthermore, a large number of surfactants with a lower concentration and a wider range of volume ratio of the two phases are suitable for macroemulsion formation.^11,12

Both low molecular weight surfactants and polymeric surfactants can be utilized to stabilize macroemulsions.¹³ Additionally, inorganic solid particles or polymer soft particles with suitable wettability can also be used as macroemulsion stabilizers, and these particle-stabilized emulsions are specially termed as “Pickering emulsions”.^2,14–18 Compared to traditional low molecular weight surfactants, amphiphilic block copolymers usually have a very low critical micelle concentration and a low diffusion coefficient.¹⁹ As a result, a lower polymeric surfactant content is required for emulsion formation. So far, many amphiphilic block polymers with different constitutes have been applied for macroemulsions in past decades.^20–24 However, to the best of our knowledge, polymeric surfactants for macroemulsions were mainly concentrated on linear amphiphilic block copolymers, and less attention was paid to amphiphilic miktoarm star polymers.²⁵

Miktoarm star polymers, also called asymmetric star polymers or hetero-arm star polymers, refer to polymers that contain a central core connected by a number of various types of polymer arms.^26,27 During the past decades, a wealth of miktoarm star polymers with varied arm constitutions have been synthesized by virtue of the great progress in polymer synthesis methodology.^28–34 Y-shaped AB₂ miktoarm polymers, as the simplest miktoarm star polymers, have received considerable attention within the past years. Synthesis strategies for Y-shaped AB₂ miktoarm polymers include atom transfer radical polymerization (ATRP),^35,36 opening polymerization (ROP),^37–39 and “click” reaction⁴⁰ as well as versatile combinations of different polymerization methods, including ATRP/ROP,^41,42 reversible addition–fragmentation chain transfer (RAFT) polymerization/ROP,⁴³ ROP/click,⁴⁴ ATRP/ROP/click,⁴⁵ living cationic polymerization/ROP, and anionic polymerization/ROP combination.^46,47 Meanwhile, self-assembly of Y-shaped AB₂ miktoarm polymers in solution also has been investigated in detail. Experimental results showed that self-assembly in solution of Y-shaped AB₂ miktoarm polymers is different from that of their linear counterparts.^37–39,41 Distinctions in self-assembly behaviors suggested that Y-shaped AB₂ miktoarm polymers should have rather different interfacial properties from their linear counterparts. However, as far as we are concerned, there have been limited reports on a macroemulsion stabilized by Y-shaped AB₂ miktoarm polymers to date.²⁵

Herein, we presented a dual responsive macroemulsion using a well-defined AB₂ miktoarm star polymer composed of poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) and polystyrene (PS) arms as polymeric emulsifiers in this study. PS–(PDMAEMA)₂ was synthesized by sequential reversible addition–fragmentation chain transfer (RAFT) polymerization of styrene monomer and atom transfer radical polymerization (ATRP) of DMAEMA monomer. The structure and the molecular weight as well as the molecular weight distribution of PS–(PDMAEMA)₂ were carefully characterized by GPC and ¹H NMR, respectively. The emulsifying performance of emulsifier and the stimuli-responses of the formed macroemulsion were investigated in detail.

2. Experimental

2.1 Materials

Trimethylolpropane was obtained from Aldrich and used without further purification. N,N-Dimethylaminoethyl methacrylate (DMAEMA) was purified by passing it through a dried basic alumina column and distilling over CaH₂ under reduced pressure prior to polymerization. Styrene was purified by reduced pressure distillation. 2-(Ethylthiocarbonothioylthio)-2-methylpropanoate (EMP) was synthesized according to procedures described in the literature.⁴⁸ 2-Bromoisobutyryl bromide (BrIB), N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA), 1,3-dicyclohexylcarbodiimide (DCC), and 4-(dimethylamino)pyridine (DMAP) were purchased from Aldrich and used as received. Azobisisobutyronitrile (AIBN) was recrystallized twice from ethanol prior to use.

2.2 Synthesis of trifunctional core

Synthesis of 2,2-bis(hydroxymethyl)butyl 2-(ethylthiocarbonothioylthio)-2-methylpropanoate (BEMP). In a typical reaction, trimethylolpropane (3.35 g, 25 mmol), DCC (4.12 g, 20 mmol), DMAP (0.49 g, 4.0 mmol) and 40 mL of fresh CH₂Cl₂ were charged into a dried round bottomed flask, and then 20 mL of CH₂Cl₂ containing EMP (4.48 g, 20 mmol) was added dropwise with stirring. After 48 h of stirring at room temperature, the mixture was filtered to remove the dicyclohexylurea product. The solvent was removed by evaporation, and the remaining product was purified by column chromatography (silica gel) using petroleum ether/ethyl acetate (6 : 1) as an eluent, affording a pale yellow oil. Yield: 5.34 g (78.5%). ¹H NMR (400 MHz, CDCl₃, δ, ppm): 4.14 (s, 2H, OCH₂), 3.59 (s, 4H, CH₂OH), 3.33–3.27 (q, 2H, SCH₂), 1.71 (s, 6H, CCH₃), 1.36–1.30 (q, 2H, CCH₂), 1.32–1.25 (t, 3H, SCH₂CH₃), 0.88–0.84 (t, 3H, CH₂CH₃).

2-Ethyl-2-((2-(ethylthiocarbonothioylthio)-2-methylpropanoyloxy)methyl)propane-1,3-diyl bis(2-bromo-2-methylpropanoate) (EPBP). In a typical reaction, BEMP (5.1 g, 15 mmol), triethylamine (8.8 mL, 60 mmol) and 25 mL fresh CH₂Cl₂ were charged into a dried round bottomed flask. The flask was immersed in an ice bath, and 10 mL of dried CH₂Cl₂ containing 2-bromoisobutyryl bromide (6.84 g, 30 mmol) was added dropwise to the flask under stirring. The mixture was stirred at 0 °C for 2 h and then at room temperature overnight. After reaction, the mixture was extracted three times with a saturated aqueous solution of sodium bicarbonate. The collected organic phase was dried over magnesium sulfate. The crude product was purified by column chromatography (silica gel) using petroleum ether/ethyl acetate (3 : 1) as an eluent, affording a pale yellow oil. Yield: 7.16 g (74.8%). ¹H NMR (400 MHz, CDCl₃, δ, ppm), 4.10 (s, 4H, OCH₂), 4.04 (s, 2H, OCH₂), 3.30–3.25 (q, 2H, SCH₂), 1.94 (s, 12H, C(Br)CH₃), 1.70 (s, 6H, CCH₃), 1.60–1.54 (t, 2H, CCH₂), 1.34–1.30 (q, 3H, SCH₂CH₃), 0.94–0.91 (t, 3H, CCH₂CH₃).

2.3 Synthesis of PS–Br₂ ATRP macroinitiator by RAFT polymerization

PS–Br₂ macroinitiator for ATRP was obtained by RAFT polymerization of St employing EPBP as a RAFT agent. In a typical reaction, St (1.248 g, 12 mmol), EPBP (0.128 g, 0.2 mmol) and AIBN (10.93 mg, 0.067 mmol) were charged into a round bottomed flask. The flask was degassed by three freeze–pump–thaw cycles and then immersed in oil bath at 75 °C for polymerization under N₂ atmosphere. After 5 h of polymerization, the reaction was quenched by diluting with THF. After multiple operations involving precipitation in methanol, filtration, and washing, the product was dried under vacuum at room temperature.

2.4 Synthesis of Y-shaped PS–(PDMAEMA)₂ miktoarm star copolymer via ATRP

PS–(PDMAEMA)₂ miktoarm star copolymer was synthesized by ATRP of DMAEMA in THF at 60 °C using PS–Br₂ and CuBr/PMDETA as macroinitiator and catalyst, respectively. In a typical reaction, 0.4 g of PS macroinitiator, CuBr (28.7 mg, 0.2 mmol), PMDETA (0.041 mL, 0.2 mmol), DMAEMA (2.51 g, 16 mmol) and 3 mL of fresh THF were charged into a round bottomed flask. The flask was degassed by three freeze–pump–thaw cycles and then immersed in an oil bath at 60 °C for polymerization under N₂. After 4 h of polymerization, the reaction was stopped by diluting with THF and then passed through a short basic alumina column to remove the metal salt. The solution was concentrated by evaporation, and the concentrated mixture was dropped into a large amount of hexane, producing a white precipitation. The precipitate was filtered, and washed with hexane. These precipitation, filtration and washing operations were repeated for three times, and the purified product was dried under vacuum at room temperature.

2.5 Generation of toluene/water (o/w) and water/toluene (w/o) emulsions stabilized by PS–(PDMAEMA)₂ miktoarm star copolymers

Typical preparation procedures of o/w and w/o emulsions were as follows: an equal volume of PS–(PDMAEMA)₂ miktoarm star copolymer toluene solution (5 mg mL⁻¹) and acidic water (pH 5) were charged in a 10 mL of glass vial at room temperature. Emulsification was performed using a XHF-D high speed disperser (Ningbo Scientz Biotechnology Co., Ltd, China) at a stirring rate of 12 000 rpm for 1 min. During homogenization, the glass vial was immersed in an ice bath to remove heat formed by this process. The emulsion droplets were observed using an optical microscope. A few drops of the diluted emulsions were placed on a glass slide and viewed. The type of emulsion was determined by conductivity using a Jenway 4510 conductivity meter.

2.6 pH- and thermo-response of o/w emulsion

The pH-response of an o/w emulsion was performed by adding a known volume of 1 mol L⁻¹ NaOH aqueous solution into an emulsion to adjust the water phase pH to about 9.0. For the broken emulsion, a known volume of 1 mol L⁻¹ HCl aqueous solution was added into the water to adjust the water phase pH to 5.0. Re-emulsification was performed by homogenization at a stirring rate of 12 000 rpm for 1 min. The thermo-response of an o/w emulsion was performed by heating the formed o/w emulsion in a water bath at 55 °C alone or with a gentle shaking by hand or with moderate magnetic stirring. All pH values of the water phase were measured by pH meter.

2.7 Characterization tests

¹H NMR spectra were recorded with a Bruker AV-400 NMR spectrometer. Molecular weights and polydispersity indexes (PDI) of polymers were determined by GPC measurements, which were performed on a Waters 1515 GPC equipped with a Waters 2414 differential refractive index detector in DMF at 80 °C with a row rate of 1.0 mL min⁻¹. Interfacial tension measurements were measured by the Wilhelmy plate method, and the experiments were performed with a Kruss tensiometer K20 equipped with a Wilhelmy slide. Optical micrographs (OM) were collected with an optical microscope (Leica, DM 4500P). A drop of the diluted emulsion was placed on a microscope slide and pictures were taken randomly from different spots of the same sample. The average size of droplets was analyzed by a MALVERN Zetasizer Nano ZS instrument. Confocal scanning laser microscopy (CSLM, Olympus FV1000 laser confocal microscope, Japan) was used to observe the primary o/w emulsion and the formed w/o emulsion by a thermo-induced inversion process. The o/w emulsion employed for a CLSM image was stabilized by 0.12 wt% of PS₃₂–(PDMAEMA₁₂₁)₂ miktoarm star copolymers. Prior to homogenization, a hydrophilic fluorescence probe of Rhodamine B (RhB) and a hydrophobic fluorescence probe of allyl-(7-nitro-benzo[1,2,5]-oxadiazol-4-yl)-amine (NBDAA) were dissolved in water and toluene, respectively. The o/w emulsion was heated in a water bath at 55 °C for 5 min with moderate stirring.

3. Results and discussion

3.1 Synthesis of Y-shaped PS–(PDMAEMA)₂ miktoarm star copolymer

In this study, Y-shaped PS–(PDMAEMA)₂ miktoarm star copolymer was synthesized using “core first” strategy. The overall synthesis route was shown in Scheme 1. As illustrated in Scheme 1, a trifunctional core of EPBP containing two ATRP initiator groups and RAFT group was first synthesized by two successive chemical processes involving the synthesis of BEMP by an esterification reaction of trimethylolpropane with EMP, and the subsequent acylation reaction between BEMP and 2-bromoisobutyryl bromide. Successful synthesis of the titled EPBP was verified by ¹H NMR studies. Fig. 1 shows the ¹H NMR spectra of EMP, BEMP, and EPBP. As displayed in Fig. 1A, the characteristic signals at δ = 3.31, 1.74, and 1.33 ppm were assigned to the protons of –SCH₂, –CCH₃ and –SCH₂CH₃ in EMP, which agrees with earlier reports.⁴⁸ The ¹H NMR spectrum of BEMP is displayed in Fig. 1B. Compared with the ¹H NMR spectrum of EMP, four groups of new resonances at δ = 4.14, 3.59, 1.31, and 0.86 ppm appeared in addition to the characteristic signals of EMP. These newly formed signals were assigned to the protons of –CH₂ next to an ester group, –CH₂OH, and –CH₂ as well as –CH₃, respectively. Based on ¹H NMR analysis, all peak integrals were consistent with the target compound. The appearance of these new resonance peaks and the consistence of peak integrals with the target compound were an indication of the successful esterification reaction between EMP and trimethylolpropane. The trifunctional core of EPBP was formed by further reaction of the produced BEMP with 2-bromoisobutyryl bromide. A new resonance peak at δ = 4.10 ppm was observed in the ¹H NMR spectrum of the resultant EPBP, as shown in Fig. 1C. This new resonance peak was assigned to the protons of –CH₂ next to –COOC(CH₃)₂Br. At the same time, another resonance peak at δ = 3.59 ppm in Fig. 1B, corresponding to the protons of –CH₂OH, completely disappeared after the acylation reaction. This suggested that the –CH₂OH groups in BEMP had been converted to –COOC(CH₃)₂Br. ¹H NMR analysis also confirmed that peak integrals were consistent with the target EPBP.

Scheme 1 Synthesis routes of BEMP, EPBP, PS–Br₂, and PS–(PDMAEMA)₂. (i) EMP, DCC/DMAP, CH₂Cl₂/room temperature. (ii) 2-Bromoisobutyryl bromide, Et₃N, CH₂Cl₂. (iii) St, AIBN/75 °C. (iv) DMAEMA, CuBr/PMDETA, THF/60 °C.

Fig. 1 ¹H NMR spectra of EMP (A), BEMP (B), and EPBP (C).

The purified trifunctional core of EPBP was then applied for the bulk RAFT polymerization of St, and the polymerization was carried out at 75 °C using AIBN as an initiator. After 5 h of polymerization, the reaction was stopped. The purified polymer was analyzed by GPC and ¹H NMR. The GPC trace of PS polymer (denoted as PS–Br₂) was shown in Fig. 2A. A symmetrical monomodal peak was an indication of a controlled RAFT polymerization of St mediated by EPBP using the described reaction conditions. GPC analysis indicated the M_n of PS–Br₂ was 4100 g mol⁻¹ with PDI of 1.13.

Fig. 2 GPC traces of PS–Br₂ (A), PS₃₂–(PDMAEMA₈₀)₂ (B), and PS₃₂–(PDMAEMA₁₂₁)₂ (C).

Fig. 3A shows the ¹H NMR spectrum of PS–Br₂, in which the respective resonances, including the bromine end groups, were clearly assigned. M_n of PS–Br₂ was calculated to be 3900 g mol⁻¹ according to the ¹H NMR integral area ratio of a peak at δ = 7.23–6.50 ppm to that at δ = 3.28 ppm. Evidently, M_n calculated from ¹H NMR analysis is approximately consistent with that from GPC result.

Fig. 3 ¹H NMR spectra of PS–Br₂ (A), PS₃₂–(PDMAEMA₈₀)₂ (B), and PS₃₂–(PDMAEMA₁₂₁)₂ (C).

Purified PS–Br₂ was then used as macroinitiator to initiate the ATRP of DMAEMA monomer to synthesize Y-shaped AB₂ miktoarm star copolymers with different PDMAEMA block lengths. ATRP of DMAEMA was carried out in THF at 60 °C. After polymerization, the polymer was purified and then analyzed by GPC. By comparison with PS–Br₂, GPC traces of PS–(PDMAEMA)₂ presented a clear shift to a higher molecular weight region, as shown in Fig. 2B and C. The resulting symmetrical GPC monomodal peak in GPC curves revealed a controlled ATRP process of DMAEMA initiated by PS–Br₂. GPC analysis indicated that the values of the M_n of the PS–(PDMAEMA)₂ product were 29 000 and 42 000 g mol⁻¹, respectively, corresponding to 1.28 and 1.25 of PDI. According to GPC analysis, the number average polymerization degree of a PS block was 32, and a PDMAEMA block was 80 and 121, respectively. As a result, the as-synthesized two Y-shaped PS–(PDMAEMA)₂ miktoarm star copolymers were thus denoted as PS₃₂–(PDMAEMA₈₀)₂ and PS₃₂–(PDMAEMA₁₂₁)₂. The ¹H NMR spectra of PS–(PDMAEMA)₂ with different PDMAEMA block lengths were displayed in Fig. 3B and C. Compared with Fig. 3A, several groups of new resonances at δ = 4.06, 2.57, 2.29, 1.84, and 0.91 ppm appeared. These resonances should be assigned to the protons of –OCH₂, –OCH₂CH₃, –N(CH₃)₂, –CCH₂ and –CCH₃ in the PDMAEMA arms, respectively.^{49 1}H NMR analysis indicated the values of M_n of the synthesized two PS–(PDMAEMA)₂ samples are 21 000 and 28 000 g mol⁻¹, respectively, which are smaller than those obtained from GPC determination.

3.2 PS–(PDMAEMA)₂ miktoarm star copolymer stabilized o/w and w/o macroemulsions

Y-shaped PS–(PDMAEMA)₂ miktoarm star copolymer has a hydrophobic PS arm and two hydrophilic PDMAEMA arms. This unique chain architecture might allow this unsymmetrical amphiphilic miktoarm star copolymer to have different emulsifying properties from those of its traditional amphiphilic linear counterpart. In order to verify this, we attempted to prepare emulsion using synthesized PS–(PDMAEMA)₂ miktoarm star copolymers as emulsifiers, where toluene was used as the oil phase by virtue of its good solubility to both PS and PDMAEMA arms. Although PDMAEMA polymer can be well dissolved in some common organic solvents, such as THF, toluene etc., PDMAEMA is capable of being preferentially solvated by a water phase, especially acidic water, which allows PS–(PDMAEMA)₂ chains to migrate from the initial toluene bulk phase to the water–oil interface during homogenization. At the water–oil interface, the PDMEMA arms were wetted by the water phase, while the PS block remained in the bulk toluene. PS–(PDMAEMA)₂ miktoarm star copolymers absorbed at the water–toluene interface will stabilize the dispersed droplets and behave like surfactants. Fig. 4 shows representative photographs of the generated macroemulsions which were generated under different conditions. Our extensive experiments showed that PS₃₂–(PDMAEMA₁₂₁)₂ showed enhanced emulsifying performances with the reduction in water pH. As shown in Fig. 4A and B, the oil phase was completely emulsified by pH 5 water with a fixed water/toluene volume ratio of 1 and a fixed PS₃₂–(PDMAEMA₁₂₁)₂ content, whereas three layers, including the upper oil layer, the middle emulsion layer, and the under water layer, were produced at higher pH values such as 6 and 7, as indicated in Fig. 4C and D. After 24 h of standing at room temperature, the emulsion thickness was 55%, 30%, and 20% relative to the total liquid layer thickness, respectively (Fig. 4B, C and D). Emulsion could not be formed when more basic water, such as pH 9 water, was employed (Fig. 4E). Optical microscope photos of emulsions stabilized by different PS₃₂–(PDMAEMA₁₂₁)₂ contents were also displayed in Fig. 4. The average size of emulsion droplets was about 3.6 μm with 0.233 of PDI (Fig. 4I), and 3.4 μm with 0.7 of PDI (Fig. 4J), corresponding to 0.12 wt% and 0.06 wt% of PS₃₂–(PDMAEMA₁₂₁)₂ content, respectively. Increasing PS₃₂–(PDMAEMA₁₂₁)₂ content was somewhat favorable to the reduction in the polydispersity instead of the average size of emulsion droplets. Meanwhile, PS–(PDMAEMA)₂ showed a decreased emulsifying performance when the PDMAEMA block length was reduced. For example, the least required PS₃₂–(PDMAEMA₈₀)₂ content was 0.10 wt% when toluene oil was completely emulsified by an equal volume of acidic water (pH 5), whereas the average size and the polydispersity of emulsion droplets were approximately the same as those of the emulsion droplets stabilized by 0.06 wt% of PS₃₂–(PDMAEMA₁₂₁)₂ (S1, ESI†).

Fig. 4 (A–H) Photographs of o/w macroemulsions generated at different emulsifier contents and pH values: (A) 0.12 wt% of PS₃₂–(PDMAEMA₁₂₁)₂ and pH 5 water; (B) 0.06 wt% of PS₃₂–(PDMAEMA₁₂₁)₂ and pH 5 water; (C) 0.06 wt% of PS₃₂–(PDMAEMA₁₂₁)₂ and pH 6 water; (D) 0.06 wt% of PS₃₂–(PDMAEMA₁₂₁)₂ and pH 7 water; (E) 0.06 wt% of PS₃₂–(PDMAEMA₁₂₁)₂ and pH 9 water; (F) 0.12 wt% of PS₃₀-b-PDMAEMA₂₅₉ and pH 5 water; (G) 0.06 wt% of PS₃₀-b-PDMAEMA₂₅₉ and pH 5 water; (H) 0.06 wt% of PS₃₂–(PDMAEMA₁₂₁)₂ and pH 7 water. (I, J and K) The optical microscopes of macroemulsion droplets shown in A, B and F, and the insets displayed in I, J, and K were the corresponding droplet size distribution histograms. Scale bar: 6 μm. All the photographs were taken after 24 h of standing at room temperature. The volume ratio of toluene to water: 1 for A–G; 0.25 for H.

The emulsifying performance of Y-shaped PS–(PDMAEMA)₂ miktoarm star copolymer was also compared with that of its linear counterpart. For this purpose, a linear di-block copolymer of PS₃₀-b-PDMAEMA₂₅₉, possessing a M_n approximately the same as that of PS₃₂–(PDMAEMA₁₂₁)₂, was synthesized by two successive ATRP processes using ethyl-2-bromoisobutanoate and CuBr/PMDETA as initiator and catalyst, respectively. The detailed synthesis and GPC characterizations are displayed in ESI (S2, ESI†). The number average polymerization degrees of PS and PDMAEMA block were 30 and 259, respectively, which was determined by GPC analysis. The obtained PS₃₀-b-PDMAEMA₂₅₉ was also used as an emulsifier for macroemulsion formation under the same conditions. Experimental observation revealed that toluene oil could not be emulsified completely by an equal volume of acidic water (pH 5) at 0.06 wt% of PS₃₀-b-PDMAEMA₂₅₉ content (Fig. 4G). Most of the toluene was separated from the emulsion after 24 h of standing. The emulsion layer thickness was about 25% relative to the total liquid layer. Only a thin emulsion layer was obtained even when the volume ratio of toluene to water was decreased to 0.25, and a separated oil layer could still be observed (Fig. 4H). The required PS₃₀-b-PDMAEMA₂₅₉ content was 0.12 wt% for the complete emulsification of toluene oil by an equal volume of pH 5 water (Fig. 4F). The average size of emulsion droplets was almost 3 μm (Fig. 4K). Obviously, the emulsion performance of PS₃₂–(PDMAEMA₁₂₁)₂ miktoarm star copolymer was higher than that of its linear counterpart of PS₃₀-b-PDMAEMA₂₅₉. The differences of self assembly in solution of Y-shaped AB₂ miktoarm star copolymers from their linear counterparts have been explained by several research groups.^41,50,51 A Y-shaped AB₂ miktoarm star copolymer with two hydrophilic arms was more inclined to self-assemble into a spherical micelle with a more compact core than its linear counterpart. Due to crowding of polymer branches at the junction point, each copolymer chain was required to occupy a larger surface area at the core–corona interface to accommodate two hydrophilic arms in an equilibrium conformation. Therefore, the micelles formed by self assembly of Y-shaped AB₂ miktoarm star copolymers have a lower aggregation number and a smaller core size compared with their linear counterpart.⁴¹ With the present system, o/w emulsions stabilized by 0.06 wt% of PS₃₂–(PDMAEMA₁₂₁)₂ and 0.12 wt% of PS₃₀-b-PDMAEMA₂₅₉ possess almost the same average droplets size (Fig. 4B and F). Thus, we can propose a reasonable hypothesis that the total emulsion droplet surface areas of the two o/w emulsions should be considered the same. Due to the larger area occupied by each Y-shaped polymer chain at the oil–water interface, a lesser amount of PS₃₂–(PDMAEMA₁₂₁)₂ emulsifier was thus required than that of its linear counterpart.

Besides the o/w macroemulsion, PS–(PDMAEMA)₂ emulsifiers could be applied for formation of w/o macroemulsion at a wider range of pH values. Our experiments revealed the formation of w/o emulsion strongly depended on the relative volume fraction of the oil phase or the water phase. For PS₃₂–(PDMAEMA₁₂₁)₂, the formed macroemulsions were a w/o type once the volume fraction of toluene was more than 52% regardless of water pH and PS₃₂–(PDMAEMA₁₂₁)₂ content (S3, ESI†). Decreasing the PDMAEMA block length led to reduction of the required volume fraction of toluene for w/o emulsion formation. For example, w/o macroemulsion was formed when the relative volume fraction of toluene was more than 50% when PS₃₂–(PDMAEMA₈₀)₂ was used as a polymer emulsifier for macroemulsion (data not shown), and this might be ascribed to increased hydrophobicity of PS–(PDMAEMA)₂ accompanying the decreasing of PDMAEMA block length.

Interfacial tension measurements were performed to gain additional insight into the activity of Y-shaped PS₃₂–(PDMAEMA₁₂₁)₂ miktoarm star copolymer and linear PS₃₀-b-PDMAEMA₂₅₉ block copolymer at the oil–water interface. The toluene–water interfacial tension experiments were carried out by the Wilhelmy plate method. Interfacial tension experiments indicated that both kinds of emulsifiers demonstrated interfacial tension lowering behavior at the toluene–water interface. For example, the toluene–water (pH 5) interfacial tension was determined to be 28.1 mN m⁻¹, which was consistent with previously reported 27.8 mN m⁻¹ of toluene–water (pH 7).^52,53 Interfacial tension decreased remarkably from 28.1 mN m⁻¹ to 1.5 mN m⁻¹ in the presence of 0.1 wt% of Y-shaped PS₃₂–(PDMAEMA₁₂₁)₂ at pH 5, whereas the toluene–water (pH 5) interfacial tension was 3.7 mN m⁻¹ in the presence of 0.1 wt% of linear PS₃₀-b-PDMAEMA₂₅₉.

3.3 Stimulus-responses of o/w emulsion stabilized by Y-shaped PS₃₂–(PDMAEMA₁₂₁)₂ miktoarm star copolymer

It is well known that PDMAEMA exhibits both pH- and thermo-responsive properties.²³ PDMAEMA aqueous solution shows reversible hydrophilic/hydrophobic transition with pH variation.²³ The pK_a of weak polyelectrolytic PDMAEMA is approximately 7 at room temperature.⁵⁴ At lower pH, protonated PDMAEMA shows more hydrophilic characteristics, whereas deprotonation of tertiary amines gives the PDMAEMA arms more hydrophobic character and presents a higher affinity for oil when pH is above its pK_a. At the same time, PDMAEMA possesses lower critical solution temperature (LCST) ranging from 32 to 60 °C, depending on its molecular weight or average polymerization degree.⁵⁵ When the solution temperature is higher than its LCST, PDMAEMA polymer dissolved in neutral or weakly basic water becomes insoluble. These pH- and thermo-dependent hydrophilic/hydrophobic transition properties have conferred upon the PDMAEMA homopolymer and PDMAEMA-containing materials many important applications in wide fields, such as drug delivery and emulsions, etc.^{17,18,56–58} For example, Armes and his colleagues¹⁷ have reported a stimulus-responsive o/w Pickering emulsion stabilized by PS latexes. Due to the stimulus-response of poly-2-(dimethylamino)ethyl methacrylate-b-polymethyl methacrylate (PDMAEMA-b-PMMA) polymer chains absorbed on the surfaces of the polystyrene latexes, the generated emulsions showed temperature-dependent inversion and pH-induced demulsification.

In the present work, the stimulus-responses of o/w emulsion stabilized by PS₃₂–(PDMAEMA₁₂₁)₂ were investigated by altering pH or temperature. As expected, the o/w emulsion displayed rapid demulsification when the aqueous pH of the original o/w emulsion (Fig. 5A) was increased to about 9 by adding 1 mol L⁻¹ NaOH aqueous solution, as shown in Fig. 5B. The demulsification resulted from increased hydrophobicity of PDMAEMA with the increase of pH. Complete phase separation was achieved after 24 h of standing at room temperature (Fig. 5C). After readjusting pH of the water to below pK_a of PDMAEMA, such as 5, the separated oil could be completely re-emulsified by water (Fig. 5D). The average size of emulsion droplets of the regenerated emulsion was almost the same as that of the primary one (Fig. 5E and F). Upon heating the o/w emulsion (the volume ratio of toluene/water was 1) in a water bath at 55 °C, the o/w emulsion demonstrated two different phenomena. The primary emulsion shown in Fig. 6A was almost completely demulsified in a short time (∼5 min) if heating was applied alone or with gentle shaking by hand (Fig. 6B). At 55 °C, the PDMAEMA block becomes insoluble in pH 7 water and shows a higher affinity for oil. The stability of the o/w emulsion thus decreased, and coalesce of the original droplets resulted in the demulsification of the original emulsion. However, the original o/w emulsion inverted to an o/w/o multiple emulsion upon heating in the presence of moderate magnetic stirring (∼600 rpm) (Fig. 6C). An early report pointed out that the inversion of an emulsion was as the result of spontaneous curvature inversion of the emulsifier at an oil–water interface.²³ Clearly, the spontaneous curvature inversion of the PS₃₂–(PDMAEMA₁₂₁)₂ emulsifier absorbed at the oil–water interface could not be performed, and extra shearing action was necessarily required. Our control experiments showed that only a thinner layer of o/w emulsion was obtained by directly stirring an equal volume of pH 7 water and toluene containing PS₃₂–(PDMAEMA₁₂₁)₂ at 55 °C either at a rate of 1000 rpm for 10 min or at a rate of 12 000 rpm for 1 min, as displayed in Fig. 6G and H. This suggested that the pre-adsorption of PS₃₂–(PDMAEMA₁₂₁)₂ emulsifier at the oil–water interface was crucial to the thermo-induced inversion. Furthermore, we also noticed that this thermo-induced inversion could not be achieved for those emulsions prepared with acidic water (Fig. 6D–F); this might be a result of the difference in configuration of the PDMAEMA chain at the oil–water interface. The o/w/o multiple emulsion droplets showed a larger average size with a broader polydispersity (Fig. 6J) than those of the original o/w emulsion (Fig. 6I). In order to demonstrate the formed o/w/o multiple emulsion, CSLM was ulitized to image the emulsion droplets. For comparison, the droplets of the primary o/w emulsion were also imaged. The droplets of the primary o/w emulsion showed merged green and red channels (Fig. 6I, inset). The dispersed green droplets perfectly distinguished them from the red water bulk phase. The green and red color arisen from dissolved NBDAA in toluene and RhB dissolved in water, which were excited by a laser with wavelengths of 446 nm and 534 nm, respectively. After inversion, the red color droplets, which entrapped many small internal droplets with green color, were dispersed in the green bulk toluene, as shown in the inset of Fig. 6J, indicating the formation of o/w/o multiple macroemulsion.

Fig. 5 (A) Photograph of 0.12 wt% of PS₃₂–(PDMAEMA₁₂₁)₂ stabilized o/w macroemulsion with pH 5 water; (B) the macroemulsion shown in (A) was broken upon increasing pH to about 9; (C) the complete phase separation of the broken emulsion shown in (B) after 24 h of standing; (D) photograph of the regenerated o/w macroemulsion at pH 5; (E and F) the optical microscope pictures of emulsion droplets shown in (A and D). Scale bar: 6 μm.

Fig. 6 (A) Photograph of 0.12 wt% of PS₃₂–(PDMAEMA₁₂₁)₂ stabilized o/w macroemulsion with pH 7 water; (B) demulsification upon heating the macroemulsion shown in (A) alone; (C) photograph of the formed o/w/o multiple macroemulsion by heating the o/w macroemulsion in a 55 °C water bath with accompanying moderate stirring; photographs of the broken macroemulsions upon heating 0.12 wt% of PS₃₂–(PDMAEMA₁₂₁)₂ content stabilized o/w macroemulsions with water having different pH values ((D) 6.5; (E) 6; (F) 5) and accompanying moderate stirring at 55 °C; directly stirring an equal volume of neutral water (pH 7) and toluene containing PS₃₂–(PDMAEMA₁₂₁)₂ at a rate of 1000 rpm for 10 min (G) or 12 000 rpm for 1 min (H) in a water bath at 55 °C; (I and J) optical microscope pictures of emulsion droplets shown in (A) and (C); insets of (I and J): CLSM images of emulsion droplets shown in (A) and (C). Scale bar: 6 μm for (I) and 100 μm for (J).

4. Conclusions

Y-shaped amphiphilic PS–(PDMAEMA)₂ miktoarm star copolymer was successfully synthesized by combining RAFT polymerization with ATRP. When applied as a polymeric surfactant for w/o and o/w macroemulsion formation, the synthesized PS–(PDMAEMA)₂ miktoarm star copolymers showed a higher emulsifying performance than that of their linear counterpart. The macroemulsion type was strongly dependent on the relative volume fraction of oil or water. Moreover, the required relative volume fraction of oil for w/o macroemulsion decreased with decreasing of the PDMAEMA block length. PS–(PDMAEMA)₂ stabilized o/w macroemulsion showed pH-reduced demulsification and thermo-induced phase inversion. However, the curvature inversion of the PS–(PDMAEMA)₂ emulsifier at the oil–water interface could not be spontaneously accomplished, and moderate stirring was necessary required. Furthermore, a thermo-induced phase inversion could only be realized for emulsions with pH 7 water, and a more acidic water was not favorable. This type of manipulated emulsion is expected to provide useful guidance in the fields of oil recovery, catalysis, and perhaps other applications.

Acknowledgements

This work was financially supported by the National Natural Scientific Foundation of China (21174118, 21404036) and Open Project of Hunan Provincial University Innovation Platform (15K123).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra16399d

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