Dual Responsive Macroemulsion Stabilized by Y-Shaped Amphiphilic

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Dual responsive macroemulsion stabilized by Y- shaped amphiphilic AB2 miktoarm star copolymers† Cite this: RSC Adv.,2015,5, 96377 Heng Li,a Duanguang Yang,a Yong Gao,*ab Huaming Liab and Jianxiong Xu*ac

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 Received 14th August 2015 macroemulsion showed pH-induced demulsiﬁcation and temperature-induced phase inversion. Accepted 30th October 2015 However, the inversion of the PS–(PDMAEMA)2 emulsiﬁer at the oil–water interface could not be DOI: 10.1039/c5ra16399d spontaneously accomplished. Furthermore, successful phase inversion was only smoothly realized for www.rsc.org/advances those emulsions with pH 7 water in the presence of a modulate stirring.

1. Introduction microemulsion structure in a particular system containing oil, water, and surfactant.10 From this point of view, preparation of Emulsions are a class of disperse systems consisting of two kinetically stable macroemulsions is relatively easy because they immiscible liquids,1 where disperse phase droplets are can be obtained by simply dispersing oil and water phases in dispersed in a continuous phase medium in the presence of the presence of emulsiers without a co-stabilizer requirement. emulsiers. Emulsions, not including Pickering emulsion,2 can Furthermore, a large number of surfactants with a lower be divided into three types: a kinetically stable macroemulsion, concentration and a wider range of volume ratio of the two 11,12 thermodynamically stable microemulsion, and an “approach- phases are suitable for macroemulsion formation. ing thermodynamically stable” nanoemulsion3 according Both low molecular weight surfactants and polymeric 13 to the droplet size; the corresponding droplet size ranges are surfactants can be utilized to stabilize macroemulsions. 0.1–5 mm, 5–50 nm and 20–100 nm, respectively.4 Because of Additionally, inorganic solid particles or polymer so particles their high stability, microemulsions and nanoemulsions have with suitable wettability can also be used as macroemulsion huge amounts of applications in food technology, personal care stabilizers, and these particle-stabilized emulsions are specially 2,14–18 and cosmetics, and drug delivery as well as materials termed as “Pickering emulsions”. Compared to traditional synthesis.5–9 However, preparation of a microemulsion is not low molecular weight surfactants, amphiphilic block copoly- easy due to a lack in general theory for predicting the mers 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, aCollege of Chemistry, Xiangtan University, Xiangtan, Hunan Province, 411105, China. many amphiphilic block polymers with different constitutes E-mail: [email protected] have been applied for macroemulsions in past decades.20–24 bKey Laboratory of Polymeric Materials & Application Technology of Hunan Province, However, to the best of our knowledge, polymeric surfactants Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan for macroemulsions were mainly concentrated on linear Province, Key Lab of Environment Friendly Chemistry and Application in Ministry of Education, Xiangtan, Hunan Province, 411105, China amphiphilic block copolymers, and less attention was paid to 25 cHunan Key Laboratory of Green Packaging & Application of Biological amphiphilic miktoarm star polymers. Nanotechnology, Hunan University of Technology, Zhuzhou, Hunan Province, Miktoarm star polymers, also called asymmetric star polymers 412007, China or hetero-arm star polymers, refer to polymers that contain † Electronic supplementary information (ESI) available. See DOI: a central core connected by a number of various types of polymer 10.1039/c5ra16399d

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26,27 arms. During the past decades, a wealth of miktoarm star DMAP(0.49g,4.0mmol)and40mLoffreshCH2Cl2 were charged  polymers with varied arm constitutions have been synthesized by into a dried round bottomed ask, and then 20 mL of CH2Cl2 virtue of the great progress in polymer synthesis methodology.28–34 containing EMP (4.48 g, 20 mmol) was added dropwise with stir-  Y-shaped AB2 miktoarm polymers, as the simplest miktoarm star ring. A er 48 h of stirring at room temperature, the mixture was polymers, have received considerable attention within the past ltered to remove the dicyclohexylurea product. The solvent was years. Synthesis strategies for Y-shaped AB2 miktoarm polymers removed by evaporation, and the remaining product was puried include atom transfer radical polymerization (ATRP),35,36 opening by column chromatography (silica gel) using petroleum ether/ethyl polymerization (ROP),37–39 and “click” reaction40 as well as acetate (6 : 1) as an eluent, affording a pale yellow oil. Yield: 5.34 g 1 versatile combinations of different polymerization methods, (78.5%). H NMR (400 MHz, CDCl3, d, ppm): 4.14 (s, 2H, OCH2), 41,42 – – including ATRP/ROP, reversible addition fragmentation 3.59 (s, 4H, CH2OH), 3.33 3.27 (q, 2H, SCH2), 1.71 (s, 6H, CCH3), 43 44 – – – chain transfer (RAFT) polymerization/ROP, ROP/click, ATRP/ 1.36 1.30 (q, 2H, CCH2), 1.32 1.25 (t, 3H, SCH2CH3), 0.88 0.84 (t, 45 ROP/click, living cationic polymerization/ROP, and anionic 3H, CH2CH3). polymerization/ROP combination.46,47 Meanwhile, self-assembly 2-Ethyl-2-((2-(ethylthiocarbonothioylthio)-2-methylpropanoy- of Y-shaped AB2 miktoarm polymers in solution also has been loxy)methyl)propane-1,3-diyl bis(2-bromo-2-methylpropanoate) investigated in detail. Experimental results showed that self- (EPBP). In a typical reaction, BEMP (5.1 g, 15 mmol), triethyl- assembly in solution of Y-shaped AB2 miktoarm polymers is amine (8.8 mL, 60 mmol) and 25 mL fresh CH2Cl2 were charged different from that of their linear counterparts.37–39,41 Distinctions into a dried round bottomed ask. The ask was immersed in in self-assembly behaviors suggested that Y-shaped AB2 mik- an ice bath, and 10 mL of dried CH2Cl2 containing 2-bromoi- toarm polymers should have rather different interfacial proper- sobutyryl bromide (6.84 g, 30 mmol) was added dropwise to the ties from their linear counterparts. However, as far as we are ask under stirring. The mixture was stirred at 0 C for 2 h and concerned, there have been limited reports on a macroemulsion then at room temperature overnight. Aer reaction, the mixture 25 stabilized by Y-shaped AB2 miktoarm polymers to date. was extracted three times with a saturated aqueous solution of Herein, we presented a dual responsive macroemulsion using sodium bicarbonate. The collected organic phase was dried over a well-dened AB2 miktoarm star polymer composed of poly(N,N- magnesium sulfate. The crude product was puried by column dimethylaminoethyl methacrylate) (PDMAEMA) and polystyrene chromatography (silica gel) using petroleum ether/ethyl acetate  – ff (PS) arms as polymeric emulsi ers in this study. PS (PDMAEMA)2 (3 : 1) as an eluent, a ording a pale yellow oil. Yield: 7.16 g – 1 d was synthesized by sequential reversible addition fragmentation (74.8%). H NMR (400 MHz, CDCl3, , ppm), 4.10 (s, 4H, OCH2), – chain transfer (RAFT) polymerization of styrene monomer and 4.04 (s, 2H, OCH2), 3.30 3.25 (q, 2H, SCH2), 1.94 (s, 12H, C(Br) atom transfer radical polymerization (ATRP) of DMAEMA CH3), 1.70 (s, 6H, CCH3), 1.60–1.54 (t, 2H, CCH2), 1.34–1.30 (q, monomer. The structure and the molecular weight as well as the 3H, SCH2CH3), 0.94–0.91 (t, 3H, CCH2CH3). molecular weight distribution of PS–(PDMAEMA)2 were carefully 1 characterized by GPC and H NMR, respectively. The emulsifying 2.3 Synthesis of PS–Br2 ATRP macroinitiator by RAFT performance of emulsier and the stimuli-responses of the polymerization formed macroemulsion were investigated in detail. PS–Br2 macroinitiator for ATRP was obtained by RAFT polymerization of St employing EPBP as a RAFT agent. In a typical 2. Experimental reaction, St (1.248 g, 12 mmol), EPBP (0.128 g, 0.2 mmol) and 2.1 Materials AIBN (10.93 mg, 0.067 mmol) were charged into a round bottomed ask. The ask was degassed by three freeze–pump– Trimethylolpropane was obtained from Aldrich and used thaw cycles and then immersed in oil bath at 75 C for poly- without further purication. N,N-Dimethylaminoethyl methac-  merization under N2 atmosphere. A er 5 h of polymerization,  rylate (DMAEMA) was puri ed by passing it through a dried the reaction was quenched by diluting with THF. Aer multiple basic alumina column and distilling over CaH2 under reduced operations involving precipitation in methanol, ltration, and  pressure prior to polymerization. Styrene was puri ed by washing, the product was dried under vacuum at room reduced pressure distillation. 2-(Ethylthiocarbonothioylthio)-2- temperature. methylpropanoate (EMP) was synthesized according to proce- 48 dures described in the literature. 2-Bromoisobutyryl bromide 2.4 Synthesis of Y-shaped PS–(PDMAEMA) miktoarm star 0 00 00 2 (BrIB), N,N,N ,N ,N -pentamethyldiethylenetriamine (PMDETA), copolymer via ATRP 1,3-dicyclohexylcarbodiimide (DCC), and 4-(dimethylamino) – pyridine (DMAP) were purchased from Aldrich and used as PS (PDMAEMA)2 miktoarm star copolymer was synthesized by – received. Azobisisobutyronitrile (AIBN) was recrystallized twice ATRP of DMAEMA in THF at 60 C using PS Br2 and CuBr/ from ethanol prior to use. 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 2.2 Synthesis of trifunctional core mmol) and 3 mL of fresh THF were charged into a round Synthesis of 2,2-bis(hydroxymethyl)butyl 2-(ethylthiocar- bottomed ask. The ask was degassed by three freeze–pump– bonothioylthio)-2-methylpropanoate (BEMP). In a typical reac- thaw cycles and then immersed in an oil bath at 60 C for tion, trimethylolpropane (3.35 g,25mmol),DCC(4.12g,20mmol), polymerization under N2.Aer 4 h of polymerization, the

96378 | RSC Adv.,2015,5, 96377–96386 This journal is © The Royal Society of Chemistry 2015 Paper RSC Advances reaction was stopped by diluting with THF and then passed emulsion and the formed w/o emulsion by a thermo-induced through a short basic alumina column to remove the metal salt. inversion process. The o/w emulsion employed for a CLSM – The solution was concentrated by evaporation, and the image was stabilized by 0.12 wt% of PS32 (PDMAEMA121)2 concentrated mixture was dropped into a large amount of miktoarm star copolymers. Prior to homogenization, a hydro- hexane, producing a white precipitation. The precipitate was philic uorescence probe of Rhodamine B (RhB) and a hydro- ltered, and washed with hexane. These precipitation, ltration phobic uorescence probe of allyl-(7-nitro-benzo[1,2,5]- and washing operations were repeated for three times, and the oxadiazol-4-yl)-amine (NBDAA) were dissolved in water and puried product was dried under vacuum at room temperature. toluene, respectively. The o/w emulsion was heated in a water bath at 55 C for 5 min with moderate stirring. 2.5 Generation of toluene/water (o/w) and water/toluene (w/o) emulsions stabilized by PS–(PDMAEMA) miktoarm star 2 3. Results and discussion copolymers 3.1 Synthesis of Y-shaped PS–(PDMAEMA)2 miktoarm star Typical preparation procedures of o/w and w/o emulsions were copolymer – as follows: an equal volume of PS (PDMAEMA)2 miktoarm star – copolymer toluene solution (5 mg mL 1) and acidic water (pH 5) In this study, Y-shaped PS (PDMAEMA)2 miktoarm star copol- “  ” were charged in a 10 mL of glass vial at room temperature. ymer was synthesized using core rst strategy. The overall Emulsication was performed using a XHF-D high speed synthesis route was shown in Scheme 1. As illustrated in disperser (Ningbo Scientz Biotechnology Co., Ltd, China) at Scheme 1, a trifunctional core of EPBP containing two ATRP  a stirring rate of 12 000 rpm for 1 min. During homogenization, initiator groups and RAFT group was rst synthesized by two the glass vial was immersed in an ice bath to remove heat successive chemical processes involving the synthesis of BEMP  formed by this process. The emulsion droplets were observed by an esteri cation reaction of trimethylolpropane with EMP, using an optical microscope. A few drops of the diluted emul- and the subsequent acylation reaction between BEMP and 2- sions were placed on a glass slide and viewed. The type of bromoisobutyryl bromide. Successful synthesis of the titled  1 1 emulsion was determined by conductivity using a Jenway 4510 EPBP was veri ed by H NMR studies. Fig. 1 shows the H NMR conductivity meter. spectra of EMP, BEMP, and EPBP. As displayed in Fig. 1A, the characteristic signals at d ¼ 3.31, 1.74, and 1.33 ppm were – – – 2.6 pH- and thermo-response of o/w emulsion assigned to the protons of SCH2, CCH3 and SCH2CH3 in EMP, which agrees with earlier reports.48 The 1H NMR spectrum The pH-response of an o/w emulsion was performed by adding of BEMP is displayed in Fig. 1B. Compared with the 1H NMR 1 a known volume of 1 mol L NaOH aqueous solution into an spectrum of EMP, four groups of new resonances at d ¼ 4.14, emulsion to adjust the water phase pH to about 9.0. For the 3.59, 1.31, and 0.86 ppm appeared in addition to the charac- 1 broken emulsion, a known volume of 1 mol L HCl aqueous teristic signals of EMP. These newly formed signals were solution was added into the water to adjust the water phase pH assigned to the protons of –CH2 next to an ester group, –CH2OH, to 5.0. Re-emulsication was performed by homogenization at 1 and –CH2 as well as –CH3, respectively. Based on H NMR a stirring rate of 12 000 rpm for 1 min. The thermo-response of analysis, all peak integrals were consistent with the target an o/w emulsion was performed by heating the formed o/w compound. The appearance of these new resonance peaks and emulsion in a water bath at 55 C alone or with a gentle the consistence of peak integrals with the target compound shaking by hand or with moderate magnetic stirring. All pH were an indication of the successful esterication reaction values of the water phase were measured by pH meter. between EMP and trimethylolpropane. The trifunctional core of EPBP was formed by further reaction of the produced BEMP 2.7 Characterization tests with 2-bromoisobutyryl bromide. A new resonance peak at d ¼ 1H NMR spectra were recorded with a Bruker AV-400 NMR 4.10 ppm was observed in the 1H NMR spectrum of the resultant spectrometer. Molecular weights and polydispersity indexes EPBP, as shown in Fig. 1C. This new resonance peak was

(PDI) of polymers were determined by GPC measurements, assigned to the protons of –CH2 next to –COOC(CH3)2Br. At the which were performed on a Waters 1515 GPC equipped with same time, another resonance peak at d ¼ 3.59 ppm in Fig. 1B, ﬀ – a Waters 2414 di erential refractive index detector in DMF at 80 corresponding to the protons of CH2OH, completely C with a row rate of 1.0 mL min 1. Interfacial tension disappeared aer the acylation reaction. This suggested that the measurements were measured by the Wilhelmy plate method, –CH2OH groups in BEMP had been converted to 1 and the experiments were performed with a Kruss tensiometer –COOC(CH3)2Br. H NMR analysis also conrmed that peak K20 equipped with a Wilhelmy slide. Optical micrographs (OM) integrals were consistent with the target EPBP. were collected with an optical microscope (Leica, DM 4500P). A The puried trifunctional core of EPBP was then applied for drop of the diluted emulsion was placed on a microscope slide the bulk RAFT polymerization of St, and the polymerization was and pictures were taken randomly from diﬀerent spots of the carried out at 75 C using AIBN as an initiator. Aer 5 h of same sample. The average size of droplets was analyzed by polymerization, the reaction was stopped. The puried polymer a MALVERN Zetasizer Nano ZS instrument. Confocal scanning was analyzed by GPC and 1H NMR. The GPC trace of PS polymer laser microscopy (CSLM, Olympus FV1000 laser confocal (denoted as PS–Br2) was shown in Fig. 2A. A symmetrical microscope, Japan) was used to observe the primary o/w monomodal peak was an indication of a controlled RAFT

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

1 Fig. 1 H NMR spectra of EMP (A), BEMP (B), and EPBP (C). Fig. 2 GPC traces of PS–Br2 (A), PS32–(PDMAEMA80)2 (B), and PS32– (PDMAEMA121)2 (C). polymerization of St mediated by EPBP using the described – 1 reaction conditions. GPC analysis indicated the Mn of PS Br2 from H NMR analysis is approximately consistent with that was 4100 g mol 1 with PDI of 1.13. from GPC result. 1 –  – Fig. 3A shows the H NMR spectrum of PS Br2, in which the Puri ed PS Br2 was then used as macroinitiator to initiate respective resonances, including the bromine end groups, were the ATRP of DMAEMA monomer to synthesize Y-shaped AB2 1 ﬀ clearly assigned. Mn of PS–Br2 was calculated to be 3900 g mol miktoarm star copolymers with di erent PDMAEMA block according to the 1H NMR integral area ratio of a peak at d ¼ lengths. ATRP of DMAEMA was carried out in THF at 60 C.   7.23–6.50 ppm to that at d ¼ 3.28 ppm. Evidently, Mn calculated A er polymerization, the polymer was puri ed and then

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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 emulsiers, 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 emulsied by pH 5 water with a xed water/toluene volume ratio of 1 and a xed

1 PS32–(PDMAEMA121)2 content, whereas three layers, including Fig. 3 H NMR spectra of PS–Br2 (A), PS32–(PDMAEMA80)2 (B), and PS32–(PDMAEMA121)2 (C). 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. Aer 24 h of standing at room temperature, the emulsion thickness was 55%, 30%, and 20% analyzed by GPC. By comparison with PS–Br2, GPC traces of relative to the total liquid layer thickness, respectively (Fig. 4B, C PS–(PDMAEMA)2 presented a clear shi to a higher molecular weight region, as shown in Fig. 2B and C. The resulting and D). Emulsion could not be formed when more basic symmetrical GPC monomodal peak in GPC curves revealed water, such as pH 9 water, was employed (Fig. 4E). Optical ﬀ – – microscope photos of emulsions stabilized by di erent PS32 a controlled ATRP process of DMAEMA initiated by PS Br2. GPC (PDMAEMA121)2 contents were also displayed in Fig. 4. The analysis indicated that the values of the Mn of the m PS–(PDMAEMA) product were 29 000 and 42 000 g mol 1, average size of emulsion droplets was about 3.6 m with 0.233 2 m respectively, corresponding to 1.28 and 1.25 of PDI. According of PDI (Fig. 4I), and 3.4 m with 0.7 of PDI (Fig. 4J), corre- – to GPC analysis, the number average polymerization degree of sponding to 0.12 wt% and 0.06 wt% of PS32 (PDMAEMA121)2 – a PS block was 32, and a PDMAEMA block was 80 and 121, content, respectively. Increasing PS32 (PDMAEMA121)2 content respectively. As a result, the as-synthesized two Y-shaped was somewhat favorable to the reduction in the polydispersity PS–(PDMAEMA) miktoarm star copolymers were thus deno- instead of the average size of emulsion droplets. Meanwhile, 2 – – – 1 PS (PDMAEMA)2 showed a decreased emulsifying performance ted as PS32 (PDMAEMA80)2 and PS32 (PDMAEMA121)2. The H NMR spectra of PS–(PDMAEMA) with diﬀerent PDMAEMA when the PDMAEMA block length was reduced. For example, 2 – block lengths were displayed in Fig. 3B and C. Compared with the least required PS32 (PDMAEMA80)2 content was 0.10 wt%  Fig. 3A, several groups of new resonances at d ¼ 4.06, 2.57, 2.29, when toluene oil was completely emulsi ed by an equal volume 1.84, and 0.91 ppm appeared. These resonances should be of acidic water (pH 5), whereas the average size and the polydispersity of emulsion droplets were approximately the same as assigned to the protons of –OCH2, –OCH2CH3, –N(CH3)2, –CCH2 49 1 those of the emulsion droplets stabilized by 0.06 wt% of PS32– and –CCH3 in the PDMAEMA arms, respectively. H NMR (PDMAEMA121)2 (S1, ESI†). analysis indicated the values of Mn of the synthesized two – – 1 The emulsifying performance of Y-shaped PS (PDMAEMA)2 PS (PDMAEMA)2 samples are 21 000 and 28 000 g mol , respectively, which are smaller than those obtained from GPC miktoarm star copolymer was also compared with that of its determination. linear counterpart. For this purpose, a linear di-block copolymer of PS30-b-PDMAEMA259, possessing a Mn approximately the same as that of PS –(PDMAEMA ) , was synthesized by two 3.2 PS–(PDMAEMA) miktoarm star copolymer stabilized o/w 32 121 2 2 successive ATRP processes using ethyl-2-bromoisobutanoate and w/o macroemulsions and CuBr/PMDETA as initiator and catalyst, respectively. The – Y-shaped PS (PDMAEMA)2 miktoarm star copolymer has detailed synthesis and GPC characterizations are displayed in a hydrophobic PS arm and two hydrophilic PDMAEMA arms. ESI (S2, ESI†). The number average polymerization degrees of PS This unique chain architecture might allow this unsymmetrical and PDMAEMA block were 30 and 259, respectively, which was

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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 mm. 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.

determined by GPC analysis. The obtained PS30-b-PDMAEMA259 junction point, each copolymer chain was required to occupy was also used as an emulsier for macroemulsion formation a larger surface area at the core–corona interface to accommo- under the same conditions. Experimental observation revealed date two hydrophilic arms in an equilibrium conformation. that toluene oil could not be emulsied completely by an equal Therefore, the micelles formed by self assembly of Y-shaped AB2 volume of acidic water (pH 5) at 0.06 wt% of PS30-b-PDMAEMA259 miktoarm star copolymers have a lower aggregation number and content (Fig. 4G). Most of the toluene was separated from the a smaller core size compared with their linear counterpart.41 emulsion aer 24 h of standing. The emulsion layer thickness With the present system, o/w emulsions stabilized by 0.06 wt% was about 25% relative to the total liquid layer. Only a thin of PS32–(PDMAEMA121)2 and 0.12 wt% of PS30-b-PDMAEMA259 emulsion layer was obtained even when the volume ratio of possess almost the same average droplets size (Fig. 4B and F). toluene to water was decreased to 0.25, and a separated oil layer Thus, we can propose a reasonable hypothesis that the total could still be observed (Fig. 4H). The required PS30-b- emulsion droplet surface areas of the two o/w emulsions should

PDMAEMA259 content was 0.12 wt% for the complete emulsi- be considered the same. Due to the larger area occupied by each cation of toluene oil by an equal volume of pH 5 water (Fig. 4F). Y-shaped polymer chain at the oil–water interface, a lesser m –  The average size of emulsion droplets was almost 3 m (Fig. 4K). amount of PS32 (PDMAEMA121)2 emulsi er was thus required – Obviously, the emulsion performance of PS32 (PDMAEMA121)2 than that of its linear counterpart. miktoarm star copolymer was higher than that of its linear Besides the o/w macroemulsion, PS–(PDMAEMA)2 emulsi- counterpart of PS30-b-PDMAEMA259. The diﬀerences of self ers could be applied for formation of w/o macroemulsion at assembly in solution of Y-shaped AB2 miktoarm star copolymers a wider range of pH values. Our experiments revealed the from their linear counterparts have been explained by several formation of w/o emulsion strongly depended on the relative 41,50,51 research groups. A Y-shaped AB2 miktoarm star copolymer volume fraction of the oil phase or the water phase. For PS32– with two hydrophilic arms was more inclined to self-assemble (PDMAEMA121)2, the formed macroemulsions were a w/o type into a spherical micelle with a more compact core than its once the volume fraction of toluene was more than 52% – linear counterpart. Due to crowding of polymer branches at the regardless of water pH and PS32 (PDMAEMA121)2 content (S3,

ESI†). Decreasing the PDMAEMA block length led to reduction toluene–water interfacial tension experiments were carried out of the required volume fraction of toluene for w/o emulsion by the Wilhelmy plate method. Interfacial tension experiments formation. For example, w/o macroemulsion was formed indicated that both kinds of emulsiers demonstrated interfacial whentherelativevolumefractionoftoluenewasmorethan tension lowering behavior at the toluene–water interface. For

50% when PS32–(PDMAEMA80)2 was used as a polymer emul- example, the toluene–water (pH 5) interfacial tension was sier for macroemulsion (data not shown), and this might be determined to be 28.1 mN m 1, which was consistent with 1 52,53 ascribed to increased hydrophobicity of PS–(PDMAEMA)2 previously reported 27.8 mN m of toluene–water (pH 7). accompanying the decreasing of PDMAEMA block length. Interfacial tension decreased remarkably from 28.1 mN m 1 to 1 Interfacial tension measurements were performed to gain 1.5 mN m in the presence of 0.1 wt% of Y-shaped PS32– – – additional insight into the activity of Y-shaped PS32 (PDMAEMA121)2 at pH 5, whereas the toluene water (pH 5) 1 (PDMAEMA121)2 miktoarm star copolymer and linear PS30-b- interfacial tension was 3.7 mN m in the presence of 0.1 wt% of – PDMAEMA259 block copolymer at the oil water interface. The linear PS30-b-PDMAEMA259.

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 mm.

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 mm for (I) and 100 mm for (J).

3.3 Stimulus-responses of o/w emulsion stabilized by Y- stimulus-responsive o/w Pickering emulsion stabilized by PS shaped PS32–(PDMAEMA121)2 miktoarm star copolymer latexes. Due to the stimulus-response of poly-2-(dimethylamino) ethyl methacrylate-b-polymethyl methacrylate (PDMAEMA-b- It is well known that PDMAEMA exhibits both pH- and thermo- 23 PMMA) polymer chains absorbed on the surfaces of the poly- responsive properties. PDMAEMA aqueous solution shows styrene latexes, the generated emulsions showed temperature- reversible hydrophilic/hydrophobic transition with pH varia- 23 dependent inversion and pH-induced demulsication. tion. The pKa of weak polyelectrolytic PDMAEMA is approxi- 54 In the present work, the stimulus-responses of o/w emulsion mately 7 at room temperature. At lower pH, protonated – PDMAEMA shows more hydrophilic characteristics, whereas stabilized by PS32 (PDMAEMA121)2 were investigated by altering deprotonation of tertiary amines gives the PDMAEMA arms pH or temperature. As expected, the o/w emulsion displayed  ﬃ rapid demulsi cation when the aqueous pH of the original o/w more hydrophobic character and presents a higher a nity for emulsion (Fig. 5A) was increased to about 9 by adding 1 mol L 1 oil when pH is above its pKa. At the same time, PDMAEMA NaOH aqueous solution, as shown in Fig. 5B. The demulsi- possesses lower critical solution temperature (LCST) ranging cation resulted from increased hydrophobicity of PDMAEMA from 32 to 60 C, depending on its molecular weight or average 55 with the increase of pH. Complete phase separation was ach- polymerization degree. When the solution temperature is ieved aer 24 h of standing at room temperature (Fig. 5C). Aer higher than its LCST, PDMAEMA polymer dissolved in neutral or weakly basic water becomes insoluble. These pH- and readjusting pH of the water to below pKa of PDMAEMA, such as  thermo-dependent hydrophilic/hydrophobic transition proper- 5, the separated oil could be completely re-emulsi ed by water (Fig. 5D). The average size of emulsion droplets of the regen- ties have conferred upon the PDMAEMA homopolymer and erated emulsion was almost the same as that of the primary one PDMAEMA-containing materials many important applications  17,18,56–58 (Fig. 5E and F). Upon heating the o/w emulsion (the volume in wide elds, such as drug delivery and emulsions, etc. 17 ratio of toluene/water was 1) in a water bath at 55 C, the o/w For example, Armes and his colleagues have reported a

96384 | RSC Adv.,2015,5, 96377–96386 This journal is © The Royal Society of Chemistry 2015 Paper RSC Advances emulsion demonstrated two different phenomena. The primary demulsication and thermo-induced phase inversion. However,  –  emulsion shown in Fig. 6A was almost completely demulsi ed the curvature inversion of the PS (PDMAEMA)2 emulsi er at the in a short time (5 min) if heating was applied alone or with oil–water interface could not be spontaneously accomplished, gentle shaking by hand (Fig. 6B). At 55 C, the PDMAEMA block and moderate stirring was necessary required. Furthermore, becomes insoluble in pH 7 water and shows a higher affinity for a thermo-induced phase inversion could only be realized for oil. The stability of the o/w emulsion thus decreased, and coa- emulsions with pH 7 water, and a more acidic water was not lesce of the original droplets resulted in the demulsication of favorable. This type of manipulated emulsion is expected to the original emulsion. However, the original o/w emulsion provide useful guidance in the elds of oil recovery, catalysis, inverted to an o/w/o multiple emulsion upon heating in the and perhaps other applications. presence of moderate magnetic stirring (600 rpm) (Fig. 6C). An early report pointed out that the inversion of an emulsion Acknowledgements was as the result of spontaneous curvature inversion of the emulsier at an oil–water interface.23 Clearly, the spontaneous This work was nancially supported by the National Natural curvature inversion of the PS32–(PDMAEMA121)2 emulsier Scientic Foundation of China (21174118, 21404036) and Open absorbed at the oil–water interface could not be performed, and Project of Hunan Provincial University Innovation Platform extra shearing action was necessarily required. 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