RSC Advances PAPER 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 emulsiers without a co-stabilizer requirement. emulsiers. 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 This journal is © The Royal Society of Chemistry 2015 RSC Adv.,2015,5, 96377–96386 | 96377 RSC Advances Paper 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 puried 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. Aer 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-dened AB2 miktoarm star polymer composed of poly(N,N- magnesium sulfate. The crude product was puried 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 emulsier and the stimuli-responses of the polymerization formed macroemulsion were investigated in detail. PS–Br2 macroinitiator for ATRP was obtained by RAFT poly- merization 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 purication. 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. Aer multiple basic