Article pubs.acs.org/Langmuir
Location and Influence of Added Block Copolymers on the Droplet Size in Oil-in-Oil Emulsions Itaru Asano,†,§ Soonyong So,‡ and Timothy P. Lodge*,†,‡ †
Department of Chemistry and ‡Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States § Chemicals Research Laboratories, Toray Industries, Inc., 9-1, Oe-cho, Minato-ku, Nagoya 455-8502, Japan S Supporting Information *
ABSTRACT: We have investigated the effect of added polystyrene-b-poly(ethylene oxide) (SO) copolymer on the stability of oil-in-oil (O/O) emulsions containing polystyrene (PS) and poly(ethylene glycol) (PEG) in chloroform (CHCl3) and directly visualized the location of SO in the emulsions by using dye-labeled SO (SO*) with confocal laser scanning microscopy (CLSM). The emulsion formed by PS/PEG/ CHCl3 = 14/6/80 (wt %) consisted of a droplet phase of PS in CHCl3 and a continuous phase containing PEG in CHCl3. SO*s with various molecular weights (Mn,SO) and volume fractions of the PS block in SO ( f PS) were prepared via living anionic polymerization and subsequent end-esterification. The effect of SO on the droplet size in the emulsions was investigated as a function of both Mn,SO and f PS. Increasing Mn,SO and decreasing f PS were effective at reducing the droplet size down to less than 1 μm, which is 100 times smaller than in the absence of SO. The location of SO*s in the O/O emulsions was further investigated by CLSM. We found that the location of SO*s changed from the droplet interior to the liquid−liquid interface and then to the continuous phase with decreasing f PS. We discuss the possible mechanism in terms of the relation of SO* location to the droplet size.
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system established by Scott.9 Interaction parameters among the three components, two immiscible polymers and solvent, as well as polymer molecular weight play key roles in the onset of phase separation.10 One of the characteristics of these emulsions is low interfacial tension (∼0.03 mN/m)11−16 relative to O/W emulsions, generally ∼30−50 mN/m. Solvent−solvent interaction between the phases is negligible due to both phases containing the same solvent. Thus, the increasing interfacial tension is attributable to the repulsion between the immiscible polymers. It has been reported that the interfacial tension is proportional to the immiscible polymer concentration.7,11−13 This appealing class of emulsions has been applied to biopolymer partitioning, food colloids, membranes, and film manufacturing.17−22 Recently, W/W emulsion polymerizations and microcapsule fabrication have received increased attention as applications with high potential.23,24 Stable W/W and O/O emulsions have great potential to expand their applications, analogous to O/W emulsions. There are two reported methodologies to prepare stable emulsions. One is the well-established use of submicrometer-sized particles,25−27 and the other is using block copolymers as stabilizers.28−30 The appropriate particles stick to the interface
INTRODUCTION Oil-in-water (O/W) emulsions are well known as solutions consisting of two immiscible liquids, i.e., an organic solvent and water. Emulsions are thermodynamically unstable systems and generally macroscopically phase separate in the absence of agitation. Therefore, stabilizing emulsions is important for enabling applications. It is well known that chemicals having hydrophilic and hydrophobic parts, such as surfactants and amphiphilic block copolymers, are able to localize at the liquid−liquid interface between oil and water phases and lead to thermodynamically stable emulsions. In addition, over the past few decades, designed amphiphilic block copolymers can play an important role in making functionalized emulsions, such as inverted water-in-oil (W/O) emulsions,1 multiple-component emulsions such as water-in-oil-in-water (W/O/W) emulsions,2,3 and microemulsions.4,5 These oil- and waterbased emulsions have been used in broad application fields, such as cosmetics, foods, pharmaceutics, coatings, and emulsion polymerization.6 There is an alternative class of emulsion formed by a homogeneous solvent with two immiscible polymers under certain conditions, typically above a certain total polymer concentration.7,8 This type of emulsion is called a water-inwater (W/W) or oil-in-oil (O/O) emulsion.7,8 Phase separation of a homogeneous solvent is promoted by unfavorable net interactions between immiscible polymers and can be explained by the extended Flory−Huggins (FH) theory for a ternary © XXXX American Chemical Society
Received: May 18, 2015 Revised: June 19, 2015
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DOI: 10.1021/acs.langmuir.5b01830 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. Synthesis route to SO* (2). SO block copolymers (1) were prepared by sequential living anionic polymerization, and subsequently the ωhydroxyl group of SO was esterified with rhodamine B acid chloride to yield SO* (2).
Table 1. Properties of Polystyrene-b-poly(ethylene oxide)-rhodamine B (SO*) polymer
Mn,PS (kg/mol)a
Mn,PEO (kg/mol)b
Mn,SO* (kg/mol)c
Đd
f PSe
RhoBCl residual (wt %)f
SO*(9-13) SO*(36-9) SO*(36-20) SO*(36-48) SO*(95-109)
9 36 36 36 95
13 9 20 48 109
22 45 56 84 204
1.02 1.01 1.14 1.05 1.04
0.45 0.82 0.68 0.47 0.52
