Article pubs.acs.org/Macromolecules
Heterografted Molecular Brushes as Stabilizers for Water-in-Oil Emulsions Guojun Xie,† Pawel Krys,† Robert D. Tilton,‡,§ and Krzysztof Matyjaszewski*,† †
Department of Chemistry, ‡Department of Biomedical Engineering, and §Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States S Supporting Information *
ABSTRACT: A series of well-defined heterografted molecular brushes with poly(ethylene oxide) (PEO) and poly(n-butyl acrylate) (PBA) side chains were synthesized via atom transfer radical polymerization (ATRP). Structural parameters including graft length, graft ratio, and backbone length were systematically tuned by using a synthetic strategy combining “grafting through” and “grafting from” methods. Coexistence of hydrophilic and hydrophobic grafts allowed these molecular brushes to stabilize water-in-oil emulsions at extremely low surfactant concentration (0.005 wt %). The effects of graft length and composition were demonstrated by tunable performance/behavior in the generation of emulsions. A comparison between molecular brushes of different conformation and their diblock analogues indicated that combining multiple hydrophilic and hydrophobic grafts in one molecule could enhance their adsorption at the interface but did not necessarily favor the formation of smaller droplets. The lowering of interfacial tension in pendant drop experiments provided evidence that the emulsifying properties of heterografted molecular brushes could be partially attributed to a decrease in the energy cost of emulsion formation.
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drimers,22−24 nano-objects from polymerization-induced selfassembly (PISA),25−33 cross-linked micelles,34−39 star polymers,40−43 and heterografted copolymers41,44−46 have also been studied as polymeric surfactants. Molecular brushes are polymers containing linear backbones with densely grafted polymeric side chains.47 RDRP techniques allow for versatile control over composition, molecular weight, and dispersity of either, or both, backbones and side chains.44,48−59 Amphiphilicity can be introduced into this type of material by having both hydrophilic and hydrophobic grafts distributed along the backbone in a blockwise (brush block copolymers)60−64 or in a statistically random manner (brush statistical copolymers).65−69 Similar to amphiphilic block copolymers that have a clear spatial segregation between hydrophilic and hydrophobic segments, brush block copolymers can afford intermolecular self-assemblies of tunable size and morphology in solution.19,20,70,71 In contrast, brush statistical copolymers were reported to be less prone to intermolecular self-assembly into micelles.72 Consequently, brush statistical copolymers can stabilize the interface by adapting Janus conformations, where the two types of side chains dissolve in different solvents. Because of the molecular structure resembling a “fusion” of diblock copolymer
INTRODUCTION Amphiphilic polymers can serve as polymeric surfactants due to the distinctly different solubility characteristics of hydrophilic and hydrophobic segments coexisting in one molecule. They find utility as materials for the preparation of many disperse and self-assembled systems in the field of (mini)emulsion polymerization,1 coatings,2,3 delivery systems,4,5 cosmetics,6 water purification,7 electronics,8 and enhanced oil recovery.9 Compared to small molecule surfactants, amphiphilic polymers allow for more diverse and flexible adjustment of properties by changing the chemical structure of monomers as well as macromolecular parameters including molecular weights, compositions, and architectures. This type of material engineering has been propelled by the development of reversible-deactivation radical polymerization (RDRP) methods, such as atom transfer radical polymerization (ATRP), due to high functional group tolerance and robust control over polymerization.10,11 Additionally, RDRP methods enable the fabrication of smart materials by incorporating functional monomers, which provide responsiveness toward external stimuli including irradiation and changes of pH, temperature, or electrolyte concentration.12−20 Because of their simple topology, block copolymers are the most frequently studied polymeric surfactants.21 However, more complex polymeric structures afford a wider range of length scales and levels of interactions than those offered by linear block copolymers. Therefore, amphiphilic den© XXXX American Chemical Society
Received: January 2, 2017 Revised: March 13, 2017
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DOI: 10.1021/acs.macromol.7b00006 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Synthesis of PEO/PBA Heterografted Copolymers
1 H NMR spectroscopy using a Bruker Advance 300 MHz NMR spectrometer with CDCl3 as a solvent. Droplets in diluted emulsions were imaged with a digital microscope camera (Bausch & Lomb Stereo Zoom 7). Size distributions of dispersed droplets were generated by image analysis using an in-house developed program and used to estimate the interfacial coverage of amphiphilic copolymers. The average hydrodynamic diameter of micelles suspected to form upon transfer of one of the polymers into water was measured by dynamic light scattering (DLS) using a high performance zeta-sizer from Malvern Instruments, Ltd. Interfacial tensions of interfaces between polymer solution and water were determined by pendant drop experiments using a Theta optical tensiometer (Biolin Scientific). Synthesis of Poly(2-bromoisobutyryloxyethyl methacrylate)-stat-poly(ethylene glycol) methyl ether methacrylate)) (P(BiBEM-stat-PEOMA)). A 25 mL Schlenk flask was charged with EBiB (4.5 μL, 31 μmol), HEMA-TMS (7.3 g, 7.7 mmol), PEOMA (1.6 g, 7.7 mmol), anisole (2.2 mL), CuBr2 (1.4 mg, 6.2 μmol), and dNbpy (27.7 mg, 67.7 μmol). The solution was degassed by three freeze−pump−thaw cycles. During the final cycle, the flask was filled with nitrogen, and CuBr (3.5 mg, 25 μmol) was quickly added to the frozen reaction mixture. The flask was sealed, evacuated, and backfilled with nitrogen five times and then immersed in an oil bath at 60 °C. Polymerization was stopped when conversion reached the targeted value as determined by 1H NMR spectroscopy. The degree of polymerization (DP) of the backbone was calculated from the molar ratio of the reacted monomer to initiator. The reaction was stopped by exposing the solution to air, diluting with methylene chloride, and filtering through an activated (neutral) alumina column to remove the copper catalyst. Unreacted monomer was removed by dialysis in MeOH/THF (50/50, vol %) solution using a dialysis membrane (Biotech, Regenerated Cellulose, MWCO 10 000). The graft copolymer was dried under vacuum to a constant mass. A 100 mL round-bottom flask was charged with the product of the last step (amount of HEMA-TMS in the last step was taken as 1.0 equiv), KF (1.2 equiv), and DTBP (0.1 equiv), and then dry THF (30 mL) was added under nitrogen. The reaction mixture was cooled down in an ice bath, followed by the injection of tetrabutylammonium fluoride (0.01 equiv) and subsequent dropwise addition of αbromoisobutyryl bromide (1.2 equiv) over the course of 20 min. The reaction mixture was then allowed to reach room temperature and was stirred for another 24 h. Afterward, the solids were filtered off, and the mixture was passed through a column filled with basic alumina. The product was reprecipitated three times in hexane and dried overnight under vacuum. The structure of the polymer was determined from the ratio of selected polymer signals: P(BiBEM) (m, broad, OCO−CH2−, 4.29−4.50 ppm) to P(PEOMA) (m, broad, O−CH2−CH2−, 3.59−3.71 ppm). All P(BiBEM-stat-PEOMA) macroinitiators were prepared according to the above procedure using the stoichiometric ratios shown in Table S1. Synthesis of Poly((2-bromoisobutyryloxyethyl methacrylate-graf t-poly(n-butyl acrylate))-stat-poly(ethylene glycol) methyl ether methacrylate) (P[(BiBEM-g-PBA)-stat-PEOMA]). A 25 mL Schlenk flask was charged with P(BiBEM198-stat-PEOMA167) (100.0 mg, containing 92.5 μmol of initiating sites), BA (6.6 mL, 46.2 mmol), anisole (0.74 mL), CuBr2 (0.5 mg, 0.002 mmol), and dNbpy (42.3 mg, 0.103 mmol). The solution was degassed by three freeze−
amphiphiles, brush statistical copolymers could have increased interaction with the interfaces. This could potentially result in higher emulsifying efficiency to generate disperse systems similar to the case of Pickering emulsions.73 Enhanced stability of miniemulsions has been reported when an amphiphilic brush copolymer synthesized via ring-opening metathesis polymerization (ROMP) was used as surfactant in place of a diblock macromonomer.72 Herein, we report the synthesis and evaluation of a series of PEO/PBA heterografted molecular brushes as emulsifiers for generation of water-in-oil emulsions. By systematically varying the molecular parameters, we explored the effects of oil phase type, graft ratio (m/p, Scheme 1), graft length, and backbone length (compactness) of the heterografted brush copolymers on emulsification behavior. Optimization of the molecular parameters allows for preparation of stable emulsions with a concentration of emulsifier as low as 0.005 wt %.
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EXPERIMENTAL SECTION
Materials. (2-Trimetylsiloxy)ethyl methacrylate (HEMA-TMS, Scientific Polymer Products), n-butyl acrylate (BA, ≥99%, Aldrich), and poly(ethylene glycol) methyl ether methacrylate with Mn = 300 or 500 (PEO0.3KMA and PEO0.5KMA, Aldrich) were purified by passing the monomer through a column filled with basic alumina to remove inhibitors. PEO2KMA (Mn = 2000, 50 wt % in water, Aldrich) was extracted with methylene chloride. The organic phase was dried with Na2SO4 and passed through a basic alumina column. PEO1KMA (Mn = 950, Aldrich) was dissolved in THF and passed through a basic alumina column. Both samples of PEOMA (Mn = 950 or 2000) were precipitated against cold hexanes or ethyl ether and dried in vacuo. Ethyl α-bromoisobutyrate (EBiB, 98%, Acros), copper(I) bromide (CuBr, 99.999%, Aldrich), copper(II) bromide (CuBr2, 98%, Acros), potassium fluoride (KF, 99%, Aldrich), tetrabutylammonium fluoride (TBAF, 1.0 M in THF, Aldrich), α-bromoisobutyryl bromide (98%, Aldrich), 2,5-di-tert-butylphenol (DTBP, 99%), triethylamine (TEA, ≥99%, Aldrich), 4,4′-dinonyl-2,2′-bipyridyne (dNbpy, 97%, Aldrich), and solvents were used as received without further purification. PEObased monofunctional macroinitiator PEO1KBiB was prepared following procedure described in the literature.74 Generation of Emulsions. The water/oil ratio was set to 1:1 by weight, and emulsions were generated using a vortex mixer (Fisher Scientific Analogue Vortex Mixer, 3000 rpm) for 30 s. Concentrations of surfactants are reported based on the mass of molecular brushes per total mass of oil and water. All copolymers were dispersed in the oil phase prior to homogenization. A drop test was conducted to confirm that the dispersed phase is water in each emulsion by placing one drop of the emulsion phase into neat water or into neat oil. All emulsion droplets readily dispersed in oil but not in water. Characterization. Apparent number-average molecular weights (M n ) and dispersity (Đ) were measured by size exclusion chromatography (SEC). The SEC was conducted with a Waters 515 pump and a Waters 2414 differential refractometer using PSS columns (SDV 105, 103, and 500 Å) with THF as eluent at 35 °C and at a flow rate of 1 mL min−1. Linear PMMA standards were used for calibration. The conversion and composition of the polymer were measured via B
DOI: 10.1021/acs.macromol.7b00006 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. Sample Information of Macromolecular Surfactants molecular parametersc sample ID
a
L1K-H16 B0.3K-H7 B0.5K-H10 S1K-H13 B1K-H6 B1K-H17 B2K-H21
composition EO1KBA5K EO0.3K*99BA2K*171 EO0.5K*111BA2K*209 EO1K*21BA4K*32 EO1K*73BA4K*242 EO1K*167BA4K*198 EO2K*175BA7K*199
mol
%PEOb 50 37 35 39 23 46 47
DPPPEOMA
DPPBiBEM
DPPBA
Mn,theoc (×10−6)
Mn,SECd (×10−6)
Đd
99 111 21 73 167 175
171 209 32 242 198 199
41 15 17 31 32 29 49
0.00638 0.407 0.566 0.152 1.13 0.960 1.65
0.00933 0.207 0.156 0.049 0.232 0.138 0.253
1.22 1.30 1.14 1.07 1.13 1.22 1.25
a XNK-HY, X = L, S, or B; (1) L represents linear copolymer, S represents star-like copolymer, and B represents brush-like copolymer. (2) NK is the molecular weight of hydrophilic (PEO-based) macromonomer used to synthesize the copolymer. (3) Y is weight percentage of hydrophilic (PEO) moieties. bNumber fraction of hydrophilic (PEO) grafts. cCalculated based on the 1H NMR spectra. dMeasured by THF SEC calibrated using PMMA standards.
pump−thaw cycles. During the final cycle, the flask was filled with nitrogen, and CuBr (7.9 mg, 0.047 mmol) was quickly added to the frozen reaction mixture. The flask was sealed, evacuated, and backfilled with nitrogen five times and then immersed in an oil bath at 70 °C. Polymerization was stopped when conversion reached the targeted value as determined by 1H NMR spectroscopy. The reaction mixture was diluted with methylene chloride and passed through a neutral alumina column to remove the catalyst. Solvent and the remaining monomer were removed by blowing air and under high vacuum. All P[(BiBEM-g-PBA)-stat-PEOMA] heterografted molecular brushes and a diblock copolymer were prepared according to the above procedure using the stoichiometric ratios shown in Table S2.
hydrophobicity.75 Similarly, the emulsifying properties of heterografted molecular brushes are expected to be largely influenced by the graft ratio, determined by the composition of the backbone. Two brush-like copolymers (B1K-H6 and B1KH17, Table 1) were used as surfactants to stabilize an emulsion of xylene and water to provide an understanding of the effect of the graft ratio. As their sample ID indicated, B1K-H6 is a brushlike copolymer with 6% of its molar mass contributed by the hydrophilic (PEO-1K) grafts. In contrast, although having similar hydrophilic (PEO-1K, DPPEO ∼ 19) and hydrophobic (DPPBA ∼ 30) grafts as B1K-H6, B1K-H17 is a brush-like copolymer with a much higher weight fraction of hydrophilic moieties (17%). As shown in Figure 1b, B1K-H17 which had 23 mol % hydrophilic grafts allowed the stabilization of water-in-xylene
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RESULTS AND DISCUSSION The combination of “grafting through” and “grafting from” was employed as the synthetic strategy for preparation of PEO/PBA heterografted molecular brushes with multiple tunable structural parameters (Scheme 1). Loosely grafted P[(PEOMA)-stat-(BiBEM)] were initially prepared via the copolymerization of PEOMA and HEMA-TMS (“grafting through”), followed by functionalization with ATRP initiators. The copolymer composition and the length of polymeric backbone were controlled by tuning the comonomer feed ratio, conversion, and the ratio of monomers to initiator. The length of hydrophilic grafts was changed by using PEOMA macromonomers with different molar mass.65 Hydrophobic PBA grafts were then grown from (“grafting from”) the PEO grafted backbone to generate heterografted molecular brushes. The compositions of the heterografted copolymers, with systematically varied molecular parameters, are summarized in Table 1. The nomenclature is explained as following for XNK-HY: (1) X = L represents a linear copolymer, X = S represents a star-like copolymer, and X = B represents a brush-like copolymer. (2) NK is the molecular weight of hydrophilic (PEO-based) macromonomer used to synthesize the copolymer. (3) Y is the weight percentage of hydrophilic (PEO) moieties. These heterografted copolymers were then evaluated as emulsifiers by generation of emulsions with a weight ratio of oil and water set at 1/1. After homogenization, all generated emulsions were determined to be water-in-oil emulsions. For all stable emulsions, no visible appearance of water layer could be observed within 1 week after the emulsions were generated. Effect of the Graft Ratio. The properties of block copolymer surfactants are governed by the ratio between hydrophilic and hydrophobic repeat units in each segment of the copolymer. For example, hydrophilic−lipophilic balance (HLB) is calculated based on the weight fraction of the hydrophilic portion of a molecule and used to the describe
Figure 1. Images of water-in-xylene emulsions stabilized by (a) B1KH6 and (b) B1K-H17. The percentage in green label is the surfactant concentration (wt %) in each emulsion.
emulsion with concentrations as low as 0.005 wt %. In contrast, an emulsion layer could hardly be found in the emulsion stabilized with 0.005 wt % B1K-H6 (Figure 1a). Herein, a higher concentration (0.02 wt %) of surfactant was needed for the formation of a stable emulsion without the presence of a neat water layer at the bottom. An optical microscope was utilized to image the droplets in the emulsion after dilution (Figure 2). The average size and size dispersity of droplets increased as the concentration of surfactant decreased. When the amount of surfactant was 0.02 wt % or higher, the droplets stabilized by B1K-H17 had narrowly dispersed size with a mean diameter about 30 μm. In C
DOI: 10.1021/acs.macromol.7b00006 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. Images of diluted water-in-xylene emulsions stabilized by heterografted copolymers B1K-H6 (a−d) and B1K-H17 (e−h) captured by a microscope camera. Percentages in green label are the surfactant concentrations (weight fraction). Scale bar: 200 μm.
Figure 3. Images of water-in-xylene emulsions stabilized by (a) B0.3KH7 and (b) B0.5K-H10. The percentage in green label is the surfactant concentration (wt %) in each emulsion.
contrast, the diameter of droplets stabilized with B1K-H6 ranged from 20 to 900 μm. (In Figures 2c and 2d, the apparent “encapsulation” of small droplets by larger ones resulted from the detection of smaller droplets underneath the larger ones.) Such difference could result from the differences in adsorption affinity of each brush macromolecule. In order to provide steric stabilization for emulsion,76 the amphiphilic polymers need to adsorb onto the newly formed oil−water interface during agitation. Since both polymeric surfactants were initially dispersed in the oil phase, the higher fraction of hydrophilic grafts in B1K-H17 could favor such interfacial adsorption, allowing stabilization with lower concentration of surfactants. Effect of the PEO Graft Length. As an example of “like dissolves like”, the solubility of PEO-based oligomer/polymer in the aqueous phase increases with the length of polymer chain due to the lower contribution of hydrophobicity from the chain end.77 Also, the gain of enthalpy upon hydration of the PEO chains contributes to the driving force for the adsorption of the amphiphilic copolymers.78 Therefore, since the length of PEO grafts could affect the affinity toward aqueous phase, we explored the effect of PEO length by comparing the emulsions stabilized by B0.3K-H7, B0.5K-H10, and B1K-H6, all containing similar weight fraction of hydrophilic moieties (Table 1). Although B0.3K-H7 and B0.5K-H10 had a higher number fraction of hydrophilic (PEO) grafts, which should favor the stabilization of emulsion by facilitating the heterografted polymer to adsorb onto the interface, the minimum surfactant concentration required to stabilize the emulsion was still much higher than that of B1K-H6 (Figure 1a). The emulsion with B0.5K-H10 was not stable, and a neat layer of water was observed until the content of surfactant increased to 0.1% (Figure 3b). When a surfactant with even shorter PEO grafts (B0.3K-H7) was used, loadings even as high as 0.1% were insufficient to stabilize the emulsion (Figure 3a). Such a trend could be explained by the increased hydrophilicity of longer PEO grafts or the increased freedom of the longer PEO chains to adsorb in an entropically suitable conformation at the oil/water interface, both of which facilitated the adsorption of heterografted copolymer onto the interface, enabling stabilization of the emulsion at lower surfactant concentration. Another grafted polymer, B2K-H21, containing longer hydrophilic (PEO-2K) grafts but similar weight fraction of hydrophilic moieties as B1K-H17, was used as the surfactant to stabilize the emulsion of xylene and water. However, further increase of the PEO graft length did not additionally improve the emulsifying efficiency. After the surfactant loading was
decreased from 0.0025% to 0.001%, only a very thin emulsion layer could be identified in the mixture stabilized by B2K-H21 (Figure 4b), in contrast to the emulsion stabilized by B1K-H17
Figure 4. Images of water-in-xylene emulsions stabilized by (a) B1KH17 and (b) B2K-H21. The percentage in green label is the surfactant concentration (wt %) in each emulsion.
(Figure 4a). Both the hydrophilic and hydrophobic grafts of B2K-H21 molecules are almost twice as long as those of B1KH17. Therefore, at a given surfactant mass concentration, the number concentration of B2K-H21 molecules is only half of that of B1K-H17, which could result in less efficient emulsion stabilization. More subtle effects may be present as well, including the possibility that these changes in molecular architecture may correlate with differences in interfacial mechanical properties, such as dilatational elasticity, or in the thin film disjoining pressure. In order to effectively adsorb onto the interfaces, the PEO moieties present in the emulsifiers need to have preferential affinity toward the aqueous phase.79 In addition to increasing the hydrophilicity by using longer PEO grafts, decreasing their solubility in the oil phase could also facilitate heterografted copolymer to adsorb to the interfaces. Cyclohexane, which is a poor solvent for PEO, was used as the oil phase in place of xylene to prepare emulsions in a manner similar to that described above. A significant increase of emulsifying efficiency was observed for all surfactants with different PEO lengths. The emulsion with 0.005 wt % B1K-H6 became stable when cyclohexane was used as the oil phase instead of xylene (Figures 1a and 5a). Additionally, the minimum required surfactant loading of B0.5K-H10 decreased from 0.1 to 0.02 wt % (Figures 3b and 5b). In contrast to the water-in-xylene emulsion with 0.1 wt % of B0.3K-H7 surfactant (Figure 3a), water-in-cyclohexane emulsion stabilized by the same amount became stable without an observable neat layer of water at the bottom (Figure 5c). D
DOI: 10.1021/acs.macromol.7b00006 Macromolecules XXXX, XXX, XXX−XXX
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6a) still had a water layer at the bottom of the vial. The higher emulsifying efficiency of B1K-H17 and S1K-H13 could result from a cooperative effect of the grafts that favored the adsorption onto interfaces. Desorption of one heterografted copolymer molecule requires simultaneous removal of many adsorbed grafts, which increases the energy barrier for desorption, similar to the case of Pickering emulsions with grafted polymers.81 A dissolution of a dry sample of L1K-H16 and S1K-H13 in water was attempted in order to analyze the solutions with dynamic light scattering. However, only in the sample prepared from L1K-H16 (0.01 wt % aqueous solution), nano-objects were detected, which had a Z-average diameter equal to 54 nm. This could indicate formation of micelles by the self-assembly of amphiphilic block copolymers. In contrast, S1K-H13 did not dissolve in water, and no nano-objects were detected. Therefore, the lower probability of emulsifier leakage into the aqueous phase might be an additional reason why heterografted copolymers allow for successful utilization of a lower surfactant loading for formation of stable emulsions. Images of droplets were also captured from emulsions with 0.01 wt % surfactants diluted by pure oil phase (Figure 7). In contrast to the emulsion formed with B1K-H17, where droplets larger than 100 μm could be easily identified, the diameters of the droplets in the emulsion stabilized with S1K-H13 were mostly smaller than 50 μm, which was similar to the emulsion prepared with L1K-H16 (Figure 7). Furthermore, image analysis was used to estimate average interfacial area stabilized by surfactant molecule (Asurf) (see Supporting Information). Asurf increased from 153 to 198 nm2/molecule when the molar mass of the heterografted polymeric surfactant was increased from 152 (S1K-H13) to 960 kg/mol (B1K-H17). Macromolecular surfactants undergo slower diffusion, as compared to small molecule surfactants.82 As a consequence, the different droplet sizes observed for the three analyzed surfactants could be a result of different diffusion rates. At a given mass concentration, B1K-H17, which diffuses more slowly due to its higher molecular weight compared to L1K-H16 and S1K-H13, could have a smaller portion of molecules diffuse onto interfaces and adsorb to stabilize interfaces during mixing procedure of a certain amount of time. Therefore, with B1KH17, a smaller area of newly formed interfaces was stabilized, which resulted in a larger size of droplets, as observed above. Also, the conformational difference between B1K-H17 and S1K-H13 could contribute to the difference in droplet sizes. As a result of steric interaction among the densely grafted side chains, the conformation of molecular brushes can be regarded as a cylinder sandwiched by two hemistars (Figure 8).80,83 Because of an environment of higher congestion, each graft of brush-like B1K-H17 within the cylindrical region allows occupation of less interfacial area compared to that of grafts at the end of backbone. Therefore, star-like surfactants can stabilize a larger interfacial area due to having fewer or no grafts in the cylindrical region. Lowering of the Interfacial Tension. An important role of the surfactant in emulsion formation is to lower the energy cost of forming new interfaces by decreasing the interfacial tension between two immiscible liquids. The lowering of xylene/water interfacial tension caused by the addition of heterografted molecular brushes was confirmed by pendant drop experiments (Figure 9). After 30 s of pendant drop formation, the interfacial tension decreased to a value substantially lower than the value for xylene/water interfaces (37.2 mN/m), although some discrepancy could be found
Figure 5. Images of water-in-cyclohexane emulsions stabilized by (a) B1K-H6, (b) B0.5K-H10, and (c) B0.3K-H7. The percentage in green label is the surfactant concentration (wt %) in each emulsion.
Effect of the Backbone Length. Heterografted copolymers have a molecular structure that resembles a side-by-side aggregation of amphiphilic block copolymers linked together. Therefore, a block copolymer L1K-H16 (Table 1) was used to demonstrate the different properties between a diblock copolymer and a heterografted copolymer (B1K-H17). Additionally, since the crowding of grafts within the same molecular brush is lower in the middle of the backbone than at its end, the conformation of grafts is not the same along the backbone. In addition, the conformation of molecular brushes could transform from star-like to brush-like as the length of the backbone increases.80 This could potentially affect the emulsifying properties of the synthesized heterografted copolymers. Therefore, a star-like heterografted copolymer with shorter (compared to B1K-H17) backbone S1K-H13 was also synthesized and tested as an emulsifier. In contrast to water-in-xylene emulsions stabilized by only 0.005 wt % B1K-H17 (Figure 1b) or S1K-H13 (Figure 6b), emulsion stabilized by the same amount of L1K-H16 (Figure
Figure 6. Images of water-in-xylene emulsions stabilized by (a) L1KH16 and (b) S1K-H13. The percentage in green label is the surfactant concentration (wt %) in each emulsion. E
DOI: 10.1021/acs.macromol.7b00006 Macromolecules XXXX, XXX, XXX−XXX
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Figure 7. Images of diluted water-in-xylene emulsions stabilized by 0.01 wt % of brush-like B1K-H17 (a), star-like S1K-H13 (b), and linear L1K-H16 (c) surfactant captured by a microscope camera. Scale bar: 200 μm.
Table 2. Xylene/Water Interfacial Tension (mN/m) for Polymer Solutions at 21 °C after 30 sa
0.01 wt % 0.1 wt % a
B0.3H7
B1KH6
B1KH17
B2KH21
L1KH16
S1KH13
27.6 21.7
28.2 26.9
29.0 23.6
27.5 22.3
28.9 15.9
25.8 22.7
The pure xylene/water interfacial tension is 37.2 mN/m.
Figure 8. Conformation of heterografted molecular brushes adsorbed on oil−water interfaces.
of heterografted molecular brush copolymers and diblock analogues indicated that combining multiple hydrophilic and hydrophobic grafts in one molecule can enhance adsorption at the interface and promote emulsifying efficiency. Star-like surfactants stabilized smaller droplets than brush-like analogues at the same surfactant mass concentration possibly due to the difference of molecular conformation or diffusivity. Pendant drop measurements indicated the formation of emulsion was facilitated by the lowering of interfacial tension, but the extent of interfacial tension reduction did not precisely correlate with emulsifying efficiency of the different polymers.
among the initial interfacial tensions (t = 0 s). This could be assigned to the uncertainty introduced during the generation of the pendant drop. The interfacial tensions for a variety of surfactants are summarized in Table 2. However, these values alone cannot predict the emulsifying efficiency since they only relate to the energy barrier for creating new interfaces.
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CONCLUSIONS PEO/PBA heterografted copolymers with multiple tunable molecular parameters were synthesized via the combination of “grafting through” and “grafting from” methods. The formed brush copolymers were used as emulsifiers to stabilize water-inoil emulsions at polymer concentrations lower than 0.01 wt %. The effects of different molecular parameters including graft length, composition, and backbone length were studied. A sufficiently high number fraction of hydrophilic grafts and long hydrophilic grafts were found to favor the emulsifying efficiency, probably due to the higher affinity toward the aqueous phase. This was supported by the improved emulsifying efficiency when cyclohexane (a poor solvent for PEO) was used instead of xylene (a good solvent for PEO) as the oil phase. A comparison between the emulsifying behaviors
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00006. Calculation of average interfacial area stabilized by surfactant, Tables S1 and S2 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (K.M.).
Figure 9. Evolution of xylene/water interfacial tension for (a) 0.01 wt % and (b) 0.1 wt % polymer solutions at 21 °C. F
DOI: 10.1021/acs.macromol.7b00006 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules ORCID
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Krzysztof Matyjaszewski: 0000-0003-1960-3402 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
We are very grateful to Dr. Jacob Mohin for the setup of microscope imaging system and to Dr. Melissa Lamson for the discussion of experimental setup. Support from the National Science Foundation via Grants DMR 1501324, DMR 1436219, and CBET-1332836 and from BSF (2012074) is acknowledged.
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DOI: 10.1021/acs.macromol.7b00006 Macromolecules XXXX, XXX, XXX−XXX
