Influence of Photo-Cross-Linking on Emulsifying Performance of the

May 20, 2014 - of 0% and 95% in response to pH change were comparatively confirmed by a combination of electrophoresis, ... so-called “Ramsden” or...
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Influence of Photo-Cross-Linking on Emulsifying Performance of the Self-Assemblies of Poly(7-(4-vinylbenzyloxyl)-4-methylcoumarin-coacrylic acid) Chenglin Yi,† Jianhua Sun,† Donghua Zhao,† Qiong Hu,† Xiaoya Liu,*,† and Ming Jiang‡ †

Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, P. R. China ‡ Key Laboratory for Molecular Engineering of Polymers, Ministry of Education, Department of Macromolecular Science, Fudan University, Shanghai 200433, P. R. China S Supporting Information *

ABSTRACT: Polymeric micelles could be used as model polymeric particulate emulsifiers to elucidate the correlation between the micellar structure and their emulsifying performance. Photo-cross-linkable and pH-responsive micelles were prepared with amphiphilic random copolymers, poly(7-(4-vinylbenzyloxyl)-4-methylcoumarin-co-acrylic acid) (PVMAA), via the self-assembly in selective-solvent DMF/H2O and then used as polymeric particulate emulsifiers to stabilize toluene-in-water emulsions. Primary micelles, based on PVMAA with 12 mol % of hydrophobic composition, were chosen as model to investigate the influence of photo-cross-linking on the emulsifying performance. The larger shrinkage degree by photo-cross-linking (SDC) the micelles have, the lower emulsifying efficiency the micelles exhibit. Furthermore, the structural transitions of micelles with SDC of 0% and 95% in response to pH change were comparatively confirmed by a combination of electrophoresis, dynamic light scattering (DLS), and transmission electron microscopy (TEM). The micelles of various states, manipulated by photo-crosslinking and pH changes, were used as emulsifiers to stabilize toluene-in-water or styrene-in-water emulsions. For the un-crosslinked micelles, polymer chains gradually protrude from micelles with pH increasing, which benefits the increase in the emulsifying efficiency of micelles. However, as pH elevated over 8, the stability of emulsions significantly decreases due to the disintegration of micelles. On the contrary, micelles with SDC of 95% keep their structural integrity and become more rigid as pH increase, leading to lower emulsifying efficiency of micelles and worse stability of the emulsions. This paper provides a new insight into the principles governing the extremely high emulsifying efficiency of polymeric particulate emulsifiers and pHdependent or pH-responsive properties of the formed emulsions.

1. INTRODUCTION

exhibit better tailorability and controllability on emulsifying performance than inorganic particulate emulsifiers. Examples include silica nanoparticles grafted with surface-active or stimuli-sensitive polymeric brushes,10−12 latexes steric-stabilized by polymers,13−15 lightly cross-linked polymeric microgels,16−18 and polymeric micelles,19−21 etc. Those polymeric particulate emulsifiers are distinguished by their high emulsifying efficiency

The colloidal particles used to prepare and stabilize emulsions, so-called “Ramsden” or “Pickering” emulsions,1,2 are particulate emulsifiers that adsorb at oil/water interface. Recently, considerable efforts have been devoted to particulate emulsifiers due to their excellent stability at oil/water interface over conventional surfactants3,4 as well as their resurgent applications in pharmaceutical uses,5 personal care products, food products,6 catalysts,7 oil industry, and fabrication of new materials.8,9 Polymeric particulate emulsifiers, capable of being designed at molecular level and prepared by diverse methods, © 2014 American Chemical Society

Received: March 21, 2014 Revised: May 7, 2014 Published: May 20, 2014 6669

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Figure 1. Schematic illustration of (a) photo-cross-linking reaction among PVMAA and (b) the structures of the PVMAA micelles and those photocross-linked.

polypeptide segments that protrude into the oil phase.28,30 To date, only few reports involve the role of the polymer chains. Tilton et al. used silica nanoparticles grafted interfaceactive poly(styrenesulfonate) brushes as particulate emulsifiers and contributed their extremely emulsifying efficiency to the penetration of the polymer chains to the liquid interface.10 Armes et al. also tried to illustrate the role played by steric stabilizer poly[2-(dimethylamino)ethyl methacrylate] in the emulsifying performance of poly(2-vinylpyridine) latexes.14 However, the exact manner in which polymer chains or segments control the interfacial performance of those polymeric particulate emulsifiers is not yet fully understood. Polymeric micelles, self-assembled from amphiphilic copolymers,31−33 display better architectural tailorability at the molecular level than the particles prepared by other methods. The desirable structures of polymeric micelles could cover the range from unimolecular micelle34 to rigid colloidal particles,35 even anisotropic Janus micelles.36−38 Hence, the self-assembled micelles appear to be good model particulate emulsifiers to elucidate the relationship between the structure and emulsifying performance of most polymeric particulate emulsifiers. In this paper, to explore the influence of the polymer chains or segments on the interfacial performance, polymeric micelles self-assembled from photo-cross-linkable amphiphilic random copolymers (RCPs), poly(7-(4-vinylbenzyloxyl)-4-methylcoumarin-co-acrylic acid) (PVMAA), as shown in Figure 1, are designed and prepared as model polymeric particulate emulsifiers for the following reasons. (i) The structure of

and/or stimuli-responsiveness, which are different from conventional, rigid particulate emulsifiers.22 The underlying mechanism of the polymeric particulate emulsifiers is not yet well established. PNIPAm microgels have garnered intense attention after the first use as pH- and temperature-sensitive emulsifiers by Ngai et al. in 200516 and have been further explored the correlation between their structural changes and their interfacial behaviors.23−25 It is well documented that the submicrometer-sized PNIPAm microgels are soft, porous, and significantly deform at the oil/water interface.25−27 Destribats et al. correlated the microgel deformability with emulsion stability and demonstrated that the softer the microgels were, the more stable the emulsions were, while the softness was inversely proportional to the cross-linking degree.25 Ngai and Richtering cooperated on examining the dynamic adsorption behaviors of PNIPAM microgels at the oil−water interface, which further confirmed the dominative role of the deformability of the microgels in their interfacial behaviors.24 The deformation was attributed to the conformational change of the polymer chains during the migration of the submicrometer-sized microgels from aqueous solution into the interface, which was rationally neglected due to the size effect. However, the role of polymer chains or segments is unneglectable to understand the underlying principles governing the emulsifying performance of proteins,28 other types of polymeric particulate emulsifiers,10 and complex mixed emulsifiers.29 For instance, the well-known interfacial unfolding behavior of proteins is exposing hydrophobic 6670

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t at 320 nm, respectively).41 This method is strictly concentrationdependent. In this paper, the shrinkage of micelles in diameter, as a subsequence of photo-cross-linking,41 was used to assess the degree of photo-cross-linking, in terms of the shrinkage degree by cross-linking (SDC). SDC was calculated by

these polymeric micelles could be simply controlled by the amphiphilicity of copolymers. Self-assembly of similar amphiphilic RCPs into spherical micelles has been well investigated.32,39 Structurally, these spherical micelles have more hydrophobic segments collapsed inner micelles and more hydrophilic segments dangling at the periphery. Here, both hydrophobic or hydrophilic segments are actually amphiphilic due to the random sequence of the polymeric chains. This structural feature endows polymeric micelles with both high emulsifying efficiency and interfacial stability.20 Meanwhile, RCPs are cheaper and more easily to be synthesized than block copolymers (BCPs). Hence RCPs micelles are more promising for industrialization. (ii) Capability of photo-cross-linking between hydrophobic coumarin moieties40−42 confers the polymeric micelles tunability over both the deformability of micelles and the restriction of the dangling polymer segments. To emphasize the role of dangling polymer segments, most soft micelles with size less than 100 nm, analogue of nanogels, will be chose as model particulate emulsifiers in this work. Crosslinking by UV irradiation in micelles could avoid the disturbance of the increasing inhomogeneity caused by the added chemical cross-linking agents. 43 Meanwhile, the absorption of UV makes these micelles being potentially applied in sunburn prevention lotions. (iii) The carboxyl groups along the main chain offer the micelles pH sensitivity. Control over pH facilitates us to investigate the swelling of the dangling polymer chains at the periphery of the micelles as well as the deformation of the micelles. In other words, the swelling behavior of the PVMAA micelles could be manipulated by the dosage of UV irradiation and pH variation. The tailored micelles with desirable structures are used as particulate emulsifiers to stabilize oil-in-water emulsions. The emulsifying characteristic would help on elucidating the underlying principle of the emulsification performance of the polymeric particulate emulsifiers.

SDC (%) =

Dt − D0 × 100% D∞ − D0

(1)

Here, D0 and Dt are the initial hydrodynamic diameter and the diameter at irradiation time t, respectively. D∞ is the diameter of micelles at the photosaturated state, over which a larger irradiation dose, i.e., a longer irradiation time, makes little difference on both the size and characteristic absorbance.44 2.4. Characterization of Micelles. Transmission Electron Microscopy (TEM). TEM images of the micelles were obtained by a JEOL JEM-2100 (HR) LaB6 transmission electron microscope with a 200 kV accelerating voltage. The TEM samples were prepared by dipping the copper grids coated with a thin polymer film into the samples solutions (mass concentration were all 0.5 mg/mL, at different pH) and then dried at room temperature. And the samples of primary micelle solutions were also prepared by the same method. Zeta Potential and Average Diameter. PVMAA micelles were characterized for size and ζ-potential using dynamic light scattering and electrophoretic mobility measurements, both of which were conducted with a combination instrument of 90Plus and ZetaPALS (Brookhaven Instruments Corporation). The size of the micelles was measured four times and give mean diameter and standard error. The ζ-potential was calculated from the electrophoretic mobility (u) using the Smoluchowski relationship ζ = ηu/ε, where it is assumed that kα ≪ 1 (where η is the solution viscosity, ε the dielectric constant of the medium, k and α are the Debye−Hückel parameter and the particle radius, respectively). ζ-potentials were averaged over 20 runs. The variance was typically within the size of the data points shown. 2.5. Preparation and Characterization of Emulsions. Emulsion Preparation. Equal volumes (4.0 mL) of oil (toluene or styrene) and micelle aqueous solution (with a micelle concentration of 2 mg/mL) at different pH values were placed in glass vials at room temperature. The mixtures were homogenized at 8000 rpm for 2 min by a XHF-D H-speed dispersator homogenizer (1 cm head) at 25 °C. The type of all emulsions was O/W, determined by a drop test as well as conductivity measurement. The emulsions were sealed and placed under quiescence condition at room temperature after homogenization. Light Microscopy. Emulsion droplets were imaged with an optical microscope (DM-BA450, Motic China Group Co., LTD) fitted with a digital camera after a 1:6 dilution in the continuous-phase liquid for better optical clarity. Droplet diameter was measured by image analysis using the “Motic Images Advanced” software. The statistical analysis of the droplet size was performed and expressed as the average ± the standard error. Scanning Electron Microscopy (SEM). An oil-phase solidification method was used to visualize the morphologies of PVMAA micelles at the oil/water interface. The method is similar to the previous report with some modifications.45 Styrene with 2.0 mol % V65 in place of toluene as the oil phase was homogenized with equal volume of micelle solution (3 mL/3 mL, the micelle solution was 2 mg/mL) at 8000 rpm for 2 min. The formed emulsions were polymerized at 40 °C for 1 week under quiescence conditions. The emulsion droplets stabilized by polymeric micelles were slowly solidified. The resultants were collected and washed by centrifugation and dried at 35 °C under vacuum for 48 h. The powder was placed on a copper stub and sputter coated with thin layers of gold and then observed with a Quanta-200 field-emission microscope that was operated at an accelerating voltage of 20 kV.

2. EXPERIMENTAL DETAILS 2.1. Materials. Copolymers PVMAA were synthesized with the molar content of hydrophobic repeat unit VM (PVM mol %) of 12, 22, and 47 mol %. The corresponding copolymers were PVMAA12, PVMAA22, and PVMAA47, respectively. More details are given in the Supporting Information. Styrene (CP, Sinopharm Chemical Reagent Co., Ltd. SCRC) was distilled under vacuum and then stored at −5 °C before use. Styrene− oil phase polymerization was initiated with V65 (2,2′-azobis(2,4dimethylvaleronitrile)) which was a gift from Qingdao Runxing Photoelectric Material Co. (LTD, Qingdao, China). Water was first deionized by reverse osmosis and purified to a resistivity of 18.2 MΩ· cm using a Millipore water purification system. Other reagents were used as received. 2.2. Preparation of PVMAA Micelles. PVMAA was dissolved in DMF with a concentration of 10 mg/mL under stirring overnight. To induce self-assembly, equal volume water was added quickly into the DMF solution of PVMAA under vigorous stirring. The solution became bluish in color, indicating the formation of self-assembled micelles. After that, an excess of water was added into the solution to “quench” the structures of the formed micelles. The suspensions were dialyzed against water for 3 days to remove DMF before use with a final mass concentration of 2.5 mg/mL. 2.3. Photo-Cross-Linking of the PVMAA Micelles. λ > 310 nm UV light (a spot UV curing system with intensity of 100 W) was utilized to irradiate PVMAA micelle solutions, giving rise to photocross-linked micelles. The degree of photo-cross-linking was always evaluated by the dimerization degree of coumarin, which monitored by UV−vis spectra and calculated with the equation DD = 1 − At/A0 (A0 and At are the initial absorbance and the absorbance at irradiation time

3. RESULTS AND DISCUSSION 3.1. Properties of the Self-Assembled Micelles. PVMAA is a typical amphiphilic random copolymer, composed of hydrophilic repeat units AA and hydrophobic, photo-cross6671

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Moreover, PVMAA12 micelles could photo-cross-link under the UV irradiation of λ > 310 nm with the absorbance at characteristic peak of 320 nm decreasing, as shown in Figure 2c, similar to the reports of Zhao et al.41 The dimerization degree is of around 60% at photosaturated state of them. Meanwhile, the size of PVMAA12 micelles decreases exponentially over irradiation time (Figure 2d), which could be fitted by an exponential decay equation:

linkable repeat units VM. During the self-assembly process of PVMAA, with the addition of water into the polymer solution in DMF, PVMAA segments or chains with larger hydrophobicity collapse faster and form the interior of micelles, while those more hydrophilic segments are residual at the periphery.39 This process is mainly governed by a delicate balance between hydrophobic and hydrophilic interactions,35 which depends on the PVM mol % of PVMAA. With the PVM mol % of PVMAA increasing, the swelling capability of the dangling PVMAA chains at the periphery of micelles was significantly weakened, demonstrated by comparing the size change of PVMAA micelles with different PVM mol % in response to pH increase, as shown in Figure S3. Furthermore, the toluene-in-water emulsion stabilized by PVMAA12 micelles exhibits the best long-term stability as shown in Figure S4. Hence, PVMAA12 micelles seem to be the best candidate as particulate emulsifiers to investigate the influence of the photocross-linking on their emulsifying performance, since it possesses the largest scale of swelling and deformation. Figure 2 shows the basic properties of the PVMAA12 micelles. The hydrodynamic diameter (dH) and polydispersity

Dt = D∞ + A × e(t0− t )/ τ

(2)

Here, D∞ is the diameter of PVMAA12 micelles at photosaturated state of 55.86 ± 0.56 nm. Other parameters are summarized in Table S3. The polydispersity indexes (μ2/Γ2) of the micelles with various irradiation times are within 0.05−0.08, indicating that there is no intramicelle photo-cross-linking occurred. 3.2. Effect of UV Irradiation on the Emulsifying Performance. PVMAA12 micelles with various SDC values were utilized to stabilize a batch of toluene-in-water emulsions. Here, the SDC was converted from the size of the PVMAA12 micelles after irradiation for a certain time, according to eq 1 with D0 and D∞ of 78.1 and 55.86 nm, respectively (Figure S5a). In this paper, emulsions are characterized in terms of the emulsifying efficiency and stability. The total toluene−water interfacial area (S) in the emulsions depends on the emulsifying efficiency of the micelles. The physical significance of S is similar to the apparent interfacial area per particle (Sapp) for Pickering emulsions, as reported by Tilton et al.10 S = 4πR2

3Voil 3

4πR

=

3Voil R

(3)

Here, R is the average oil droplets radius and Voil is the volume of oil incorporated in the creamy layer. As shown in Figure 3a, all the oil phase is incorporated into the creamy layer, indicating that Voil is a constant. Thus, according to eq 3, S is inversely dependent on the average oil droplet size, namely, that the smaller size the emulsion droplets are of, the higher emulsifying efficiency the micelles have. The size of the emulsion droplets significantly increases as the SDC increases (Figure 3b). Apparently, the emulsifying efficiency of micelles is reduced by the increase of SDC. It is probably due to the restriction of the swelling dangling polymer chains of the micelles reducing the interfacial area per micelle occupy. To clarify the underlying principle, especially the roles that the dangling polymer chains play, the micelles with SDC of 0% and 95% are compared in both their structural transition and emulsifying performance in response to pH changes. 3.3. Structural Transition of the Micelles in Response to pH Change. Poly(acrylic acid) dissolved in water always exhibits a pKa of around 5. pKa of carboxyl groups is environmentally dependent, demonstrated by Gough et al.46 and Grzybowski et al. 47 A hydrophobic environment surrounding the AA component will suppress dissociation of the protons from AA.46,48 According to the architecture of primary PVMAA12 micelles, at a given pH, the dangling polymer segments at the periphery will ionize more easily than the segments at the interior, generating larger electrostatic repulsion. As pH increases, the deprotonation of carboxyl groups gradually propels from the periphery to the interior. The swelling behavior of micelles is governed by a delicate balance between the increasing electrostatic repulsion and the attraction force by hydrophobic interaction among the polymer

Figure 2. Properties of the PVMAA12 micelles. (a) Autocorrelation function and size distribution curve for primary PVMAA12 micelles with hydrodynamic diameter of 78.1 nm and polydispersity index of around 0.07. (b) TEM image of the primary PVMAA12 micelles. (c) Photo-cross-linking of micelles traced by UV spectra with the micelle solution of 3 mL 0.08 mg/mL. Inset is the plot of dimerization degree of micelles against irradiation time. (d) The decrease in micelle size over irradiation time could be fitted by the exponential decay function (eq 2) with parameters are summarized in Table S3. The kinetic study by DLS was measured with 3 mL, 0.5 mg/mL micelle solution, which was filled into a quartz cell and photo-cross-linked by λ > 310 nm irradiation from a spot UV curing system (100 W) under mild stirring. All the samples were kept at room temperature during UV irradiation with a cooler.

index (μ2/Γ2) of the formed primary PVMAA12 micelles are ∼78.1 nm and ∼0.07, respectively, indicating the excellent monodispersity. They are spherical with a diameter of 30−50 nm by TEM. The significant shrinkage in size after dehydration suggests that PVMAA12 micelles are analogues of nanogels. 6672

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although the increase in size of micelles could be fitted by Boltzmann curves (details in Supporting Information). For the un-cross-linked micelles, below pH 8, the decreases in both zeta potential and scattering intensity of micelles with a smaller polydispersity (μ2/Γ2) as pH increases (Figure 4, circle), indicate that the ongoing ionization gradually generates negative charges and accelerates the swelling of polymer chains from the periphery to the interior of micelles. Notably, even for the micelles of extremely swollen state at pH 7.35, the polymer segments at periphery protruding out blur the edge of the micelles, which still maintain their structural integrity, as shown in Figure 5c. As pH elevated over 8, there is a distinct jump from −41.62 mV at pH 8.12 to −27.77 mV at pH 9.08 along with the varying trend of the apparent zeta potential in response to pH increase. The jump probably originates from two aspects: (1) the extremely swollen micelles disintegrate into small pieces of aggregates carrying less net charges, resulting in the jump of apparent zeta potential. (2) The reassembly of polymer chains generates aggregations with an extremely swollen and “hairy” structure, which possess a significantly increased hydrodynamic resistance in aqueous solution under a given electric field,49 leading to a smaller absolute value of apparent ζ-potential. The reassemblies with larger size are proved by TEM in Figure 5d. The mixture of the disintegrated smaller pieces and reassemblies make the μ2/Γ2 jump to a plateau around 0.15 (Figure 4c). Comparatively, the photo-cross-linked micelles (SDC = 95%) keep their structural integrity over the entire pH range, proved by both the stable μ2/Γ2 less than 0.10 (Figure 4c) and the TEM images in Figure 5e−h. The zeta potential decreases until pH approach to boundary 8 and then goes into a plateau (Figure 5d). The plateau indicates the stationary of deprotonation of the carboxyl groups along the polymer segments at the periphery of micelles. Despite the stationary, as pH further elevated over 8, the increasing electrostatic

Figure 3. Emulsifying performance of the PVMAA12 micelles with various SDCs. (a) Appearance and (b) optical microscope images of the toluene-in-water emulsions stabilized by the micelles with various SDCs, including 0%, 48%, and 95%. The emulsions were incubated 1 day after the homogenization of equal volumes of toluene and micelle solution. The concentration of micelle solution was 2.0 mg/mL in the absence of salt.

chains as well as covalent bonds by photo-cross-linking (for cross-linked micelles only). Obviously, the structural transitions of micelles could be divided by the boundary pH of around 8, especially for un-cross-linked micelles as shown in Figure 4,

Figure 4. pH-responsive properties of the PVMAA12 micelles with SDC of 0% (circle) and 95% (star). (a) The enlargements of micelles are fitted by the Boltzmann equation with parameters summarized in Table S4. Plots of (b) scattering intensity, (c) polydispersity index, and (d) zeta potential of the micelles are drawn against the pH increase. All the concentration of the micelle solutions were 0.25 mg/mL in the presence of 10 mM NaCl. 6673

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Figure 5. TEM images of the PVMAA12 micelles at various pHs with SDC of 0% and 95%, respectively. (a−d) The micelles with SDC of 0% at pH 3.11, 5.09, 7.35, and 9.08, respectively. As comparison, (e−h) are the micelles with SDC of 95% at pH 2.95, 5.32, 7.25, and 9.45. All the scale bars are 50 nm.

Figure 6. Properties of emulsions stabilized by PVMAA12 micelles with SDC of 0% and 95% at various pHs. (a) Appearance of the batches of toluene-in-water emulsions incubated 3 weeks after homogenization with equal volumes of toluene and micelle aqueous solution (4 mL/4 mL); the micelle concentrations were 2.0 mg/mL with 10 mM NaCl. (b) Plots of the statistical size of emulsion droplets against pH increase with different incubation times. The mean diameters of the emulsion droplets from the corresponding emulsions in (a) were counted through the optical microscope images.

Recalling the structural transition of the un-cross-linked micelles, it could be presumed that there are two distinct principles governing the emulsifying performance of micelles divided by the same boundary pH 8. This presumption is supported by the statistical analysis of the size of emulsion droplets in response to pH increase. Figure 6b (sphere) shows that the size of the emulsion droplets stabilized by un-crosslinked micelles (SDC = 0%) gradually decrease after incubation for 5 h. It indicates that the swelling of micelles increase their emulsifying efficiency according to eq 3, probably due to that the more swollen the micelles are, the larger interfacial area per micelle occupied. Note that, after incubation for 21 days, the size of the emulsion droplets significantly increases at pH > 8 while those basically unchanged at pH from 3.0 to 7.5 (Figure 6b, triangle). The distinct discrepancy in the stability of emulsions is attributed to the disintegration of micelles as pH elevated over 8. At pH > 8, the disintegrated micelles stabilize the emulsions as macromolecule emulsifiers and form smaller emulsion droplets that facilely merge into larger ones over time.

repulsion at the interior of the micelles could overcome the restriction of the polymer chains by both hydrophobic interaction and photo-cross-linking, resulting in a slight increment of size (Figure 5a, star). 3.4. Emulsifying Performance of the Micelles. Batches of toluene-in-water emulsions were prepared using micelles with SDC of 0% and 95% as emulsifiers at various pHs. The properties of the emulsions are shown in Figure 6. Obviously, pH of around 8 divides each batch of emulsions into two parts. Figure 6a shows that the creamy layer of emulsions stabilized by un-cross-linked micelles (SDC = 0%) is gradually reduced as pH increase from 3.0 to 7.5 and then jumps to and maintains at a largest value as pH elevated over 8, without oil separated out over the whole pH range. As a comparison, the creamy layer of emulsions stabilized by cross-linked micelles (SDC = 95%) maintains at the same value as pH increase from 3.0 to 7.5, and then oil begins to separate out as pH elevated over 8; finally, total phase separation occurs at pH 10.4. 6674

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with SDC of 95% is smaller than that by micelles without photo-cross-linking, probably due to the larger electrostatic repulsion force among those photo-cross-linked micelles, evidenced by zeta potential analysis (Figure S5b). Note that total phase separation occurs at pH 10.4 of the emulsions stabilized by micelles with SDC of 95%. They also could be used to stabilize pH-responsive Pickering emulsion, which could undergo several cycles of demulsification and emulsification triggered by the addition of alkali or acid (Figure S6). Comparatively, the emulsion stabilized by un-cross-linked micelles show no pH-responsive property, probably due to the irreversibly structural transition of the micelles at oil/water interface. This phenomenon is similar to that reported by Fujii et al.19

At pH < 8, it is very stable for the emulsion, even at pH 7.5, which the micelles are of an extremely swollen state and exhibit the largest efficiency. Probably, the amphiphilic dangling chains of the swollen micelles could easily adopt the best conformation to stabilize biphase interface, and the micelles work like “Janus” particles at the interface. More evidence could be found in the SEM images of the polymerized styrene-inwater emulsions stabilized by the PVMAA12 micelles with SDC of 0% and 95% at various pHs. Figure 7b shows there is full

4. CONCLUSION Photo-cross-linkable and pH-responsive micelles self-assembled from amphiphilic random copolymers poly(7-(4-vinylbenzyloxyl)-4-methylcoumarin-co-acrylic acid) (PVMAA) in selective solvent have been prepared and used as polymeric particulate emulsifiers to investigate the influence of photo-cross-linking on their emulsifying performance. Photo-cross-linking among hydrophobic coumarin moieties brings restriction of the dangling polymer chains at the periphery of the primary micelles, then decreases the interfacial deformability of the micelles, and diminishes their emulsifying efficiency in turn. Further systematic comparisons between the un-cross-linked micelles and highly photo-cross-linked micelles focus on both their structural transitions and emulsifying performance in response to pH increase. It could be concluded that the capability of swelling and conformational changing of the dangling polymer chains at the periphery of micelles plays an important role in their emulsifying efficiency and stability. This work provides a new insight into the principle governing the extremely high emulsifying efficiency and pH-dependent or pHresponsive properties of the emulsions stabilized by polymeric particulate emulsifiers. Significantly, it may help on understanding in challenging areas where the polymers and particulate emulsifiers cooperate,50,51 such as cosmetic and food industry.

Figure 7. SEM images of polymerized styrene-in-water emulsions stabilized by the PVMAA12 micelles with SDC of 0% and 95% at different pHs.

coverage of nanopapilla on the surface of polymerized styrene beads at pH 5.0, implying the integrity of the micelles at the interface. As a contrast, Figure 7d shows that there are some “pancake-like” protuberances arising from a smooth surface of the beads at pH 9.5. Probably, smooth area is stabilized by the smaller fragments or free polymers from the disintegrated micelles while the protuberances are from the reassemblies. Comparatively, both the size and the standard error of the emulsion droplets stabilized by micelles with SDC of 95% are larger than those by micelles with SDC of 0% and gradually become larger as pH increase from pH 4.0 to pH 9.4 after incubation for 5 h (Figure 6b, circle). While incubating for 21 days, the droplets size of this batch of emulsions totally become larger, suggesting worse stability of the emulsions compared with those stabilized by un-cross-linked micelles. Probably, photo-cross-linking restricts the dangling chains and prevents their conformational adjustment when the cross-linked micelles at the toluene−water interface. Meanwhile, cross-linked micelles become more rigid as pH increase due to the enhanced electrostatic repulsion and are hardly to adsorb at the toluene−water interface, exhibiting lower emulsifying efficiency and worse stability of the emulsions over time. Figures 7f and 7b provide further evidence that, at pH of around 5, the coverage of the emulsion droplets by micelles

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and characterization of the PVMAA polymers; characterizations of the structures and emulsifying performance of the primary PVMAA micelles with different compositions; properties of the PVMAA12 micelles with various doses of UV irradiation; pH-responsive behavior of the emulsion stabilized by PVMAA12 micelles with SDC of 95%. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the National Nature Science Foundation of China (NSFC) (under Grants 20974041, 21174056, and 50973044). 6675

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