Dual pH- and Light-Responsive Amphiphilic Random Copolymer

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Dual pH- and Light-Responsive Amphiphilic Random Copolymer Nanomicelle as Particulate Emulsifier to Stabilize the Oil/Water Interface Feng Wang, Xiaoyun Yu, Zhouxiaoshuang Yang, Hui Duan, Zhijiao Zhang, and Hui Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05065 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 5, 2018

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The Journal of Physical Chemistry

Dual

pH-

and

Light-Responsive

Amphiphilic

Random

Copolymer Nanomicelle as Particulate Emulsifier to Stabilize the Oil/Water Interface

Feng Wang, a Xiaoyun Yu, a Zhouxiaoshuang Yang, a Hui Duan, a Zhijiao Zhang, a and Hui Liu*a, b a

College of Chemistry and Chemical Engineering, Central South University, 932

South Lushan Road, Changsha 410083, Hunan, P. R. China b

Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese

Resources, Central South University, Changsha 410083, Hunan, P. R. China

*Corresponding author: Hui Liu (E-mail: [email protected]) Tel: +86 731 88879616 Fax: +86 731 88879616 Postal address: College of Chemistry and Chemical Engineering, Central South University, 932 South Lushan Road, Changsha 410083, Hunan, P. R. China

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ABSTRACT: Dual pH- and light-responsive nanomicelles self-assembled from amphiphilic

random

poly

(methacrylic

vinylbenzyloxyl)-4-methylcoumarin)

acid-co-methyl

methacrylate-co-7-(4-

(P(MAA-co-MMA-co-VM))

have

been

successfully prepared and employed as particulate emulsifiers to stabilize the oil/water interface. The self-assembling behavior of copolymers in selective solvent is explored. The structural transitions of CP55 micelles based on copolymer with 55 mol% hydrophilic MAA units are investigated by the dosage of UV irradiation and pH variation. As pH increases, the ionization of the carboxyl groups leads to the swelling of micelles. Both the size and polydispersity index of cross-linked micelles are far larger than those of un-cross-linked micelles under alkaline conditions. The comparison of the configuration of un-cross-linked micelles and cross-linked micelles at the oil/water interface is investigated by scanning electron microscopy (SEM). The results show that the ability of swelling and conformational changing in respond to external pH- and light-triggers largely affected the emulsifying performance of particulate emulsifiers. Especially for emulsions stabilized by cross-linked micelles, oil begins to separate out as pH elevated over 7.3 on account of the restricted conformational adjustment resulted from the increasing inter-particle cross-linking and electrostatic repulsion. The findings may be useful for challenging industrial areas where organic macromolecules and surfactants cooperate.

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1. INTRODUCTION Pickering emulsions have been extensively used in many areas such as food industry, biocatalysis, personal care products, and fabrication of new materials.1-8 As a multiphasic dispersion system, emulsions typically consist of three main components: water phase, oil phase, and emulsifier.3,9 Various types of particulate emulsifiers, including inorganic particles,10-13 organic particles,14-19 polymer-modified hybrid particles,20-26 microgels,2,27-33 and self-assembled polymeric micelles,7,34-43 have been used to control the stability and type of Pickering emulsions (briefly summarized in Table 1). Additionally, Pickering emulsions have low toxicity and good biocompatibility as a result of reducing the use of conventional surfactants.3 Table 1. A Brief Summary of Particulate Emulsifiers Particulate emulsifiers Inorganic particles

Organic particles

Polymer-modified hybrid particles

Microgels

Self-assembled polymeric micelles

Particles

Emulsion type

References

Graphene oxide

O/W

10

Carbon nanotube

W/O

11

Laponite clay

O/W

12

Janus CuO/CuS colloids

O/W

13

Cellulose nanocrystals

O/W

14 and 15

Protein

O/W

16

Poly(2-vinylpyridine) latexes

W/O

17

Polystyrene latexes

W/O or O/W

18 and 19

Polymer-grafted silica nanoparticles or brushes

W/O or O/W

20-22

Polymer-modified Fe3O4 nanoparticles

O/W

23 and 24

Polymer-grafted cellulose nanocrystals

O/W

25

Polymer-grafted lignin nanoparticles

O/W

26

Poly(N-isopropylacrylamide)-based microgels

W/O or O/W

2 and 27-31

Poly(2-(tert-butylamino) ethyl methacrylate) microgels

O/W

32

Poly(4-vinylpyridine)-silica nanocomposite microgels

O/W

33

Block copolymer micelles

W/O or O/W

34-36

Random or alternating copolymer micelles

W/O or O/W

7 and 36-43

Over the past decade, the self-assembled polymeric micelles have been recognized 3



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as an excellent model to elucidate the relationship between the emulsifying performance and structure of particulate emulsifiers, on account of the facile controllability and tailor-ability of their structure. A kind of linear triblock copolymer micelle was employed as particulate emulsifier to stabilize 1-undecanol-in-water emulsions, which was the pioneering work of Armes et al. in 2005.34 Afterwards, interest in the surface activity and ease of emulsification of the self-assembled polymeric micelles is steadily gaining momentum. Zhang at al.35 used self-assembled nanoparticles

as

emulsifiers

to

control

the

Pickering

emulsions.

Since

poly(2-(dimethylamino)ethyl methacrylate) side chains were deprotonated under alkaline conditions and the nanoparticles tended to be wetted by the oil phase, a stable oil-in-water (O/W) emulsion was inverted to a water-in-oil (W/O) emulsion when the pH was changed from pH 3-5 to pH 8-9. With further increase of pH value to 11-12, the nanoparticles exhibited hydrophilicity again due to the adsorption of the hydroxyl ions on the nanoparticles surfaces. Phase inversion occurred once again and O/W emulsions were formed. Yi and Liu et al.36 synthesized a series of random copolymer poly(acrylic acid-co-styrene) (P(AA-co-St)) and block copolymer poly(acrylic acid)-b-polystyrene (PAA-b-PSt) with similar chemical composition but different chain

microstructure.

Compared

with

PAA-b-PSt

self-assembled

micelles,

P(AA-co-St) micelles had better interfacial performance and were more tailorable and controllable. Further systematic comparison of the emulsifying performance between P(AA-co-St) micelles and PAA-b-PSt micelles at various pHs confirmed that the existing hydrophobic segment at the micelle peripheries provided interfacial activity 4


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for P(AA-co-St) micelles, and thus enhanced their emulsifying efficiency. In our previous study, a series of polymeric micelles self-assembled from linear amphiphilic random copolymers were employed as particulate emulsifiers. The correlation between the oil-water interfacial property and emulsifying performance of polymeric micelles was deeply investigated.38-41 We found that self-assembled micelles based on random copolymers were interfacially active. Different from the clear core-shell structure of block copolymer micelles, for random copolymer micelles, there were hydrophilic segments inside the micellar hydrophobic core while hydrophobic segments also existed in the micellar hydrophilic surface. The micellar structure and hydrophilicity/hydrophobicity largely affected the emulsifying performance of particulate emulsifiers. Further endeavors are recommended to investigate the underlying principle of stimuli-responsive random copolymer micelles as particulate emulsifiers. In this work, to explore the self-assembling behavior and oil-water interfacial property of stimuli-responsive random copolymer micelles, dual pH- and light-responsive random poly (methacrylic acid-co-methyl methacrylate-co-7-(4vinylbenzyloxyl)-4-methylcoumarin) (P(MAA-co-MMA-co-VM)) was designed and prepared. The structure of the resultant nanomicelles was tailored by the amphiphilicity of copolymers. In our cases, both hydrophilic and hydrophobic segments are actually amphiphilic in virtue of the random sequence of the monomers. This structural feature endows polymeric micelles with both high emulsifying efficiency and interfacial stability.42 Moreover, the photo-cross-linkage of coumarin 5



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units results in inter-chain cross-linking and inter-particle cross-linking in the micelle emulsifiers, and the dangling copolymer segments are normally restricted by the cross-linked inter-chain network. Cross-linking by ultraviolet (UV) irradiation in micelle aqueous dispersion not only avoid the use of chemical cross-linking agents but also impel the potential application of UV absorption in sunscreen products. In addition, the carboxyl groups along the main chain confer the micelles pH sensitivity. The swelling behavior and emulsifying performance of the resultant nanomicelles were investigated by the dosage of UV irradiation and pH variation.

2. EXPERIMENTAL SECTION 2.1.

Materials.

The

light-responsive

monomer

7-(4-vinylbenzyloxyl)-4-methylcoumarin (VM) was synthesized according to the previous report,40 and the corresponding 1H NMR spectrum was shown in Figure S1. 1

H NMR (400 MHz, DMSO): δ (ppm) 7.68-7.70 (d, 1H), 7.44-7.52 (m, 2H),

7.02-7.07 (m, 2H), 6.71-6.78 (m, 1H), 6.22 (s, 1H), 5.83-5.88 (dd, 1H), 5.27-5.29 (dd, 1H), 5.23 (s, 2H), 2.39 (s, 3H). The pH-responsive monomer methacrylic acid (MAA, 99%, Adamas) was distilled under vacuum and then stored in a cool and dry area. Methyl methacrylate (MMA, 99%, Adamas) was passed through a column of basic alumina to remove inhibitors before use. Dioxane (99.5%+, Greagent), styrene (St, 99%, TCI), 2,2'-azobis(isobutyronitrile) (AIBN, 98%+, Adamas), tetrahydrofuran (THF, 99%), N,N-dimethylformamide (DMF, 99.5%+), n-hexane, paraffin oil, n-heptane, cyclohexane, benzene, and toluene were all used as received. NaOH (0.1 6


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M) solution and HCl (0.1 M) solution were used to adjust the pH value of the aqueous dispersions. 2.2. Typical Copolymerization Procedure. A series of amphiphilic random copolymers were synthesized by free radical solution polymerization. Monomers including MAA, VM, and MMA were added into dioxane in a dry Schlenk tube (1.33 M in dioxane). The copolymerization reaction was carried out under a high-purity nitrogen atmosphere at 60 ℃ for 24 h using AIBN (molar ratio: [Monomers]/[AIBN] = 50/1) as initiator (Scheme 1a). The obtained copolymer P(MAA-co-MMA-co-VM) was isolated by pouring the reaction mixture into an excess amount of cold n-hexane, and further purified by dissolution in THF and precipitation in n-hexane three times. The final products were dried under vacuum for three days. Scheme 1. (a) Synthesis of Random Copolymer P(MAA-co-MMA-co-VM); (b) Schematic Illustration of the Self-Assembly Process of Copolymers, and the pHand Light-Responsive Behavior of the Resultant Nanomicelles; (c) Pickering Emulsions with Nanomicelles as Particulate Emulsifiers

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2.3. Preparation of Self-Assembled Nanomicelles. P(MAA-co-MMA-co-VM) nanomicelles were prepared via a water-induced micellization method (Scheme 1b). Typically, 100 mg of P(MAA-co-MMA-co-VM) was first dissolved in 15 mL of DMF and then stirred overnight to form a clear solution. 15 mL of water was added quickly into the solution under vigorous stirring. The color of the solution showed a bluish opalescence, indicating the formation of self-assembled nanomicelles. After 12 h, the mixture was quenched into an excess amount of water, followed by dialysis (molar 8


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mass cutoff: 7000) against water for three days to remove DMF. To explore the self-assembling behavior of amphiphilic random copolymer, the absorbance or turbidity change of the copolymer solution (in DMF) at 621 nm was detected by UV-Vis spectrophotometry as water was added dropwise. 2.4. Pickering Emulsions with Nanomicelles as Particulate Emulsifiers. Briefly, equal volumes (4 mL/4 mL) of the oil phase (paraffin oil, n-heptane, cyclohexane, benzene, toluene, styrene) and the P(MAA-co-MMA-co-VM) nanomicelle aqueous dispersion were placed in a 20 mL glass vessel. The mixture was homogenized at 7000 rpm for 3 min by a XHF-DY H-speed dispersator homogenizer (Scheme 1c). The obtained emulsion was sealed and placed under quiescence condition at room temperature after homogenization. The type of all emulsions was O/W, determined by drop tests. The emulsifying efficiency of O/W emulsions was assessed by the emulsion ratio which was defined as the volume of creamy layer over the whole volumes of oil and water phase. The higher the emulsion ratio, the better the emulsifying efficiency. 2.5. Solidifying Polystyrene-in-Water Emulsion Droplets. The emulsion droplets were solidified to investigate the configuration and property of amphiphilic copolymer nanomicelles at the oil/water interface. St with the initiator AIBN (molar ratio: [St]/[AIBN] = 50/1) was employed as oil phase to prepare emulsions using P(MAA-co-MMA-co-VM)

nanomicelles

as

particulate

emulsifiers.

After

emulsification at 7000 rpm for 3 min, the oil phase in the emulsion droplet was solidified by the polymerization of St at 60 ℃ for 24 h. White solidified 9



polystyrene-in-water droplets were collected by centrifugation and dried under vacuum. 2.6. Analytical Techniques and Characterization. The gel permeation chromatographic (GPC) analysis of the copolymers was performed on a Waters515 GPC apparatus with DMF + 0.05 M LiBr as the eluent (the addition of LiBr was used for quenching acrylic acid groups of MAA so that the acid groups did not be adsorbed on the column). DMF was pumped through the system at a fixed flow rate of 1 mL·min-1, and the standard polystyrene was used for the calibration of samples. The 1

H NMR spectroscopic analysis was performed on a Bruker AVANCE III 400 MHz

nuclear magnetic resonance instrument. Fourier transform infrared spectroscopy (FTIR) measurement was recorded on a Nicolet model 6700. The UV-Vis analysis was performed on a UV-2600 spectrometer. Transmission electron microscopy (TEM) was conducted on Tecnai G2 20S-Twin at an acceleration voltage of 100 kV. The size of the self-assembled nanomicelles was measured by dynamic light scattering (DLS) using Malvern Zetasizer Nano ZS90. Scanning electron microscopy (SEM) was carried out using a Quanta FEG 250 microscope operating at 10kV. Solidified emulsion droplets were placed on a copper platform and sputter-coated with gold to minimize sample-charging problems.

3. RESULTS AND DISCUSSION 3.1. Amphiphilic Random Copolymers P(MAA-co-MMA-co-VM). In order to modulate the hydrophilicity and hydrophobicity of copolymers under the same 10


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cross-linkable monomer VM content ([VM]/[Monomers] molar ratio was fixed), the synthesis of copolymers P(MAA-co-MMA-co-VM) was carried out via free radical solution polymerization by adjusting the feed ratio of [MAA]/[MMA]. The chemical structure of the amphiphilic random copolymers was illustrated in Scheme 1a. The MAA content of the P(MAA-co-MMA-co-VM) samples used in this study was 45% (CP45), 55% (CP55), and 65% (CP65), respectively (see Table 2). The 1

H NMR spectrum of CP55 was presented as a typical example in Figure 1. It was

clear that the carbon-carbon double bond peaks of each monomer had disappeared. The characteristic signals of VM unit at peaks 11 and 15, MMA unit at peak 6, and MAA unit at peak 3 validated that P(MAA-co-MMA-co-VM) had been successfully synthesized. The actual molar ratio among MAA, MMA, and VM units was determined through comparison of the integrals of peak 3, 6, and 11. Further supporting verification was conducted by FTIR (see Figure S2).

Figure 1. 1H NMR spectrum of P(MAA-co-MMA-co-VM). 11



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Table 2. Detailed Characterization of P(MAA-co-MMA-co-VM) n(MAA) : n(MMA) : n(VM) MAA mol% b

copolymers

Mn c

Mw

PDI

a

feed ratio

actual ratio

CP45

13 : 15 : 4

13.6 : 12.5 : 4.0

45%

16200

26200

1.62

CP55

18 : 10 : 4

17.0 : 9.8 : 4.0

55%

11900

23400

1.97

CP65

21 : 7 : 4

19.6 : 6.4 : 4.0

65%

9200

19800

2.15

Note: a The actual molar ratio of the copolymers was calculated by 1H NMR. b The molar content of hydrophilic MAA units in the copolymers. c The Mn and PDI of the copolymers were determined by GPC, using DMF + 0.05 M LiBr as the eluent (flow rate: 1 mL·min-1) and polystyrene as the standard.

3.2. Nanomicelles. The self-assembling behavior of P(MAA-co-MMA-co-VM) into nanomicelles using the selective solvent DMF/H2O was monitored by the change in absorbance of the copolymer solutions with the addition of water. Figure 2 showed that, for all the samples, the absorbance was initially low and approximately invariable, and the copolymer dispersion was clear. The solubility of selective solvent to the hydrophobic segment decreased with the addition of water. When the water was added to a certain amount, the absorbance or turbidity increased dramatically, implying that the copolymer chains started to aggregate and form nanomicelles. This point was defined as the critical water content (CWC). As could be seen in Figure 2a, the CWC increased with the increasing MAA mol% of P(MAA-co-MMA-co-VM). A rational explanation was that a higher MAA content generated a stronger hydrophilicity. Accordingly, a larger amount of water was needed to give a stronger hydrophobic interaction to drive the microphase separation of micelle formation. 12


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Figure 2b showed that CWC increased with the increasing pH, which was attributed to the state of the carboxyl groups in the MAA component.44 The enhanced ionization of the carboxyl groups under alkaline conditions caused a more hydrophilic copolymer chains, resulting in the increase of CWC.

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Figure 2. (a) Plots of the absorbance of the CP45, CP55, and CP65 copolymers (in DMF solutions) at λ=621 nm with the volume fraction of H2O. (b) Plots of the absorbance of the sample CP55 with the volume fraction of H2O in different pHs. The initial concentration of copolymer solutions in DMF is 5 mg·mL-1.

P(MAA-co-MMA-co-VM) random copolymers with three compositions were self-assembled into spherical nanomicelles without core-shell structure as shown in Figure 3, which spontaneously indicated that hydrophobic units of random copolymers also existed at the micellar hydrophilic surface. The size and polydispersity of micelles were detected by DLS (Figure 4). The hydrodynamic diameter (Dh) of nanomicelles changed from 79 nm of CP45 to 101 nm of CP55, and then 104 nm of CP65. The mean particle sizes of CP55 and CP65 were slightly larger than CP45. These variations were in good agreement with TEM images. In addition, the polydispersity of CP45 was worse than that of CP55 and CP65 on account of partial inter-particle aggregation in CP45 micelles.

Figure 3. TEM images of CP45, CP55, and CP65 micelles. The micelle concentration is 0.5 mg·mL-1. 14


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Figure 4. Size distribution of CP45, CP55, and CP65 micelles. The micelle concentration is 0.2 mg·mL-1.

The resultant nanomicelles could photo-cross-link through a cycloaddition reaction under the UV irradiation of λ > 310 nm, and the absorbance of coumarin units at around 320 nm decreased continuously with the increasing irradiation time (as demonstrated in Figure S3).45,46 After the UV irradiations at λ = 365 nm for 0 h (un-cross-linked) or 3 h (cross-linked), CP55 nanomicelles with various pHs were characterized and utilized to stabilize a batch of paraffin oil-in-water emulsions. Figure 5a showed the size and distribution of CP55 micelles under the eight typical 15



conditions. When CP55 micelles was at a given pH, the dangling copolymer segments of the micellar surface ionized more easily than the segments of the micellar interior, bringing larger electrostatic repulsion. As pH increased, the ionization of the carboxyl groups was gradually extended from the surface to the interior. When the electrostatic repulsive force overcame the hydrophobic interaction force, the micelles would swell and the Dh would become large (see Figure 5b). After careful investigation, the Dh of cross-linked micelles (Figure 5b, star) was slightly smaller than that of un-cross-linked micelles (Figure 5b, circle dot) under acidic and neutral conditions, which meant that the inter-chain cross-linking induced the shrinkage of the whole nanomicelles under the UV irradiation (analogous to the findings by Liu and Jiang et al.42). However, a sharp increase in the Dh of cross-linked micelles was observed under alkaline conditions (Figure 5b, star). Excessive swelling of the dangling copolymer segments of the micellar surface facilitated cross-linking among the various nanomicelles to form a dense network structure with poorer deformability and larger size. Meanwhile, the polydispersity index of cross-linked micelles increased with the increasing pH under alkaline conditions. In contrast, the un-cross-linked micelles displayed good monodispersity within the entire pH regulation range (inset in Figure 5b), and the structural transitions of nanomicelles were stable.

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Figure 5. The size distribution (a) and hydrodynamic diameter (b) of CP55 aqueous dispersions with UV irradiation for 0 h (circle dot) and 3 h (star) at various pH values. Inset is the plot of the corresponding polydispersity index against the pH increase. The micelle concentration is 0.2 mg·mL-1. 17



3.3. Emulsions. Three types of primary P(MAA-co-MMA-co-VM) nanomicelles were employed as particulate emulsifier to stabilize paraffin oil-in-water emulsions, as shown in Figure 6a. For all the cases, there was nearly no oil separation in the emulsions, showing excellent surface activity and high stability even after incubating ten days due to the favorable amphiphilicity and suitable swellability. More evidence could be found in the SEM images of the solidified polystyrene-in-water emulsions stabilized by the primary nanomicelles (Figure 7). There was full coverage of regular spherical nanoparticles on the surface of the solidified polystyrene beads (SP beads) in Figure 7b and Figure 7c, indicating the integrity of CP55 and CP65 nanomicelles at the oil/water interface. Additionally, the spheres provided steric hindrance and prevented the emulsion droplets from coalescence. Comparatively, only few spherical micelles were adsorbed or embedded in the interface in Figure 7a, while some micron-sized hills arose from a smooth surface of the SP beads. As mentioned in the TEM images of CP45 micelles, the micron-sized hills probably was formed by the aggregation of CP45 micelles, reducing the emulsifying efficiency of particulate emulsifiers (as suggested by the fact that the height of the emulsion creamy layer stabilized by CP45 micelles was lower than that by CP55 or CP65 micelles (see Figure 6a)).

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Figure 6. (a) The emulsifying performance of CP45, CP55, and CP65 micelles. Appearance of the paraffin oil-in-water emulsions incubated 2 h (left) and 240 h (right) after homogenization with equal volumes of paraffin oil and micelle aqueous dispersion (4 mL/4 mL). (b) Emulsions stabilized by CP55 micelles using paraffin oil, n-heptane, cyclohexane, benzene, and toluene (from left to right) as the oil phase. The micelle concentration is 2.0 mg·mL-1 in the absence of salt.

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Figure 7. SEM images of solidified polystyrene-in-water emulsion droplets stabilized by the primary CP45, CP55, and CP65 micelles. For example, the labels a1-a3 represents the different magnifications of the samples. In addition, CP55 nanomicelle was chosen as model to investigate the effect of the oil type on the emulsification ability. Paraffin oil, n-heptane, cyclohexane, benzene, and toluene with different polarities were employed as the oil phases to prepare Pickering emulsions, and the appearance of emulsions incubated ten days after homogenization was shown in Figure 6b. Almost all emulsions had the same emulsion ratio, and no oil separated out, implying high emulsifying efficiency and stability. The 20


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emulsion type was all O/W regardless of the oil polarity. These results revealed that the polarity of oil had no effect on the emulsifying performance of CP55 nanomicelles, and Pickering emulsions could be stabilized by CP55 nanomicelle using a wide range of oils, which was significant in further potential applications. The configuration of CP55 nanomicelles at the oil/water interface depended on the micelles prior to homogenization and also played a vital role in emulsion properties. Based on the pH- and light-responsive analysis of CP55 micelles mentioned above, batches of paraffin oil-in-water emulsions were prepared using CP55 micelles with UV irradiation for 0 h and 3 h as emulsifiers at various pHs, as shown in Figure 8. The initial micelle was bluish solution with a pH of 6.5, and the state of carboxyl ionization induced a change in the color of the solution when acid or alkali was added. Figure 8a showed that the emulsion ratio of emulsions stabilized by un-cross-linked micelles (UV 0 h) was increased as pH increased from 2.5 to 5.0 and reached the largest value at pH 5.0, and then gradually reduced as pH increased from 6.5 to 11.8. There was no oil separation over the whole pH range. As a contrast, The emulsion ratio of emulsions stabilized by cross-linked micelles (UV 3 h) maintained around 80% as pH increased from 2.5 to 6.5, and then oil began to separate out as pH elevated over 7.3 (Figure 8b), suggesting that the stability of emulsions was broken by addition of alkali. Both the size and polydispersity index of CP55 micelles with UV irradiation for 3 h were far larger than those of CP55 micelles with UV irradiation for 0 h under alkaline conditions (see Figure 5b). In other words, cross-linked micelles became more rigid (poor deformability) with the elevating pH due to the increasing 21



inter-particle cross-linking and electrostatic repulsion, and they were hardly adsorbed at the paraffin oil/water interface, displaying lower emulsifying efficiency and worse stability of the emulsions over time.

Figure 8. The emulsifying performance of CP55 micelles with UV irradiation for 0 h (a) and 3 h (b) at various pH values. All the batches of emulsions are incubated for ten days after homogenization with equal volumes of paraffin oil and micelle aqueous dispersion (4 mL/4 mL). The micelle concentration is 2.0 mg·mL-1 with 10 mM NaCl.

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Figure 9. SEM images of solidified polystyrene-in-water emulsion droplets stabilized by the sample CP55 with UV irradiation for 0 h (a) and 3 h (b) at various pH values. Next, the comparison of the configuration between un-cross-linked and cross-linked micelles at the oil/water interface was further investigated by SEM. Figure 9 showed the morphologies of different solidified polystyrene-in-water emulsion droplets. Compared with SP beads stabilized by primary CP55 nanomicelles (Figure 9(a3-a4)), great stacking of nanoparticles was observed at pH 3.0 (Figure 9(a1-a2)), indicating that the micelles could be well-wetted by styrene as a result of the higher lipophilicity of the MAA side chains under acidic conditions. The un-cross-linked micelles were extremely swollen and easily decomposed at pH 10.0. Some disintegrated fragments or free polymers actually acted as macromolecular surfactant, and the others reassembled into large composite particles which could be extremely deformed and spread out to occupy larger oil/water interface area (Figure 23



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9(a5-a6)). Moreover, the surface of SP beads stabilized by cross-linked CP55 micelles was rough and mussy (Figure 9(b1-b4)). Probably, photo-cross-linking restricted the free motion of dangling chains and prevented their conformational adjustment, causing the disordered stacking. It was noted that no SP beads was observed at pH 10.0 and the cross-linked micelles could not stabilize the oil phase (Figure 9(b5-b6)). A rational explanation was that the obtained emulsion at pH 10.0 phase-separated, coalesced, and solidified before the emulsion reached its most thermodynamically stable state. The stability of emulsions significantly decreased and even demulsification was observed, which was in good agreement with the appearance of the emulsions stabilized by cross-linked CP55 micelles under alkaline conditions (see Figure 8b). The enhanced swelling of the dangling copolymer segments of the micellar surface facilitated cross-linking among the various nanomicelles to form a dense network structure with poorer deformability and larger size (as pH increases). The resulting “rigid” micelles did not be absorbed at the oil/water interface, leading to lower emulsifying efficiency of micelles and worse stability of the emulsions.

4. CONCLUSIONS Self-assembled polymeric micelles have been recognized as an excellent particulate emulsifier to stabilize Pickering emulsions. Dual pH- and light-responsive nanomicelles

self-assembled

from

amphiphilic

random

copolymers

P(MAA-co-MMA-co-VM) in selective solvent DMF/H2O have been prepared to elucidate the correlation between the structure and emulsifying performance of 24


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particulate emulsifiers. The self-assembling behavior of P(MAA-co-MMA-co-VM) is firstly studied. The results show that CWC of copolymers increases with the increasing MAA mol% and pH. The swelling behavior and emulsifying performance of the resultant nanomicelles are investigated by the dosage of UV irradiation and pH variation. As pH increases, the ionization of the carboxyl groups is gradually extended from the micellar surface to the interior, resulting in the swelling of micelles. The Dh of cross-linked micelles (UV 3 h) is slightly smaller than that of un-cross-linked micelles (UV 0 h) under acidic and neutral conditions. While both the Dh and polydispersity index of cross-linked micelles are far larger than those of un-cross-linked micelles under alkaline conditions. The enhanced swelling of the dangling copolymer segments of the micellar surface facilitates cross-linking among the various nanomicelles to form a dense network structure with poorer deformability and larger size (as pH increases). Pickering emulsions could be stabilized by the primary nanomicelles with excellent surface activity using a wide range of oils. Further systematic investigation on the comparison of un-cross-linked and cross-linked CP55 micelles is focused on both emulsifying performance and oil-water interfacial behavior. It could be concluded that the ability of swelling and conformational changing of the dangling copolymer segments of the micellar surface plays an important influence on their emulsifying efficiency and stability. This work provides a new insight into the underlying principle of stimuli-responsive random copolymer micelles as particulate emulsifiers.

25



ASSOCIATED CONTENT Supporting Information 1

H NMR spectrum of VM, FTIR spectra of copolymers, and photo-cross-linking of

nanomicelles (UV spectra) (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Hui Liu: 0000-0002-4792-9285 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by Natural Science Foundation of China (Grant No. 21376271), Natural Science Foundation of Hunan Province (No. 2015JJ2174), the Fundamental Research Funds for the Central Universities of Central South University (No. 2017zzts783), the Hunan Provincial Science and Technology Plan Project, China (No. 2016TP1007), the Undergraduates Innovative Training Foundation of Central South University (CX20170047, ZY20170735), and Open-End Fund for the Valuable and Precision Instruments of Central South University. 26


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TOC Graphic

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Dual pH- and Light-Responsive Amphiphilic Random Copolymer

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