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ACS Omega
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10.1021/acsomega.3c09315
Article
Morphology and Emulsification of Poly(N-2-(methacryloyloxy) ethyl pyrrolidone)-b-poly(benzyl methacrylate) Assemblies by Polymerization-Induced Self-Assembly
Cheng Shuozhen †‡
Wang Jun §∥
Li Chunhui †
He Sixian §∥
Liu Yashuang †
Wang Yan §∥
Dong Jinfeng *†
https://orcid.org/0000-0002-7277-0130
Li Xuefeng *†
† College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P. R. China
‡ Chemistry Metrology Division, Hubei Institute of Measurement and Testing Technology, Wuhan 430200, P. R. China
§ Oil & Gas Technology Research Institute, Changqing Oilfield Company, Xi’an 710018, China
∥ National Engineering Laboratory for Exploration and Development of Low-Permeable Oil and Gas Fields, PetroChina Changqing Oilfield Company, Xi’an 710018, China
* Email: jfdong@whu.edu.cn.
* Email: lixuefeng@whu.edu.cn.
22 08 2024
03 09 2024
9 35 3691736925
22 11 2023
30 07 2024
07 06 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

In this work, a series of amphiphilic diblock copolymers poly(N-2-(methacryloyloxy) ethyl pyrrolidone)-b-poly(benzyl methacrylate) (PNMPm-b-PBzMAn) were developed by the dispersion polymerization method in ethanol. The polymerization-induced self-assembly (PISA) behaviors were studied systematically, and a comprehensive structure–property relationship was also established. Two distinct PISA tendencies were observed, which was mainly depended on the polymerization degree m of PNMP segment. When m is small such as 39 and 55, morphological transitions from spherical to vesicle-like assemblies via wormlike ones upon increasing n commonly happen regardless of the solid content. Alternatively, spherical assemblies became the sole morphology for PNMP64-b-PBzMAn block copolymers because of the excellent solvophilicity of the PNMP64 segment. Attributing to the amphiphilicity of PNMPm-b-PBzMAn block copolymers, PNMPm-b-PBzMAn assemblies by PISA are a type of excellent Pickering emulsifiers. These assemblies prefer to stabilize O/W Pickering emulsions as confirmed by the confocal laser scanning microscopy method, and the effects of polymerization degree of PBzMA segment or morphologies of PNMPm-b-PBzMAn assemblies are finite.

National Natural Science Foundation of China 10.13039/501100001809 21773174 National Natural Science Foundation of China 10.13039/501100001809 22072109 document-id-old-9ao3c09315
document-id-new-14ao3c09315
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pmc1 Introduction

In the past decades, studies on the development and self-assembly behaviors of polymeric amphiphiles become one of the most attractive topics because they show superiority to conventional surfactants in many physicochemical properties such as the lower dosage and higher efficiency, better stability and membrane intensity, and richer self-assembly behaviors.1−3 These amphiphilic block copolymers are widely used in various fields including biotechnology, the development of functional materials, drug-controlled release systems, catalysis, and so forth.1−6 It is well-known that inorganic solid particles with specific surface wettability are excellent emulsifiers to stabilize emulsions, namely, Pickering emulsions,7,8 which show better stability than classic emulsions stabilized by surfactants. Recent studies show that assemblies of amphiphilic block copolymer can also be employed as Pickering emulsifiers,9,10 and even multiple Pickering emulsions might be generated using a single block copolymer emulsifier,11 which extensively enlarge the content and application potential of emulsions.12−14 Attributing to the development of synthetic chemistry, the structures of amphiphilic block copolymers were diversified much. Although numerous synthetic methods of block copolymers were reported,15−17 the polymerization-induced self-assembly (PISA) method is a shining star that accomplishes the development and preparation of block copolymer assemblies simultaneously.18 Currently, the corresponding reaction requirement of PISA including the type of monomers, solvents, and the polymerization methods is experimentally studied well, and the major advantage and progress of PISA as well.19−23

The abundant self-assembly behavior is one of the most interesting characteristics of PISA, spherical, wormlike, and vesicle-like, and even other higher ordered assemblies can be generated.24−28 The formation and growth of assemblies by PISA show high similarity to those of the conventional surfactants,26 i.e., the morphological transitions from spherical to vesicle-like assemblies via wormlike ones are commonly reported through adjusting the polymerization degree of solvophobic segment in PISA systems, which also follow the well-known molecular packing parameter.24,25,29 In addition, factors such as the polymerization degree, the alterable compression, solvation, and mobility of solvophobic segment, and the steric hindrance of solvophilic segment might also momentously affect the morphology of assemblies by PISA,18,26,30,31 and thereby resulting in attractive indeterminacy during the PISA processes. For example, Armes and co-workers studied the PISA behaviors of PSMA-PBzMA diblock copolymers by employing the in situ small-angle X-ray scattering (SAXS) technique.32 Although spherical, wormlike, and vesicle-like nano-objects were formed for PSMA13-PBzMAx series diblock copolymers by the gradual growth mechanism. However, spheres became the sole morphology in the PSMA31-PBzMAx and PSMA18-PBzMAx dispersions even increasing x to 2000, in which the excellent solvophilicity of the PSMA segment with the larger polymerization degree was critical.

Our group has reported some pyrrolidone-based amphiphilic diblock copolymers developed by the RAFT method, which not only show abundant self-assembly behaviors in the selected solvents but also display interesting stimuli responses.33−35 For example, poly(N-(2-methacrylaoyxyethyl)-pyrrolidone)-b-poly(methyl methacrylate) (PNMP-b-PMMA) could form thermal-response organogels in isopropanol because of the disassembly of 3D micellar networks at the higher temperature.35 Similar amphiphilic copolymers were alternatively prepared by the PISA method by other groups, and rich self-assembly behaviors were observed.36,37 Recently, a new family of pyrrolidone-based amphiphilic diblock copolymers, poly(N-2-(methacryloyloxy) ethyl pyrrolidone)-b-poly(benzyl methacrylate) (PNMP-b-PBzMA), were reported. The morphologies of PNMP50-b-PBzMAn assemblies show gradual transition from spherical to vesicle-like assemblies via wormlike ones.38 However, a comprehensive illustration about the structure–property relationship of PNMP-b-PBzMA diblock copolymers is still unknown.

In this work, PNMPm-b-PBzMAn amphiphilic diblock copolymers were synthesized by the reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization method in ethanol, and their PISA behaviors were studied systematically by various techniques including dynamic light scattering (DLS), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) measurements. To establish the structure–property relationship of PNMPm-b-PBzMAn diblock copolymers, factors such as the polymerization degree m and n of PNMP and PBzMA segments, respectively, and the solid content on the morphologies of assemblies were considered carefully. Moreover, the emulsification of PNMPm-b-PBzMAn assemblies by PISA was evaluated in detail. The comprehensive physiochemical properties of PNMPm-b-PBzMAn diblock copolymers provide solid basics for their application potential in the related fields fundamentally.

2 Experimental Section

2.1 Materials

2-Cyanopropyl-2-dithiobenzoate (CPDB) was synthesized according to the reported method.27 2, 2′-Azobis(isobutyronitrile) (AIBN, 99%) was purchased from Shanghai HATECH Co. Ltd. and recrystallized in ethanol twice. N-hydroxyethyl pyrrolidone and benzyl methacrylate (BzMA, 98%) were purchased from TCI Development Co., Ltd. (Shanghai). Methacryloyl chloride (98%, Shanghai HATECH Co. Ltd.) was distilled under reduced pressure before use. Dimethylformamide (DMF, 99%, Shanghai HATECH Co. Ltd.) was distilled under reduced pressure, which was mixed with 0.05 mol·L–1 NaNO3 (99%, Shanghai HATECH Co. Ltd.) and then filtered on 0.2 μm polytetrafluoroethylene filters for gel permeation chromatography (GPC). Nile red (99%) was obtained from Sigma-Aldrich. All other solvents and reagents were purchased from commercial sources and were used as received.

2.2 Synthesis of PNMPm-b-PBzMAn Diblock Copolymer Dispersions

PNMPm-b-PBzMAn block copolymer dispersions were synthesized according to the following procedure (Scheme 1):38

Scheme 1 Synthetic Route of PNMPm-b-PBzMAn Block Copolymer Dispersions

The macromolecule chain-transfer reagent of PNMP (macro-CTA PNMP) was synthesized by the RAFT polymerization in ethanol at 60 °C first and then followed by a RAFT-mediated ethanol dispersion polymerization at 70 °C.

2.2.1 Synthesis of N-2-(Methacryloyloxy)ethyl Pyrrolidone (NMP) Monomer

NMP monomer was synthesized and purified by the previous reported methods,35 yield: 67%. 1H NMR (ppm, CDCl3): 6.11 and 5.60 (2H, CH2=CCH3), 4.29 (2H, COOCH2CH2), 3.6 (2H, COOCH2CH2), 3.5 (2H, NCOCH2CH2 CH2 in the pyrrolidone ring), 2.42 (2H, NCOCH2CH2CH2 in the pyrrolidone ring), 2.05 (2H, NCOCH2CH2CH2 in the pyrrolidone ring), and 1.95 (3H, CH2=CHCH3).

2.2.2 Synthesis of Macro-CTA PNMP

The synthetic route of macro-CTA PNMP was reported in detail elsewhere,35 and the representative formulation of macro-CTA PNMP39 was described as following: NMP (40.0 g, 203.04 mmol), CPDB (1.07 g, 4.84 mmol), AIBN (0.26 g, 1.58 mmol), and CH3CH2OH (80 mL) were charged in a 200 mL Schlenk flask capped with rubber septa. Subsequently, the homogeneous solution was deoxygenated by purging with highly pure nitrogen gas for 30 min and then reacted at 60 °C for 10 h under stirring in a thermostatic oil bath. The monomer conversion at 93% was determined by 1H NMR analysis employing the internal standard of 1, 3, 5-trioxane. The crude product was diluted by dichloromethane and then poured into diethyl ether to precipitate at least twice. The final product was dried in a vacuum oven at 45 °C to give a pink powder. The mean degree of polymerization was calculated using 1H NMR to be 39 by comparing the integrated aromatic proton signals of CPDB at 7.3–8.0 ppm to that of the methylene carbonyl signals at 4.06 ppm. DMF GPC analysis indicated the Mn of 7064 g·mol–1 and a Mw/Mn of 1.01 [vs a series of near-monodisperse poly(methyl methacrylate) calibration standards]. Macro-CTA PNMP55 and macro-CTA PNMP64 were synthesized by the same routes, and detailed structural information is provided in Figure S1 and Table S1.

2.2.3 Synthesis of PNMPm-b-PBzMAn Diblock Copolymer Dispersions in Ethanol

A typical synthesis of PNMP39-b-PBzMA115 diblock copolymer dispersion at the solid content of 20 wt % was described as following: macro-CTA PNMP39 (0.250 g; 0.0325 mmol), BzMA (0.658 g; 3.74 mmol; target n = 115), and AIBN (1.066 mg; 6.50 μmol; dissolved at 0.1% w/w in ethanol; CTA/AIBN molar ratio = 5.0) were dissolved in ethanol (2.748 g) in a 15 mL Schlenk flask capped with rubber septa. Subsequently, the homogeneous solution was deoxygenated by purging with highly pure nitrogen gas for 30 min and then reacted at 70 °C for 24 h under stirring in a thermostatic oil bath. The final monomer conversion was determined by 1H NMR spectroscopy analysis by integrating the PBzMA peak (CH) at 5.0–4.7 ppm to the vinyl peaks (CH2) of BzMA monomers at 5.17 ppm. The final reaction dispersion was diluted approximately 10-fold in CDCl3 and showed a conversion of BzMA monomers about 93% (Figure S2). DMF GPC analysis indicated a Mn of 24,430 g·mol–1 and Mw/Mn of 1.02. The kinetic results (Figure S3a,c) show that the conversion of BzMA monomers increases gently at the initial 5 h and then follows a rapid and linear increase process because of the formation of monomer-swollen copolymer particles during the micellar nucleation process.38 The linear evolution of molecular weight with conversion indicates a well-controlled pseudoliving RAFT polymerization (Figure S3b,d).

Other PNMPm-b-PBzMAn diblock copolymer dispersions were synthesized by the same routes at the solid content of 10, 15, 20, 25, and 30 wt %, and totally 56 different PNMPm-b-PBzMAn diblock copolymer dispersions were developed. All PNMPm-b-PBzMAn diblock copolymers were characterized by GPC and 1H NMR systematically, and the detailed molecular information is summarized in Figure S4 and Table S2.

2.3 Characterization of PNMPm-b-PBzMAn Diblock Copolymers

1H NMR spectra were recorded on a 400 MHz Bruker-BioSpin spectrometer using CDCl3. GPC measurements were performed at 35 °C using DMF (containing 0.05 M NaNO3) as the eluent at a flow rate of 1.0 mL·min–1. The column set consisted of two MZ-SD plus 5 μm columns (500 Å and linear); Wyatt Optilab DSP Interferometric refractometer and Wyatt DAWN EOS multiangle laser light scattering detector with a helium–neon laser light source (λ = 685 nm), K5-flow cell, and a broad range of scattering angle from 45 to 160° were employed. The molecular weight and polydispersity data were determined using the Wyatt ASTRA software package. The refractive index increment of the polymer solution (dn/dc) was measured using an Optilab DSP refractometer at a wavelength of 685 nm.

2.4 Preparation of PNMPm-b-PBzMAn Assembly-Stabilized Emulsions

The aqueous dispersions of PNMPm-b-PBzMAn assemblies were obtained directly by diluting the corresponding PISA solutions under stirring until homogeneous. All emulsions were prepared using an IKA Ultra-Turrax T-18 homogenizer equipped with a 10 mm dispersing tool, in which the mixture of dodecane and block copolymer micelle solution with the total volume of 2 mL was homogenized for 3 min at 25 °C. The oil phase containing Nile red and the fluorescent probe-solubilized PNMPm-b-PBzMAn aqueous dispersions were prepared according to the reported procedure.9

2.5 Characterization of PNMPm-b-PBzMAn Diblock Copolymer Assemblies

DLS was performed on a Zetasizer ZEN 3600 (Malvern, U.K.) with a 173° back scattering angle and He–Ne laser (λ = 633 nm) at 25 °C. Each dispersion was diluted by ethanol into 0.20 wt % before measurements.

TEM measurements were conducted with a JEOL JEM-100CXII transmission electron microscope at an accelerating voltage of 120 kV. Samples for TEM observation were prepared as following: the diluted dispersions (0.20 wt % in ethanol, 10.0 μL) were dropped to the carbon-coated grids, and the excess solution was blotted with filter paper after 1 min and then dried under ambient conditions.

SEM observations were performed on an FEI Zeiss Sigma SEM. The samples for SEM observations were prepared by casting a drop of diluted dispersion (0.20 wt % in ethanol, 5.0 μL) on a polished silicon wafer, followed by drying under ambient conditions, and gold was coated on the sample surface with 80 s sputtering time and 30 mA current.

Light microscopy images were performed on an Olympus BX 51 microscopy (Japan). Fluorescence microscope images of emulsion droplets were recorded with the Ultraview Vox spinning disc confocal system (PerkinElmer) equipped with a Yokogawa CSU-X1 spinning disc head and EMCCD camera (Hamamatsu C9100-13) and coupled with a Nikon Ti-E microscope. Confocal image acquisition and analysis were performed with Volocity software (PerkinElmer) according to the reported procedure.9

3 Results and Discussion

3.1 Effect of Polymerization Degree n of PBzMA Segment

Figure 1a–c shows the macro-appearance of PNMP39-b-PBzMAn diblock copolymer assemblies synthesized at the solid contents of 10, 20, and 30 wt %, respectively. Each series of samples shows a similar transition tendency, transition from optical transparency to opaque gradually via a viscous intermediate state upon increasing n, indicating the microstructural change of assemblies. For example, the dispersion of PNMP39-b-PBzMA96 (Figure 1a) can be inverted without any perturbation; however, the viscous region would be enlarged at the higher solid content. Viscosity results (Figure S5) show that the viscosity of PNMP39-b-PBzMAn dispersions with a small or large n, i.e., PNMP39-b-PBzMA45 and PNMP39-b-PBzMA180, is very low (∼1 mPa·s). In contrast, those of PNMP39-b-PBzMA85, PNMP39-b-PBzMA96, and PNMP39-b-PBzMA105 become very high, showing typical shear-thinning behaviors as non-Newton fluids. These characters of amphiphilic homologues often suggest the morphological transition of assemblies from spherical to vesicle-like via wormlike shapes.29,31

Figure 1 Appearance of PNMP39-b-PBzMAn dispersions prepared by PISA at the solid content of 10 (a), 20 (b), and 30 wt % (c) in ethanol at 70 °C, and the inset number represents the corresponding polymerization degree n of PBzMA segment. Typical size distributions of them prepared at solid contents of 10 (d), 20 (e), and 30 wt % (f).

Figure 1d–f shows the corresponding size distributions of diluted PNMP39-b-PBzMAn dispersions at different solid contents, which exhibit similarity. The radius of assemblies (Rh) is increased from tens to hundreds of nanometers upon increasing n, which is consistent with the transition from spherical to vesicle-like via wormlike assemblies mentioned above. To make clear evidence, samples were studied using the electric microscopy techniques. Figure 2 shows the representative TEM and SEM images of PNMP39-b-PBzMAn assemblies generated at 20 wt %. Spherical assemblies are formed in the PNMP39-b-PBzMA52 dispersion (Figure 2a). Alternatively, linear, branched, and even entangled wormlike assemblies of PNMP39-b-PBzMA82 become the majority (Figure 2b). For PNMP39-b-PBzMA108, both wormlike and vesicle-like assemblies are formed as observed (Figure 2c). In addition, the jellyfish assemblies (the inset image in Figure 2c) are the intermediate states of assemblies, which were widely observed in similar PISA systems during the wormlike to vesicle-like morphology transition process.31,39,40 Further increasing n to 191, mainly vesicles are visible (Figure 2d). Undoubtedly, the results confirm the gradual morphological transition of assemblies from spherical to vesicle-like via wormlike shapes.41,42

Figure 2 TEM images of PNMP39-b-PBzMA52 (a), PNMP39-b-PBzMA82 (b), and PNMP39-b-PBzMA108 (c), and SEM image of PNMP39-b-PBzMA191 (d). Inset image in (c) is the corresponding SEM image of PNMP39-b-PBzMA108.

Similar morphological transitions of assemblies also happened for PNMP39-b-PBzMAn dispersions synthesized at other solid content (Figures S6 and S7). Figure 3 illustrates the morphological dependence between the polymerization degree of the PBzMA segment and the solid content of PNMP39-b-PBzMAn diblock copolymer dispersions. Obviously, the morphologies of PNMP39-b-PBzMAn assemblies are mainly depended on n, and the solid content affects a little. The transitions of aggregates from spherical to vesicle-like assemblies via wormlike ones along with the increase of n are the major characteristic. That is to say, the increased solvophobic PBzMA segment benefits the formation of assemblies with larger size, which also follows the molecular parameter theory well.29

Figure 3 Dependence of aggregates on the polymerization degree of the PBzMA segment and the solid content in the PNMP39-b-PBzMAn diblock copolymer dispersions prepared by PISA in ethanol at 70 °C. S, W, and V represent spherical, wormlike, and vesicle-like assemblies, respectively.

We also noticed the enlarged viscous region of PNMP39-b-PBzMAn dispersions at the higher solid content (Figure 1a–c), and wormlike assemblies were often formed in these dispersions. To clarify the microstructural difference caused by the solid content, assemblies of PNMP39-b-PBzMAn block copolymers with n around 83 ± 2 formed at different solid content were studied and are shown in Figure 4. The morphology of PNMP39-b-PBzMA85 assemblies prepared at the solid content of 10 wt % is short rods with the size of about several hundred of nanometers. For the PNMP39-b-PBzMA82 dispersion synthesized at 20 wt % solid content, linear, branched, and entangled wormlike shapes are formed. Once the solid content was increased to 30 wt %, very long and entangled wormlike assemblies of PNMP39-b-PBzMA83 about several micrometers became majority. Since assemblies with different morphologies by PISA were often resulted from the initially formed spheres by agglomeration, destruction, and recombination processes, as confirmed by SAXS and TEM techniques.26,31,32 The higher solid content is certainly beneficial to the formation of wormlike assemblies with longer length. Thus, the enlarged viscous region can be attributed to the formation of wormlike assemblies with longer length.

Figure 4 TEM images of PNMP39-b-PBzMA85 (a), PNMP39-b-PBzMA82 (b), and PNMP39-b-PBzMA83 (c) prepared at 10, 20, and 30 wt % solid content, respectively.

3.2 Effect of Polymerization Degree m of PNMP Segment

Figure 5 shows the macro-appearance and size distribution of PNMP55-b-PBzMAn and PNMP64-b-PBzMAn series block copolymers, which were developed at 20 wt % solid content. Nearly no difference can be distinguished from the appearance of PNMP55-b-PBzMAn (Figure 5a) and PNMP39-b-PBzMAn (Figure 1b). Both undergo similar transitions from optical transparency to turbidity via a viscous region along with the enlarged n, and the average sizes of assemblies are increased from about 20 nm to hundreds of nanometers (Figures 1e and 5b). TEM and SEM images of PNMP55-b-PBzMAn dispersions (Figure 6) show the morphological transition from spherical to vesicle-like assemblies via wormlike ones, confirming the self-assembly similarity between PNMP55-b-PBzMAn and PNMP39-b-PBzMAn.

Figure 5 Appearance and typical size distribution of PNMP55-b-PBzMAn (a,b) and PNMP64-b-PBzMAn (c,d) diblock copolymer dispersions prepared by PISA at the solid content of 20 wt % in ethanol at 70 °C, and the inset image in (d) represents the dependence of averaged size on n.

Figure 6 TEM images of PNMP55-b-PBzMA86 (a), PNMP55-b-PBzMA127 (b), and SEM images of PNMP55-b-PBzMA141 (c) and PNMP55-b-PBzMA193 (d) diblock copolymer dispersions synthesized at the solid content of 20 wt % in ethanol at 70 °C.

However, the behaviors of PNMP64-b-PBzMAn series become significantly different so that all dispersions keep thinning states except the increased turbidity (Figure 5c). DLS results (Figure 5d) show that the average radius Rh of assemblies is about 25 nm when n is below 80, whereas it increases linearly upon further increasing n as observed in other PISA systems.32,43 TEM images of PNMP64-b-PBzMA64 (Figure 7a), PNMP64-b-PBzMA125 (Figure 7b), and PNMP64-b-PBzMA173 (Figure 7c) show that only spherical assemblies are formed. This suggests that the morphologies of PNMP64-b-PBzMAn assemblies are independent of n, and the increased turbidity is caused by the enlarged size. Interestingly, some irregular spheres such as the spindle-like (inset image in Figure 7b) and triangle (inset image in Figure 7c) ones are also distinguishable, suggesting that larger spherical assemblies should result from the fusion of initially formed ones.

Figure 7 TEM images of PNMP64-b-PBzMA64 (a), PNMP64-b-PBzMA125 (b), and PNMP64-b-PBzMA173 (c) dispersions prepared at the solid content of 20 wt % in ethanol at 70 °C. The inset images correspond to the area signed by the circle in (b,c), respectively.

The general self-assembly behaviors of PNMPm-b-PBzMAn block copolymers prepared at the constant solid content of 20 wt % can be preliminary illustrated by Figure 8 based on the relationship between m and n. The diagram presented here strongly suggests two distinct tendencies. When m is small (<55), the morphological transitions from spherical to vesicle-like assemblies via wormlike ones are happened upon increasing n. When m arrives at 64, only spherical assemblies are generated regardless of n. Similar dependences of the self-assembly behaviors on the polymerization degree of solvophilic segment are widely reported in PISA systems previously.26,31,32,43−45 The processes relate to the altered assembly manner of assemblies from kinetically controlled to thermodynamically controlled during the PISA process because the PNMP segment with larger m would increase the solubility and solvophilicity of PNMPm-b-PBzMAn block copolymers, which form spherical micelles spontaneously.38

Figure 8 Dependence of self-assemblies on the polymerization degree of PNMP and PBzMA segments prepared at the constant solid content of 20 wt % by PISA in ethanol at 70 °C. S, W, and V represent spherical, wormlike, and vesicle-like assemblies, respectively.

3.3 Emulsification of PNMPm-b-PBzMAn Assemblies

The emulsification of PNMP39-b-PBzMAn assemblies was evaluated in the dodecane/water systems, in which PNMP39-b-PBzMAn aqueous dispersions were obtained through diluting 30 wt % PISA dispersions by water. During the emulsification processes, the concentration of PNMP39-b-PBzMAn (cEmulsifier), the oil/water volume ratio (VO/VW), and the shear rate were kept constant at 0.5 wt %, 1/1, and 12,000 rpm, respectively. Generally, stable and gel-like O/W emulsions can be formed at the top phase for all PNMP39-b-PBzMAn emulsifiers. Light microscopy images show that PNMP39-b-PBzMAn-stabilized emulsions were well-distributed, and no significant difference can be distinguished with the average size of about tens of micrometers (Figure S8). The original morphologies of PNMP39-b-PBzMA47, PNMP39-b-PBzMA83, and PNMP39-b-PBzMA197 are spherical, wormlike, and vesicle-like, respectively, emulsions stabilized by them were studied further by the confocal laser scanning microscopy (CLSM) method (Figure 9a–c). Since Nile red was dissolved in dodecane before emulsification, the fluorescent and dispersed droplets evidence the formation of O/W emulsions. This also suggests that the type of emulsion is independent of the polymerization degree n of the PBzMA segment. In other words, there is no immediate relationship between the type of emulsion and the morphology of assemblies as reported previously,11 and the dominant hydrophilicity of PNMP39-b-PBzMAn assemblies might be the major cause.

Figure 9 CLSM images of emulsions stabilized by PNMP39-b-PBzMA47 (a,d), PNMP39-b-PBzMA83 (b,e), and PNMP39-b-PBzMA197 (c,f) assemblies, respectively. Bars: 100 μm.

To lighten the interfacial layer of emulsions, PNMP39-b-PBzMAn assemblies stabilized emulsions were also studied by the developed CLSM method,9 in which Nile red was solubilized in PNMP39-b-PBzMAn assemblies before emulsification (Figure 9d,e). In this case, the fluorescence mainly came from the oil/water interfacial layers instead of dispersed dodecane droplets (Figure 9a–c). The remarkable interfacial fluorescence of emulsion droplets confirms that PNMP39-b-PBzMAn assemblies must remain and participate in the emulsification, suggesting the formation of Pickering emulsions. Otherwise, the interfacial layer should be indistinguishable, along with the disassembly of PNMP39-b-PBzMAn assemblies. Figure 10 shows the normalized fluorescent intensity (IN) versus the normalized radius (r) curves of the representative emulsion droplets from no. 1 to no. 6 along the singed direction, where r and IN are the data divided by the radius of the emulsion droplet and the maximum fluorescent intensity along the signed direction, respectively.10 If Nile red was dissolved in dodecane, the fluorescent intensity of the inner oil droplet was very strong, whereas it was weakened remarkably, and the interfacial fluorescence became dominant once Nile red was alternatively solubilized in assemblies. The strong interfacial fluorescence certainly confirms the presence of PNMPm-b-PBzMAn assemblies at the oil/water interfaces of emulsions, indicating the formation of Pickering emulsions rather than normal emulsions. Thus, PNMPm-b-PBzMAn assemblies developed by PISA are excellent Pickering emulsifiers.

Figure 10 Normalized fluorescent intensity (IN) vs the normalized radius (r) curves of emulsion droplets along the signed direction.

4 Conclusions

In summary, this work developed a series of PNMPm-b-PBzMAn diblock copolymers by the RAFT dispersion polymerization method, and the corresponding PISA behaviors of them were studied in detail. Morphologies of PNMPm-b-PBzMAn assemblies were characterized systematically and established a comprehensive structure–property relationship. Accordingly, the morphological dependence of PNMPm-b-PBzMAn assemblies on the solid content and the polymerization degree of PNMP and PBzMA segments were clarified. Generally, the solid content affects the self-assembly behaviors a little, whereas the effect of the polymerization degree of the PBzMA segment is significant. In the PNMP39-b-PBzMAn and PNMP55-b-PBzMAn series, morphological transitions from spherical to vesicle-like via wormlike assemblies are commonly happened upon increasing n, resulting from the gradual fusion of initially formed spherical assemblies. Alternatively, the spherical shape became the major morphology of assemblies for PNMP64-b-PBzMAn series block copolymers because of the excellent solvophilicity of the PNMP64 segment. We also noticed the excellent emulsification of PNMPm-b-PBzMAn assemblies, which preferred to stabilize O/W Pickering emulsions as confirmed by the CLSM method, in which the polymerization degree of the PBzMA segment or the morphologies of PNMPm-b-PBzMAn assemblies is finite. Thus, PNMPm-b-PBzMAn might shed application potential in fields of drug-controlled release, the development of nano/micro-materials as a template, the enhancement of crude oil recovery, and so forth.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c09315.Molecular information and methods for the calculation of the polymerization degree m and n of macromolecule chain-transfer reagent of macro-CTA PNMPm and PNMPm-b-PBzMAn block copolymers from GPC and 1H NMR results, rheological responses and morphologies of PNMP39-b-PBzMAn series block copolymer dispersions prepared at different solid contents, and light microscopy images of emulsions stabilized by PNMP39-b-PBzMAn block copolymers (PDF)

Supplementary Material

ao3c09315_si_001.pdf

The authors declare no competing financial interest.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (NSFC 22072109 and 21773174). We are grateful to the Core Facility of Wuhan University for the electron microscopy and CLSM measurements.
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