Letter Cite This: ACS Macro Lett. 2017, 6, 1315-1319
pubs.acs.org/macroletters
Core−Shell SiO2‑PS Colloids with Controlled Eccentric Ratio Yitong Li†,‡ and Bing Liu*,† †
State Key Laboratory of Polymer Physics and Chemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100149, China S Supporting Information *
ABSTRACT: Precisely controlling microstructure of colloidal particles is crucial for their applications. Core−shell colloids have been extensively synthesized and used in past decades. However, controlling the location of cores in core−shell particles remains a challenge. To address this problem we explored the synthesis of SiO2-PS core−shell colloids by using a simple system containing only core particles, monomer, initiator, and water/ethanol and found the increase of ethanol/water ratio can induce a structure transition sequence from eccentric to concentric to eccentric to concentric to eccentric. Furthermore, we illustrate that the eccentric ratios of SiO2-PS core−shell colloids, that is, the location of SiO2 cores in the whole particles, can be precisely controlled by a two-step polymerization procedure. It is anticipated that our results can widen the application of core−shell colloids, especially after the introduction of functionality for core or shell materials.
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whole hybrid particle to the difference between the radii of the core and the whole hybrid particle, can be controlled, and core−shell particles with expected eccentric ratios can be obtained by a two-step polymerization procedure. To illustrate this concept, we selected monodisperse SiO2 nanoparticles (NPs) as the core and polystyrene as the shell, an often studied system in the literature. To grow PS shells onto SiO2 cores, we employed emulsion polymerization and dispersion polymerization techniques. Differently from previous studies,8,10,24−28 our system consisted of only SiO2 seeds, styrene (St), potassium persulfate (KPS), and solvent. Monodisperse SiO2 particles with a diameter of 405 nm (measured by TEM, polydispersity σ = 5.3%: standard deviation divided by the mean value in this paper) were first synthesized according to the standard Stöber method29 (Figure S2) and modified with 3-(trimethoxysilyl) propyl methacrylate (MPS) to introduce CC double bonds onto the surface (method 1). Fourier transform infrared spectroscopy (FT-IR) confirmed the successful modification (Figure S3a). To obtain the amount of surface double bonds, a thermal gravimetric analysis (TGA) was performed (Figure S3b,c). From the weight loss of the modified SiO2 particles, we obtained an estimated value of 2.7 double bonds per nm2. MPS-modified SiO2 was dispersed in water containing a given amount of St. The polymerization was initiated by injection of KPS aqueous solution at 75 °C. In this condition, the obtained core−shell particles show slightly eccentric structure, observed from regular TEM imaging (Figures 1a
he synthesis of core−shell colloidal particles has been advanced significantly in past decades because of their extensive applications in catalysis, adsorption, separation, energy storage and conversion, drug release, target therapy, and optics.1−15 Core−shell structures combine two types of materials or components together so that a single particle has multifunctional or special properties.16 It is worthwhile to notice that core−shell particles have been used for fabrication of matrix-free thermoplastic elastomer composites and for the solution of simultaneous optical manipulation and 3D visualization of dense colloidal suspension.17,18 The core material is generally regarded to locate at the center of the whole hybrid particle, and the shell material uniformly attaches on the core surface. In this case, core−shell colloidal particles usually have a concentric structure.2 However, new development in material sciences has focused more attention on colloids, even spheroidal colloids with new structures, such as eccentric core−shell colloids.19−22 These structures have been shown recently to serve as optical probes to study rotational dynamics of spherical colloids.23 The synthesis of both concentric and eccentric particles has been widely explored in the literature;2,4 however, precise controlling of the location of a core particle in a core−shell particle has hardly been addressed and still remains a challenge. Such controlling is useful for tailoring colloidal interactions for new assembly structures and exploring new possible applications of core−shell particles. Herein we report on the synthesis of inorganic/polymer core−shell colloidal particles and show that the location of the core particle, from the center to the edge, can be controlled by simply adding ethanol to regular solvent water. Furthermore, we show that an eccentric ratio (E), defined by the ratio of the distance between the center of the core and the center of the © XXXX American Chemical Society
Received: August 19, 2017 Accepted: November 1, 2017
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DOI: 10.1021/acsmacrolett.7b00629 ACS Macro Lett. 2017, 6, 1315−1319
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ACS Macro Letters
Figure 2. (a) Cumulative probability analysis of eccentric ratios of core−shell particles obtained in solvents with increasing ethanol/water ratios. Black curves (symbol: circle, square, and triangle) show the simulated curves with random orientational distribution with designed E values. (b) Diagram of the formed core−shell SiO2−PS structures as a function of ethanol/water ratios.
Figure 1. TEM images showing SiO2−PS core−shell particles with different eccentric ratios. They are synthesized in increasing ethanol/ water ratio (v/v): (a) 0/10.0; (b) 0.6/9.4; (c) 3.0/7.0; (d) 6.0/4.0; (e) 7.0/3.0; (f) 8.0/2.0; (g) 8.5/1.5; (h) 9.0/1.0; and (i) 9.6/0.4. Scale bars are 200 nm in (a), (c−e), (h), and (i) and 500 nm in (b) and (f− g).
with E = 0, it will be a straight line with cumulative probability value equal to 1. We assume that our particles have a distribution close to a random distribution of orientations when they were dropped onto TEM grids. Then for each sample, we measured and calculated projected E values for 50 random particles. Their cumulative probability curves were plotted in Figure 2a. From these curves, we can clearly see that the core− shell particles synthesized in water without ethanol show a wide distribution of E values. When a small amount of ethanol was added (E/W = 0.6/9.4), the eccentric degree decreased, and the distribution of E values became narrow. More ethanol resulted in an increase of eccentric degree. At E/W = 7.0/3.0, the obtained particles showed very small eccentric degree as the curve is close to the simulated curve of E = 0. When E/W > 8.5/1.5, the obtained particles showed big eccentric degree and curves close to the simulated curve of E = 1. We also noticed that at E/W = 9.4/0.6 the curve implied that the particles had uniform orientations, which was possibly because the particles became slightly nonspherical. To more clearly see the structure transition, we plotted the obtained structure as a function of ethanol/water values in Figure 2b. As shown in Figure 2b, it can be roughly divided into five regions according to ethanol/water ratios. In region A, eccentric structure with broad distribution was observed. In region B, the predominant structure was concentric. By comparing to the simulated curve of E = 0.25, we estimated an average E value less than 0.25 in this region (Figure 2a). In region C, all the formed structures were eccentric again, and they have bigger eccentric degree than those in region A. In region D, the formed structures had a good concentric degree with an estimated E value of about 0.1. In region E, the particles were eccentric, and the eccentric degree was greater than those in region A and region C. Next we tracked the evolution of structures with polymerization time for regions B, C, D, and E by following one E/W ratio in each region. The results are summarized in Figure S5. In regions B and C, PS first formed an almost homogeneous
and S4a). We found the addition of ethanol has a big effect on the formed core−shell structure. When the added ethanol was only a small amount (ethanol/water, E/W = 0.6/9.4) (Figures 1b and S4b), more concentric core−shell particles were observed. In comparison, a similar system with the presence of sodium dodecyl benzenesulfonate (SDBS) leads to more eccentric core−shell structure.28 When E/W increased to 3.0/ 7.0, the obtained core−shell structure predominantly became eccentric (Figures 1c and S4c). This change stopped at the E/ W value of 6.0/4.0 (Figures 1c, 1d, and S4c−S4e). It was noticed that the eccentric degrees decrease with increasing ethanol. Interestingly, when E/W reached 7.0/3.0, the formed core−shell structures became concentric again (Figures 1e, 1f, S4f, and S4g). In this case, the core−shell particles show good uniformity. With further increasing of E/W (>8.5/1.5), the formed core−shell structures became eccentric again (Figures 1g−1i and S4h−S4j). The eccentric degrees again increase with increasing ethanol/water values. The boundary between eccentric and concentric for high ethanol content is between 8.0/2.0 and 8.5/1.5. In nonaqueous ethanol we did not obtain core−shell particles; it was possibly due to low solubility of KPS in ethanol. To accurately obtain an E value for each particle is difficult due to two main reasons. One is that only two-dimensional (2D) projection of particles can be obtained by regular TEM measurement, and another is that particles may take different orientations on TEM grids. In principle, only those particles which oriented parallel to the image plane can be used for accurate evaluation of E values. If a sample has a distribution of E values, this method will encounter a problem. Instead, we analyzed the eccentric degree of core−shell particles with a method based on cumulative probability. For particles with E = 1, a random distribution on orientations results in a cumulative probability curve as shown in black in Figure 2a. For particles 1316
DOI: 10.1021/acsmacrolett.7b00629 ACS Macro Lett. 2017, 6, 1315−1319
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ACS Macro Letters thin layer on SiO2 within a short time (
