Adsorption of Silica Nanoparticles onto Poly(N-vinylpyrrolidone

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Adsorption of Silica Nanoparticles onto Poly(Nvinylpyrrolidone)-functionalized Polystyrene Latex Hua Zou, and Xia Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03977 • Publication Date (Web): 23 Jan 2017 Downloaded from http://pubs.acs.org on January 24, 2017

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Adsorption of Silica Nanoparticles onto Poly(N-vinylpyrrolidone)-functionalized Polystyrene Latex

Hua Zou,* Xia Wang*

School of Materials Science and Engineering, University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai 200093, China * Corresponding authors. Tel.: +86 21 55270632; fax: +86 21 55270632. E-mail addresses: [email protected], [email protected].

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ABSTRACT This paper presents a more general method to prepare silica-coated polystyrene (PS) particles minimal excess silica by adsorption in highlighting the role of poly(N-vinylpyrrolidone) (PVP). The method is based on the addition of small silica nanoparticles onto submicrometer-sized near-monodisperse polymer latex particles under the circumstances of monolayer silica coverage of the latex surface. Either cationic or anionic initiator could be used in the PVP-involved emulsion polymerization to prepare PS particles, and the adsorption was conducted successfully at either acidic or basic condition. Neither cationic initiator nor basic condition is prerequisite for the adsorption process, which should be related to the much stronger interaction between PVP and silica surface. This method is expected to substantially extend the adsorption conditions of the polymer-silica colloidal nanocomposite syntheses.

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1. INTRODUCTION Since the early 1990s, colloidal polymer-inorganic nanocomposite particles have been extensively studied by both academic and industrial researchers since such colloidal nanocomposite particles have a wide range of applications, such as high-performance coatings.1-8 The syntheses of the colloidal nanocomposites typically involve in situ heterophase polymerization, and more specially, by emulsion polymerization of various vinyl monomers in the presence of silica nanoparticles with tiny size, resulting a core-shell morphology with polymer core and silica shells. However, there is often a problem for this in situ polymerization route, i.e., relatively poor silica incorporation efficiency, which means many free silica nanoparticles exists besides of the desired polymer-silica nanocomposite particles. The excess silica nanoparticles are always unfavorable to the characterizations and applications of the resulting nanocomposite particles.

To address this problem, several protocols to polymer-silica colloidal nanocomposite particles with high silica incorporation efficiency by in situ polymerization route have been developed.9-15 For example, Tiarks et al. prepared polymer dispersions by miniemulsion polymerization of vinyl monomers such as styrene, butyl acrylate and methyl methacrylate in the presence of silica nanoparticles,9 in which almost all silica nanoparticles were incorporated into the nanocomposite particles due to the particular polymerization mechanism. However, wide applications of the miniemulsion polymerization method are always restricted by the facilities required. Another example

with

almost

100%

silica

incorporation

efficiency

was

poly(2-vinylpyridine)(P2VP)-silica colloidal nanocomposite particles with P2VP cores and silica shells. These nanocomposite particles were prepared by emulsion polymerization of 2-vinylpyridine at 60 oC in the presence of 20 nm silica nanoparticles using a cationic initiator.10 The main drawback of this formulation involves that costly and highly odorous 2-vinylpyridine is used as the monomer. A further example with up to 95% silica incorporation efficiency was polystyrene-silica 3


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nanocomposite particles.12 These nanocomposite particles were also prepared by emulsion polymerization at 60 oC in the presence of 20 nm silica nanoparticles using a cationic initiator. However, silica nanoparticles modified with hydrophilic glycerol groups were used here, which added to the cost.

Alternatively, colloidal polymer-inorganic nanocomposites can be prepared by physical methods.16 In this regard, “heterocoagulation” and “heteroflocculation” are often used to describe the mixed particulate systems. Strictly speaking, “heterocoagulation” is generally used to describe irreversible contact through electrostatic

interactions

between

particles

with

opposite

charges;

while

“heteroflocculation” is an association between particles involving weaker reversible adsorption.17 However, it seems the expressions of “heterocoagulation” are very trendy in literature with the definition not strictly followed. In the context of polymer-silica nanocomposite particles, Bleier and Matijević reported the heterocoagulation of 255 nm poly(vinyl chloride) latex particles and 13.5 nm silica nanoparticles in 1978.18 Luna-Xavier et al. investigated the heterocoagulation of positively charged, 33 nm poly(methyl methacrylate) nanoparticles onto negatively charged, 90 nm silica particles.19 More recently, Balmer et al. reported that polymer-silica nanocomposite particles with minimal free silica nanoparticles could be prepared by physical adsorption (heteroflocculation) of 20 nm silica nanoparticles onto larger poly(ethylene glycol) methacrylate (PEGMA)-stabilized P2VP latex particles at approximately pH 10 under monolayer coverage conditions.20-23 The amount of silica nanoparticles required to form core-shell particles with little or no free silica could be calculated.20 It is believed the presence of PEGMA is crucial for the heteroflocculation, as the PEGMA chains located on the surface of P2VP particles by chemical grafting can interact with the silica nanoparticles. Similarly, PEGMA-stabilized polystyrene (PS)-silica core-shell nanocomposite particles were also prepared by heteroflocculation.22 Cationic or neutral initiator but not anionic initiator could be used to synthesize the PS latex particles. 4


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Figure 1. Scheme of the formation of PS-silica nanocomposite particles by adsorption of silica nanoparticles onto large PVP-functionalized polymer latex particles.

In the current work, we report a more general approach to prepare core-shell polystyrene-silica nanocomposite particles with minimal free silica nanoparticles by adsorption of silica onto large PS particles. PS was selected as a ‘model’ polymer since it can be readily synthesized with high monodispersity at different sizes,24 which is important for accurate calculation of the number of silica particles for monolayer silica coverage of the polymer particle surface. In addition, the morphology of this rigid polymer can be readily observed by scanning electron microscopy. Different from previous work,22 the PS latexes were stabilized by poly(N-vinylpyrrolidone) (PVP), whose role in the adsorption process is highlighted. The PVP-functionalized PS latex particles could be synthesized with either cationic or anionic initiator, and the adsorption experiments could be conducted at either acidic or basic condition (see Figure 1). This method is expected to substantially extend the adsorption conditions of the colloidal nanocomposite syntheses.

2. EXPERIMENTAL SECTION 2.1 Materials. Styrene,

2,2'-azobis(2-methylpropionamidine)

dihydrochloride

(AIBA)

and

ammonium persulfate (APS) were purchased from Aldrich. Styrene was purified with a basic column filled with alumina and the two initiators were used as received. PVP (K29, average Mw = 58 kDa) from Fluka was used as a non-ionic polymeric stabilizer. 5


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The 20 nm silica nanoparticles (Bindzil 2040, 41.63 wt % by test) were provided by AkzoNobel in the form of an aqueous sol. 1 M aqueous NaOH and HCl solutions were used to adjust the pH value. Milli-Q water was used in all experiments.

2.2 Synthesis and Purification of PVP-functionalized PS Latex Particles. The near-monodisperse, PVP-functionalized PS particles were prepared by emulsion polymerization as follows:25,26 PVP (1.0 g) were dissolved in H2O (80 g) in a 250-ml three-necked flask equipped with a mechanical stirring apparatus (Eurostar 20 digital, IKA), a nitrogen gas inlet and a condenser, followed by addition of 10.0 g of styrene with high-speed stirring (500 rpm). After stirring with nitrogen gas bubbling for at least 30 min at 200 rpm, the mixture was heated to 70 °C. The polymerization was initiated by injection a solution of 100 mg of AIBA or APS solution in 10 g of H2O and continued for 24 h.

To remove residual styrene and free PVP, the resulting PS latex was purified by centrifugation at 13000 rpm for 30 min. The supernatants were decanted with care and the precipitates were redispersed with deionized water. The procedures were conducted for five times.

2.3 Dynamic Light Scattering (DLS) The particle sizes and size distributions were measured by DLS using a Zetasizer Nano ZS (Malvern, UK) at 25 oC instrument equipped with a He-Ne solid-state laser (633 nm). All data were reported by the Malvern software.

2.4 Adsorption of Silica sol onto PS Latex. Silica sol with proper weight was added to a known weight of dilute dispersion of purified PS latex particles with known solid content. The weight of aqueous silica sol was determined based on a well-established relationship between the number of silica nanoparticles per polymer core and the silica packing efficiency (P = 69 %).20 For 6


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example, 0.083 g of aqueous silica sol (41.63 wt %, containing 3.80×1015 silica nanoparticles) was added to 10 g of the 399 nm diluted PS latex dispersion (1.222 wt %, containing 3.14×1012 silica nanoparticles). KOH or HCl was used to pre-adjust the pH of the polymer dispersion to 10 or 5 prior to addition. The mixture was vigorously agitated with a vortex mixer (IKA, Germany) at 2000 rpm for 30 s followed by shaking on a roller mixer for at least 1 h.

2.5 Aqueous Electrophoresis Aqueous electrophoresis measurements were conducted on a Zetasizer Nano ZS (Malvern, UK). HCl or NaOH was used to change the pH to desired value.

2.6 Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). SEM pictures were recorded on a FEI Inspect F microscope. The dried samples were sputter-coated with gold before observation. The morphology of the particles was also evidenced by TEM on a Philips CM100 microscopy with sample loaded on a copper grid.

2.7 X-ray Photoelectron Spectroscopy (XPS). The resulting colloidal nanocomposite particles were purified by repeated centrifugation-redispersion cycles (5,000 rpm for 30 min) with the supernatants being decanted and the precipitates being redispersed with deionized water. The procedures were repeated (typically eight cycles) until all free silica nanoparticles were removed, as checked by TEM. The purified nanocomposite particles and also the purified PS particles were dried under vacuum and then subjected to XPS study using a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic Al X-ray source (10.0 mA, 15 kV).

3. RESULTS AND DISCUSSION 7


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3.1 Characterization of PVP-functionalized PS Particles.

Figure 2. SEM pictures of PVP-functionalized PS particles synthesized with AIBA initiator (PVP-AIBA-PS particles) at (a) low and (b) high magnifications, and PVP-functionalized PS particles synthesized with APS initiator (PVP-APS-PS particles) at (c) low and (d) high magnifications.

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Figure 3. XPS survey spectrum of (a) PVP-AIBA-PS particles and (b) corresponding PS-silica nanocomposite particles prepared at pH 10.

Following well-established procedures,25,26 near-monodisperse PS latexes were first synthesized by emulsion polymerization with cationic AIBA initiator and PVP (denoted as PVP-AIBA-PS particles, see Figure 2a and 2b). DLS study on the purified particles suggested the mean diameter was 399 nm. PVP is a frequently-used amphiphilic nonionic polymer.27,28 It is well-documented that PVP acted as a stabilizer by a steric mechanism during the polymerization process, generating 9


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PVP-functionalized polymer particles with hydrophilic PVP on their surfaces.25,29,30

In this current work, the PVP stabilizer located on the polymer particle surfaces was examined by XPS study. This is evidenced by the peak of the O 1s arising only from the PVP chains, as shown in Figure 3a. Furthermore, the PVP coverage percentage on the PVP-AIBA-PS particle surfaces can be estimated with the O% result by the following equation:31 % surface PVP = (% surface O)×100/(% O of the PVP)

(1)

where the XPS % O of the PVP (C6NO) is 15.7%, and the % surface O was obtained from the XPS data (9.0%). Thus, the % surface PVP on the PVP-AIBA-PS particles was calculated to be 53.4%.

Similarly, near-monodisperse 352 nm PS particles (denoted as PVP-APS-PS particles, see Figure 2c and 2d) were synthesized by emulsion polymerization with an anionic APS initiator and PVP.

Table 1 summarizes the hydrodynamic particle diameter and zeta potential data obtained for PVP-AIBA-PS particles and PVP-APS-PS particles under different pH conditions.

Table 1. Summary of Hydrodynamic Particle Diameter and Zeta Potential Data Obtained for PVP-AIBA-PS, PVP-APS-PS and PS-Silica Nanocomposite Particles Under Different pH Conditions. particles

hydrodynamic

PDI

particle diameter (nm)

zeta

potential zeta

potential

at pH 5

at pH 10

PVP-AIBA-PS

399

0.03

24.2

-1.8

PVP-APS-PS

352

0.03

-29.8

-35.9

SiO2

20

0.18

-22.2

-51.7

10


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PS-SiO2a

455

0.06

-15.1

-22.7

PS-SiO2b

403

0.07

-24.8

-43.6

a

prepared at pH 10 based on PVP-AIBA-PS.

b

prepared at pH 10 based on PVP-APS-PS.

3.2 Estimation for the Number of Silica Nanoparticles per PS Core. In order to optimize the amount of silica nanoparticles needed to form nanocomposite particles with minimal free silica, it was necessary to calculate the number of silica nanoparticles per polymer particle (N), which is expressed as follows:32

N=

m1 ρ2 r23 m2 ρ1r13

(2)

where m1, ρ1 and r1 are the mass, density and radius of the silica nanoparticles, respectively, and m2, ρ2 and r2 are those of the PS particles, respectively.

As mentioned earlier, Armes et al.20 has established a relationship between N and the packing efficiency (P), as shown below: P=

Nr12

(3)

4(r2 + r1 )2

For monolayer silica coverage, a P value of 69% was estimated based on experimental results. In this study, to make P = 69%, N was calculated to be 1211 for 399 nm PVP-AIBA-PS particles, and N was calculated to be 955 for 352 nm PVP-APS-PS particles.

3.3 Adsorption Based on PS particles Prepared with AIBA Initiator. The synthesis of nanocomposite particles with aqueous silica sols as the starting materials is preferentially carried out at pH 9-10, simply because silica is highly anionic and stable at this pH range.12 As for heteroflocculation, P2VP-silica and PS-silica composite particles have been reported to be readily prepared by adding silica sol to AIBA-initiated PEGMA-stabilized P2VP and PS particles at pH 10, 11


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respectively.22

In this work, nanocomposite particles were first prepared by mixing PVP-AIBA-PS particles with silica sols at pH 10. SEM was used as a direct tool to identify the formation of core-shell nanocomposite particle. Obviously, highly monodisperse PS-silica nanocomposite particles were formed (see Figure 4a, Figure 4b and Table 1). It should be mentioned that the nanocomposite particles showed better monodispersity than those obtained through in situ polymerization due to no interruption of silica in reaction media.33 Furthermore, the appearance is comparable to that obtained by in situ polymerization.34

Figure 4. SEM pictures of PS-silica nanocomposite particles prepared at different pH values based on PVP-AIBA-PS particles. (a) pH 10 at low magnification and (b) pH 10 at high magnification, (c) pH 5 at low magnification and (d) pH 5 at high 12


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magnification.

As is well-documented,20,35 both the silica and the PVP-AIBA-PS latex are anionic at pH 10 (see Table 1), indicating that the formation of nanocomposite is not due to electrostatic driving force, but due to the interaction between the PVP and silica. This is reasonable because adsorption between PVP and silica in aqueous solution readily took place by considering the polymer thermodynamics.15,36,37

The solution pH is important for adsorption because it affects the surface characteristics of both the PS particles and silica nanoparticles. In this regard, it is reported that P2VP-silica particles prepared by adsorption at pH 5 were less well-defined as compared with the case of pH 10, which may be ascribed to the poor stability of silica nanoparticles at low pH.20 However, there is also a report on stable poly(methyl methacrylate) particles coated with silica nanoparticles at pH 5.5.38 The SEM pictures of the nanocomposite particles prepared by mixing PVP-AIBA-PS particles with silica sols at pH 5 are shown in Figure 4c and 4d. Clearly, PS-silica nanocomposite particles were also obtained in this case. At this pH, the silica nanoparticles are still negatively charged but the latex is positively charged (see Table 1), indicating that besides interaction between the PVP and silica, electrostatic driving force may provide additional contribution to the adsorption.

3.4 Adsorption Based on PS Particles Prepared with APS Initiator.

13


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Figure 5. SEM pictures of PS-silica nanocomposite particles prepared at pH 10 based on PVP-APS-PS particles at (a) low magnification and (b) high magnification.

Figure 6. SEM pictures of PS-silica nanocomposite particles prepared at pH 5 based on PVP-APS-PS particles at (a) low magnification and (b) high magnification.

It is well known that the type of initiator has a crucial effect on the physical nature of the polymer latex produced in dispersion or emulsion polymerization. At both pH 10 and pH 5, PS particles prepared with APS initiator exhibited negative zeta potentials (see Table 1) resulting from the strong anionic SO42- groups,35 while the silica sols also exhibited anionic surface charge.20 It was reported that the formation of nanocomposite particles were unsuccessful by mixing silica with an anionic 4,4'-azobis(4-cyanovaleric acid) (ACVA)-initiated PEGMA-stabilized PS latex at pH 14


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9-10,22 presumably because the mutual electrostatic repulsion between PS and silica was larger than the interaction between PEGMA and silica. However, in this work, when PVP instead of PEGMA was used as the stabilizer, nanocomposite particles were prepared by mixing the PVP-APS-PS latex and silica at either pH 10 or pH 5 (see Figure 5 and Figure 6, respectively). It should be noted that both ACVA and APS are anionic initiators; ethanol-soluble ACVA is suitable for dispersion polymerization and water-soluble APS is often used in emulsion polymerization. This clearly demonstrates the crucial role of PVP in the nanocomposite particles formation.

It is noteworthy that there were seemingly few partially naked PS particles observed for the PS-silica nanocomposite particles prepared at pH 10 based on PVP-APS-PS particles, while almost all PS particles were coated by silica nanoparticles at pH 5. This should be explained by the fact that the electrostatic repulsion between PS and silica is larger at higher pH value (since both of them become more negative at pH 10, as shown in Table 1), which weakens the interaction between PVP and silica. In spite of this, this work is meaningful because it substantially broadens the adsorption conditions of such colloidal nanocomposite syntheses.

Given the differences resulting from PEGMA and PVP, it is necessary to examine the characters of the two compounds. As a macromonomer widely used in emulsion or dispersion polymerization,39,40 PEGMA is actually a derivative of poly(ethylene oxide) (PEO). For reference, the adsorption between PEO/PVP and silica nanoparticles has been well comparatively studied as model systems in literature, providing useful information on the roles of PEGMA and PVP in the adsorption process. It is widely proved that PVP interacts much more strongly with silica surface than PEO.41-45 For PEGMA, hydrogen bonds between its ethylene oxide units and the SiOH groups on the silica surface are responsible for the adsorption.46 For PVP, strong hydrogen bonds form between its carbonyl groups and the SiOH groups.36 Furthermore, the adsorption between the two polymers and silica nanoparticles could be quantitatively 15


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presented by the net segmental adsorption energy. The values were determined to be 4 kBT for PVP47,48 and

~0.5 kBT for PEO, respectively, indicating a much stronger

interaction between PEO and silica42,43,49 In addition, Pattanaik and Bhaumik proposed an acid-base adsorption mechanism to understand the interaction between PVP and silica surfaces,50 in which silica is considered as a Bronsted acid due to its SiOH groups and PVP is considered as a Lewis base due to its negatively charged carbonyl group formed by resonance within the PVP ring. Therefore, the stronger interaction between PVP and silica is best explained by the fact that ionic bonding is normally stronger than hydrogen bonding.44

3.5 Characterization of the PS-Silica Nanocomposite Particles.

16


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Figure 7. TEM pictures of (a) PVP-AIBA-PS particles and (b) corresponding PS-silica nanocomposite particles prepared at pH 10, and (c) PVP-APS-PS particles and (d) corresponding PS-silica nanocomposite particles prepared at pH 10.

To further examine the morphology of the nanocomposite particles, the PVP-AIBA-PS particles and corresponding PS-silica nanocomposite particles prepared at pH 10 were observed by TEM. Comparing the TEM pictures in Figure 7a and 7b indicates that the silica nanoparticles were located on the surface of the latex with few free silica nanoparticles existing, which corresponded to a near-monolayer silica coverage. Thus, time-consuming centrifugations are unnecessary to a great extent. Meanwhile, Figure 7c and 7d show the TEM pictures of the PVP-APS-PS particles and corresponding PS-silica nanocomposite particles prepared at pH 10, respectively. It seems some nanocomposite particles were not fully coated, which is in agreement with the SEM observations. In this case, more free silica nanoparticles may exist. One reviewer suggested that the observed particle morphology may not reflect reality, i.e., TEM study could indicate PS core and SiO2 nanoparticle shell morphology even though the SiO2 particles were not adsorbed onto the PS particle surface, since the free SiO2 particles dispersed in continuous aqueous media tend to attach onto the PS particles due to capillary force during drying process on TEM grid. To clarify this point, the PS-silica nanocomposite particles based on PVP-AIBA-PS particles after purification by centrifugal washing were also checked by TEM. As shown in Figure S1, the silica nanoparticles were indeed located on the surface of the PS particles after all free silica nanoparticles had been removed. Meanwhile, the existence of some free SiO2 nanoparticles in Figure 7d indicated that the effect of the capillary force during drying process of the TEM sample was not that strong.

The PS-silica nanocomposite particles were further characterized with an emphasis on these prepared at pH 10 based on PVP-AIBA-PS, which had a well-defined core-shell morphology. The diameter of the composite particles increased from 399 nm to 455 17


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nm after adsorption, as determined by DLS. Surface characterizations of the nanocomposite particles were conducted by both aqueous electrophoresis and XPS. As shown in Table 1, the PS-silica composite particles had negative zeta potentials at both pH 5 and 10, suggesting a silica-rich surface.12 XPS is well-suited to assess the elemental compositions of the particle surface. In this study, C signal and Si signal were used as markers for polymer and silica, respectively.50 As shown in Figure 3b, the presence of silica on the composite particle surface was evidenced from the peak at 100 ev (Si 2p), and the presence of polymer component on the particle surface was evidenced by the C 1s signal. This indicates that the packing of the silica nanoparticles was imperfect, i.e., there were gaps between the silica particles. This is similar to the case of heteroflocculation between PEGMA-stabilized polymer particles and silica.20 By comparing the % Si of the PS-silica nanocomposite particles (25.0%) and that of the silica nanoparticles (37.9%)22 obtained from XPS analysis (note that only semi-quantitative information can be provided), a silica surface coverage of 66.0% could be roughly calculated, which is closed to the theoretical value.

4. CONCLUSIONS In this work, a more general preparative approach to silica-coated PS nanocomposite particles with minimal free silica nanoparticles by adsorption is presented, with stressing the importance of PVP. Submicrometer-sized, near-monodisperse PS latexes were prepared by emulsion polymerization using either cationic or anionic initiator in the presence of PVP first, and then adsorption of 20 nm silica nanoparticles onto the PVP-functionalized PS latexes was conducted at either acidic or basic condition. PS-silica core-shell nanocomposite particles were formed successfully in each case. Thus cationic initiator is unnecessary for the polymerization process, and basic conditions is unnecessary for the adsorption process, providing PVP is used as the polymerization stabilizer, which has a much stronger interaction with silica surface than PEO. Therefore, the adsorption conditions of such colloidal nanocomposite syntheses could be substantially extended. Considering that PS presented here is a 18


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model polymer, this method should be readily applicable to other vinyl polymers.

Acknowledgments This authors thank Shanghai Municipal Education Commission (Program of Young Eastern Scholar from Shanghai Institutions of Higher Learning, QD2015014), Science and Technology Commission of Shanghai Municipality (Shanghai

Pujiang

Program, 15PJ1406400) and National Natural Science Foundation of China (51503123) for funding of this work. The University of Shanghai for Science and Technology is also thanked for providing financial support.

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Figure 1. Scheme of the formation of PS-silica nanocomposite particles by adsorption of silica nanoparticles onto large PVP-functionalized polymer latex particles. 254x85mm (96 x 96 DPI)


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Figure 2a. SEM picture of original PVP-functionalized PS particles prepared with AIBA initiator (PVP-AIBA-PS particles) at low magnification. 276x238mm (72 x 72 DPI)


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Figure 2b. SEM picture of original PVP-functionalized PS particles prepared with AIBA initiator (PVP-AIBA-PS particles) at high magnification. 276x238mm (72 x 72 DPI)


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Figure 2c. SEM picture of PVP-functionalized PS particles synthesized with APS initiator (PVP-APS-PS particles) at low magnification. 276x238mm (72 x 72 DPI)


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Figure 2d. SEM picture of PVP-functionalized PS particles synthesized with APS initiator (PVP-APS-PS particles) at high magnification. 276x238mm (72 x 72 DPI)


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Figure 3a. XPS survey spectrum of PVP-AIBA-PS particles. 289x203mm (150 x 150 DPI)


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Figure 3b. XPS survey spectrum PS-silica nanocomposite particles prepared at pH 10 based on PVP-AIBA-PS particles. 289x203mm (150 x 150 DPI)


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Figure 4a. SEM picture of PS-silica nanocomposite particles prepared at pH 10 based on PVP-AIBA-PS particles at low magnification. 276x238mm (72 x 72 DPI)


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Figure 4b. SEM picture of PS-silica nanocomposite particles prepared at pH 10 based on PVP-AIBA-PS particles at high magnification. 276x238mm (72 x 72 DPI)


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Figure 4c. SEM picture of PS-silica nanocomposite particles prepared at pH 5 based on PVP-AIBA-PS particles at low magnification. 276x238mm (72 x 72 DPI)


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Figure 4d. SEM picture of PS-silica nanocomposite particles prepared at pH 5 based on PVP-AIBA-PS particles at high magnification. 276x238mm (72 x 72 DPI)


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Figure 5a. SEM picture of PS-silica nanocomposite particles prepared at pH 10 based on PVP-APS-PS particles at low magnification. 276x238mm (72 x 72 DPI)


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Figure 5b. SEM picture of PS-silica nanocomposite particles prepared at pH 10 based on PVP-APS-PS particles at high magnification. 276x238mm (72 x 72 DPI)


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Figure 6a. SEM picture of PS-silica nanocomposite particles prepared at pH 5 based on PVP-APS-PS particles at low magnification. 276x238mm (72 x 72 DPI)


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Figure 6b. SEM picture of PS-silica nanocomposite particles prepared at pH 10 based on PVP-APS-PS particles at high magnification. 276x238mm (72 x 72 DPI)


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Figure 7a. TEM picture of PVP-AIBA-PS particles. 300x300mm (72 x 72 DPI)


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Figure 7b. TEM picture of PS-silica nanocomposite particles prepared at pH 10 based on PVP-AIBA-PS particles. 300x300mm (72 x 72 DPI)


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Figure 7c. TEM picture of PVP-APS-PS particles. 300x300mm (72 x 72 DPI)


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Figure 7d. TEM picture of PS-silica nanocomposite particles prepared at pH 10 based on PVP-APS-PS particles. 300x300mm (72 x 72 DPI)


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Table of Contents Graphic 254x85mm (96 x 96 DPI)


Adsorption of Silica Nanoparticles onto Poly(N-vinylpyrrolidone

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