In-situ monitoring of silica shell growth on PS-b-P4VP micelles as templates using DLS

In-situ monitoring of silica shell growth on PS-b-P4VP micelles as templates using DLS

Polymer xxx (2016) 1e7 Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer In-situ monitoring of si...

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Polymer xxx (2016) 1e7

Contents lists available at ScienceDirect

Polymer journal homepage: www.elsevier.com/locate/polymer

In-situ monitoring of silica shell growth on PS-b-P4VP micelles as templates using DLS Andriy Horechyy a, *, Bhanu Nandan b, Aruni Shajkumar a, Petr Formanek a, Jarosław Paturej a, c, Manfred Stamm a, d, *, Andreas Fery a, d, * a

Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Strasse 6, Dresden 01069, Germany Department of Textile Technology, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Institute of Physics, University of Szczecin, Wielkopolska 15, 70451 Szczecin, Poland d €t Dresden, Physical Chemistry of Polymer Materials, Dresden 01062, Germany Technische Universita b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 May 2016 Received in revised form 25 July 2016 Accepted 27 July 2016 Available online xxx

Polystyrene-block-poly(4-vinylpyrdine) (PS-b-P4VP) block copolymer (BCP) was used as a template for the preparation of [email protected] core-shell particles using an acid catalyzed sol-gel process. The process of silica shell formation and development of shell morphology were studied using a combination of in-situ dynamic light scattering (DLS) and transmission electron microscopy (TEM). The results obtained reveal that shell formation and growth principally involve the following stages: (I) sol assembly around BCP micelles; (II) hydrolysis-condensation reaction accelerated by the protonated P4VP corona of BCP micelles; (III) shell densification; and (IV) shell growth. Present work provides insight into the solgel process which takes place in systems containing “reactive” templates, such as protonated PS-b-P4VP micelles, and discloses the mechanism and pathways of silica shell formation. We demonstrate that the whole process can be effectively monitored in-situ using conventional DLS. The results are of significant importance for fabrication of targeted core-shell nanostructures. © 2016 Elsevier Ltd. All rights reserved.

Keywords: In-situ DLS Sol-gel Core-shell particles Block copolymer micelles PS-b-P4VP

1. Introduction Among various templates, self-assembled block copolymer (BCP) structures are often used for the synthesis of novel functional nanomaterials and provide many benefits [1e5]. Having their periodicities in nanometer scale, these structures have been used for the fabrication of various “smart” materials, such as membranes for selective separation and purification, vesicles for drug delivery, controlled release materials, porous catalysts or proton exchange membranes, etc [6]. The degree of polymerization (N), volume fraction of the constituting blocks (4) and Flory-Huggins interaction parameter (c) determine the periodicity and morphology of self-assembled structures at equilibrium. Alteration of experimental conditions, for instance, solvent selectivity, also influence characteristics of self-assembled structures [7,8]. These selfassembled structures can be stabilized, e.g. by cross-linking of

* Corresponding authors. Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Strasse 6, Dresden 01069, Germany. E-mail addresses: [email protected] (A. Horechyy), [email protected] (M. Stamm), [email protected] (A. Fery).

one BCP component, and then disintegrated using an appropriately selected solvent to produce isolated hairy particles [9,10]. If the difference in solubility of BCP constituents in given solvent is large enough, it is possible to obtain isolated hairy particles using the socalled selective solvent approach, which does not require any additional stabilization (cross-linking). Through an appropriate selection of experimental conditions and precursors, multicomponent materials or multifunctional particles with controlled localization of functionalities can be prepared [5,6,11,12]. Polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymers are often used for the preparation of various functional nanostructures. Due to the reactive nature of pyridine units, P4VPbased block copolymers can be complexed with small molecules, metal precursors, or directly loaded with nanoparticles [13e16]. The lone pair of electrons renders the 4VP unit a strong ligand able to coordinate with electron-deficient species, such as transition metals [17]. The ability of 4VP units to undergo hydrogen bonding is also well known and has been widely exploited in various supramolecular systems. In addition, in protonated or quaternized state 4VP units can electrostatically interact with charged molecules, particles and substrates [18]. At the same time, comparably high c

http://dx.doi.org/10.1016/j.polymer.2016.07.080 0032-3861/© 2016 Elsevier Ltd. All rights reserved.

Please cite this article in press as: A. Horechyy, et al., In-situ monitoring of silica shell growth on PS-b-P4VP micelles as templates using DLS, Polymer (2016), http://dx.doi.org/10.1016/j.polymer.2016.07.080

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A. Horechyy et al. / Polymer xxx (2016) 1e7

value for PS/P4VP pair [19] allows disassembly of the matrix forming components with selective solvent without affecting the shape of domains formed by the minority component. The feasibility of using non-crosslinked PS-b-P4VP hairy particles as templates was demonstrated in our previous works. In particular, we have shown that PS-b-P4VP hairy particles, either cylindrically or spherically shaped, can be coated with silica or titania shell and further converted into hollow structures by oxidative pyrolysis [20,21]. Experimental work on preparation of core-shell type particles via a sol-gel process on P4VP-based polymeric templates was also carried out by other research groups in the past [22e25]. Liang and co-workers used symmetric PS-b-P4VP for the fabrication of PS-bP4VP/SiO2 core-shell particles by acid catalyzed sol-gel synthesis [22]. Depending on whether selective or non-selective solvent was used as reaction medium, spherical particles having silica in the core or coated with a silica shell were obtained. In the case of PSselective solvent (THF), silica deposition took place within the protonated P4VP core of the micelles. In the case of non-selective solvent (DMF), micellization of PS-b-P4VP was induced upon addition of aqueous oxalic acid followed by silica shell formation atop of PS-b-P4VP micelles. Another example of using PS-b-P4VP micellar templates for the fabrication of various nanoobjects, such as gold or titania nanodots and nanowires, was recently reported by Cho and co-workers [23]. The authors used THF/ethanol and THF/ water solvent mixtures of different ratios to obtain PS-b-P4VP micellar structures of various morphologies. Different aggregation morphologies, such as cylindrical and spherical micelles (PS core), vesicular structures, or reverse spherical micelles (P4VP core) were obtained, depending on the type of added solvent (ethanol or water) and solvent ratio. Apart from block copolymer micelles, spherical microparticles prepared from pyridine-based random copolymers or P4VP homopolymer were also used as templates for silica shell deposition. Zhang and co-workers used P4VP microspheres for fabrication of hierarchically structured core-shell particles under acidic conditions, which were then converted into hollow silica microspheres [24]. In other work, the same group reported on the preparation of polymer/silica hybrid particles using PS-co-P4VP microspheres as templates [25]. There are several advantages of template-assisted sol-gel processes over conventional acid or base catalyzed processes. In particular, by using templates various hybrid particles with predetermined size, shape and shell thickness can be prepared relatively easily. The core-shell particles can be further converted into hollow structures or yolk-shell type particles by pyrolytic removal or by selective etching [26]. Regarding “reactive” templates, those can be considered as such having building blocks, functional groups or other structural components which are chemically involved in sol-gel process. As will be discussed further, PS-b-P4VP micelles represent such a “reactive” case because of the localized catalytic effects of the protonated pyridine units. Although existing reports describe the structural and morphological aspects of templatederived core-shell particles, to the best of our knowledge, there are no studies on kinetics and mechanism of the sol-gel process which occurs in the presence of “reactive” templates, like PS-bP4VP micelles or P4VP based particles. It is well known that kinetics and mechanism of acid- or basecatalyzed sol-gel reactions depend on the temperature, pH, type of catalyst (acid/base), and composition of the reaction medium [27e29]. At a pH close to the isoelectric point of silica, the rate of the condensation reaction is very slow and the gelation time is high. Gelation time increases with an increase of precursor/solvent (alcohol) ratio. The variation of gelation time with respect to different precursor/water ratios is non-monotonic with a minimum which, in turn, depends on the precursor/solvent ratio and pH.

Thus, changes in pH and/or composition of reaction medium affect the kinetics and mechanism of the sol-gel process and, subsequently, may alter properties of the formed silica particles. Such changes, which may locally occur in the vicinity of PS-b-P4VP micelles upon addition of acidic silica sol, would alter the kinetics of the sol-gel reaction as compared to the rest of the solution. A thorough understanding of the sol-gel reaction taking place in the presence of “reactive” templates is necessary for the synthesis of core-shell nanostructures with pre-defined characteristics and properties. In the present work, we combined in-situ DLS and transmission electron microscopy to investigate the sol-gel process carried out in the presence of PS-b-P4VP micelles and to understand the mechanism and pathways of silica shell formation. 2. Experimental 2.1. Materials PS-b-P4VP block copolymer with Mn ¼ 18500 g/mol and Mn ¼ 40500 g/mol for PS and P4VP, respectively, and a polydispersity index of 1.10, was purchased from Polymer Source (Canada). The calculated volume fractions of the PS and P4VP blocks were 0.3 and 0.7 respectively. Hydrochloric acid (37 wt%) was obtained from Merck. Tetraethyl orthosilicate (TEOS) was obtained from Sigma Aldrich. The solvents (chloroform, absolute ethanol, and methanol, analytical grade) were purchased from Acros Organics. Prior to use all solvents were filtered through a 0.2 mm pore size PTFE membrane filter to remove particulate impurities. Carbon coated TEM grids (300 mesh) were purchased from Plano GmbH (Germany). Highly polished single-crystal silicon wafers of {100} orientation were used as substrates. Silicon wafers were cleaned with dichloromethane in an ultrasonic bath for 20 min and then further in a 1:1:1 v/v mixture of 29% ammonium hydroxide (Acros), 30% hydrogen peroxide (Merck) and deionized water for 1.5 h at 65  C, rinsed several times with water and finally dried under argon flow. 2.2. Characterization SEM images were obtained using Neon40 FIB-SEM workstation (Carl Zeiss Microscopy GmbH, Germany) operated at 3 kV. The samples prepared on silicon substrates were viewed under the SEM without any additional coating. Tapping mode AFM imaging was performed using a Dimension 3100 Scanning force microscope (Digital Instruments, Inc., Santa Barbara, CA) using silicon cantilevers with a resonance frequency of 60e70 kHz and a tip radius of ca. 10 nm. Conventional and energy filtered TEM images (EFTEM) were obtained using Libra120 transmission electron microscope (Carl Zeiss Microscopy GmbH, Germany) equipped with an Omegatype energy filter and operated at 120 kV. EFTEM imaging was performed by 3-windows method using 20 eV window width. For elemental mapping, K-ionization edges of N and C, and L23-ionization edge of Si were used. Specimens for TEM imaging were prepared on the carbon coated copper grids by drop casting method followed by immediate rinsing with absolute ethanol. Dynamic light scattering (DLS) measurements were performed at 25  C using Zetasizer Nano S (ZEN 1600, NIBS Technology, Malvern Instruments, UK) equipped with 4 mW He-Ne-laser (632.8 nm, scattering angle 173 ). 2.3. Synthesis of [email protected] core-shell particles via sol-gel PS-b-P4VP block copolymer was dissolved in chloroform to give a homogeneous solution with BCP concentration of 0.5 mg/mL and filtered through a 0.2 mm pore size PTFE membrane filter. An

Please cite this article in press as: A. Horechyy, et al., In-situ monitoring of silica shell growth on PS-b-P4VP micelles as templates using DLS, Polymer (2016), http://dx.doi.org/10.1016/j.polymer.2016.07.080

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equivalent volume of methanol was added dropwise to the above solution and the mixture was stirred for 1 h. Next, both solvents were slowly removed on a rotary evaporator until constant mass was attained. The solid residue was re-dispersed in methanol and kept under reflux for 24 h to obtain PS-b-P4VP micellar solution. The final concentration of PS-b-P4VP micelles in methanol was 0.01 wt %. If required, the BCP concentration was adjusted by adding methanol (minor weight loss during reflux). Silica sol was prepared separately by mixing 1.2 mL of TEOS with 1.25 mL of methanol, 1.5 mL of deionized water and 0.75 mL of 0.2 M aqueous HCl. The mixture was stirred for 30 min at room temperature and filtered through a 0.2 mm pore size PTFE membrane filter before mixing with the PS-b-P4VP micellar solution. Deposition of silica shell on PS-b-P4VP micelles was done by mixing 1 g of BCP micellar solution with 227 mL of freshly prepared silica sol. was added and intensively mixed for 20 s. 2.4. In-situ DLS monitoring of silica shell formation The whole procedure was carried out in disposable DLS cuvettes without stirring. After adding of silica sol to the PS-b-P4VP micelles, the reaction mixture was intensively mixed for 20 s. The cuvette was then placed in Z-sizer and continuous DLS measurements were started. The interval between successive measurements was 2 min, whereas the time interval between addition of silica sol and the first DLS record was approximately 3.5 min which is the time required for mixing of components and temperature equilibration. The refractive indices and viscosity were set as specified for methanol (n ¼ 1.326, h ¼ 0.5476 cP) and polystyrene latex (n ¼ 1.590). For each measurement point, ten autocorrelation functions of 10 s data collection time per scan were averaged and evaluated by the Dispersion Technology Software (DTS) appendant to Zetasizer Nano S. DTS includes cumulant analysis and multimodal size distribution algorithm NNLS, which have been used for the calculation of hydrodynamic particle size, PDI and particle size distributions. 3. Results and discussion It is obvious that properties of core-shell particles largely depend on the characteristics of the shell, such as thickness and morphology, its compactness and density. Thus, monitoring of shell formation during reaction using a simple and widely accessible analytical technique will be very helpful and informative for predicting and controlling the final properties of core-shell particles. DLS is obviously one of the most widely accessible techniques for measuring particle size. The main focus of the present work was to monitor silica shell formation using acidic silica sol as a precursor and block copolymer micelles as the reactive template using this technique. We used asymmetric PS-b-P4VP block copolymer with PS block as minority component. In P4VP-selective solvents, such as methanol or ethanol, this BCP forms micelles composed of collapsed PS core and swollen P4VP corona. Previously, we have shown that cylindrical micelles of PS-b-P4VP block copolymer can be used as templates for the deposition of SiO2 or TiO2 shell via an acidic solgel process [20,21]. Upon heating (or by long-time aging) cylindrical BCP micelles are transformed into spherical ones. The latter can also be coated with a shell giving uniform [email protected] core-shell particles. Representative SEM, TEM, AFM topography images and corresponding height profile of [email protected] particles isolated from the reaction mixture are shown in Fig. 1. Initially, we tried to monitor the changes in [email protected] particle size with DLS by collecting them from the reaction mixture at different time

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intervals during silica shell deposition. However, most of the DLS results obtained from such isolated and further re-dispersed particles differ from each other. In fact, when [email protected] coreshell particles were centrifuged and then re-dispersed again, a substantial increase of polydispersity index (PDI) and broadening of size distribution peaks were often detected. This is due to the presence of particle clusters and aggregates which are formed during the centrifugation step when particles are compactly packed. The presence of such aggregates (as displayed in Fig. 1) hindered DLS monitoring of particle size evolution. Subsequently, we combined in-situ DLS experiments with electron microscopy imaging in order to overcome the difficulties described above. As templates, we used spherical PS-b-P4VP micelles, which are suitable for monitoring the process with DLS. It should be noted that due to the presence of several components in the reaction mixture (water, TEOS, and products of hydrolysis and condensation) the particle size obtained from DLS experiments should be considered as apparently measured size, which differs from the actual hydrodynamic particle diameter (except those referred for PS-b-P4VP micelles). Fig. 2a shows the apparent size, dapp, (measured by intensity) and PDI of PS-b-P4VP micelles and [email protected] particles as a function of time before (red) and after addition of acidic silica sol (black). The initial size of PS-b-P4VP micelles was 126.7 ± 1.7 nm (measured by intensity) and PDI of 0.083 ± 0.017. Immediately after addition of acidic sol dapp steeply increased up to 155 nm and then gradually decreased, reaching a plateau after ca. 1.5 h after addition of sol. The particle size evolution was monitored further for a period of several days. The results of long-time behavior of dapp are shown in Fig. 2b. As can be seen, after a gradual decrease, until the plateau is established, the apparent particle size increases, but with a considerably slower rate as compared to the previous stage. Notably, the PDI remained within the same range (between 0.05 and 0.10) before and after silica sol addition, as well as after prolonged reaction time. Moreover, all time-dependent DLS experiments reveal a mono-modal particle size distribution during the whole course of the DLS measurements, indicating that no clustering or particle aggregation occurs (Fig. 3). The evolution of apparent particle size with time, as shown in Fig. 2, suggests that several simultaneous processes occur in the system. In what follows, we will discuss the observed changes in particle size in more detail. There are several reasons which may induce the initial steep increase of apparent particle size upon addition of acidic silica sol. First of all, due to the protonation of pyridine units and electrostatic repulsion, the neighboring P4VP chains of the micellar corona are expected to stretch as compared to their initial state in methanol. In their protonated form, P4VP chains become more hydrophilic. The protonated P4VP chains cause additional solvation of micellar corona, particularly with water present in the reaction mixture [30,31]. Additional solvation and possible enrichment with water subsequently lead to an increased apparent particle size. Fig. 4 shows (a) TEM image and (b-d) the EFTEM elemental maps of silicon, carbon, and nitrogen performed on [email protected] core-shell particles deposited from the reaction mixture immediately after addition of silica sol. As it can be seen, at the early stage of shell formation, particles are surrounded by a silica-rich irregularly shaped corona. When particles are closely located with respect to each other, coronas merge suggesting their deformable (soft) nature. The EFTEM results explain the initial steep increase of particle size after addition of silica sol as observed by DLS measurements. Upon mixing with PS-b-P4VP micelles, the silica sol accumulates immediately around protonated P4VP corona. Micelles surrounded

Please cite this article in press as: A. Horechyy, et al., In-situ monitoring of silica shell growth on PS-b-P4VP micelles as templates using DLS, Polymer (2016), http://dx.doi.org/10.1016/j.polymer.2016.07.080

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Fig. 1. (a) Overview SEM, (b) close view TEM, and (c) topography AFM images of [email protected] particles isolated from the reaction mixture after 24 h of shell formation; (d) height profile of [email protected] core-shell particles across the line on the AFM image (c).

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Fig. 2. (a) Apparent particle size, dapp, (rectangles) and PDI (triangles) of PS-b-P4VP micelles in methanol (red symbols) and [email protected] particles (black symbols) measured by intensity and PDI (triangles) as a function of time. The results were obtained by in-situ DLS during the initial period of silica shell formation; (b) Long-time behavior of dapp (rectangles) and PDI (triangles) for [email protected] particles obtained by DLS during continuous sol-gel process. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

by sol will diffuse slower as compared to BCP micelles, which subsequently leads to an increased apparent particle size. There are also several other parameters which influence the measured apparent particle size. The presence of ions and the total ionic strength of the medium are known factors affecting the particle diffusivity by changing the thickness of the electric double layer. Low ionic strength medium produces an extended double layer of ions around particles, reduces diffusion and results in a larger apparent hydrodynamic particle size. Higher ionic strength compresses the electrical double layer and hence, reduces the apparent hydrodynamic particle size. Finally, the presence of water, TEOS, and products of its hydrolysis and condensation affect the viscosity and refractive index of the reaction medium, which subsequently influences apparently measured particle size. Currently, we are carrying out systematic experiments to understand the impact of above parameters. These studies are beyond the scope of the present work and will be discussed elsewhere. After the initial steep increase, the apparent particle size gradually decreases and then reaches a plateau. During this stage, template-directed hydrolysis-condensation of silica precursor determines the hydrodynamic behavior of particles. Protonated and, plausibly, water-enriched P4VP coronas act as multiple catalytic sites for hydrolysis-condensation reaction. Under these conditions, condensation and subsequent networking of hydrolyzed precursor should be more probable. In other words, in the vicinity of PS-bP4VP micelles, sol-gel reaction occurs much faster as compared to the rest of solution. These assumptions were further supported by TEM images shown in Fig. 5 aed. A substantial difference in particle morphology

at the beginning of the shell formation (Fig. 5 a, b) and after reaching the plateau (Fig. 5 c, d) is clearly observed. Thus, considering TEM results, the gradual decrease of particle size observed by DLS can be explained as a result of locally accelerated condensation of sol and formation of a network-like silica layer around BCP micelles. The latter, so far loose in nature, acts as a cross-linker and reduces the conformational entropy of P4VP chains. In addition, solvent molecules which initially occupy the P4VP corona are gradually replaced by condensing silica (entropically favorable). Both effects increase the diffusion rate and subsequently reduce the apparently measured particle size. Although the presence of the shell becomes quite well visible at the plateau, particles retain their irregular shape (Fig. 5 d) indicating that at this stage the condensation process is still not complete. As was expected, the increase of reaction time induces densification of the shell: in the following 15 h particles adopt a well-defined core-shell structure, as demonstrated in Fig. 5 e, f. Within this period, particles are slightly reduced in size (faintly seen but still detectable in TEM images - compare the appearance of particles in Fig. 5c and e). Notably, at this stage, the hydrodynamic behavior of core-shell particles shows an opposite trend. After reaching the plateau, continuous slow increase of the apparent particle size was revealed by DLS (see Fig. 2b). We suppose that such a monotonic and slow increase of hydrodynamic particle size might be a result of the gradual change in properties of the reaction medium itself. Continuing hydrolysis-condensation of remaining TEOS and already hydrolyzed species leads to the decrease in viscosity and, subsequently, to the increase of apparently measured particle size. When reaction time was increased further, the silica

Please cite this article in press as: A. Horechyy, et al., In-situ monitoring of silica shell growth on PS-b-P4VP micelles as templates using DLS, Polymer (2016), http://dx.doi.org/10.1016/j.polymer.2016.07.080

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Fig. 4. (a) Bright field TEM image and (ced) elemental maps of [email protected] core-shell particles deposited from the reaction mixture on TEM grid immediately after addition of silica sol: (b)eSi map, (c)ecarbon map, (d)enitrogen map.

Please cite this article in press as: A. Horechyy, et al., In-situ monitoring of silica shell growth on PS-b-P4VP micelles as templates using DLS, Polymer (2016), http://dx.doi.org/10.1016/j.polymer.2016.07.080

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Fig. 5. Overview (top) and high-magnification TEM images (bottom) TEM images of [email protected] core-shell particles deposited from the reaction mixture on TEM grid (a, b) immediately after addition of silica sol, and (c, d) after 3 h, (e, f) after 18 h, and (g, h) after 2 days of silica shell formation.

shell becomes smooth and more uniform (Fig. 5 g, h). Moreover, during this time period the shell continued to grow, as can be concluded from the appearance of an additional outer layer (Fig. 5 h). As can be seen, the changes in the particle size monitored by DLS and observed by TEM show an opposite trend. According to TEM, particles adopt the largest size at the reaction stage which corresponds to the plateau region, that is when the [email protected] particles have the smallest hydrodynamic size. In opposite, after reaching plateau particles appear visually smaller, while apparent hydrodynamic particle size increases. This contradiction can be explained as a result of two complementary effects: (a) an increased electron density of the SiO2-rich but still loosely networked micellar corona leads to the visually larger particles (TEM); (b) networking and densification of the shell reduce the apparent hydrodynamic particle size (DLS). After reaching the plateau, the hydrodynamic particle size increases mainly due to the changes in the environment (reduction of viscosity), whereas actual increase of the particle size because of the shell growth is negligibly small. The whole process of silica shell formation is schematically shown in Fig. 6. In the presence of “reactive” PS-b-P4VP micelles, formation and growth of the shell principally involve the following

stages: (I) sol assembly around BCP micelles; (II) hydrolysiscondensation reaction accelerated by the protonated P4VP corona; (III) shell densification; and (IV) shell growth. The whole process of silica shell formation can be monitored in-situ using conventional DLS. Though the results were shown and discussed above disclose solely the time evolution of the shell formation, they already provide distinctive and important information regarding the pathway, stages and possible mechanism of the sol-gel process which takes place in the presence of “reactive” templates, such as PS-b-P4VP micelles. The template method also provides a direct way for the encapsulation of various “active” species, such as quantum dots or catalytically active nanoparticles, and fabrication of so-called yolk-shell particles. The reactive template method might be also valuable for other materials prepared by the sol-gel process, especially when the control over the particle size and shape is challenging [32,33]. We belive, that obtained results will be important for producing core-shell and hollow particles with predetermined properties. By quenching the reaction at a particular stage it should be possible to alter and tune the structure and properties of the shell, such as shell thickness and porosity, pore size distribution, shell permeability and selectivity. Other parameters, such as pH, temperature, the amount and composition of

Fig. 6. Schematics depicting four stages of silica shell formation using acidic sol-gel process and spherical PS-b-P4VP micelles as reactive templates. More details are given in the text.

Please cite this article in press as: A. Horechyy, et al., In-situ monitoring of silica shell growth on PS-b-P4VP micelles as templates using DLS, Polymer (2016), http://dx.doi.org/10.1016/j.polymer.2016.07.080

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added sol, the type of solvent are also expected to have an influence on the shell formation. These experiments are currently in progress and will be published elsewhere. 4. Conclusions The growth of silica shell around spherical PS-b-P4VP micelles acting as “reactive” templates was investigated using in-situ DLS and TEM. Our results suggest that the whole process of silica shell formation passes through several well-defined stages. The first stage involves the preferential accumulation of sol around the micellar corona. This is accompanied by a steep increase of hydrodynamic particle size. In the next stage, the protonated P4VP chains accelerate the hydrolysis-condensation reaction of silica precursor accumulated around BCP micelles and formation of loose silica network. Subsequently, condensation and densification of the silica network occurs, which results in the formation of particles with distinct core-shell morphology. In the latter stage, the silica shell grows further. The whole process can be monitored in-situ using conventional DLS. The present elucidation of the mechanism of silica shell growth will be of significant value for the fabrication of core-shell, yolk-shell and hollow particles with targeted shell characteristic. Acknowledgements This research was supported by Deutsche Forschungsgemeinschaft (Projects STA324/1-1 and HO5526/1-1) and a grant from Department of Science and Technology, India (SB/S1/PC-016/2013). References [1] S. Park, J.Y. Wang, B. Kim, T.P. Russell, Nano Lett. 8 (2008) 1667e1672. [2] S. Vignolini, N.A. Yufa, P.S. Cunha, S. Guldin, I. Rushkin, M. Stefik, K. Hur, U. Wiesner, J.J. Baumberg, U. Steiner, Adv. Mater. 24 (2012) OP23eOP27. [3] B. Sarkar, P. Alexandridis, Prog. Polym. Sci. 40 (2015) 33e62. [4] N.C. Bigall, B. Nandan, E.B. Gowd, A. Horechyy, A. Eychmüller, ACS Appl. Mater. Interfaces 7 (2015) 12559e12569.

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Please cite this article in press as: A. Horechyy, et al., In-situ monitoring of silica shell growth on PS-b-P4VP micelles as templates using DLS, Polymer (2016), http://dx.doi.org/10.1016/j.polymer.2016.07.080

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