Self-Assembly Behavior of Hairy Colloidal Particles with Different

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Self-assembly behavior of hairy colloidal particles with different architectures: mixed vs. Janus Alina Kirillova, Georgi Stoychev, Leonid Ionov, and Alla Synytska Langmuir, Just Accepted Manuscript • DOI: 10.1021/la503455h • Publication Date (Web): 08 Oct 2014 Downloaded from http://pubs.acs.org on October 10, 2014

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Self-assembly behavior of hairy colloidal particles with different architectures: mixed vs. Janus A. Kirillova1,2, G. Stoychev1,2 , L. Ionov1,2 and A. Synytska1,2* 1

2

Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Str. 6, 01069 Dresden, Germany

Technische Universität Dresden, Physical Chemistry of Polymer Materials, 01062 Dresden, Germany

KEYWORDS: Self-assembly, Janus particles, supracolloidal, anisotropic, multifunctional

Abstract. In this paper we investigated the aggregation and assembly behavior of hairy coreshell particles with different architectures consisting of a hard silica core and soft polymer brush shells. We varied the nature of the polymers which form the shell: we used different hydrophilic positively

(poly(2-(dimethylamino)ethyl

methacrylate)

-

PDMAEMA)

and

negatively

(polyacrylic acid - PAA) charged polymers, uncharged hydrophilic (polyethylene glycol - PEG) polymers, and hydrophobic (poly(laurylmethacrylate) - PLMA, polystyrene - PS) polymers. We synthesized particles covered by polymer of one sort (homogeneously coated particles) as well as Janus particles (two polymers are grafted to the opposite sides of the core) and investigated/compared the aggregation behavior of different fully covered particles, their mixtures, and Janus particles.

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Introduction. Self-assembly is the basic way of building complex structures in nature.1 There is a number of self-assembling systems such as a lipid bilayer, DNA double helix, proteins with secondary and ternary structure, etc.2-4 Self-assembly occurs on any scale from singular atoms and molecules to molecular aggregates, colloidal particles, and larger systems such as groups of animals, solar systems, and galaxies.4-12 Self-assembly of colloidal particles is a particularly fascinating topic because they are located at the border between nano- and macro-worlds. With the help of colloidal particles one can transfer rules of self-assembly on the nanoscale level for the design of macroscopic structures. A variety of new building blocks of different shapes, compositions, patterns and functionalities is now available for the creation of new superstructures.13 In fields such as crystal engineering14 and the design of porous materials15,16 such superstructures of specially designed building blocks could be of great interest. Very interesting from experimental and theoretical points of view is the self-assembly of asymmetric colloidal particles such as Janus particles.17-20 Janus particles (named after the twofaced Roman god) have attracted significant attention due to their multifunctionality and resemblance to the molecular amphiphiles, such as surfactants, phospholipids, and block copolymers. Due to their unique anisotropy, Janus particles have found their applications in different fields, including stabilization of emulsions,18,21 chemical catalysis,22 drug delivery,23 display technology,24 etc. Considerable advances in the field of Janus particle self-assembly have been made by Granick and co-workers.25,26,27,28 In his work, Granick has used micrometer-sized Janus and multiblock colloidal particles with different patch sizes to induce their assembly and study its mechanism. This was achieved by employing such interparticle interactions as hydrophobic attraction and


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electrostatic repulsion.25,26,27,28 Janus spheres were selectively decorated with hydrophobic domains, usually by depositing a thin metal film onto a particle monolayer25 or through sequential glancing angle deposition.27 It was shown that, for instance, triblock Janus spheres could be induced to self-assemble into a colloidal kagome lattice.27 In another example Janus spheres with hemispherical hydrophobic attraction formed kinetically favored isomers, which fused into fibrillar triple helices.25 In addition, some theoretical studies have been performed on the examples of Janus particle self-assembly.29,30 Recently, H. Shin and K. S. Schweizer have proposed a study of the rich crystalline phase behavior of amphiphilic Janus colloids using a new formulation of the self-consistent phonon theory.30 However, all these examples refer to the so-called “bare” colloidal particles, and there are very few studies devoted to the investigation of “hairy” Janus particles – particles with grafted polymer shells.31, 32 Here, we investigate and discuss the self-assembly of hairy Janus particles decorated with different functional polymers, such as hydrophobic and hydrophilic, charged and uncharged, positively and negatively charged ones.

EXPERIMENTAL SECTION Materials: Tetraethylorthosilicate (TEOS, Fluka, 99%), ammonia solution (NH4OH, Acros, 28-30% solution), ethanol abs. (EtOH, VWR, 99.9%), 3-aminopropyltriethoxysilane (APS, ABCR, 97%), α-bromoisobutyryl bromide (Aldrich, 98%), α-bromoisobutyric acid (BiBA, Aldrich, 98%), anhydrous dichloromethane (Fluka), triethylamine (Fluka), fluorescein o-acrylate (FA, Aldrich, 97%), copper(II) bromide (Aldrich, 99.999%), tin(II) 2-ethylhexanoate (Aldrich, 95%),

tris(2-pyridylmethyl)amine

(TPMA,

Aldrich,

98%),

N,N,N′,N′′,N′′-

Pentamethyldiethylenetriamine (PMDTA, Aldrich, 99%), anhydrous N,N-dimethylformamide


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(DMF, Aldrich, 99.8%), ethyl α-bromoisobutyrate (EBiB, Aldrich, 98%), toluene (Aldrich, 99.8%), acetone (Aldrich, 99.5%), chloroform (Aldrich, 99.8%), hydrochloric acid (Aldrich, 36.5-38.0%), methanesulfonic acid (Aldrich, 99.5%), diethyl ether (Aldrich, 99.7%), paraffin wax (mp 53-57 °C, Aldrich), N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC, Aldrich), N-hydroxysuccinimide (NHS, Aldrich), hexane (Aldrich, 95%), potassium chloride (Aldrich), and sodium hydroxide (Aldrich) were used as received.

Poly(tert-butyl

acrylate), poly(lauryl methacrylate) and polystyrene were purchased from Polymer Source and used without further purification. 2-(dimethylamino)ethyl methacrylate (DMAEMA, Aldrich, 98%), poly(ethylene glycol) methyl ether methacrylate (PEGMA, Mn: 475, Aldrich), tert-butyl acrylate (tBA, Aldrich, 98%), styrene (Aldrich, 99%), and lauryl methacrylate (LMA, Aldrich) were filtered prior to the polymerization through basic, neutral and acidic aluminium oxides.

Fluorescence microscopy: Fluorescence images were obtained using an Axio Imager.A1m microscope with a 100× objective (Carl Zeiss, Göttingen, Germany) equipped with a Hg lamp. For data acquisition a standard FITC (excitation: HQ 480/40; dichroic: Q 505 LP; emission: HQ 535/50, Chroma Technology) filter set in conjunction with a Photometrics Cascade II: 512 camera (Visitron Systems GmbH) and a MetaMorph imaging system (Universal Imaging, Downingtown, PA) were used. The images were statistically treated by counting the clusters consisting of a different number of particles. Overall, an area of more than 68890 µm2 was examined for each sample. The thickness of the dispersion layer under the cover slip was 62 µm. Since the size of the particles was 800 nm, they were not limited in their movement, and only very big aggregates could be pinned on the surface of the glass.


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Scanning Electron Microscopy (SEM): All scanning electron microscopy (SEM) images were acquired on a NEON 40 EsB Crossbeam scanning electron microscope from Carl Zeiss NTS GmbH, operating at 3 kV in the secondary electron (SE) mode. In order to enhance electron density contrast, samples were coated with platinum (3.5 nm) using a Leica EM SCD500 sputter coater.

Transmission Electron Microscopy (TEM) and Cryo-TEM: Transmission electron microscopy (TEM) images and cryogenic TEM images were taken with a Libra 120 cryo-TEM from Carl Zeiss NTS GmbH equipped with a LaB6 source. The acceleration voltage was 120kV and the energy filter with an energy window of 15 eV was used. The 200 nm JP samples for cryo-TEM were prepared as follows. First, PDMAEMA was stained (quaternized) with CH3I to enhance the contrast in TEM. Next, the JPs were dispersed in water (0.5 mg/ml) by ultrasonication for 20 minutes. Prior to the analysis 3.5 µl of the sample were taken, blotted and vitrified in liquid ethane at -178°C. Ultimately, an approx. 200 nm thick ice film was examined in the TEM. CryoTEM was used for the estimation of the thickness of polymer brushes. Statistical analysis of different particles (approx. 30 particles) resulted in a standard deviation of 3 nm for the PAA polymer and 7 nm for the PDMAEMA polymer.

Electrokinetic

Measurements:

The

pH-dependent

electrokinetic

measurements

(via

electrophoresis) of the particles in dispersion were carried out with a Zetasizer Nano ZS from Malvern Instruments Ltd. and an MPT-2 autotitrator. For all measurements, the particles were suspended in a solution of 10-3M KCl in water (10 mg/24 ml or 0.42 mg/ml). The pH of the


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prepared suspensions was controlled by adding 0.1 M KOH or HCl aqueous solutions. Three measurements were recorded for each sample at each pH value.

Synthesis of Monodisperse SiO2 particles: 800 nm sized silica particles were synthesized using a multistep hydrolysis-condensation procedure of TEOS in ammonia hydroxide-ethanol solution based on Stöber approach.33 In brief, TEOS was added sequentially via syringe into a mixture of ethanol and ammonia solution in a polypropylene bottle. The particles produced within one step of synthesis were used as seeds for the next step. Each reaction was carried out by stirring the mixture at 500 rpm overnight at room temperature (starting from the last addition of TEOS). Subsequently, the dispersion with particles of the desired size was separated by centrifugation to yield monodisperse silica spheres. Purified particles were dried in a vacuum oven under reduced pressure at 60 °C.

Synthesis of Bicomponent Polymeric Janus Particles (JPs): Preparation of colloidosomes was done by a wax-water Pickering emulsion approach described elsewhere.34,35 Colloidosomes were prepared with 200 and 800 nm APS-modified silica spheres. For the immobilization of the ATRP-initiator, 355 mg EDC dissolved in 2.5 ml water and 208 mg α-bromoisobutyric acid and 18 mg NHS dissolved in 2.5 ml water were cooled down to 1°C and then combined. The initiator suspension was added to a flask containing 800 mg colloidosomes in 30 ml water cooled down to 1°C. The reaction was carried out for 22 hours under stirring at 1°C temperature. Afterwards, the modified colloidosomes were filtered off, washed with pure water to remove unattached particles and dried in a vacuum oven at 25 °C overnight. Ultimately, wax was dissolved in hexane and initiator-covered particles were used for polymerization.


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Grafting

of

PDMAEMA

and

PEGMA

Using

Surface-Initiated

ATRP:

Poly(2-

(dimethylamino)ethyl methacrylate) (PDMAEMA) was grafted on the initiator-modified particles as follows: 6 mg TPMA dissolved in 2 ml anhydrous DMF, 38 µl CuBr2 (0.1 M solution in DMF), 0.15 µl EBiB, 3 ml DMAEMA and 2 wt% fluorescein o-acrylate were added to a test tube containing initiator-modified silica particles (500 mg). After the tube was sealed with a rubber septum and purged with argon, 100 µl Sn (II) 2-ethylhexanoate in 1 ml DMF were injected. The polymerization was carried out under continuous stirring at 70 °C in a water bath for 120 minutes. Further, particles with the grafted polymer were washed by centrifugation in DMF and ethanol 8 times and dried under vacuum at 60 °C. A similar procedure was used for grafting of poly(ethylene glycol) methacrylate (PEGMA) onto the initiator-modified particles. In short, 3 ml ethanol, 3 ml PEGMA 475, 60 µl PMDTA (0.5 M solution in DMF), 60 µl CuBr2 (0.1 M solution in DMF), 2 wt% fluorescein o-acrylate and 0.15 µl EBiB were added to the particles. The mixture was sonicated and purged with Ar, followed by the injection of 200 µl of ascorbic acid (1 M solution in DMF). The polymerization was performed under continuous stirring at room temperature for 24 hours. Further, particles with the grafted polymer were washed by centrifugation in ethanol 8 times and dried under vacuum at 60 °C.

Grafting of Carboxy-Terminated PtBA, PLMA and PS Using “Grafting to” Approach: The “grafting to” approach was utilized to graft the second polymer on JPs modified by the first polymer (PDMAEMA or PEGMA).

Silica particles with the grafted first polymer were

dispersed in 20 mL of 1 wt% solution of either carboxy-terminated poly(tert-butyl methacrylate) (PtBA), poly(lauryl methacrylate) (PLMA) or polystyrene (PS) and stirred for 2 h. Next, the solvent was evaporated and the particles were annealed at 150 °C overnight. The ungrafted


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polymer was removed by multiple cycles of redispersing of particles in an appropriate solvent and subsequent centrifugation. The PtBA on the PDMAEMA/PtBA Janus particles was hydrolyzed with methanesulfonic acid to yield polyacrylic acid (PAA) and, as a result, PDMAEMA/PAA Janus particles.

Fully covered PDMAEMA, PAA, PEG, PS, PLMA particles: Particles fully covered with PDMAEMA, PAA, PEG, PS or PLMA were prepared using silica particles fully coated with the ATRP-Initiator. The grafting of PDMAEMA, PtBA, PEG, PS or PLMA onto the initiatormodified particles was done using the surface-initiated ATRP procedure, as described above. PtBA was then hydrolyzed with methanesulfonic acid to yield PAA.

By maintaining the same conditions during the synthesis of JPs (e.g. conditions for ATRP), reproducible results are obtained, for example, concerning particle size or the polymer brush thickness. The IEP for particles obtained in different batches varies only up to pH ± 0.3 that indicates a good reproducibility of the synthesis.

Self-assembly experiments. In order to observe the self-assembly behavior of bicomponent Janus particles, different sets of dispersions were prepared. All of the dispersions had a concentration of 1 mg/ml. Oppositely-charged polyelectrolyte-modified JPs (PDMAEMA/PAAJP) were dispersed either in water or in a salt solution (5 mM KCl). The pH value was adjusted to 2, 6 and 9, respectively, with the help of hydrochloric acid and sodium hydroxide. Reference particles, fully covered with either PDMAEMA or PAA (PDMAEMA-FC and PAA-FC respectively), were dispersed in analogous media. Amphiphilic JPs (PEG/PLMA-JP) were


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dispersed either in water or in toluene. Charged/hydrophobic JPs (PDMAEMA/PLMA-JP and PDMAEMA/PS-JP) were dispersed in water, salt solution (5 mM KCl) and toluene. Reference particles, fully covered with PEG, PLMA, PS or PDMAEMA (PEG-FC, PLMA-FC, PS-FC and PDMAEMA-FC respectively), were dispersed in water and toluene. The prepared dispersions were ultrasonicated for 10 minutes prior to microscopy investigations. The self-assembly behavior of Janus and homogeneously covered particles was explored in dispersions by means of fluorescence microscopy and in dry state using scanning electron microscopy (SEM). The number of clusters used to obtain the graphs depends strongly on the investigated system. In case of large aggregates (for example, PDMAEMA/PAA-JP at pH 6), a total of 80 clusters could be found in 10 fluorescence microscopy images. In case of intermediate aggregation (PEG/PLMA-JP, PDMAEMA/PLMA-JP, PDMAEMA/PS-JP), typically 200-500 clusters were counted. In case of single particle prevalence (for example, PEG-FC in H2O), up to 1200 particles and particle clusters were counted.

RESULTS AND DISCUSSION Characterization of Janus particles. Four kinds of Janus particles (JPs) with different properties were synthesized by sequential grafting of polymers on each side of core particles using a recently developed approach (Table 1).34 The first system are oppositely charged JPs covered on one side by poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA, positively charged, hydrophilic) and on the other side by polyacrylic acid (PAA, negatively charged, hydrophilic). The second kind of JP system is amphiphilic, the particles are covered by polyethylene glycol (PEG, hydrophilic) and poly(lauryl methacrylate) (PLMA, hydrophobic).


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Third and fourth kinds of JPs are also amphiphilic, covered by hydrophilic and charged PDMAEMA and a hydrophobic polymer which is either soft PLMA or hard polystyrene (PS).

The prepared Janus particles were thoroughly characterized by means of SEM, TEM and electrokinetic measurements (Table 1). SEM clearly shows that the Janus ratio of the prepared particles is 1:2, i.e. 1/3 of the surface is covered by a polymer grafted by the “grafting to” approach, and 2/3 of the surface are covered by a polymer grafted by the “grafting from” approach. Visualization of synthesized Janus particles by SEM reveals differences in their morphology (Figure 1). For example, sides covered with PDMAEMA, PAA and PLMA appear smoother than sides covered with PEG or PS. Thickness of the polymer layers was evaluated using TEM by examining one side hairy Janus particles obtained after the grafting of the first polymer. The thickness of the second polymer layer, which was grafted via the “grafting to” approach, was always 5-7 nm as obtained from TGA results (not shown, Table 1). Fully covered particles were also characterized by means of zeta-potential measurements (electrophoresis) and TGA (Table 1). In fact, the IEP of Janus particles is always in between that of the corresponding monocomponent particles, which is an indication of the Janus character. The grafting density of the polymers on the silica particle surface is typically 0.2 chains nm-2.

Table 1. Janus particle samples and their properties

System

Component

Properties

Layer thickness

PDMAEMA

positively charged, hydrophilic

7 nm

PAA

negatively charged, hydrophilic

PEG

uncharged hydrophilic

PDMAEMA/PAA-JP PEG/PLMA-JP

IEP

4.4 12 nm

4.7


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PLMA

hydrophobic soft

PDMAEMA


PLMA

hydrophobic soft

PDMAEMA


PS

hydrophobic hard

PDMAEMA-FC

PDMAEMA


12 nm

9.9

PAA-FC

PAA

negatively charged, hydrophilic

15 nm

50%) remains non-aggregated. Fully covered PDMAEMA particles demonstrate an opposite scenario (Figure 5b, Figure 6b). They almost do not form aggregates at pH 2 and 6, when the polymer is strongly charged (IEP = 10, pKa = 7.3-7.5) and swollen. Similarly to fully covered PAA particles, fully covered PDMAEMA particles form very large aggregates (>60%) when the polymer chains are uncharged and collapsed, which happens at high pH values (pH = 9). At pH =9 some amount of particles (< 10 %) remains non-aggregated.

Mixing of PAA and PDMAEMA fully covered particles leads to a more complex aggregation behavior (Figure 5c, Figure 6c). There is a coexistence of individual particles (ca 60%) and aggregates at extreme pH values (pH = 2 and pH = 9). The agglomerates at low pH values are most probably formed by PAA-covered particles, the agglomerates at high pH values are formed


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by PDMAEMA-covered ones. The individual particles at high pH values are therefore PDMAEMA-covered ones, the individual particles at low pH values are therefore PAA-covered ones. The mixed particles form large aggregates (> 80%) at pH 6 and the fraction of individual particles remains negligible (< 10%). Both polymers are charged at these conditions and interact electrostatically with each other, which leads to agglomeration.

Behavior of Janus particles is similar to the behavior of particle mixtures (Figure 5d, Figure 6d). Many large aggregates are formed at pH 6 when both polymers are charged and electrostatic interactions occur. At low pH values we observed individual particles (ca 20%), agglomerates formed by 2-5 particles (ca 60%) as well as large agglomerates (8-20 particles, ca 20%). We believe that small agglomerates are formed by Janus particles, which are oriented by their nonswollen part, which is PAA, towards each other.


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Figure 5. Fluorescence microscopy images of fully covered PAA (a) and PDMAEMA (b), their mixture (c), and PDMAEMA/PAA-JP (d) at different pH values. Scale bars: 20 µm


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Figure 6. Aggregation behavior of charged colloidal particles (fully covered PAA (a) and PDMAEMA (b), their mixture (c), and PDMAEMA/PAA-JP (d)) depending on the pH value. Solid points represent the number distribution of clusters, empty points – the mass distribution. Scale bars: 20 µm. Orange polymer in the cartoons – PDMAEMA, green – PAA

We also found that PDMAEMA/PAA-JP tend to form chain-like aggregates in pure water dispersions that is because of the Janus character of the particles (Figure 7). Other kinds of particles (fully covered particles and their mixtures) do not form such chains. We believe that particle chains are formed when the PDMAEMA-covered side of one particle sticks to the PAAcovered side of another particle. We observed that unordered aggregates are formed in a salty solution. In fact, ions screen charged groups and reduce coulombic interactions leading to a lower contrast between the two polymers that most probably reduces specificity of binding of particles to each other.


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Figure 7. Examples of structures formed by PDMAEMA/PAA-JP depending on the pH and ionic strength (Fluorescence microscopy images). Chain-like structures are observed in pure water. Scale bars: 5 µm

Next, we investigated the behavior of amphiphilic JPs (PEG/PLMA-JP, PDMAEMA/PLMAJP and PDMAEMA/PS-JP) in water and toluene (Figure 8). We varied the hydrophilic polymers: in PEG/PLMA-JP the hydrophilic polymer is uncharged (PEG), in PDMAEMA/PLMA-JP the hydrophilic polymer is charged (PDMAEMA). We also varied the hydrophobic polymer: in PDMAEMA/PLMA-JP the hydrophobic polymer is soft (PLMA, Tg = -35°C), in PDMAEMA/PS-JP the hydrophobic polymer is hard (PS, Tg = 105°C).

We first tested the aggregation behavior of colloidal particles fully covered by each of the polymers – PEG, PDMAEMA, PS, PLMA in water and toluene (Figure 8-9). We observed that PEG-modified fully covered particles are dispersible in both water and toluene. PDMAEMA-


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modified ones are dispersible in water and form aggregates in toluene. PLMA- and PS –modified particles are dispersible in toluene and form large aggregates (non-dispersible) in water.

We found that PEG/PLMA-JPs are highly dispersible in both water and toluene, while a small amount of chain-like aggregates is formed. Dispersibility of PEG/PLMA-JP in toluene can easily be explained by the solubility of polymers in water and toluene. Dispersibility of PEG/PLMAJP in water where PLMA is not soluble can be explained by the fact that the area of the particles´ surfaces covered by PEG is considerably larger than that covered by PLMA. As a result, PEG forms a shell around the particle core that prevents particle aggregation. Small amount of particles form chain-like aggregates due to the sticking of PEG/PLMA JPs by their PLMA sides (Figure 10).

PDMAEMA/PS-JP and PDMAEMA/PLMA-JP demonstrate very similar aggregation behavior: aggregates with similar size distribution are observed in all solvents (pure water, salty water and toluene, Figure 8-9). In fact, PDMAEMA is a hydrophilic polymer, which swells in water, but, contrary to PEG, does not swell in toluene. This limits the dispersability of these particles in toluene, and JPs most probably form aggregates where they touch each other by their PDMAEMA sides. A similar situation is observed in water. In this case, hydrophobic polymers PLMA and PS prevent the dispersing of JPs, and they form aggregates where individual JPs touch each other by their hydrophobic sides (PS and PLMA).


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Figure 8. Fluorescence microscopy images of fully covered PEG (PEG-FC, a), PDMAEMA (PDMAEMA-FC, b), PLMA (PLMA-FC, c), and PS (PS-FC, d) and amphiphilic Janus particles (PEG/PLMA, e); PDMAEMA/PLMA, f); PDMAEMA/PS, g)) in different media. Scale bars: 20 µm


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Figure 9. Aggregation behavior of reference fully covered PEG (PEG-FC, a); PDMAEMA (PDMAEMA-FC, b), PLMA (PLMA-FC, c), PS (PS-FC, d) and amphiphilic Janus particles (PEG/PLMA-JP (e), PDMAEMA/PLMA-JP (f), and PDMAEMA/PS-JP (g)) in DI water (upper panels) and toluene (lower panels) (solid symbols – number distribution / open – mass


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distribution). Orange polymer in the cartoons – PDMAEMA, green – PLMA or PS. Scale bars: 20 µm

Figure 10. Typical examples of structures formed by amphiphilic JPs depending on the media (Fluorescence microscopy images). Snake-like structures observed under all conditions. Scale bars: 5 µm Very interesting is that similar to PDMAEMA/PAA-JP, PDMAEMA/PLMA-JP and PDMAEMA/PS-JP form chain-like aggregates in pure water dispersions and in toluene, which is because of the Janus character of the particles (Figure 10). A proposed scenario for such aggregation behavior is illustrated in Figure 11. PEG/PLMA-JP and PDMAEMA/PLMA-JP


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Janus particle have one hydrophilic side, which is swollen in water, and one hydrophobic side. As result these particles tend to stick to each other by their hydrophobic sides in water in order to minimize unfavorable contacts between PLMA and water. This leads to formation of linear chain-like agglomerates. The character of these agglomerates depends however on the kind of Janus particles. PEG/PLMA-JP form thin one-particle chains.

PDMAEMA/PLMA-JP and

PDMAEMA/PS-JP form thicker chains. We believe that PEG is highly swollen and doesn’t allow the particles to come too close to each other with their PLMA sides. As result the area of possible contact between hydrophobic sides is reduced that leads to formation to linear chains (Figure 11 a). In case of PDMAEMA/PLMA-JP and PDMAEMA/PS-JP a more complex aggregation behavior is observed due to the fact that PDMAEMA is swollen in water and positively charged. We believe that the particles still tend to touch by their hydrophobic sides; however, there are possibilities for them to have more neighboring particles due to the large size of the polymer patches. In case of ‘stickier’ PLMA larger structures are formed (Figure 11 b). When the electrostatic repulsion of the PDMAEMA sides is hindered by increasing the ionic strength, even larger linear structures are formed. A similar mechanism was observed in the work of Granick et al., where they used the same effects to construct linear 3D helical structures from amphiphilic particles, however, without hairy morphologies.24


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b)

a)

Figure 11. Proposed structure of the chain-like structures the formed by PEG/PLMA-JP (a) and PDMAEMA/PLMA-JP (b) Janus particles in DI water. Fluorescence microscopy images (analogous to Fig. 10) and representative cartoons

Conclusions In this paper we investigated the assembly behavior of different kinds of hairy core-shell particles consisting of a hard silica core and a soft polymer brush shell. We varied the nature of the polymers constituting the shell: we used different hydrophilic positively (poly(2(dimethylamino)ethyl methacrylate) - PDMAEMA) and negatively (polyacrylic acid - PAA) charged, as well as uncharged hydrophilic (polyethylene glycol - PEG) polymers and hydrophobic polymers (poly(laurylmethacrylate) - PLMA, polystyrene - PS). We synthesized particles covered by polymer of one sort (homogeneously coated particles) as well as Janus particles (two polymers are grafted to the opposite sides of the core). Next, we investigated and compared the aggregation behavior of different one side hairy Janus particles, their mixtures, and Janus particles. We found that homogeneously-coated particles of one sort formed aggregates in solvents or at pH values where the polymer chains forming the shell are not soluble or collapsed.


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The mixture of oppositely charged particles (PAA and PDMAEMA) and PAA-PDMAEMA Janus particles demonstrate very similar aggregation behavior in aqueous environment at different pH values: a small amount of aggregates was observed at extreme pH values (pH = 2 or pH = 9) when the polymer chains of at least one sort are collapsed. Very strong agglomeration was observed at pH = 6 when polymer chains of both sorts (PDMAEMA and PAA) are charged and tend to form interpolyelectrolyte complexes. However, the difference between the aggregation behavior of mixed particles and Janus ones is in the shape of the aggregates: a mixture of particles forms typically unordered large aggregates, while Janus particles tend to form chain-like structures. Amphiphilic hairy Janus particles covered by hydrophilic and hydrophobic polymers (PDMAEMA/PLMA and PDMAEMA/PS) form aggregates in water and organic solvents (toluene). Similarly to oppositely charged particles, these aggregates tend to form chain-like structures.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (A.S.) Tel.: +49 (0351) 4658 327 Fax: +49 (0351) 4658 474


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources DFG (Grant SY 125/4-1)

ACKNOWLEDGMENTS The authors are thankful to DFG (Grant SY 125/4-1) and IPF for financial support.

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SYNOPSIS (Word Style “SN_Synopsis_TOC”). If you are submitting your paper to a journal that requires a synopsis, see the journal’s Instructions for Authors for details.


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Self-Assembly Behavior of Hairy Colloidal Particles with Different

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