Microcolloidal Architectures from Janus Seeds by ATRP

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Growth of Nano-/Micro- Colloidal Architectures from Janus Seeds by ATRP Chengjun Kang, and Andrei Honciuc Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02946 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 13, 2018

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Chemistry of Materials

Growth of Nano-/Micro- Colloidal Architectures from Janus Seeds by ATRP Chengjun Kang, Andrei Honciuc* Institute of Chemistry and Biotechnology; Zurich University of Applied Sciences, Einsiedlerstrasse 31, 8820 Waedenswil, Switzerland. ABSTRACT: In the natural world, seeds grow into plants and the seed diversity ensures the significant vegetation heterogeneity. Here, we show the growth of colloidal structures from starting seed nanoparticles by controlled radical polymerization, which resembles the natural processes of plant growth from seeds. Specifically, nano-/micro-sized architectures with a surprising diversity can be “grown” from snowman-type Janus nanoparticle seeds (JNPS) by atom transfer radical polymerization (ATRP) technique. The current approach aims at concentrating ATRP initiators asymmetrically in the bulk of one JNPS lobe. After initiating the polymerization, the addition of monomers promotes JNPS growth into asymmetric nano-/microcolloidal architectures. Depending on the types of the JNPS and on the growth conditions, the grown architectures could adopt dish-, basket-, cocoon-, flower-, helmet-, mushroom-, dumpling- and pumpkin-like geometries. Additionally, the surfaces of these grown architectures could be controlled to have smooth-, islands- and grouped islands-like nanostructures. This method providing an alternative approach for synthesizing anisotropic colloids with complex geometries and tunable surface morphologies, enrich the variety of colloidal particle synthetic families.

Anisotropic nano-/micro-particles represent an important avenue for scientific research1–5 to understand the importance of asymmetries in nature and to create new nano-materials with a multitude of functionalities, geometries and surface patterns6,7. Synthesis of colloidal particles with controllable shapes and surface patterns is thus an important topic for scientists. The past decades have brought great progress in particle synthesis. Colloidal particles with snowman-8,9, discs-10,11, crescent moon-12, pistachio-12 and lens-shaped13 geometries, among others, have been synthesized. However, the direct synthesis of colloids with complex or even hierarchical structures is rare. Additionally, the synthesized colloids often showed smooth surfaces, the colloids with tunable surface nanostructures, as prevalent in natural world14,15, have not been achieved. Atom transfer radical polymerization (ATRP) is a technique that has been widely used for synthesizing narrow polydispersed polymers or block copolymers with well-defined structures16–18. ATRP initiators are often halogen species and are most frequently distributed in two ways. The first way is to disperse initiators in a solvent19,20, which then grow into individual polymer chains. Secondly, initiators are immobilized on a substrate surface21–23, these grafted ATRP initiators partly

lose their freedom and grow into “polymer brushes”. It is evident that the distribution of ATRP initiators will largely affect the way the polymer chains grow. A third and less explored way is the incorporation of the ATRP initiators entirely in the bulk of a solid polymeric nanoparticle, in which case the polymer chain growth is significantly constrained. Finally, if the ATRP initiators are incorporated in an asymmetric particle, such as JNPS, the constrained polymer chain growth becomes directional. Here, by distributing initiators in only one lobe of a Janus nanoparticle, we take a new approach to the synthesis of polymeric anisotropic nano-/ micro- sized architectures with complex geometries and tunable surface nanostructures. The basic idea of our current method is as follows: in contrast to the ATRP initiators that dispersed in a solution or attached on a surface, we concentrate ATRP initiators in one lobe (bulk and surface) of a snowmanshaped Janus nanoparticle, forming JNPS. After the initiation, the continuous addition of monomers into the ATRP initiators containing lobe promotes the directional growth of JNPS into larger polymeric architectures. The first step is to make narrow polydispersed JNPS containing concentrated ATRP initiators in one lobe. Seeded emulsion polymerization is a technique that has

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been frequently used to fabricate Janus nanoparticles 24– 26. In this method, the phase separation of the second monomer from a seed NPs results in the formation of a new lobe27–30. Inspired by this idea, we used ATRP initiators as the monomer to form the second lobe on seed NPs by phase separation. In the present study, we used the mixture of siloxane derivatives, namely (3mercaptopropyl) trimethoxysilane (MTS) and ATRP initiator containing 2-bromo-2-methyl-N-(3(trimethoxysilyl) propyl) propanamide (BMTP)31,32. The reason we use MTS as the matrix for the ATRP initiator is that the MTS mixes well with the BMTP in any ratio, and these mixtures exhibit phase separation on different types of seed NPs. More importantly, the bonding forces between the MTS and the BMTP molecules are strong enough for becoming solid particles and prevent being dissolved, however, weak enough to be expanded by the addition of monomers during polymerization process, so as to be further grown into a large variety of polymeric nano-/ micro- sized architectures. It should be noted that compared to the conventional seeded emulsion polymerization, in which the second monomer swells seed particles, then extruded under elevated polymerization temperature, the siloxane derived monomers only attach on the seed particle surface without swelling process. This hypothesis is proved by the fact that the “swelling-extrusion” mechanism will inevitably result in entangled and interpenetrated polymer chain networks between the two lobes, subsequently, these two lobes are strongly bonded together and cannot be separated by mechanical means. In contrary, the “surface attaching and phase separation” mechanism is more likely in the case of the siloxane that will not result in interpenetrated polymer networks between the two lobes, therefore, JNPS can possibly be separated into two individual lobes by mechanical means. This is consistent with our observation that the two lobes of JNPS can be separation under strong ultrasonication conditions (Figure S1). There are several parameters allowing us to tune the growth of the polymeric nano-/ micro- sized architectures. First, we have examined the influences of the size and shape of the MTPS/BMTP-lobe on the polymeric colloidal architectures formation. For this, we keep other JNPS parameters constant in this specific set of experiments and we fixed the volume ratio between the MTS and the BMTP at 3: 2; cross-linked NPs containing 90% poly(tert-butyl acrylate) (PtBA) and 10% polystyrene (PS) was used as the seed NPs (Figure S2). Because of the wide applicability and fast polymerization speed in aqueous environment, 2-hydroxylethyl methacrylate (HEMA) was used as the monomer for promoting the growth of colloidal of JNPS into nano-/ micro- sized architectures33,34. SEM images show that the

mixture of MTS/BMTP can form Janus lobe on the seed NPs surface, the lobe size can be controlled by the feed ratio between the seed NPs and monomer amount (Figure 1). For 1.0 g seed NPs, 0.5 ml MTS/BMTP mixture generates Janus lobe with diameter of 159 ± 5.1 nm (Figure 1 b, c). These lobes further grow into disc-like geometries with diameters of 1.3 ± 0.1 µm after the ATRP polymerization of HEMA. The obtained discs have smooth surfaces with few wrinkles, which might be formed during the drying of process. In most cases (> 90%), each disc has a seed nanoparticle in the center, forming a mushroom-like structure (Figure 1 d, e). On the other hand, by increasing the amount of the MTS/BMTP mixture to 2.5 ml per 1.0 g seed NPs, two effects were observed on the newly formed Janus lobe. The first effect was that the lobe size increased from 159 ± 5.1 nm to 423 ± 16.1 nm (Figure 1 f, g). Secondly, because the bigger Janus lobe contained more ATRP initiators, the polymeric colloidal architectures were also bigger, e.g. colloidal architectures as big as 3.9 ± 1.2 µm could be grown. Another observation is that the bigger MTS/BMTP Janus lobes grow into colloidal architectures with increased curvature and a basket-like geometry. Since the basket-like architectures formed during the polymerization process without any influence of other external factors, or templating, the only possible matter that fills the hollow area is the polymerization medium, namely, water and monomer. These basket-like colloidal architectures have smooth surfaces with minor shallow wrinkles, which may be produced by the drying process. Analogue to the disk-like colloidal architectures, a seed NPs is contained inside of each basket. The above observations indicate that the size and shape of the initiator containing Janus lobe have significant influence on the geometries of the grown polymeric colloidal architectures. Control experiments showed that the JNPS synthesis conditions, such as pH and polymerization time, did not have any obvious influences on their growth properties (Figure S2 & S3). It should be pointed out that the basket-like colloidal structures have been obtained previously by radiation miniemulsion polymerization,35 however, their cavity structures formed before the polymerization process via templating. In contrast, in the current work the cavity structure does not exist before the ATRP polymerization, but instead, the polymerization itself leads to the formation of cavity/basket architectures via a “seed-growth” mechanism.


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Chemistry of Materials seed NPs inside, with a dumpling-like geometry (Figure 2d, h, l).

Figure 1. (a) Scheme showing the method used to grow colloidal architectures from JNPS by ATRP. (b, c) JNPS containing one small lobe with ATRP initiators. (d, e) Disclike colloidal architectures showing one nanoparticle in the middle produced from (b, c). (f, g) JNPS with one big ATRP initiators containing Janus lobe. (d, e) Basket-like colloidal architectures produced from (d, e).

We subsequently examined the influence of the ratio between the MTS and BMTP on the growth of colloidal architectures. In order to realize the JNPS growth, the bonding force between the MTS and BMTP molecules should be weaker than the inner expansion strength generated by the polymerization of monomers, which in turn, is determined by the ATRP initiator concentration in JNPS. To examine this issue, we synthesized PtBAMTS/BMTP JNPS with the same MTS/BMTP lobe sizes, but different in the BMTP concentration (Figure S4). Specifically, JNPS with BMTP volume concentration of 12.5 %, 25 %, 40%, 50 % were synthesized. Fourier Transform Infrared (FTIR) spectroscopy confirmed different initiator concentrations in these JNPS (Figure S5). After the ATRP polymerization of HEMA, we found that JNPS containing only 12.5% BMTP in the MTS/BMTP lobe cannot grow into colloidal architectures (Figure 2a, e). As the BMTP concentration increased to 25 %, JNPS can grow into colloidal architectures. These experiments indicate that the JNPS growth can only occur when the initiator concentration is high enough, such that sufficient expansion force can be generated. Additionally, we found that JNPS with BMTP concentrations of 25%, 40% and 50 % can grow into different structures (Figure 2b-d). Specifically, JNPS with 25% BMTP concentration grow into solid hemispheres with the seed NPs sitting on the top of the shallow concave side (Figure 2b,f, j). As for the JNPS with 40% BMTP, the curvature of the grown architectures significantly increased adopting a basketlike structure (Figure 2c, j, k). The curvature of the architectures grown from JNPS (BMTP 50%) increased further so that the outer shell layer folded, wrapping the

Figure 2. (a-d) Scheme showing the cross section of JNPS with different BMTP volume concentration, (a) 12.5%, (b) represents the monomers 25%, (c) 40%, (d) 50%, consumed in the MTS/BMTP lobe during the polymerization. (e-h) Scheme showing the cross section of the colloidal architectures grown from the JNPS a-d, respectively. (i-l) SEM images of the colloidal architectures grow from the JNPS a-d, respectively.

It is evident that the BMTP concentration significantly influences the curvature of the grown colloidal architecture. This phenomenon can be explained by the relative speed between the monomer diffusion and monomer consumption during the ATRP polymerization process. When the initiator-BMTP concentration is low (25%), the monomer consumption speed is slow in the polymerization process, such that the monomer diffusion rate is high to provide sufficient monomers inside the growing MTS/BMTP lobe, thus, every part in the MTS/BMTP lobe grow evenly (Figure 2b), given grown architectures quite similar to the MTS/BMTP lobe on JNPS. As the BMTP concentration increases, the monomer consumption speed increases accordingly during the polymerization process. In this case, the monomer diffusion rate cannot provide sufficient monomers needed for the polymerization. Those points that are closer to the MTS/BMTP lobe surface would be easier to access to the monomers (Figure 2c, d), so they would grow faster than the points deeper inside of the MTS/BMTP lobe. This difference in the polymerization speed between the outer and deeper layer of the MTS/BMTP lobe generates inner forces, which in turn bend the growing colloidal architectures. The higher the BMTP concentration is, the larger the difference between polymerization speed between the outer and deeper layers of the MTS/BMTP lobe, which eventually lead to



the formation of colloidal architectures with larger curvatures. In the following section, we show the tuning of colloidal surface nanostructures by altering the surface chemistry of the seed NPs. The surface chemistry of seed NPs is an important parameter for the seeded emulsion polymerization. The seed NPs surface chemistry influences the phase separation of the second monomer, and therefore, altering the interaction strength between the seed and the newly formed Janus lobe. Previously, we found that the complete phase separation between the initiator containing lobe and the seed NPs, the polymerization of these JNPS leads to architectures with smooth surfaces. Alternately, if the phase separation is incomplete and the attraction forces between the seed NPs and the initiator lobe are stronger, then larger inner expanding forces are required for the growing MTS/BMTP lobe to detach from the seed NPs surface. This counter action might generate interesting surface nanostructures of the grown colloidal architectures. To test this hypothesis, instead of PtBA, we used poly (ethyl methacrylate) (PEMA) as the seed NPs, which is less hydrophobic as compared to the PtBA nanoparticles, as a result, less phase separation and stronger Janus lobe/seed NPs interactions can be expected (Figure S6).

Figure 3. Tuning of the surface nanostructures of the colloidal architectures. (a, b) Scheme showing the synthesis of disc- and basket-like colloidal architectures with different surface nanostructures by using different seed NPs. (c, d) SEM images of disc-like colloidal architectures containing PEMA seed NPs. (e, f) Star-like colloidal architectures produce from JNPS with PMMA as the seed NPs. (g, h) Basket-like colloidal architectures with island-like surface nanostructures produced from JNPS with PEMA as the seed NPs. (i, j) Pumpkin-like colloidal architectures with special surface nanostructures produced by JNPS with PMMA as the seed.

PEMA-MTS/BMTP JNPS with different lobe sizes were obtained by controlling the feeding ratios between the

PEMA seed NPs and the MTS/BMTP mixture (Figure S6). Analogue to the polymerization of the PtBA-MTS/BMTP JNPS, the polymerization of the PEMA-MTS/BMTP JNPS with small and big MTS/BMTP lobes grow into disc- and basket-like colloidal architectures, respectively. However, instead of the smooth surfaces, the PEMA-MTS/BMTP JNPS grow into colloidal architectures with rather rough, islands-like surface nanostructures (Figure 3 a, c, g). These surface nano-structuring is unlikely due to the drying forces as in Figure 1, but possibly because of the stronger expanding forces required for the growing MTS/BMTP lobe to detach from the PEMA seed surface. We hypothesize that during the polymerization process, because of the expansion in dimension, the MTS/BMTP lobe is prone to detach from the seed NPs surface. When the interaction strength between the seed NPs surface and the MTS/BMTP lobe is weak, as in the case of PtBA NPs, the detachment of the growing MTS/BMTP is easily achieved, giving a smooth surface morphology on the grown colloidal architecture. As the attraction strength between the seed NPs and the MTS/BMTP lobe increases, the detachment of the growing MTS/BMTP lobe become difficult, as a result, internal stress formed between the seed NPs and the MTS/BMTP interfaces, which distorts the surface morphology of the grown colloidal architecture. This was verified next by using even more polar poly(methyl methacrylate) (PMMA) seed NPs. SEM images show that the phase separation of PMMA-MTS/BMTP JNPS is not as complete as the PtBA and PEMA NPs (Figure S6), indicating even larger attraction strength. It is no surprising that when the PMMA-MTS/BMTP JNPS with small MTS/BMTP lobes, the MTS/BMTP lobe expansion generated forces is insufficient for detaching from the PMMA surfaces, but instead splitting into several parts covering on the seed NPs surface, forming flower-like structures (Figure 3 e, f). Increasing the MTS/BMTP lobe size will significantly increase the expansion force during the their growth, which gives the possibility for the MTS/BMTP lobe to detach from the PMMA NPs surface. From Figure 3 we can see the pumpkin-like colloidal architectures obtained from the PMMA-MTS/BMTP JNPS with larger MTS/BMTP lobe sizes. As compared the previous two cases, islandslike nanostructures also formed on the surface of the grown colloidal architectures, however, these islands are separated into several groups by lines running from one pole of the colloid to the other, forming pumpkin-like grouped island nanostructures. Broken “pumpkins” are occasionally encountered (Figure S7), which show the void inside without observing the seed PMMA NPs. This observation could be explained as the MTS/BMTP growth, the PMMA seed NPs were torn apart and form several symmetry chambers.


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Chemistry of Materials The growth kinetics of polymeric colloidal architectures is next examined. ATRP growth kinetics has been extensively studied36,37. For a well-controlled ATRP, the initiation is much faster than the polymer chain growth rate, such that polymer chains with narrow molecular weight polydispersity can be expected. In the present study, we used ATRP technique to grow colloidal architectures from initiator concentrated in JNPS and because of this special initiator placement, the study of their growth kinetics is necessary. The growth kinetics of the nano-/microcolloidal architectures was studied by monitoring their size with SEM at different times (Figure 4). The following phenomena were observed. Firstly, in the initial 2 h polymerization, most of the seed JNPS were swollen by the monomer and diameter increased from 429 ± 20 nm to 501 ± 33 nm. Within this period of time, no grown architectures were observed. As the polymerization prolonged to 3.3 h, around 5% of grown basket-like architectures were observed, co-existing with the swelled JNPS.

Figure 4. Kinetic study the growth of colloidal architectures from the PEMA-MTS/BMTP JNPS. (a) Initiation efficiency changes with the polymerization time. (b) The change of the PEMA-MTS/BMTP JNPS sizes with polymerization time (red), the MTS/BMTPJNPS formed basket-shaped colloidal architectures grow with polymerization time (black). (c-g) SEM images showing different initiation efficiency with polymerization time. (h-l) Cartoons represent the SEM images of c-g.

As the polymerization continued, the amount of the swelled JNPS gradually decreased along and the percentage of the grown basket-like architectures increased. In this process, the size of the swollen JNPS remained constant (Figure 4b(red)), while the dimension of the grown architectures gradually increased (Figure 4b(black)). As polymerization time extended to 12 h, no swollen JNPS remained and all have grown into the basket-like architectures. The initiation speed of JNPS is far less than their growth rate, which implies that an

initiation energy barrier exists. The slow JNPS initiation as compared to their growth rate implies a broad size distribution of the basket-like architectures. This hypothesis is proven by the direct SEM observations, e.g. as the polymerization time of 6 h, the size of the grown basket-like architecture is 2.0 ± 1.1 μm. However, as the initiation approaches completion, the polydispersity of colloidal architectures becomes narrower; for example, after 12 h, the size of the basket-like capsules became 2.5 ± 0.5 μm. Because of the slow initiation speed as compared to growth speed, the growing JNPS exhibit a wide polydispersity. One can find JNPS at different stages of growth that co-exist, which makes it difficult to directly capture the evolution of the individual polymeric colloidal architectures at different polymerization times. However, we could easily identify colloidal architectures at different growth stages and put them together in Figure 5. It should be emphasized that the growth of individual colloidal architectures is not only due to surface initiated polymerizations, but instead, polymerization is also initiated from the inside of JNPS. After the polymerization of the outer most layers of initiators, the swelling will facilitate the diffusion of monomers and polymerization from the bulk of the Janus lobe. Subsequently, all initiators in the JNPS seed bulk can participate in the polymerization. It is reasonable to expect that the polymerizations closer to JNPS surface enjoys longer growth time and easier access to the monomers, therefore, they may have longer chains as compared to polymers grown from the inside of JNPS (Figure 5).

Figure 5. Growth of an individual colloidal architecture: (a) Cartoon depicting a typical JNPS. (b) Magnification of a small part of (a, i-ii), which depicts the polymerization and diffusion of monomers from the outer layer into the inside of JNPS. (c) SEM image of a single JNPS. (d-h) SEM images showing JNPS at different stages of growth, these images were taken at the same polymerization time.

Our current technique also shows great potential for synthesizing complex colloidal structures, which are, otherwise, difficult to be fabricated by other methods. Complex structures can be made possible by controlling



the number of Janus lobes loaded with ATRP initiators. To illustrate this, we synthesized water molecule-like JNPS containing two seed NPs with a yield of 20%38 (Figure 6b). After the polymerization, each seed NPs result in a cavity forming a helmet-like architecture (Figure 6 a, c, d). On the other hand, JNPS containing one seed NP with two MTS/BMTP Janus lobes can be fabricated by controlling experimental conditions as previously reported39. Each of these MTS/BMTP lobes could independently grow into a basket-like architecture. Depending on the relative size of the MTS/BMTP lobes, the obtained colloidal architecture could have different geometries. Specifically, JNPS containing two MTS/BMTP lobes with similar sizes can be synthesized with a yield of 12 % (Figure 6e, Figure S8). These JNPS could grow into a cocoon-like colloidal architecture (Figure 6 a, f, g), such that the openings of the two basket-like architectures closely attach with each other. On the contrary, JNPS with one MTS/BMTP lobe is significantly larger than the other can be synthesized with a yield of 15 % (Figure 6h, Figure S7). In this case, the smaller MTP/BMTP lobe will grow into a disc-like colloid, which is incorporated into the inner side of a larger lobe grown basket-like architecture (Figure 6 a, i, j), forming a disc-colloid in basket-hierarchical structure. In addition to the individual and hierarchical complex colloidal architectures, we found that the basket-like colloids can self-assembly into even larger structures simply by drying in air, such that ‘baskets’ are closely packed together; over 90 % baskets have their openings oriented toward one direction. The reason why these colloids adopt an oriented configuration is not clear yet, further study on these basked-shaped architectures assembly is in progress.

close contact with each other. (h) JNPS containing one seed NPs and two ATRP initiator loaded Janus lobes with different sizes. (i, j) colloids produced from (h), which consist of one basket-like colloid architecture containing a disc-like colloid with a spherical seed NPs inside. (k) Basket-like colloid containing one spherical NPs inside. (l, m) Basket-like colloid assembly upon drying. The different colours in the cartoon image intend to show colloidal architectures that are grown from different initiation Janus lobes. These colloidal architectures have the same chemical compositions.

In summary, we have used ATRP to synthesize anisotropic nano-/microcolloidal architectures under surfactant-free conditions and in large-scale amount. This approach expanded the scope of ATRP, previously used mainly for producing narrowly polydisperse polymer solutions or polymer brushes on surfaces. This approach is also different from the previous colloids fabrication methods, such as emulsion polymerization and its derivatives. One distinct advantage of the current method is that the colloidal surface nanostructures can be tuned, in sharp contrast to the emulsion polymerization produced colloids with only smooth surfaces. Additionally, the current method is especially interesting as it generates a variety of asymmetric colloidal architecture, from discs- to basket-like with different surface roughness. Hierarchical colloidal architectures can also be produced by controlling the distribution of the ATRP initiators. This work opens a new path toward the controlled growth of a large variety of asymmetric colloidal particles with unconventional geometries and functionalities.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental methods and synthesis procedures, including different JNPS synthesis and characterization, ATRP polymerization procedures (PDF).

AUTHOR INFORMATION Corresponding Author * Andrei Honciuc. Email: [email protected] Figure 6. Synthesis of hierarchical colloidal architectures. (a) Scheme showing the synthesis of hierarchical colloidal architectures. (b) JNPS containing two seed NPs. (c, d) Helmet-like colloids produced from (b). (e), JNPS containing one seed NPs and two ATRP initiator loaded Janus lobes with similar sizes. (f, g) colloids produced from (e), which are formed by two basked-like colloids with their opening in

ORCID Chengjun Kang: 0000-0003-0208-2954 Andrei Honciuc: 0000-0003-2160-2484

ACKNOWLEDGMENT We gratefully acknowledge the financial funding from Metrohm Foundation (Herisau, Switzerland).


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Microcolloidal Architectures from Janus Seeds by ATRP

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