Polymeric Janus nanoparticles templated by block copolymer thin films

Abstract

We report a novel approach to synthesize well-defined polymeric Janus nanoparticles by combining the self-assembly of block copolymers in thin films and surface modification by polymer grafting.

---

Janus particles, named after the doubled faced Roman god “Janus”, are compartmentalized particles comprised of two or more surface regions with different chemistries or polarities. This broken symmetry gives rise to unique properties such as the ability to have directional interactions, which makes this class of compound potentially useful in a wide range of applications, including chemical and biological sensors,¹ phase transfer agents,² switchable devices,³ drug carriers,⁴ emulsion stabilizers⁵ and others.⁶ In this context, the ability of block copolymers to self-assemble into complex nanostructures makes them very interesting building blocks for the synthesis of such nanosized particles. However, since direct self-assembly of block copolymers into anisotropic particles is challenging, dedicated strategies or systems are required to break the symmetry and generate anisotropic particles. Most of these strategies take advantage of triblock terpolymers forming particular morphologies or nanostructures either in bulk^7,8 or in selective solvents.^9,10

In this communication, we report on a method to produce well-defined polymeric Janus particles using a nanostructured thin film made from a diblock copolymer as template. The proposed strategy is highlighted in Scheme 1. In the first step a nanostructured block copolymer thin film with regularly spaced nearly monodisperse nano-domains is formed by spin-coating followed by solvent annealing. Afterwards, those nano-domains are functionalized with alkyne groups and selectively cross-linked. These alkyne functions further allow the grafting of an azido-terminated homopolymer onto the functionalized nano-domains through copper-catalyzed alkyne–azide cycloaddition (CuAAC). Since the nano-domains are cross-linked and the grafting only occurs on top of them, well-defined and asymmetric Janus nanoparticles are obtained after dissolution of the thin film.

Scheme 1 Formation of Janus nanoparticles by grafting of a homopolymer onto a block copolymer thin film.

A polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer has been used to obtain the initial nanostructured thin film (see Experimental part in ESI†). A copolymer with a P4VP volume fraction of 0.24 has been selected in order to target a thin film with P4VP cylinders embedded in a PS matrix (Scheme 1). In order to obtain the desired Janus nanoparticles, the thickness of the accordingly prepared thin film has however been kept small (Scheme 1). Practically, the concentration of the spin-coated polymer solutions and the conditions of the spin-coating have been tuned to obtain films with a thickness comprised between 20 nm and 50 nm. After spin-coating the films have been subjected to solvent annealing with 1,4-dioxane vapors for 12 h to promote microphase separation between PS and P4VP chains. Since the diameter of the targeted cylinders (15–25 nm) is expected to be in the same range as the film thickness (20–50 nm), the obtained nanostructured thin films can be seen as P4VP dots regularly distributed in a PS matrix, rather than true P4VP cylinders. This assumption is experimentally confirmed in Fig. 1 where, after solvent annealing, dot-like nanodomains are clearly evidenced. This is ascribed to the formation of P4VP dot-like nanodomains embedded in a PS matrix as previously described elsewhere for similar systems.^11,12

Fig. 1 Atomic force microscopy (AFM) height and phase pictures of a 30 nm thick PS-b-P4VP film annealed for 12 h in 1,4-dioxane vapor.

In the next step, the pyridine moieties located into the P4VP domains have been partially quaternized with 4-bromo-1-butyne vapors in order to introduce alkyne groups into the P4VP nano-domains (Scheme 2).

Scheme 2 PS-b-P4VP functionalization by alkyne groups, cross-linking with a diiodo derivative and grafting of α-methoxy-ω-azido-poly(ethylene glycol) through copper-catalyzed alkyne–azide cycloaddition.

In the conditions used in this work, the degree of functionalization of the pyridine groups of the P4VP domains is around 10–15%, as estimated by ¹H NMR of dissolved thick P4VP films functionalized in the same conditions than the ones used for the thin films (see Fig. S1†).

After functionalization with alkyne groups, the P4VP nano-domains have been selectively cross-linked by reacting the remaining non-functionalized pyridine units with 1,4-diiodobutane vapors using a procedure previously described by others.^13–16 At this point, it is important to mention that neither the alkyne functionalization nor the cross-linking affects the thin film morphology as verified by AFM (Fig. S2†). The degree of cross-linking has been estimated from X-ray photoelectron spectroscopy (XPS) measurements. Indeed, after cross-linking, a peak appears at 402 eV which corresponds to the 1s electrons coming from the quaternized nitrogen atoms of the cross-linked pyridine units (Fig. S3†). After 24 h of exposure to 1,4-diiodobutane vapor a degree of cross-linking around 50% is obtained, which is enough to ensure a good integrity of the formed objects in solution.¹⁷ This is further confirmed by the formation of well-defined nanoparticles with an apparent hydrodynamic radius (R_h,app) of 55 nm (see ESI† for more details) after dissolution of a cross-linked PS-b-P4VP film in a good solvent of both blocks, such as DMF (Fig. 4A).

The grafting of a homopolymer onto the top of the P4VP domains has been achieved through copper-catalyzed alkyne–azide cycloaddition (CuAAC) between the alkyne functionalized P4VP and α-methoxy-ω-azido-poly(ethylene glycol) (PEG-N₃). To avoid the dissolution of the PS-b-P4VP thin film during the grafting process, the reaction has been carried out in a methanol–water (1/1 v/v) mixture. As shown in Fig. 2, the “dot-like” surface morphology of the initial PS-b-P4VP film is deeply modified after the grafting step. Indeed, an ill-defined morphology similar to the one observed for PEG brushes grafted onto flat surfaces is rather observed, although some dot-like features can be hardly visualized in the background.¹⁸ In contrast, when the film is immersed in a methanol–water mixture that doesn't contain PEG-N₃, no significant change in the surface morphology of the film has been observed (Fig. S4†). The presence of PEG onto the film after the grafting step is further confirmed by XPS analysis which shows a significant increase (from 0.09 to 0.25) in the O/C ratio after the grafting step (Fig. 3). The wetting properties of the film are also affected by the grafting. Indeed, as expected, the water contact angle of a PEG-grafted film is shifted towards smaller values, indicating a more hydrophilic surface (Table 1). Another indication of the successful grafting of PEG is the slight increase (≈5 nm) in the thickness of the film after the grafting step.

Fig. 2 AFM height and phase pictures of the PEG-grafted PS-b-P4VP thin film (35 nm thick).

Fig. 3 XPS wide scans of PS-b-P4VP films before and after grafting of PEG.

Table 1 Static water contact angles for pristine, alkyne functionalized and PEG-grafted PS-b-P4VP thin films

	PS-b-P4VP	Functionalized PS-b-P4VP	PEG-grafted PS-b-P4VP
θ (°)	83	99	68

---

Finally, the non-centrosymmetric Janus nanoparticles have been recovered by dissolution of the PEG-grafted films in a good solvent of the PS blocks forming the film matrix. The particles have been characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic force microscopy (AFM). DLS measurements reveal, as expected, the presence of well-defined nanosized particles with a R_h,app of 60 nm and a polydispersity index of 0.18 (see ESI† for more details). Their R_h,app is also slightly bigger than the one of the nanoparticles formed from a cross-linked but ungrafted PS-b-P4VP film (Fig. 4). The asymmetric architecture of the obtained nanoparticles is demonstrated by TEM images. Indeed, a less contrasted area, compatible with a PEG domain, is visibly attached on only one side of the cross-linked P4VP core, which appears dark in the unstained TEM images due to the presence of iodine and bromine atoms in its structure (Fig. 4A and B). The contrasted TEM pictures (Fig. 4C and D) also reveal the non-symmetrical corona of the formed nanoparticles. In those pictures, the P4VP core and the PS corona have been stained by RuO₄ vapors, while the PEG chains are unstained. In comparison, no similar asymmetrical features are observed on the TEM images of particles obtained from ungrafted PS-b-P4VP films (Fig. S6†). AFM pictures of the obtained Janus nanoparticles deposited onto a silicon wafer do not reveal any asymmetry (Fig. S7†). This can be explained by the preferential absorption of the PEG corona onto the Si substrate which leaves only the PS corona visible from the top, a behavior previously observed for other polymeric Janus particles.⁸

Fig. 4 Size distributions of the particles obtained from PEG-grafted PS-b-P4VP and ungrafted PS-b-P4VP films in solution in DMF (top). TEM micrographs (bottom) showing the asymmetric structure of the nanoparticles obtained from PEG-grafted films without staining (A and B) and stained with RuO₄ (C and D).

Another characteristic feature of Janus particles is their ability to assemble into larger nanostructures when placed in a poor solvent of one face.⁸ In our case, when water (a poor solvent of the PS face) is added to a solution containing PEG-grafted nanoparticles in DMF the formation of larger (R_h,app = 300 nm) and quite well-defined objects (PDI = 0.2) has been observed (Fig. 5 and S8† for a TEM picture of those objects). This could be ascribed to the formation of clusters of Janus nanoparticles with the PS insoluble corona sticking together. In comparison addition of water to a suspension of ungrafted particles only leads to the formation of very large (R_h,app ≈ 1 μm) and ill-defined aggregates (PDI = 0.5) that precipitate out of the solution with time (Fig. S9†).

Fig. 5 Size distribution of the particles obtained from PEG-grafted PS-b-P4VP films in solution in a DMF-water mixture (1/1 v/v).

In summary we have developed a novel strategy to produce polymeric Janus nanoparticles using a nanostructured block copolymer thin film as template to achieve desymmetrization. The obtained nanoparticles present a narrow size distribution since both the thickness of the thin film and the lateral size of the nanodomains are well-defined. This method could be reproduced with block copolymers of different length and/or films with different thicknesses allowing thus the creation of nanoparticles with different sizes and aspect ratios. Moreover, Janus nanoparticles of various chemical natures could be generated by changing the nature of the block forming the matrix of the film and/or the polymer grafted onto the nano-domains.

Acknowledgements

EP and JPB are grateful to FRIA for financial support. BE and JFG are grateful to the ESF P2M.

Notes and references

1. L. Y. Wu, B. M. Ross, S. Hong and L. P. Lee, Small, 2010, 6, 503–507.
2. Y. Song and S. Chen, Langmuir, 2014, 30, 6389–6397.
3. S. Berger, A. Synytska, L. Ionov, K.-J. Eichhorn and M. Stamm, Macromolecules, 2008, 41, 9669–9676.
4. P. A. Suci, S. Kang, M. Young and T. Douglas, J. Am. Chem. Soc., 2009, 131, 9164–9165.
5. A. Walther, M. Hoffmann and A. H. E. Müller, Angew. Chem., Int. Ed., 2008, 47, 711–714.
6. A. Walther and A. H. E. Müller, Chem. Rev., 2013, 113, 5194–5261.
7. A. Walther, X. André, M. Drechsler, V. Abetz and A. H. E. Müller, J. Am. Chem. Soc., 2007, 129, 6187–6198.
8. R. Erhardt, A. Böker, H. Zettl, H. Kaya, W. Pyckhout-Hintzen, G. Krausch, V. Abetz and A. H. E. Müller, Macromolecules, 2001, 34, 1069–1075.
9. P. A. Rupar, L. Chabanne, M. A. Winnik and I. Manners, Science, 2012, 337, 559–562.
10. A. H. Gröschel, A. Walther, T. I. Löbling, J. Schmelz, A. Hanisch, H. Schmalz and A. H. E. Müller, J. Am. Chem. Soc., 2012, 134, 13850–13860.
11. E. B. Gowd, B. Nandan, M. K. Vyas, N. C. Bigall, A. Eychmüller, H. Schlörb and M. Stamm, Nanotechnology, 2009, 20, 415302.
12. B. Bharatiya, J.-M. Schumers, E. Poggi and J.-F. Gohy, Polymers, 2013, 5, 679–695.
13. R. Saito, A. Fujita, A. Ichimura and K. Ishizu, J. Polym. Sci., Part A: Polym. Chem., 2000, 38, 2091–2097.
14. P. S. Chinthamanipeta, Q. Lou and D. A. Shipp, ACS Nano, 2010, 5, 450–456.
15. R. C. Hayward, B. F. Chmelka and E. J. Kramer, Adv. Mater., 2005, 17, 2591–2595.
16. R. Saito, Macromolecules, 2001, 34, 4299–4301.
17. K. Ishizu, T. Ikemoto and A. Ichimura, Polymer, 1999, 40, 3147–3151.
18. R.-V. Ostaci, D. Damiron, S. Capponi, G. Vignaud, L. Léger, Y. Grohens and E. Drockenmuller, Langmuir, 2008, 24, 2732–2739.

---

Footnote

† Electronic supplementary information (ESI) available: Materials, procedures, synthesis and characterization of thin films and nanoparticles. See DOI: 10.1039/c5ra05290d

---