Asymmetric deformation of swollen microspheres on a water surface

Abstract

We demonstrate an approach to fabricating asymmetric particles simply by assembly of swollen spheres on a water surface and evaporation of the swelling agent. The swelling process plasticizes the polymer spheres, rendering them deformable. After assembly, the spheres are subject to the surface tension force of water, leading to the deformation of the spheres. Asymmetric particles can be collected after evaporation of the swelling agent.

---

Introduction

Polymer particles with anisotropic shapes have attracted substantial research interest because of their potential applications as building blocks for supramolecular structures,¹ as emulsifiers of Pickering emulsions² and in the fabrication of superhydrophobic surfaces.³ Various types of shape anisotropic particles have been synthesized by using polymer spheres as precursors. Dimers,⁴ multiplets,⁵ spheres with dimples,⁶ disklike particles⁷ and particles with other morphologies^8,9 have been synthesized by triggering phase-separation of the swollen spheres during the seeded polymerization processes. Patchy colloids have been realized by controlled deformation of biphasic colloidal particles¹⁰ and by templating against sphere clusters.¹¹ Ellipsoidal particles have been fabricated though stretching 3D arrays of particles embedded in a polymer matrix.¹² Particles with different morphologies have been achieved by plasma treatment¹³ or reactive ion etching³ of the top surface of 2D sphere arrays. Recently, asymmetric colloids have been produced through deformation of a 2D array of spheres self-assembling at an oil–hydrogel interface.¹⁴

In this article, we propose a facile approach of transforming spheres into anisotropic shapes simply by assembly of swollen spheres on a water surface and evaporation of the swelling agent. As illustrated in Fig. 1, first, certain amount of solvent is introduced into an emulsion of polystyrene (PS) or crosslinked PS (CPS) spheres to swell the spheres. Subsequently, the emulsion is mixed with propanol and dropped on the water surface with a syringe. The swollen PS spheres self-assemble on water surface to form a monolayer automatically upon contact with water. The spheres are subject to surface tension force of water, which stretches the particles laterally at the three-phase contact line. After evaporation of solvent from the spheres, asymmetric particles with unique shapes are formed.

Fig. 1 The scheme of fabricating asymmetric particles on a water surface.

Experimental

Materials

Styrene (≥99.0%), divinylbenzene (DVB, 80.0%), pyrrole (≥98.0%), and potassium persulfate (KPS, ≥99.0%) were purchased from Sigma-Aldrich. FeCl₃ (≥99.0%), xylene (≥99.0%), H₂SO₄, n-propanol and H₂O₂ were purchased from Sinopharm Chemical Reagent. All the chemicals were used as received without further purification.

Asymmetric deformation of spheres on a water surface

PS spheres of 850 nm and PS spheres crosslinked with DVB of 370 nm were synthesized with an emulsifier-free emulsion polymerization method.¹⁵ After washing by repeated centrifuge and redispersion in de-ionized (DI) water for three times, emulsions with 6 wt% PS or CPS spheres were prepared and put into 10 ml screw-cap glass vials, with 4 g in each vial.

For PS spheres, 0.24 g, 0.48 g, 0.72 g or 1.2 g xylene (m_x/m_PS, the weight ratios of xylene over the unswollen spheres: 1, 2, 3 or 5, respectively) was introduced into the emulsions. For CPS spheres, 0.24 g or 0.72 g xylene (m_x/m_PS: 1 or 3, respectively) was introduced to the emulsions. The addition of xylene was under 800 RPM magnetic stirring for all samples and after which the vials were sealed and stirred for 24 h. Then the emulsion was mixed thoroughly with equal volume of propanol under ultrasonication. Subsequently, a 100 μl sharp-end syringe was used to spread emulsion on the water surface, during which the tip of the syringe was just in contact with the water-surface.^16,17 In a typical process, 30 μl emulsion was injected onto the water surface in around 1 min manually. The spheres self-assembled on the water surface and a monolayer was formed. Xylene swelling in the spheres was allowed to evaporate at room temperature (20 °C) or 60 °C for 12 h. A microscopic cover glass was treated with piranha solution and washed with copious DI water. The glass slide was inserted obliquely into the water and lifted the particle array from water.

Coating the water-exposed side of spheres with polypyrrole (PPy)

To coat the bottom surface of PS spheres with PPy,¹⁸ after the emulsion was mixed with propanol, it was spread with a syringe on 20 ml of 0.04 wt% aqueous pyrrole solution (for CPS spheres, 0.08 wt% aqueous pyrrole solution was used) in a 50 ml beaker. After the self-assembly of spheres on water surface, 2 M FeCl₃ solution (Fe³⁺/pyrrole molar ratio was fixed at 2.33 for all experiments)¹⁹ was introduced from the bottom of the beaker with a syringe and the polymerization was allowed to proceed for 24 h at room temperature.

Characterization

The as-prepared samples were characterized with FE-SEM (S-4800, Hitachi Co., Japan) and samples were sputtered with an Au layer prior to examination. The feature sizes of the particles were estimated by software analysis (Smile View 2.2) of the SEM images. The values reported were the averages of 50 measurements of different particles. The size measurement of the swollen spheres was carried out on ZetaPALS (Brookhaven) potential and particle size analyzer.

Results and discussions

Asymmetric deformation of PS spheres

Marangoni effect^16,20 has long been employed to spread particle suspensions on a water surface.^21,22 In the present case, after PS spheres are swollen with xylene, the emulsion is mixed with propanol, which decreases the surface tension of emulsions. During the injection of the propanol-containing emulsion on water surface, the syringe tip was just in contact with water, allowing the formation of meniscus. The curved meniscus buffers the vertical flow to prevent breaking of water surface. Water surface tension is larger than that of the emulsion, causing the emulsion spreading outward immediately on the water surface upon contacting with water. The spreading facilitates the evaporation of solvent and layering of spheres on water surface. After evaporation of solvent at room temperature, monolayer of deformed particles can be collected. Four deformed PS particles, APS-1, APS-2, APS-3 and APS-5 were obtained, which are named after the weight ratios of xylene added into the emulsions over the unswollen spheres, m_x/m_PS.

As shown in Fig. 2a, APS-1 maintained spherical shape without obvious shape change when m_x/m_PS was 1. A small spherical ridge appeared on each particle of APS-2 and the top spherical cap were flattened (Fig. 2b). When m_x/m_PS was increased to 3, the deformed particles, APS-3 has been transformed to asymmetric flattened particles and the diameter of the particles enlarged from the original 850 nm to 1080 nm, as illustrated in Fig. 2c and d. At the same time, the uniformity of APS-3 decreased, with a standard deviation (S.D.) of 6.6%, larger than that of the original spheres, with a S. D. of 3%. When m_x/m_PS was further increased to 5, the final asymmetric particles were more flattened, with an enlarged diameter 1240 nm and a standard deviation of 12.3%. Some dimples are formed on the air-exposed side of samples APS-2, APS-3 and APS-5, as indicated by the arrows in Fig. 2b, c and e, which might be due to phase-separation of swelling agent with the polymer particles during evaporation.⁹

Fig. 2 SEM images of the particles APS-1 (a), APS-2 (b), APS-3 (c and d) and APS-5 (e and f), respectively. (f) is a sample scratched with a sharp needle. Scale bars, 500 nm.

Asymmetric deformation of CPS spheres

CPS spheres of 370 nm were also employed in the fabrication of asymmetric particles. ACPS-1 and ACPS-3 are fabricated from CPS emulsions with m_x/m_PS of 1 and 3, respectively. A tiny spherical ridge could be observed on the particles of ACPS-1, as indicated by the arrow in Fig. 3b. With the increase of xylene amount, ACPS-3 evolved to an asymmetrical shape, as indicated by Fig. 3c and d. Their diameter enlarged to 430 nm, undergoing a 16% increase from original sphere, which is less than the 27% increase for uncrosslinked spheres with the same amount of xylene, due to the decreased deformability induced by crosslinking.

Fig. 3 SEM images of the original CPS spheres (a), the deformed particles ACPS-1 (b), ACPS-3 (c and d) and the particles fabricated at 60 °C ACPS-1e (e and f) and ACPS-3e (g and h). Scale bars: 500 nm.

The deformation experiments were also conducted at an elevated temperature to investigate the influence of temperature. After the CPS spheres assembled on water surface, the containers were transferred to a 60 °C oven to evaporate the swelling agent. ACPS-1e and ACPS-3e are fabricated from CPS emulsions with m_x/m_PS of 1 and 3, respectively. For ACPS-1e, an obvious spherical ridge was produced, as illustrated in Fig. 3e and f. The average diameter of spherical ridge is 390 nm, slightly larger than the original sphere diameter. ACPS-3e were observed to be asymmetric particles of 445 nm, as illustrated in Fig. 3g and h. For both emulsions, the increase of temperature leads to greater deformation of the particles. Additionally, although the original CPS spheres have a standard deviation of 3.1%, similar to that of the PS spheres, all the four particles evolved from CPS spheres have a standard deviation of less than 5%, smaller than the counterpart fabricated from the uncrosslinked samples. It suggests that crosslinking of spheres helps keeping the uniformity of particles during the shape transformation.

The mechanism of the asymmetric deformation

To understand the driving force of the shape deformation, after self-assembly of the swollen PS spheres on a water surface, the water-exposed side of spheres was coated with PPy. Fig. 4a and b show the particles fabricated from the PS emulsions with m_x/m_PS of 1 and 3, respectively. Fig. 4c shows the particles obtained from the CPS emulsion with m_x/m_PS of 3. From Fig. 4a, it can be observed that most part of the particle surface is covered by PPy except a small spherical cap in the top, which suggests that the major part of swollen spheres is immersed in water while a cap is exposed in air. From Fig. 4b and c we can observe that the horizontal PPy film formed between the particles on water surface, denoted by the arrows, is at the same level with the spherical ridgeline of the particles,¹⁸ which indicates that the positions of the ridgeline and the three-phase contact line coincide with each other. Additionally, when emulsions of spheres swollen with different amounts of solvent are directly dripped on glass slide and allowed to dry, the spheres maintain their original spherical shape (ESI Fig. S1†). These phenomena hint that the shape transformation of the particles is caused by the surface tension force of water.

Fig. 4 SEM images of the PS particles with bottom surface coated with PPy, obtained from PS emulsions with m_x/m_PS of 1 (a) and 3 (b), from CPS emulsion with m_x/m_PS of 3 (c). (d) An illustration of the forces acting on a point at the three-phase contact line. γ_sa, γ_sl and γ_la are the surface tension forces of solid–air interface, solid–liquid interface and liquid–air interface, respectively. f is the tensile force from adjacent mass points within the PS sphere. (e) The shape transformation of the particle due to surface tension forces. (f) The volume contraction caused by solvent evaporation under the resultant force of three interfacial forces, γ_net. Scale bars: 500 nm.

As illustrated in Fig. 4a, the average diameter of the spherical caps exposed in air for the particles obtained from the PS emulsion with m_x/m_PS of 1 are measured to be 685 nm. The volume ratio of the cap to the bottom part immersed in water can be estimated to be 1 : 8.3 (the calculation is shown in ESI†). Since the spheres are held at the interface by surface tension force,^18,23 on the order of 10⁻⁷ N, much stronger than the gravitational force, on the order of 10⁻¹⁵ N (eqn (S1) and (S2), ESI†), the volume ratio of the two parts of the spheres mainly depends on the water contact angle of polymer spheres instead of the gravity of spheres. Thus, increase of m_x/m_PS is not supposed to alter the relative proportions of the two parts of the swollen spheres. Consequently, when water surface tension force stretches the PS spheres along the three-phase contact line to flatten them, the top cap with a much smaller volume becomes much flatter than the bottom part.

The size of the unswollen and swollen spheres were measured on particle size analyzer (Fig. S4†). For the unswollen PS sphere and CPS sphere, the measured sizes are 868 nm and 382 nm, both larger than the size determined from SEM image, probably due to the electric double layer in the emulsion. The swollen PS spheres in emulsions with m_x/m_PS of 1, 2, 3 and 5 were measured to be 976 nm, 1017 nm, 1041 nm and 1048 nm, respectively, corresponding to calculated volumes of 1.42, 1.61, 1.72 and 1.76 times of unswollen volume, respectively. The swollen CPS spheres in emulsions with m_x/m_PS of 1 and 3 had sizes of 424 nm and 443 nm, respectively, corresponding to volumes of 1.37 and 1.56 times of unswollen volume. With the increase of m_x/m_PS, the sphere diameter increases, suggesting absorbing of more xylene, and the rate of increase decreases. The volumes of the swollen spheres are much smaller than the sum of unswollen sphere and xylene added per sphere, which suggests that the spheres could not absorb all the xylene in the emulsion. Except that the spheres in emulsions with m_x/m_PS of 5 have a polydispersity index (PDI) of 0.063, all other spheres have PDI of 0.005, which indicates the uniformity of the swelling.

The glass transition temperature (T_g) of PS was reported to be ∼100 °C (ref. 24) and T_g of CPS is normally higher than PS because crosslinking restricts their molecular mobility. Since the experimental temperature (20 °C) is far below the T_g of them, the unswollen PS and CPS spheres are in a hard glassy state. The PS spheres still maintain their initial shapes after assembly on a water surface and being subject to interfacial force (Fig. S1a†). The addition of solvent enlarges the free volume of PS particles, enhancing the polymer segmental mobility and leading to a depressed T_g. The T_g of a polymer–solvent mixture can be predicted by the Kelley–Bueche (KB) equation:²⁵

where T_gp, T_gs, ϕ_p, ϕ_s, α_p and α_s are the T_g, volume fractions, thermal expansion coefficients of polymer and solvent, respectively. Through the increase of ϕ_s, the T_g of spheres can be successfully suppressed to be lower than 20 °C,²² transforming the spheres from glassy state to deformable rubbery state. The more solvent is added, the lower the T_g and the larger the deformability of spheres.

A deformation mechanism of the swollen spheres is proposed as illustrated in Fig. 4. A mass point at the three-phase contact line is subjected to the surface tension forces of solid–air interface, solid–liquid interface, liquid–air interface, denoted as γ_sa, γ_sl, γ_la, respectively, with values of 29.8 mN m⁻¹, 24 mN m⁻¹ and 71.9 mN m⁻¹, respectively according to previous reports.²⁶ γ_sa and γ_sl could not balance γ_la, resulting in a nonzero net force, γ_net, pulling the sphere along the three-phase contact line.

For a rigid unswollen sphere or sphere in emulsion of low m_x/m_PS, the tensile force from adjacent mass point, f, is large enough to resist γ_net, and the spherical shape is maintained. For a sphere in emulsion of higher m_x/m_PS, f could not counterbalance γ_net and the sphere is stretched along the three-phase contact line. The stretch causes the increase of f and appearance of ridges, changing the direction of γ_sa and γ_sl and causing a decrease in γ_net (Fig. 4e). Consequently, a balanced state could be reached. The lateral stretch is also energetically favourable since the water–air surface area is reduced. In the meantime, the evaporation of solvent results in the contraction of the sphere under the surface tension forces, also contributing to the flattening of particles (Fig. 4f). The evaporation of solvent elevates T_g of the particles and causes the loss of deformability. If the amount of swelling agent is limited, only a spherical ridge is formed. Increase of m_x/m_PS enhances the deformability of spheres, leading to the horizontal augmentation of particles along three-phase contact line and formation of asymmetric particles. The more solvent is used to swell the sphere, the more flattened the asymmetric particles are.

Conclusions

We have developed a facile approach for the fabrication of asymmetric particles. PS or CPS spheres are swollen with a solvent and then self-assemble on water surface. The addition of solvent plasticizes the polymer spheres, suppressing their T_g and rendering the spheres deformable. During the evaporation of solvent, the spheres are subject to surface tension force of water, which stretches the particles laterally along the three-phase contact line. Consequently, asymmetrically flattened particles are formed after solvent evaporation. Crosslinking of spheres contributes to a better uniformity of the asymmetric particles.

Acknowledgements

We thank the National Natural Science Foundation of China (No. 51302109), Natural Science Foundation of Jiangsu Province of China (BK20130144) and MOE&SAFEA for the 111 Project (B13025) for financial support.

References

1. A. Walther and A. H. E. Müller, Chem. Rev., 2013, 113, 5194; E. Duguet, A. Desert, A. Perro and S. Ravaine, Chem. Soc. Rev., 2011, 40, 941; S. C. Glotzer and M. J. Solomon, Nat. Mater., 2007, 6, 557; Z. W. Mao, H. L. Xu and D. Y. Wang, Adv. Funct. Mater., 2010, 20, 1053; I. D. Hosein, M. Ghebrebrhan, J. D. Joannopoulos and C. M. Liddell, Langmuir, 2010, 26, 2151; S.-M. Yang, S.-H. Kim, J.-M. Lim and G.-R. Yi, J. Mater. Chem., 2008, 18, 2177; B. Peng, F. Smallenburg, A. Imhof, M. Dijkstra and A. van Blaaderen, Angew. Chem., Int. Ed., 2013, 52, 6709.
2. Y. Zhu, S. Zhang, Y. Hua, H. Zhang and J. Chen, Ind. Eng. Chem. Res., 2014, 53, 4642; J.-W. Kim, D. Lee, H. C. Shum and D. A. Weitz, Adv. Mater., 2008, 20, 3239.
3. S.-H. Kim, S. Y. Lee and S.-M. Yang, Angew. Chem., Int. Ed., 2010, 49, 2535.
4. J.-W. Kim, R. J. Larsen and D. A. Weitz, J. Am. Chem. Soc., 2006, 128, 14374; E. B. Mock, H. De Bruyn, B. S. Hawkett, R. G. Gilbert and C. F. Zukoski, Langmuir, 2006, 22, 4037.
5. M. S. E.-A. J. W. V. H. R. Sheu, J. Polym. Sci., Part A: Polym. Chem., 1990, 28, 629; J.-W. Kim, R. J. Larsen and D. A. Weitz, Adv. Mater., 2007, 19, 2005; J.-G. Park, J. D. Forster and E. R. Dufresne, Langmuir, 2009, 25, 8903.
6. S.-H. Kim, A. D. Hollingsworth, S. Sacanna, S.-J. Chang, G. Lee, D. J. Pine and G.-R. Yi, J. Am. Chem. Soc., 2012, 134, 16115.
7. T. Fujibayashi and M. Okubo, Langmuir, 2007, 23, 7958.
8. L. Wang, L. Xia, G. Li, S. Ravaine and X. S. Zhao, Angew. Chem., Int. Ed., 2008, 47, 4725.
9. L. Tian, X. J. Li, P. P. Zhao, X. Chen, Z. Ali, N. Ali, B. L. Zhang, H. P. Zhang and Q. Y. Zhang, Macromolecules, 2015, 48, 7592.
10. S. Sacanna, M. Korpics, K. Rodriguez, L. Colon-Melendez, S.-H. Kim, D. J. Pine and G.-R. Yi, Nat. Commun., 2013, 4, 1688.
11. Y. Wang, Y. Wang, X. Zheng, G.-R. Yi, S. Sacanna, D. J. Pine and M. Weck, J. Am. Chem. Soc., 2014, 136, 6866.
12. Y. Lu, Y. D. Yin, Z. Y. Li and Y. N. Xia, Langmuir, 2002, 18, 7722; C. C. Ho, A. Keller, J. A. Odell and R. H. Ottewill, Polym. Int., 1993, 30, 207; M. K. Klein, N. R. Saenger, S. Schuetter, P. Pfleiderer and A. Zumbusch, Langmuir, 2014, 30, 12457; A. Mohraz and M. J. Solomon, Langmuir, 2005, 21, 5298; T. Ding, Z.-F. Liu, K. Song, K. Clays and C.-H. Tung, Langmuir, 2009, 25, 10218.
13. Q. Yan, F. Liu, L. Wang, J. Y. Lee and X. S. Zhao, J. Mater. Chem., 2006, 16, 2132.
14. B. J. Park and E. M. Furst, Langmuir, 2010, 26, 10406.
15. S. E. Shim, Y. J. Cha, J. M. Byun and S. Choe, J. Appl. Polym. Sci., 1999, 71, 2259.
16. J. T. Zhang, L. L. Wang, D. N. Lamont, S. S. Velankar and S. A. Asher, Angew. Chem., Int. Ed., 2012, 51, 6117.
17. Z. Y. Cai, D. H. Kwak, D. Punihaole, Z. M. Hong, S. S. Velankar, X. Y. Liu and S. A. Asher, Angew. Chem., Int. Ed., 2015, 54, 13036.
18. S. Fujii, M. Kappl, H. J. Butt, T. Sugimoto and Y. Nakamura, Angew. Chem., Int. Ed., 2012, 51, 9809.
19. S. P. Armes, Synth. Met., 1987, 20, 365.
20. L. E. Scriven and C. V. Sternling, Nature, 1960, 187, 186.
21. J. Yu, Q. Yan and D. Shen, ACS Appl. Mater. Interfaces, 2010, 2, 1922; A. Kosiorek, W. Kandulski, P. Chudzinski, K. Kempa and M. Giersig, Nano Lett., 2004, 4, 1359; Y. Li, N. Koshizaki, H. Wang and Y. Shimizu, ACS Nano, 2011, 5, 9403.
22. L. Zheng, Z. H. Ma, C. Geng and Q. F. Yan, Part. Part. Syst. Charact., 2013, 30, 812.
23. N. Vogel, J. Ally, K. Bley, M. Kappl, K. Landfester and C. K. Weiss, Nanoscale, 2014, 6, 6879.
24. J. T. Seitz, J. Appl. Polym. Sci., 1993, 49, 1331.
25. F. N. Kelley and F. Bueche, J. Polym. Sci., 1961, 50, 549.
26. D. Y. Kwok, C. N. C. Lam, A. Li, K. Zhu, R. Wu and A. W. Neumann, Polym. Eng. Sci., 1998, 38, 1675; Y. Li, J. Q. Pham, K. P. Johnston and P. F. Green, Langmuir, 2007, 23, 9785.

---

Footnote

† Electronic supplementary information (ESI) available: SEM images of dried swollen spheres not subject to surface tension force, the calculation of the volumes of the two parts of a sphere. See DOI: 10.1039/c6ra05259b

---