60th Annual Report on Research 2015

  Figure 1a shows the schematics of the synthetic route we have followed. We first crosslink pre-synthesized polystyrene spheres based on seeded emulsion polymerization. The CPS are then swollen by toluene droplets which are pre-emulsified in water, forming a second lobe. Under the optical microscopy (Inset of Figure 1b), we observe dumbbell shape particles on which the second lobe is much larger than the original one and appears to be filled with toluene. In the final step, we remove the excess amount of toluene by quickly centrifuging the dumbbells solution in ethanol. What surprises us is that the dumbbells do not change back to the spherical shape of CPS as the toluene lobe is removed completely and we obtain highly monodisperse dimers (Figure 1b). Moreover, one of the two lobes is consistently buckled, while the other surface is smooth, forming the rough-smooth dimers. Further experiments show that some of the uncrosslinked polymer chains (typically located in the core of CPS) migrate to the newly-formed and toluene-swollen lobe. When the toluene removal speed is fast (e.g., via centrifugation), there is little time for the polymer chains in the second lobe to migrate back and a significant second lobe is preserved. On the CPS lobe, there is a mismatch in terms of the elastic moduli between the shell and core, where the shell modulus is higher than the core. Therefore, mechanical buckling develops during the fast removal of toluene, leaving rough surfaces on the original lobe.

Figure 1 Dimers formed by swelling CPS with toluene. (a) The schematics. (b) A large field of view of the dimers after toluene extraction. Scale bar: 2 µm.

The rough-smooth dimers made from our toluene-swelling-extraction method exhibit surprising anisotropy in surface charge distribution as well. The original CPS particles are negatively charged because of the addition of sodium 4-styrene sulfonate monomer during synthesis. When we mix our dimers with a positively charged dye - Rhodamine 6G, we find much stronger fluorescence intensity on the larger lobe (Figure 2a). This indicates that the negative charges remain on the original lobe and they do not migrate to the second lobe. In contrast, we find that dimers produced via the conventional method, i.e., styrene-swelling-polymerization, have symmetric charge distribution on both lobes regardless of the size ratio (Figure 2b).

Figure 2 Surface charge characterization on dimers. (a) The bright-field and corresponding fluorescent images of polystyrene dimers, coated with Rhodamine 6G. (b) Fluorescent images of conventional dimers indicating uniform surface charge distribution. Scale bar: 5 µm.

  Using toluene as a swelling agent can also help create functionalities on the second lobe. For example, we have tried to incorporate pH-responsive polymers into dimers. We mix different amounts of methacrylic acid with toluene, which is then used to swell CPS particles. Figure 3a shows the SEM images of dimers after polymerizing MAA and removing excess toluene. It is clear that the MAA polymerization induces significant mechanical buckling when the MAA concentration is relatively high (≥10 vol%). The morphology difference between the original and second lobes in Figure 3a-i indicates possible interfacial anisotropy where the buckled lobe consists of primarily PMAA and the original (spherical) lobe is primarily polystyrene. To test this, we add the dimers in a 50:50 vol% mixture of hexane (with dye Oil Red O) and water at different pH and vortex the solution for five minutes. After that, the vials are kept still on bench. And the emulsion stabilities are examined. As shown in Figure 3b-i, at pH 2, the carboxylic groups in PMAA are protonated. Therefore, both lobes of the dimers are hydrophobic and they prefer to stay in the oil phase. At pH 7, some of the carboxyl groups on dimer surfaces are partially deprotonated since the pKa of PMAA is ~5.5. Therefore, more particles can be suspended in the water phase. The particles are, however, not amphiphilic enough to stabilize emulsions for long time. Therefore, oil and water are phase-separated. At pH 12, essentially all carboxylic groups are deprotonated and the PMAA lobes are hydrophilic. The contrast in hydrophobicity between the polystyrene and PMAA lobes becomes significant enough so that oil-in-water emulsions (Figure 3b-ii) can be stabilized over a period of at least one month. In comparison, the dimers without PMAA incorporated in the second lobe do not show any pH responsiveness and cannot stabilize emulsions either. Therefore, our toluene-swelling method can promote the surface modification of the second lobe selectively, especially for hydrophilic and stimuli-responsive polymers, which is a significant challenge in dimer synthesis.

Figure 3 Synthesis and interfacial activity of pH responsive dimers. (a) Dimers with different amount of MAA incorporated during toluene swelling. Scale bar: 2 µm. (b) (i) Interfacial activity of dimers incorporated with 12.5 vol% MAA at different pH in hexane-water mixture. The dye Oil Red O is added in the hexane phase. (ii) Optical image of oil-in-water emulsions stabilized by dimers at pH 12. Scale bar: 100 µm. (iii) Dimers without MAA incorporated in a hexane-water mixture.

  Overall, this research funded by ACS-PRF allows the support of one Ph. D. student. Two papers based on this research have been published on the ACS Applied Materials and Interfaces and Langmuir, respectively.