# Synthesis of Polystyrene Particles with Precisely Controlled Degree of Concaveness
## 3 Results and Discussions
### Synthesis and Characterization of PS Particles with Controllable Concave  
Shape of polymeric particle is fixed once they are synthesized, mainly due to the fact that common polymers, such as PS and PDVB, have higher glass transition temperatures than room temperature. The key procedure in our synthesis is to induce the shape change in a controlled manner. This is achieved by a clever choice of solvent mixtures. We choose a good solvent for PS, DMF, to soften the particle, and more importantly, to dissolve the non-crosslinked PS chains in the particle [18; 29]. After removing the dissolved PS chains by centrifugation and wash, we found the particles buckled and the concaves enhanced due to the loss of the material. To demonstrate this dissolution effect, we examine the dimple PS particles with and without the centrifugation and wash. The results are shown in the SEM images in Figure 3. The fluffy solid material, distributed randomly on or near the unwashed particles, is believed to be the dissolved PS chains (Figure 3b). These polymer chains were collected during the wash and centrifugation step. By drying supernatant in the centrifugation tube after the particles are separated, we obtain the same fluffy solid material. We then checked the material by infrared spectroscopy and \({}^{1}\)H NMR, and it is indeed polystyrene (see Figures S1 and S2). It is natural to expect that with increasing fraction of DMF in the solvent mixture, more PS chains are dissolved by DMF. However, pure DMF can dissolve and destroy the dimple PS particles (Figure 4). To avoid complete dissolution, instead of using pure DMF we mixed it with water, a poor solvent for the polystyrene. Then by changing the fraction of the good solvent, DMF in the solvent mixture, we can change the amount of non-crosslinked PS chains dissolved and the level of buckling, and hence achieve precise control of the concaveness. The SEM images in Figure 5 demonstrate the successful implementation of the method. As the fraction of DMF increases, the degree of concaveness increases. Meanwhile, the size of the particle and the concaveness remain uniform in each sample. Figure 1: Schematic of (**A**) Polymerization of styrene with programmed feeding of cross-liker, DVB to form dimple PS particles; (**B**) formation of PS particles with controlled degree of concaveness. Solution 1: styrene, DVB and ethanol; Solution 2: styrene, AIBN, PVP and ethanol. Figure 2: SEM images of PS dimple particles. Scale bar: (**a**) 5 \(\upmu\)m; (**b**) 2 \(\upmu\)m.  
To quantify the effect of the DMF/Water mixing ratios on the degree of concaveness, we have used the maximum size of the open end of the concave as an indication of the concaveness. This maximum size is statistically calculated based on the size information obtained from the SEM images. Figure 6 shows the size of concave as a function of the DMF volume ratio (\(R=V_{\mathrm{DMF}}/V_{\mathrm{water}}\)) in the solvent mixture. The initial size of the concave is 500 \(\pm\) 20 nm. The overall trend of the size shows that the degree of concaveness increases smoothly and then reached a plateau with increasing \(R\). Specifically, when \(R\) is increased from 0 to 4, the length of the concave reaches 2.3 \(\pm\) 0.1 \(\upmu\)m. Then the size of  
Figure 4: SEM image of the concave PS particles after exposure to pure DMF. Most of the particles are dissolved and collapsed by the good solvent DMF. Scale bar: 2 \(\upmu\)m.  
Figure 5: SEM images of PS particles with controlled degree of concaveness. The particles are produced by suspending them in DMF and water mixtures with different volume ratios, \(R=V_{\mathrm{DMF}}/V_{\mathrm{water}}\). Form (**a**–**f**), \(R\) is 0:10, 2:8, 4:6, 6:4, 8:2, 9:1 respectively. Scale bar: 2 \(\upmu\)m.  
Figure 3: SEM images of the concave PS particles with (**a**) and without (**b**) the centrifugation and wash. The fluffy materials sticking to the dimple particles in (**b**) are believed to be the dissolved PS chains.