# Textured and Hierarchically Constructed Polymer Micro- and Nanoparticles
## 4 Discussion  
### Textured Nanoparticles  
The experiments showed that with MMA as well as DVB particles, textured surfaces can be produced. In the case of MMA, the particle diameter was about 400 nm, having surface substructures of about 60 nm. We previously reported flower-like PMMA particles of similar shape but larger size: particles about double the size and substructures about the four-fold size [34]. Those particles were obtained from a monomer phase that was 99% MMA + 1% EGDMA + 21.3 mM AIBN and the carrier phase was water with 0.11-0.56 g/L (1-5 mM per monomer unit) poly-N-vinylpyrrolidone (PVP) using the same equipment at 97 \({}^{\circ}\)C. The explanation is that smaller particles formed at earlier stages of the polymerization and grew and aggregated to larger ones, which was stimulated by PVP. However, the presence of extra surfactant, SDS, or CTAB in relevant concentrations prevented aggregation, yielding only spherical shapes. This is in contrast to the actual observation, where textured particles were found in the absence of PVP but with considerable amounts of SDS. The main differences between the experiments are:  
* The dilution effect of the monomer due to the high toluene content;
* Lower temperature and AIBN concentration;
* The presence of methanol in the carrier phase. The last factor is assumed to be of minor importance since it generates perfectly round particles [35; 36]. Nevertheless, it can play a role because methanol facilitates the passage of MMA into the aqueous phase. We assumed that due to factors 1 and 2, the polymerization rate was considerably lower, providing the particles more time and a higher probability to aggregate despite the electrically repulsive effect between them caused by SDS. Aggregation was further enhanced since the extra solvent toluene prolongs the time of the sticky gel phase until solidification. In the case of DVB, typical particles were about 150 nm in size, having 20 nm wide substructures. Here, the above-reported effects for MMA were observed (see Figure 3) when ionic detergents or polyelectrolytes were present. This is in line with the above assumption of polymerization rate and gel time because of the higher initiator concentration used and the faster solidification due to two double bonds per molecule instead of one. The use of nonionic surfactants was therefore essential for highly textured particles. Compared to ionic surfactants such as SDS or CTAB those substances are usually applied in higher dosages, although their critical micelle concentrations are very low (Bri52: 0.067 uM [38], Span20: 23 uM [39]). This may explain the small size of the observed substructures. In summary, we state that our particles are textured but not wrinkled and one order of magnitude smaller than those of [PERSON] et al. [11]. They agree fairly with our earlier model of flower-like particles [34]. The proposed mechanism is as follows: In the droplet generation unit (hole-plate device), the velocities of both the carrier and monomer phase are high (about 250 and 500 mm/s, respectively) compared to the hole dimensions (10 um). Thus, the jetting monomer phase is rapidly dispersed into very small droplets. The droplet size is modulated by the surfactant, which decreases the surface tension between the phases. Especially with nonionic tensides, the size is further reduced due to micelle formation. These droplets may also shrink due to a certain solubility of the monomer phase in the excess carrier phase but stay fluid until they enter the heating unit. When polymerization starts inside the droplets, they become increasingly gel-like, which is accelerated by the addition of crosslinkers. During this sticky gel interval, particles come into contact due to Brownian motion and van der Waals attraction, forming larger aggregates. The aggregation number is expected to be dependent on:  
* Temperature: When temperature increases, the gel interval decreases but the Brownian motion increases.