# Textured and Hierarchically Constructed Polymer Micro- and Nanoparticles
## 4 Discussion
### Hierarchically Constructed Particles  
The design principle of the hierarchically structured three-level system by in situ photopolymerization of acrylamide monomer solution with dispersed poly-(MMA/TPGDA) composites in it was successfully demonstrated.  
Compared to Section 4.1, the obtained PMMA nanoparticles were roughly round-shaped with indications of surface substructures (Figure 5). This lower degree of texture can be attributed to the different synthesis conditions (higher temperature: 97 \({}^{\circ}\)C vs. 85 \({}^{\circ}\)C, monomer content 97% + 3% (MMA + GCDMA) vs. 38%, and initiator concentration 30 mM vs. 11.6 mM, and the presence of polyelectrolytes) and agrees with the model discussed above. For the approach of hierarchical assembly, the exact shape of these PMMA particles is of minor importance. However, it is crucial to create forces between these nanosized particles and the next higher size range. Surface functionalization with coupled molecular ligands or surfactants gives the ability to create specific characteristics: hydrophobic or hydrophilic properties as well as positive or negative charge. We achieved this by electrostatic interactions. Other methods might use covalent or hydrogen bonds. Unfortunately, we cannot directly quantify the surface charge of the poly-TPGDA particles because they are too large for zeta-potential measurements. Hence, it is difficult to compare them with PMMA nanoparticles in terms of surface potential. However, electrostatic assembly of particles of such different sizes was reported to be successful [15; 16]. [PERSON] et al. [15] used silica-micro- and alumina-nanoparticle multilayers coated with polyelectrolytes as starting materials, whereas in [16], submicron silica particles were grafted with polyacrylic acid and poly(2-dimethylamino)ethyl methacrylate, separately, and subsequently assembled. Here, we showed that the combination of polyelectrolyte-coated PMMA nanoparticles and chemically modified poly-TPGDA microparticles is also a successful approach.  
Packaging micrometer composite particles in even larger hydrogel particles has some additional benefits:  
* Particles can be stored, portioned, weighed, and handled more easily.
* These aggregates can be dried and re-wet without damaging the composites inside.
* The hydrogel structure allows for the diffusion of molecules in- and outside and thereby mechanically shields the composites.  
These properties are especially important if the core particles are further modified for catalytic or sensor purposes. The polyacrylamide-methylenebisacrylamide copolymer conforms to this, being used as a standard hydrogel in life sciences. Because of the high polymerization rate, acrylamide is particularly suitable for in situ photopolymerization. The problem of sedimentation of the precursor particles in monomer solution was mitigated, though not resolved completely, by adding rheologic modifiers. Such components remain unchanged in the hydrogel matrix and may deteriorate its physical and chemical properties. A more elaborate method would involve integrating them chemically into the polymer matrix or using more viscous pre-polymers.  
Conceptualization, J.M.K. and K.-P.K.; methodology, R.M.-S., X.Z., and J.M.K.; software, X.Z.; validation, J.M.K. and K.-P.K.; formal analysis, J.M.K.; investigation, R.M.-S. and X.Z.; resources, J.M.K.; data curation, R.M.-S., X.Z., and K.-P.K.; writing--original draft preparation, K.-P.K. and R.M.-S.; writing--review and editing, K.-P.K. and J.M.K.; visualization, R.M.-S.; supervision, J.M.K.; project administration, J.M.K.; funding acquisition, J.M.K. All authors have read and agreed to the published version of the manuscript.  
The work is partially supported by DFG (K01403/39-3).