PILP can also be formed for organic molecular crystals. For these systems, they are much more stable against crystallization compared to their inorganic counterparts. This is likely an effect of the lower lattice energy of organic systems resulting in reduced crystallization tendency. Organic PILP has been described for glutamic acid, lysine, and histidine (Wohlrab, Cölfen, & Antonietti, 2005) as well as an organic azo pigment (Ma et al., 2009). In these cases, the PILP phase could be isolated and its composition analyzed (Wohlrab et al., 2005), showing that a substantial amount of the polymer is included in the PILP phase. What is needed for the generation of the organic PILP phase is a strongly interacting polymer. In case of charged amino acids, this can be a polyelectrolyte with opposite charge. An example is polyethyleneimine, which generates PILP with glutamic acid. Or in case of the azo pigment, a block copolymer was used with one block modified with parts of the pigment molecule, which also maintained a strong interaction between the polymer additive and the crystallizing molecules. When PILP droplets of organic molecules are left to crystallize without further manipulation, they can form porous microspheres as shown in Fig. 13.9. The pigment example also shows that the amount of strongly interacting polymer is very low. In that case, the molar ratio of the polymer to crystallizing molecule was 1:430. If PILPs are used to generate thin films, these films can have a hierarchical structure and can also be mesocrystalline (Jiang, Gong, Volkmer, Gower, & Coelfen, 2011; Jiang et al., 2013). The biggest disadvantage of PILP is that it is only present in a very small region of the phase diagram, as shown in the phase diagram of the glutamic acid–polyethyleneimine system (Jiang, Gower, Volkmer, & Coelfen, 2012). Since this is the solvent-rich region in the phase diagram with only a small amount of the crystallizing component, it is not possible to generate large amounts of PILP to use them in large-scale applications.

Ethylene Diamine Tetraacetate Acid Tetrasodium Salt (EDTA-4Na)

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Figure 13.9. SEM images at different magnifications of pigment microspheres of nanoplates obtained in mixed solvents of water and isopropanol (50:50, v/v) via a block copolymer ABABA-acetoacetyl-PEI-b-PEG. The molar ratio of PY181 and ABABA-acetoacetyl-PEI-b-PEG was about 430:1.

Figure reproduced from Ma et al. (2009) with permission of Wiley-VCH.