Among different possible morphologies, capsules (i.e., core–shell particles with
one liquid core) are often targeted morphologies for the protection and encapsulation of substances. When nanocapsules are produced from the emulsion–solvent
evaporation process, the liquid core material is usually non-functional. However,
functional non-solvents can also be used, for example in the form of self-healing
agents [31, 32] or of pH-responsive non-solvent [91]. The encapsulation of Grubbs
catalysts of monomers for a self-healing reaction based on ring-opening metathesis
polymerization by the mild emulsion–solvent evaporation method was found to be
advantageous over other methods. Although silica nanocapsules with a hydrophobic liquid core are porous and therefore cannot be used as fillers in a hydrophobic
matrix [92], it was not possible to encapsulate Grubbs catalysts in nanocapsules
fabricated by free-radical polymerization in miniemulsion polymerization [91].
Furthermore, whereas in interfacial step-growth polymerization the functional
units in the monomers needed to form the polymer shell can react with sensitive
products such as catalysts [94], the polymer building the nanocapsules shell can be
relatively chemically inert. For the pH-responsive core, tertiary amines with long
alkyl chains were embedded as liquid core in nanocapsules and could be released to
the continuous phase after protonation of the amine [91]. The diffusion of the core
out of the nanocapsules allowed for an unprecedented chemical transformation of
the liquid core from hydrophobic to aqueous. Finally, swollen PMMA nanocapsules
prepared by the emulsion–solvent evaporation technique could be elongated to a
core–shell ellipsoidal shape in an electrospinning jet [95].
2.3 Effect of the Nanoconfinement
Because the polymer nanoparticles prepared by the emulsion–solvent evaporation
process are highly pure, they are ideal samples for investigating the effect of
nanoconfinement on polymer properties such as crystallization. For the crystallization of poly(ethylene oxide) nanoparticles after evaporation of water from inverse
miniemulsions, a large supercooling was detected compared to the bulk material
[13]. Upon removal of water from the dispersion, the loosely packed poly(ethylene
oxide) lamellae slid apart, possibly generating single crystals. Recently, the crystallization of semicrystalline polymers such as syndiotactic and isotactic polystyrene as well as poly(L-lactic acid) (PLLA) was investigated [41]. For all polymers,
a decrease in crystallinity with decreasing particle diameter was observed. Both
syndiotatic and isotactic polystyrene nanoparticles showed anisotropic structures
because the crystallization induced a deformation of the otherwise spherical
particles (Fig. 7a, b). For PLLA, it was even possible to control the crystallinity
by heating the aqueous dispersion because PLLA cold-crystallizes below 100
C.
The extent of cold-crystallization upon heating the PLLA particles was found to be
larger for smaller particles than for the larger ones (Fig. 7c).
Recent Advances in the Emulsion Solvent Evaporation Technique for the. . .
339
one liquid core) are often targeted morphologies for the protection and encapsulation of substances. When nanocapsules are produced from the emulsion–solvent
evaporation process, the liquid core material is usually non-functional. However,
functional non-solvents can also be used, for example in the form of self-healing
agents [31, 32] or of pH-responsive non-solvent [91]. The encapsulation of Grubbs
catalysts of monomers for a self-healing reaction based on ring-opening metathesis
polymerization by the mild emulsion–solvent evaporation method was found to be
advantageous over other methods. Although silica nanocapsules with a hydrophobic liquid core are porous and therefore cannot be used as fillers in a hydrophobic
matrix [92], it was not possible to encapsulate Grubbs catalysts in nanocapsules
fabricated by free-radical polymerization in miniemulsion polymerization [91].
Furthermore, whereas in interfacial step-growth polymerization the functional
units in the monomers needed to form the polymer shell can react with sensitive
products such as catalysts [94], the polymer building the nanocapsules shell can be
relatively chemically inert. For the pH-responsive core, tertiary amines with long
alkyl chains were embedded as liquid core in nanocapsules and could be released to
the continuous phase after protonation of the amine [91]. The diffusion of the core
out of the nanocapsules allowed for an unprecedented chemical transformation of
the liquid core from hydrophobic to aqueous. Finally, swollen PMMA nanocapsules
prepared by the emulsion–solvent evaporation technique could be elongated to a
core–shell ellipsoidal shape in an electrospinning jet [95].
2.3 Effect of the Nanoconfinement
Because the polymer nanoparticles prepared by the emulsion–solvent evaporation
process are highly pure, they are ideal samples for investigating the effect of
nanoconfinement on polymer properties such as crystallization. For the crystallization of poly(ethylene oxide) nanoparticles after evaporation of water from inverse
miniemulsions, a large supercooling was detected compared to the bulk material
[13]. Upon removal of water from the dispersion, the loosely packed poly(ethylene
oxide) lamellae slid apart, possibly generating single crystals. Recently, the crystallization of semicrystalline polymers such as syndiotactic and isotactic polystyrene as well as poly(L-lactic acid) (PLLA) was investigated [41]. For all polymers,
a decrease in crystallinity with decreasing particle diameter was observed. Both
syndiotatic and isotactic polystyrene nanoparticles showed anisotropic structures
because the crystallization induced a deformation of the otherwise spherical
particles (Fig. 7a, b). For PLLA, it was even possible to control the crystallinity
by heating the aqueous dispersion because PLLA cold-crystallizes below 100
C.
The extent of cold-crystallization upon heating the PLLA particles was found to be
larger for smaller particles than for the larger ones (Fig. 7c).
Recent Advances in the Emulsion Solvent Evaporation Technique for the. . .
339