surfactant are of outmost importance for the determination of the final particle
morphology. In the case of nanoparticles of a binary blend of a hole-transporting
and an electron-transporting polymer, the composition of both phases (i.e., the
distribution of one polymer in the other one in the two phases) followed the
prediction of the Flory–Huggins theory [56]. The quantum efficiency of devices
produced with the nanoparticle blend were found to be improved compared to other
methods [57].
However, what happens if G s cannot be minimized and is stuck in a local
minimum on the energy landscape? In this case, the morphology is kinetically but
not thermodynamically stable. In addition, other factors such as the crystallization
of polymers can also significantly influence the particle morphology [41, 58]. Chen
et al. showed that the viscosity of the liquids played a significant role in the
morphology of particles composed of polystyrene (PS) and poly(methyl methacrylate) (PMMA) [59]. Indeed, PMMA partially encapsulated PS for high molecular
weight polymers whereas the contrary was observed for low molecular weight
polymers. Okubo et al. investigated the effect of different stabilizers on the morphology of PS/PMMA particles [33]. Particles stabilized with poly(vinyl alcohol)
displayed small dimples whereas for SDS acorn and spherical structures with
increasing amount of SDS were observed. Both phenomena were explained by
the interplay of solvent evaporation and stabilization by the surfactant. Although
high amounts of SDS stabilized both the PS and the PMMA interfaces to water
equally well, this was not the case for smaller amounts. Therefore, bowl-like
PMMA shells were formed, in which the PS slowly hardened upon further evaporation of the solvent. As the PS contracted because of the ongoing evaporation of
solvent, bowl- or dimple-like structures were obtained. These effects were not
observed when dichloromethane was used instead of toluene as solvent to be
evaporated. This was explained by the fact that toluene is preferentially partitioned
in the PS phase, which is not the case for dichloromethane. The molecular weights
of the PS and PMMA were also found to play a role on the morphology
[34]. Whereas the interfacial tension polymeric droplet against aqueous phase
was not dependent on the molecular weight, the interfacial tensions between PS
and PMMA in the droplets increased with increasing molecular weight and
snowman-like particles could be obtained for high molecular weight polymers.
Besides the well-known core–shell and inverse core–shell [60], or acorn and
Janus morphologies obtained with polymers with similar polarities [33], other
interesting structures can be formed. Half-spherical structures are accessible
by removing the liquid in acorn structures formed with a polymer and a liquid
[40, 61]. Onion-like structures are created from block copolymers, for which the
phase separation into lamellae causes layered structures that follows the curvature
of the particle where they are confined [37, 40, 62]. The diameter of the particle is in
this case also very important. Indeed, if the particle size is low enough, core–shell
or Janus structures can be obtained [40]. A large variety of different structures were
predicted from simulations on diblock copolymer/homopolymer blends [63], startriblock copolymers in spherical nanopores [64], or on diblock copolymers under
different confinements [65–67], among which several have already been prepared
336
R.H. Staff et al.
morphology. In the case of nanoparticles of a binary blend of a hole-transporting
and an electron-transporting polymer, the composition of both phases (i.e., the
distribution of one polymer in the other one in the two phases) followed the
prediction of the Flory–Huggins theory [56]. The quantum efficiency of devices
produced with the nanoparticle blend were found to be improved compared to other
methods [57].
However, what happens if G s cannot be minimized and is stuck in a local
minimum on the energy landscape? In this case, the morphology is kinetically but
not thermodynamically stable. In addition, other factors such as the crystallization
of polymers can also significantly influence the particle morphology [41, 58]. Chen
et al. showed that the viscosity of the liquids played a significant role in the
morphology of particles composed of polystyrene (PS) and poly(methyl methacrylate) (PMMA) [59]. Indeed, PMMA partially encapsulated PS for high molecular
weight polymers whereas the contrary was observed for low molecular weight
polymers. Okubo et al. investigated the effect of different stabilizers on the morphology of PS/PMMA particles [33]. Particles stabilized with poly(vinyl alcohol)
displayed small dimples whereas for SDS acorn and spherical structures with
increasing amount of SDS were observed. Both phenomena were explained by
the interplay of solvent evaporation and stabilization by the surfactant. Although
high amounts of SDS stabilized both the PS and the PMMA interfaces to water
equally well, this was not the case for smaller amounts. Therefore, bowl-like
PMMA shells were formed, in which the PS slowly hardened upon further evaporation of the solvent. As the PS contracted because of the ongoing evaporation of
solvent, bowl- or dimple-like structures were obtained. These effects were not
observed when dichloromethane was used instead of toluene as solvent to be
evaporated. This was explained by the fact that toluene is preferentially partitioned
in the PS phase, which is not the case for dichloromethane. The molecular weights
of the PS and PMMA were also found to play a role on the morphology
[34]. Whereas the interfacial tension polymeric droplet against aqueous phase
was not dependent on the molecular weight, the interfacial tensions between PS
and PMMA in the droplets increased with increasing molecular weight and
snowman-like particles could be obtained for high molecular weight polymers.
Besides the well-known core–shell and inverse core–shell [60], or acorn and
Janus morphologies obtained with polymers with similar polarities [33], other
interesting structures can be formed. Half-spherical structures are accessible
by removing the liquid in acorn structures formed with a polymer and a liquid
[40, 61]. Onion-like structures are created from block copolymers, for which the
phase separation into lamellae causes layered structures that follows the curvature
of the particle where they are confined [37, 40, 62]. The diameter of the particle is in
this case also very important. Indeed, if the particle size is low enough, core–shell
or Janus structures can be obtained [40]. A large variety of different structures were
predicted from simulations on diblock copolymer/homopolymer blends [63], startriblock copolymers in spherical nanopores [64], or on diblock copolymers under
different confinements [65–67], among which several have already been prepared
336
R.H. Staff et al.