of curvature of the block layers is much higher; therefore the layers adopt an onionlike morphology to conserve the total volume.
In the case of nanocapsules consisting of low molecular weight PS 76 -bPMMA 79 , patchy structures were observed in SEM and TEM (Fig. 57). In SEM,
these patches with a diameter of 15 nm appear bright whereas they appear dark in
TEM. The patchy structure is due to the weak phase separation characteristic for
values of 10.5 > χN > 6. The same patchy structures were also observed on the
nanoparticles made of low molecular weight PS-b-PMMA. XPS measurements on
nanoparticles and nanocapsules made of low molecular weight PS-b-PMMA show
that both PS and PMMA are located on the surface. PMMA constitutes the main
part (74.8% on nanocapsules and 77.7% on nanoparticles), which is probably due to
the higher polarity of the PMMA. From the XPS measurements it can be deduced
that the surface of the PS 76 -b-PMMA 79 nanoparticles contains slightly more
PMMA than PS 76 -b-PMMA 79 nanocapsules does, whereas more PMMA is
observed on nanocapsules than on nanoparticles for P(S 838 -b-MMA 945 ). Patchy
structures were not only observed on the surface of the nanoparticles and
nanocapsules, but also on the inside (Fig. 57). The images show that there is no
well-defined lamellar structure as observed in nanoparticles and nanocapsules made
of high molecular weight PS-b-PMMA. There is an irregular sequence of brighter
and darker domains, which can be assigned to PMMA and PS, respectively.
Nanoparticles, half-spherical nanoparticles, and nanocapsules show this structure.
Roughly, the size of these domains can be determined to be between 10 and 20 nm.
They exhibit approximately the same size as the patches observed on the surface of
the nanoparticles and nanocapsules with SEM and TEM. In the case of the
nanocapsules, cross-sections were prepared. Therefore, the patches outside of
the nanocapsules are due to cuts apart from the equator of the capsules through
the capsular wall.
The examples show the influence of the confinement on polymers. Nanoparticles
and nanocapsules of PS-b-PMMA with different molecular weights were prepared.
Their morphology could be precisely tuned by changing the amount of surfactant
and hexadecane in double confinement. Also, the wall thickness of the capsules
could be controlled by the amount of hexadecane employed during formation using
a miniemulsion process with subsequent solvent evaporation.
In order to better understand the experimentally observed microphase separation
of block copolymers in confinement, it is instructive to consider an even simpler
system theoretically. Therefore, we consider a mixture of 50% A and 50% B
monomers in spherical confinement. The resulting structures depend on the interfacial tensions between the two species and the respective tensions between the
monomers and the wall. One would expect a Janus-type structure, as observed
in simulations of simple binary mixtures [222, 223], if the tensions towards the
confining wall are equal (resulting in a contact angle of 90
according to Young’s
equation). Alternatively, core–shell structures emerge if one type of particle is
strongly preferred and wets the wall. Sickle-like structures arise for finite contact
angles of less than 90
.
182
K. Binder et al.
In the case of nanocapsules consisting of low molecular weight PS 76 -bPMMA 79 , patchy structures were observed in SEM and TEM (Fig. 57). In SEM,
these patches with a diameter of 15 nm appear bright whereas they appear dark in
TEM. The patchy structure is due to the weak phase separation characteristic for
values of 10.5 > χN > 6. The same patchy structures were also observed on the
nanoparticles made of low molecular weight PS-b-PMMA. XPS measurements on
nanoparticles and nanocapsules made of low molecular weight PS-b-PMMA show
that both PS and PMMA are located on the surface. PMMA constitutes the main
part (74.8% on nanocapsules and 77.7% on nanoparticles), which is probably due to
the higher polarity of the PMMA. From the XPS measurements it can be deduced
that the surface of the PS 76 -b-PMMA 79 nanoparticles contains slightly more
PMMA than PS 76 -b-PMMA 79 nanocapsules does, whereas more PMMA is
observed on nanocapsules than on nanoparticles for P(S 838 -b-MMA 945 ). Patchy
structures were not only observed on the surface of the nanoparticles and
nanocapsules, but also on the inside (Fig. 57). The images show that there is no
well-defined lamellar structure as observed in nanoparticles and nanocapsules made
of high molecular weight PS-b-PMMA. There is an irregular sequence of brighter
and darker domains, which can be assigned to PMMA and PS, respectively.
Nanoparticles, half-spherical nanoparticles, and nanocapsules show this structure.
Roughly, the size of these domains can be determined to be between 10 and 20 nm.
They exhibit approximately the same size as the patches observed on the surface of
the nanoparticles and nanocapsules with SEM and TEM. In the case of the
nanocapsules, cross-sections were prepared. Therefore, the patches outside of
the nanocapsules are due to cuts apart from the equator of the capsules through
the capsular wall.
The examples show the influence of the confinement on polymers. Nanoparticles
and nanocapsules of PS-b-PMMA with different molecular weights were prepared.
Their morphology could be precisely tuned by changing the amount of surfactant
and hexadecane in double confinement. Also, the wall thickness of the capsules
could be controlled by the amount of hexadecane employed during formation using
a miniemulsion process with subsequent solvent evaporation.
In order to better understand the experimentally observed microphase separation
of block copolymers in confinement, it is instructive to consider an even simpler
system theoretically. Therefore, we consider a mixture of 50% A and 50% B
monomers in spherical confinement. The resulting structures depend on the interfacial tensions between the two species and the respective tensions between the
monomers and the wall. One would expect a Janus-type structure, as observed
in simulations of simple binary mixtures [222, 223], if the tensions towards the
confining wall are equal (resulting in a contact angle of 90
according to Young’s
equation). Alternatively, core–shell structures emerge if one type of particle is
strongly preferred and wets the wall. Sickle-like structures arise for finite contact
angles of less than 90
.
182
K. Binder et al.