More recently, another type of hierarchical porous materials has been fabricated
via a kinetically controlled two-step process derived from a combination of
BCP SA and spinodal decomposition, so-called spinodal-decomposition-induced
macro- and mesophase separation plus extraction by rinsing (SIM
2 PLE) [48].
A strongly segregating amphiphilic BCP (e.g., polystyrene-block-polyethylene
oxide, PS-b-PEO) and a short oligomer (e.g., o-PEO) were chosen to prepare a
blend from solution. Upon solvent evaporation, the BCP/oligomer blend separated
into an oligomer-rich phase and a BCP-rich phase. In the BCP-rich phase, the
oligomer selectively swelled the PEO block because the oligomer energetically
prefers the PEO block to the PI block. Selective removal of the oligomer in the
resulting blend by rinsing with a selective solvent retained the structure of the two
distinctive phases and resulted in hierarchical pore formation: macropores from the
oligomer-rich phase and mesopores from the BCP-rich phase by rising away the
oligomer in both phases. Furthermore, varying the quench depth into the miscibility
gap by varying the evaporation speed via temperature allowed selection of different
BCP mesostructures via different degrees of BCP swelling with the oligomer. There
are several experimental requirements for induction of such spinodal decomposition. First, the amphiphilic BCP and the additive should be enthalpically repulsive
to generate two phases in the blend. Second, the volume fractions of BCP and
additive need to be in a regime where enthalpic repulsive forces dominate the
entropic driving force of mixing. Lastly, the resulting structure from the SA is
highly dependent on SA kinetics. Thus, the experimental conditions related to the
kinetics of SA, such as temperature and solvent choice, need to be carefully
controlled. First results suggested, however, that the process, which is dependent
on general thermodynamic considerations, may be quite general [48].
This section has summarized fabrication techniques for novel polymeric materials
with multiple structural characteristics. These techniques in part offer flexibility in the
choice of materials for structure direction, including oxides, semiconductors, and
metals. Thus, the techniques provide a useful toolbox for synthesis of various
nanostructured materials. The next section outlines in more detail the specific classes
of nanostructured materials that have been derived from BCP SA.
5 Classes of Block Copolymer-Derived Hybrid
Nanomaterials
A BCP is a good tool for the design of nanostructured materials, as discussed in
the previous sections. It offers a flexible platform for a variety of nanomaterials
with functionalities and robustness since one can combine BCP SA with a sol–gel
process, nanoparticle synthesis, chemical vapor deposition, electrodeposition, or
electroless deposition. Thus, in principle, the structural and material diversity
obtained from the aforementioned techniques is substantial. This section introduces
specific classes of material prepared from BCP-directed SA by the Wiesner group.
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K. Hur and U. Wiesner
via a kinetically controlled two-step process derived from a combination of
BCP SA and spinodal decomposition, so-called spinodal-decomposition-induced
macro- and mesophase separation plus extraction by rinsing (SIM
2 PLE) [48].
A strongly segregating amphiphilic BCP (e.g., polystyrene-block-polyethylene
oxide, PS-b-PEO) and a short oligomer (e.g., o-PEO) were chosen to prepare a
blend from solution. Upon solvent evaporation, the BCP/oligomer blend separated
into an oligomer-rich phase and a BCP-rich phase. In the BCP-rich phase, the
oligomer selectively swelled the PEO block because the oligomer energetically
prefers the PEO block to the PI block. Selective removal of the oligomer in the
resulting blend by rinsing with a selective solvent retained the structure of the two
distinctive phases and resulted in hierarchical pore formation: macropores from the
oligomer-rich phase and mesopores from the BCP-rich phase by rising away the
oligomer in both phases. Furthermore, varying the quench depth into the miscibility
gap by varying the evaporation speed via temperature allowed selection of different
BCP mesostructures via different degrees of BCP swelling with the oligomer. There
are several experimental requirements for induction of such spinodal decomposition. First, the amphiphilic BCP and the additive should be enthalpically repulsive
to generate two phases in the blend. Second, the volume fractions of BCP and
additive need to be in a regime where enthalpic repulsive forces dominate the
entropic driving force of mixing. Lastly, the resulting structure from the SA is
highly dependent on SA kinetics. Thus, the experimental conditions related to the
kinetics of SA, such as temperature and solvent choice, need to be carefully
controlled. First results suggested, however, that the process, which is dependent
on general thermodynamic considerations, may be quite general [48].
This section has summarized fabrication techniques for novel polymeric materials
with multiple structural characteristics. These techniques in part offer flexibility in the
choice of materials for structure direction, including oxides, semiconductors, and
metals. Thus, the techniques provide a useful toolbox for synthesis of various
nanostructured materials. The next section outlines in more detail the specific classes
of nanostructured materials that have been derived from BCP SA.
5 Classes of Block Copolymer-Derived Hybrid
Nanomaterials
A BCP is a good tool for the design of nanostructured materials, as discussed in
the previous sections. It offers a flexible platform for a variety of nanomaterials
with functionalities and robustness since one can combine BCP SA with a sol–gel
process, nanoparticle synthesis, chemical vapor deposition, electrodeposition, or
electroless deposition. Thus, in principle, the structural and material diversity
obtained from the aforementioned techniques is substantial. This section introduces
specific classes of material prepared from BCP-directed SA by the Wiesner group.
276
K. Hur and U. Wiesner