384
11 Templated Systems
gyroids as well as simple, modulated and perforated lamellae [207]. For electrochemical applications, diblock copolymers with cylindrical guest material embedded in the
host polymer are of highest importance. Templates prepared from such soft composites are analogous to nanoporous track-etched and PAA membranes, but their characteristic size parameters fall in a different range. Besides, micellar [208] and gyroid
[209] templates have also been used.
The notation of the diblock copolymers follows the B–b–X scheme in which B is
the percolating and matrix-forming majority component, whereas X is the dispersed
(granular or fibrous) constituent, and “b” stands for identifying the first list element
as block. For template formation with nanochannels, electrically conductive materials are coated with a thin layer of the components of the diblock copolymers. The
formation of the thin surface layer usually takes place with spin coating followed by
the solvent evaporation. The structure of the diblock copolymer does not form spontaneously as the mixture is drained but requires annealing above the glass formation
temperature of both components. For the orientation of the nanocolumn-forming
minority component, an electric field perpendicular to the surface is often applied.
Then, the chemical bonds in the material of the nanofibres are partly destroyed either
chemically or with irradiation. This makes it possible to dissolve the fibres with a
suitable solvent. An advantage of the process is that the further steps do not involve
any etching process by using highly reactive media that can destroy the substrate;
hence, metals other than noble ones can be used as substrate. The template thus
produced is mesoporous since the diameter of the polymer fibres to be removed
usually falls in the diameter range of at most a few tens of nanometres.
Concerning the typical materials used for the preparation of diblock
copolymer templates (DCTs) [210–214], polystyrene (PS, 55–70%) and
poly(methylmetacrylate) (PMMA) as block-forming and fibre-forming components,
respectively, are commonly used. Key parameters for the formation of a uniform
diblock structure are the molecular weight of the polymers and their polydispersity
(desired values: ~1–5 × 10
4 Daltons and <1.1, respectively). For the spin-coating
step, a dilute solution of the polymeric ingredients in toluol is used (concentration: 0.5–3 wt.%). The subsequent annealing process at 150–190 °C for 1–2 days is
carried out with the application of an electric field perpendicular to the substrate. The
field strength applied is around 30–400 V μm
–1 . Without an electric field, lamellar
segregation may take place instead of cylindrical domains. The exposure the diblock
copolymer to UV light has a twofold impact since it degrades the PMMA domains and
simultaneously cross-links the PS matrix. Thereafter, the partly destroyed PMMA can
be dissolved, e.g., in acetic acid, hence developing the nanoporous structure with a
15–35-nm nanopore diameter and an interwire spacing about 150% of the pore diameter. Various other copolymer components are also applied for preparing nanoporous
systems for electrochemical applications [215–217]. Some surface images of the
thus produced templates can be seen in Fig. 11.10. Due to both the soft nature of the
templates and their sub-micrometre thickness, cross-sectional SEM images are too
difficult to make for either the empty of metal-filled templates obtained from diblock
copolymers.
11 Templated Systems
gyroids as well as simple, modulated and perforated lamellae [207]. For electrochemical applications, diblock copolymers with cylindrical guest material embedded in the
host polymer are of highest importance. Templates prepared from such soft composites are analogous to nanoporous track-etched and PAA membranes, but their characteristic size parameters fall in a different range. Besides, micellar [208] and gyroid
[209] templates have also been used.
The notation of the diblock copolymers follows the B–b–X scheme in which B is
the percolating and matrix-forming majority component, whereas X is the dispersed
(granular or fibrous) constituent, and “b” stands for identifying the first list element
as block. For template formation with nanochannels, electrically conductive materials are coated with a thin layer of the components of the diblock copolymers. The
formation of the thin surface layer usually takes place with spin coating followed by
the solvent evaporation. The structure of the diblock copolymer does not form spontaneously as the mixture is drained but requires annealing above the glass formation
temperature of both components. For the orientation of the nanocolumn-forming
minority component, an electric field perpendicular to the surface is often applied.
Then, the chemical bonds in the material of the nanofibres are partly destroyed either
chemically or with irradiation. This makes it possible to dissolve the fibres with a
suitable solvent. An advantage of the process is that the further steps do not involve
any etching process by using highly reactive media that can destroy the substrate;
hence, metals other than noble ones can be used as substrate. The template thus
produced is mesoporous since the diameter of the polymer fibres to be removed
usually falls in the diameter range of at most a few tens of nanometres.
Concerning the typical materials used for the preparation of diblock
copolymer templates (DCTs) [210–214], polystyrene (PS, 55–70%) and
poly(methylmetacrylate) (PMMA) as block-forming and fibre-forming components,
respectively, are commonly used. Key parameters for the formation of a uniform
diblock structure are the molecular weight of the polymers and their polydispersity
(desired values: ~1–5 × 10
4 Daltons and <1.1, respectively). For the spin-coating
step, a dilute solution of the polymeric ingredients in toluol is used (concentration: 0.5–3 wt.%). The subsequent annealing process at 150–190 °C for 1–2 days is
carried out with the application of an electric field perpendicular to the substrate. The
field strength applied is around 30–400 V μm
–1 . Without an electric field, lamellar
segregation may take place instead of cylindrical domains. The exposure the diblock
copolymer to UV light has a twofold impact since it degrades the PMMA domains and
simultaneously cross-links the PS matrix. Thereafter, the partly destroyed PMMA can
be dissolved, e.g., in acetic acid, hence developing the nanoporous structure with a
15–35-nm nanopore diameter and an interwire spacing about 150% of the pore diameter. Various other copolymer components are also applied for preparing nanoporous
systems for electrochemical applications [215–217]. Some surface images of the
thus produced templates can be seen in Fig. 11.10. Due to both the soft nature of the
templates and their sub-micrometre thickness, cross-sectional SEM images are too
difficult to make for either the empty of metal-filled templates obtained from diblock
copolymers.