Biomedical Engineering Reference
In-Depth Information
this way were reported by Nakahama in 1988 using a Poly(styrene-
b
-isoprene) combination [48]. There are now at least four copolymer
systems amenable to selective etching of a single component to
produce mesoporous materials: hydrolysis of polyesters [49],
hydrofluoric acid cleavage of akylsilxoanes [50], ozonolysis of
polydienes [51, 52], and UV-etching of poly(methylmethacrylate)
[16]. A selective etching functionality can also take the form of a
single cleavable linkage in the copolymer, such that the one phase is
freed from the other and can be subsequently removed by selective
solvents [53].
A summary of the reported, selectively sacrificial diblock
copolymers and relevant etching methods used to produce porous
materials is shown in Table 2.1. Rather few of these systems have
yet been explored as electrochemical film templates, largely owing
to difficulties in microphase alignment. A substrate-wetting layer
as thin as a single copolymer domain (≤10 nm) can be sufficient to
effectively block electronic contact with the working electrode and
prevent replication [54].
So far the most widely studied copolymers studied as precursors
for porous electrochemical film templates are poly PS-
b
-PMMA and
PFS-
-PLA.
Poly(styrene)-
b
-(methylmethacrylate) (PS-
b
-PMMA):
b
The
first system to be used to form a porous electrochemical template
by Thurn-Albrecht
in 2000 remains the most widely
studied copolymer film template [55]. PMMA is removed by
et al.
dosage
for a 1 μm thick film) and the degradation products rinsed away
in a selective solvent such as acetic acid. The majority polystyrene
component undergoes a cross linking reaction during UV exposure
that can render the template insoluble in organic solvents.
Poly(4-fluorostyrene-
b
-poly(lactide)
photodegradation under 254 nm UV irradiation (25 J/cm
2
Poly
(lactic acid) presents a particularly interesting sacrificial copolymer
component as it is very simple to degrade under mild chemical
conditions — namely soaking the material in a 0.05M NaOH
(PFS-
b
-PLA):
(aq)
solution [49]. In PFS-
-PLA, the fluorinated styrene majority block is
intended to improve the dielectric contrast between the two blocks
for ease of electric field alignment in comparison to the more widely
available poly(styrene-
b
b
-poly(lactide) (PS-
b
-PLA) [56].
Search WWH ::
Custom Search