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Adapted from "Spatial and Orientation Control of Cylindrical Nanostructures in ABA Triblock Copolymer Thin Films by Raster Solvent Vapor Annealing", ACSNano,
This nanosciece paper
describes a
novel approach to annealing polymeric thin films, particularly block copolymer
thin films. Self-assembly is a
significant phenomenon in these materials because they open the floodgates for
designer nanoscale materials for nanoporous membranes, lithographic masks, and
nanopatterning/templating applications (the last two have huge implications for
the electronics industry). These
three nanotechnologies often exploit morphologies often found in AB diblock and
ABA triblock copolymers (spheres, gyroid, and lamellae) because the
thermodynamics of bulk self-assembly is relatively well established. The morphology of bulk
block copolymers is influenced by three major factors: the degree of
polymerization (N), the interaction
parameter (c),
and the volume fraction of the blocks (f).
Adapted from "Spatial and Orientation Control of Cylindrical Nanostructures in ABA Triblock Copolymer Thin Films by Raster Solvent Vapor Annealing", ACSNano,
Jonathan E. Seppala, Ronald L. Lewis, III, and Thomas H. Epps, VOL. 6 ’ NO. 11 ’ 9855–9862 ’ 2012)
With thin films, surface energy
becomes an additional factor; it can be exploited by thermal annealing to
facilitate copolymer self-assembly via the bestowment of mobility to amorphous
regions that are trapped upon casting.
However, thermal annealing is limited to copolymer systems where the
components have similar γ‘s and are thermally insensitive. Another technique is solvent vapor
annealing (SVA), which grants mobility by effectively reducing the Tg of the copolymers. It is a powerful technique, but is
limited to small quantities that are typical of research labs.
The objective is to devise a faster
method for large-scale production of block copolymer (BCP)
nanotechnologies. The method must
enable control over morphology and orientation of BCP thin films. The authors propose raster solvent
vapor annealing (RSVA): solvent vapor from a bubbler system is directed onto a
BCP surface, which then creates a SVA zone. The zone is modified/expanded by a motorized stage moving in
a raster fashion.
RSVA was performed with a THF-rich vapor stream in single or
multiple passes over a 100 nm thick poly(styrene-b-isoprene-b-styrene)
(SIS) film, with domains of 29 nm.
The RSVA speeds ranged from 500 µm/s to 3 µm/s. The RSVA process swelled the
films due to hydrolysis, so the film thickness was measured by spectral
reflectometry. The swelling
increased the thickness to 160 nm, and eventually dried down to nearly the
original thickness, although some samples were reported to have residual
solvent.
Several approaches
to RSVA were performed. One was
single-pass, where the stage moved under the nozzle once, with only the speed
varying. The as-cast film had a lamellar structure consisting of cylinders
oriented parallel to the substrate, but with minimal long-range order. Varying the speed affected the
ordering of the lamellar cylinders.
The slower the speed, the longer the order-range; at 10 µm/s, the
cylinders have mostly perpendicular orientation. The 10 µm/s speed corresponds to an annealing time of 50
s. The cylinders even looked
slightly swollen, which was confirmed by azimuthally integrated 1D profiles from
FFTs of the AFM images. Even reducing the nozzle diameter
still induced the ^cylinders,
although at lower speeds.
The post-RSVA morphology is an
imbroglio of competing forces. The lower g for polyisoprene
(32.0 mJ/m2 vs. 40.7 mJ/m2), the majority block, enables
wetting of both the free and substrate surfaces, which leads to the propensity
for the cylinders to possess parallel orientation. High RSVA speeds do not change the orientation (see 2a–2c)
because the surface energy difference was too large for entropy to take
effect. The slow raster allowed
enough solvation to lower the differences in g,
which lets a) entropic effects to manifest, and to compensate for the
stretching experienced by the cylinders during swelling and deswelling.
Briefly,
Seppela et al tried two more
approaches. One is multiple passes
under the nozzle. Retracing the
RSVA pathways altered the cylindrical orientation toward perpendicularity. The other is a crossed-path approach;
two orthogonal passes cross each other, and the result is a domain dominated by
perpendicular cylinders. Both
approaches are supported by crisp AFM phase images.
This
is a wonderfully written paper, but it helps that I have significant background
in polymer chemistry. Note that
only THF was used in the vapor stream; it’s natural to ask if this approach has
been done with other solvents, and other BCPs. dTHF = 18.1
(MPa)1/2, dpolyisoprene
= 16.2 (MPa)1/2, dpolystyrene
= 18.6 (MPa)1/2, so
utilizing similar solubilities is a probable reason for the RSVA setup
described in the paper. One should
also ask if this technique can be done for other self-assembling polymeric thin
films. Seppela et al noted that this
annealing method can be altered according to slit geometries, solvent quality,
and substrate temperature–indications of much promise for RSVA.