PhD Thesis
Self-Assembly of Block Copolymers in External Fields
Alexander Böker (01/2002-01/2002)
Support: Axel H. E. Müller
Summary
The influence of external fields on the
microdomain structure of block copolymers has been studied. Both surface
fields and electric fields have been considered.
The first part describes the synthesis and
characterization of ABC triblock copolymers aiming towards the
generation of
tailor-made substances for the controlled patterning of surfaces on
nanometer
scale. It is demonstrated that thin and ultrathin films of polystyrene-b-poly(2‑vinyl-pyridine)‑b-poly(methyl
methacrylate) and
polystyrene-b-poly(2-hydroxyethyl
methacrylate)-b-poly(methyl
methacrylate) block copolymers on silicon wafers reveal regular surface
patterns with worm‑, stripe- or island‑like morphologies. The
characteristic
spacings can be controlled via the molecular weight of the different
blocks of
the respective copolymers and the film thickness.
Thin films
(~ 20 nm thickness)
prepared by dip-coating from a polymer solution were found to exhibit a
phase-separated worm‑like surface morphology that presumably only
consists of
PS and PMMA microdomains with a characteristic lateral length scale
similar to
the bulk period L0. The
generation of such a striped surface pattern can be explained by complete coverage of the silicon
oxide surface by PHEMA or P2VP, resulting in a thin film
structure that
consists of a homogeneous layer of the middle block adsorbed at the
substrate
covered with a laterally microphase-separated surface layer of PS and
PMMA
microdomains. The proposed
model for this morphology is in agreement with recent self-consistent
field
calculations.
In the case of the ultrathin films (thickness
<
7 nm), our results
demonstrate that adsorption of a block copolymer as an ultrathin film
leads to
a periodic surface domain structure (stripes), where both polar blocks
(B and
C) adsorb to the surface. Due to significant stretching of the adsorbed
blocks
the spacings between the domains are large for the rather low molecular
weight
block copolymers. The lateral dimensions correlate well with the
molecular
dimensions of the A and B/C blocks according to recently derived
scaling laws.
In the second part of this thesis external
electric fields are used to create macroscopically oriented bulk
samples.
In order to circumvent limitations associated with the application of
external
fields to melts of high molecular weight block copolymers and
multiblock
copolymers of complex architecture, a new solvent-based procedure
is
introduced, i.e. the block copolymer microdomains are aligned by
application of
an electric field (E ~ 1 ‑ 2 kV/mm)
during
solvent casting of bulk samples.
In order to elucidate the dominating parameters
and governing mechanisms, the microdomain orientation kinetics of
concentrated
block copolymer solutions exposed to a DC electric field is
investigated by
time-resolved synchrotron small-angle X-ray scattering (SAXS) at the
ID02A
beamline at the European Synchrotron Radiation Facility (ESRF) in
Grenoble,
France. As a first model system, a lamellar polystyrene-b-polyisoprene
block copolymer dissolved in toluene is used. The
orientation kinetics follows a single exponential behavior with
characteristic
time constants varying from a few seconds to some minutes depending on
polymer
concentration, temperature, electric field strength, and system size.
Furthermore, two mechanisms governing the electric
field alignment of a lamellar block copolymer from concentrated
solutions are
identified. It is shown that depending on the segregation power (c µ fP, c µ 1/T)
a single mechanism
dominates the orientation process, i.e. in a weakly segregated system
(low
concentration or high temperature) the migration
of boundaries prevails, whereas a stronger phase separated system
(high
concentration or low temperature) predominantly exhibits rotation
of grains.
In addition, the orientation kinetics slows
down with increasing polymer concentration,
which can be correlated to the respective solution viscosity and the
mechanism
of orientation. Moreover, the influence of the electric
field strength on the orientation kinetics is determined,
including a threshold value below which no electric field induced
orientation
could be achieved on the time scale of the experiment. The time
constants of
the fastest processes were in the range of 0.5 sec, reaching a
final
orientation described by order parameters of up to P2 = -0.35.
Finally, the variation of temperature yields control
of the governing
mechanisms at a fixed polymer concentration.
In additional studies, the dielectric contrast
of the block copolymer components was varied systematically (PS-b-PI,
PS-b-PMMA, PS-b-PtBMA, PS-b-PHEMA-b-PMMA,
PS-b-P2VP). It is found that a high dielectric contrast leads to
faster
alignment kinetics (e.g. the time constants of the fastest processes
for a PS-b-P2VP
diblock system in THF are in the range of 0.3 sec) and reduces the
threshold field strength (around 200 V/mm for PS-b-P2VP).
Furthermore, it could be shown that the
interplay between degree of phase-separation, solution viscosity and
dielectric
contrast is crucial to decide if a given polymer/solvent system can be
used for
electric field-induced microdomain alignment. For example, it was found
that
PS-b-PtBMA shows electric field-induced orientation
of the
microdomains while PS-b-PMMA does not. This can be
explained by the
larger interaction parameter cST
compared to cSM
leading to a phase-separated solution at lower viscosities. In a
similar way,
the introduction of a high dielectric constant middle block (PHEMA)
into a PS-b-PMMA,
which additionally enhances phase separation, is shown to be the key to
creating a well-performing methacrylate-based
block copolymer system for electric field induced alignment from
solution.
Finally, we could show that the even more
complex lamellar and core-shell cylindrical PS-b-P2VP-b-PtBMA
high molecular weight triblock copolymer systems could be oriented by
virtue of
an electric field from solution.
In summary, it was demonstrated that electric
field alignment of block copolymer domains from solution is a powerful
tool to
generate highly anisotropic bulk block copolymer samples. The large
variety of
parameters which we can control allows us to further improve the
preparation of
macroscopically aligned melt samples via solvent casting in the
presence of an
electric field.