| dc.description.abstracteng | Synthetic polymers on surfaces are excellent models to mimic biological
systems which are highly sophisticated in their responsive behaviour to
environmental changes. To expand the application possibilities of smart
polymer surfaces as modern devices like sensors, microfluidic channels
or even next-generation computer chips, the development of new surface
analyzing techniques is required. In the context of this thesis, tethered
polymers were synthesized and comprehensively analyzed using metal
induced energy transfer (MIET) to establish this technique in polymeric
surface science. With the gained insight of surface-tethered polymers, the
generation of nanostructures mediated by constrained dewetting of homopolymers
was investigated.
MIET is a novel method that allows for the measurement of accurate
distances at the scale of macromolecular systems using fluorescence lifetime
imaging. This technique offers the possibility to investigate single molecules
attached to a silicon dioxide spacer on a thin gold layer. To exploit this
potential, the preparation of structurally well-defined polymer chains is indispensable.
Using reversible addition-fragmentation chain transfer (RAFT)
polymerization, the goal of synthesizing immobilized polymer chains of
desired grafting density, molar mass and end-group functionalization could
be accomplished. Specifically, by the thorough investigation of different
reaction conditions, optimum polymerization parameters were established
to prepare MIET substrates with tethered poly(2-(dimethylamino)ethyl
prop-2-enoate) (p(DMAEMA)), a pH-responsive polymer. This achievement
provided the basis for exploring the scope of MIET in collaboration with
the Enderlein group (Third Institute of Physics, University Göttingen).
For the first time, MIET measurements could successfully be conducted
for polymer layers with grafting densities that range from individual tethered
chains to high density polymer brushes. It was observed, that the
layer thickness increases with increasing grafting density, which is in agreement
with theoretical descriptions known from the literature. In order to explore the range of applicability of the MIET technique, p(DMAEMA) was
exposed to aqueous solutions at different pH values and measurements
were performed to investigate the responsive polymer properties. Here,
monitoring of repulsive segment-segment interactions for small grafting
densities and additional repulsive interactions between adjacent polymer
chains was achieved. The results of the MIET measurements were also
confirmed by spectroscopic ellipsometry.
Within the second project of this thesis, it could be demonstrated that
only a few tethered homopolymer chains are required to generate tailored
nanostructured surfaces. The systems of choice were linear and four-arm
star polystyrene samples prepared by RAFT polymerization on gold substrates.
Without activation or chemical modification of the gold substrate,
polymeric molecules of both architectures could be immobilized only via
their sulfur-containing RAFT group. Subsequently, by means of an optimized
dewetting technique, a variety of spherical and worm-like micelles
as well as network structures were obtained. The nanostructures were
specifically tuned by the adjustment of different grafting densities and the
solvent quality during the dewetting procedure.
As the visual evaluation of the formed morphologies can be misleading,
a strategy to quantitatively analyze the differently shaped domains was developed.
An unambiguous characterization was achieved using Minkowski
quantities and completed by the specification of the obtained domains
through calculation of circularities.
Furthermore, the assembly of gold nanoparticles on planar gold substrates
featuring polystyrene layers. Gold nanoparticles stabilized in aqueous
solution could be precisely arranged within a preformed polymeric
nanostructure. The latter prevented the uncontrolled aggregation of the
gold nanoparticles, and instead mediated the formation of a dense nanoparticle
monolayer. In contrast, the immersion of uniform polystyrene layers
in dispersions of gold nanoparticles in toluene yielded multicomponent
aggregates of tunable size, which could be controlled by the number of
tethered polymer molecules. | de |