Molecules at interfaces
Behavior and properties of polymers at interfaces and surfaces
In attempting to reduce the size of functional devices, the thickness of polymer films has reached values even smaller than the diameter of the unperturbed macromolecules. However, despite enormous efforts for more than a decade, our understanding of the origin of some puzzling properties of such thin films is still not satisfactory and several peculiar observations remain mysterious. For example, under certain conditions, such films show negative expansion coefficients or show undesirable rupture although they are expected to be stable. Some of these extraordinary effects can be related to residual stresses within the film, resulting from the preparation of these films from solution by fast evaporation of the solvent. Consequently, depending on thermal history and ageing time, such films show significant changes even in the glassy state, which we quantify by dewetting experiments and corresponding theoretical studies. Identifying the relevance of frozen-in polymer conformations gives us a handle for manipulating and controlling properties of nanometric thin polymer films.
In this context, entropic energy contributions have to be explicitly emphasized as they represent a key difference between polymers and simple molecules. A basic understanding of the influence of conformational changes is expected to explain the behavior of polymers at interfaces.
Typical questions that are investigated in the group concern topics of:
- wetting and dewetting, adhesion and friction
- stability and morphological changes due to intermolecular interactions
- aging and relaxation behavior of polymers in restrictive spaces
Schematic presentation of dewetting at A) a non-slipping and B) a slipping substrate.
C) Characteristic temporal evolution of dewetting (hole growth) for a 50nm thick polystyrene film (Mw = 52 kg/mol) at 120°C for 5, 30 and 60min, respectively. The size of each of the three images is 59 × 47 μm2.
3D-view (measured by atomic force microscopy) of a typical hole obtained by dewetting a polystyrene film on a PDMS-coated substrate at temperatures close to the glass transition of PS. Typical 3D-cross-section of B) the asymmetric shape of the rim at early stages and comparatively low temperatures and C) the more symmetric shape at late stages or at high temperatures.
Typical result for dewetting of a PDMS film on top of a densely grafted brush of end-functionalised PDMS molecules. A) Optical micrograph (top view) of dewetting. B) Temporal evolution of dewetted distance d and the width w of the rim, schematically represented in the inset.
Evolution of the residual stress relaxation time τ with dewetting temperature (T) for various chain lengths (as indicated). At low temperatures, τ follows the VTF behaviour. However, τ is significantly lower than the reptation time and also does not depend on molecular weight.
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