Klaus Schröter

Unveiling the molecular mechanism of self-healing in a telechelic, supramolecular polymer network.

Reversible polymeric networks can show self-healing properties due to their ability to reassemble after application of stress and fracture, but typically the relation between equilibrium molecular dynamics and self-healing kinetics has been difficult to disentangle. Here we present a well-characterized, self-assembled bulk network based on supramolecular assemblies, that allows a clear distinction between chain dynamics and network relaxation. Small angle x-ray scattering and rheological measurements provide evidence for a structurally well-defined, dense network of interconnected aggregates giving mechanical strength to the material. Different from a covalent network, the dynamic character of the supramolecular bonds enables macroscopic flow on a longer time scale and the establishment of an equilibrium structure. A combination of linear and nonlinear rheological measurements clearly identifies the terminal relaxation process as being responsible for the process of self-healing.

Block copolymer nanotubes by melt-infiltration of nanoporous aluminum oxide.

N The use of shape-defi ning hard templates containing arrays of cylindrical nanopores, such as self-ordered nanoporous anodic aluminum oxide (AAO), [ 1 , 2 ] is a well-established synthetic approach to one-dimensional (1D) nanostructures. [ 3 ] However, generating specifi c functionalities requires control over mesoscopic structure formation processes occurring in the confi ned geometry of nanorods or nanotube walls, such as crystallization and phase separation. [ 4 ] The self-assembly of block copolymers (BCPs) in nanopores having hard confi ning pore walls has been considered as an attractive access to 1D nanostructures exhibiting mesoscopic fi ne structures as a second hierarchical structure level. Their nanoscopic domain structures could be converted into polymeric scaffolds containing mesoporous structures by post-infi ltration process steps including selective degradation of one of the blocks [ 5 ] or selective swelling of one of the components. [ 6 , 7 ] Since nanotubes can be exploited as nanoscopic containers, pipelines or separation media, it is highly desirable to fabricate tubular nanostructures, the walls of which consist of microphase-separated BCPs. Hence, potential functionalities of nanotubes and of BCPs could be combined. Thus, for example, nanotubes with walls composed of multiple concentric layers having different properties could be accessible in this way. Catalytic systems, micro reactorscharge storage systems, pipelines or sensors consisting of hallow/tubular nanostructures with multilayered walls may show superior properties as compared to device architectures based on corresponding soilid rodlike nanostructures. [ 8 ]

Thermal response in the splitting region of the dynamic glass transition

Abstract Calorimetric and shear responses are, for the first time, measured in the splitting region of the dynamic glass transition in a glass former. The results can be interpreted in terms of a breakdown of cooperativity with a characteristic length of order one nanometer.

Investigation of the different stable states of the cantilever oscillation in an atomic force microscope

We present an algorithm which allows us to identify all possible stable states of the cantilever oscillation of an AFM operated in the intermittent contact mode within the harmonic approximation. The oscillatory states are qualified as quasi-free, net-attractive and net-repulsive solutions. Using a generic model for the tip-sample interaction the influence of a number of important experimental parameters on the state of oscillation is systematically studied. The analysis gives conditions under which an AFM can be operated in a chosen state. As an exemplary experimental application we compare selected measurements on a semicrystalline polymer acquired in the net-repulsive and the net-attractive mode with simulations based on the approach introduced here. The experiments indicate that a small indentation below one nanometer in the net-attractive mode is enough to produce phase contrast.

Onset of the dynamic glass transition in poly(n butyl methacrylate)

Specific heat Cp, dynamic shear modulus G∗, and dielectric compliance e∗ are measured in the splitting region a ↔ α + β of poly(n butyl methacrylate). The characteristic length of the glass transition is estimated from the calorimetric data to be about one nanometer before the breakdown of cooperativity between 15 and 21°C. The merging of the dielectric α and β traces is between 70 and 80°C, more than fifty degrees higher.

Stability of the T∞FT < T∞α relation between Vogel temperatures of flow and glass transition against determination variants

Dynamic shear measurements in the frequency range from 10−4 to 500 rad/s at the flow and main transition of a polydisperse poly(vinyl acetate) and a monodisperse polystyrene sample are presented. For both samples the Vogel temperature of the flow transition T∞FT is smaller than the Vogel temperature of the main transition T∞α, independent of the criteria used for data evaluation. The difference between the two Vogel temperatures corresponds to results for samples with other molecular weight and polydispersity from the literature. The T∞FT<T∞α relation is discussed in terms of short (α) and long (FT) dynamic glass transitions in entangled polymers. The relation is explained by preaveraging of the energy landscape for the long flow transition by the short glass transition.

The synthesis and characterization of a nematic liquid crystalline network, 3. Network formation, dynamic and quasistatic mechanical behaviour

We monitor a crosslinking reaction forming a nematic network made from a main-chain liquid crystalline prepolymer, consiting of statistical distributed segments of tert-butylhydroquinone, sebacic acid, 4-hydroxybenzoic acid and itaconic acid using rheological measurements. We show that the percolation model describes the dynamical scaling properties of such systems. A power law behaviour of the storage and loss modulus at the gel point over the whole frequency axis is an evidence for it. The estimated exponent n is in good agreement with the theoretical predicted value of 0.25. The stress-strain properties of the synthesized networks can be described with a model which combines elasticity, yield phenomena and plasticity.