The SANS technique if coupled to macroscopic characterization methods like rheology, provides a powerful tool for the study of bulk materials in external fields. Our research activities focus on the structural response of polymers upon extension and shear deformation in space and time in in-situ experiments.
Theological measurements provide the necessary link between mechanical properties and molecular structure as determined from scattering methods. The dynamic moduli efficiently probe the relaxation time spectrum in polymer melts. Properties such as relaxation processes or plateau moduli are discussed within the tube picture for topological interactions and ideally show up as more or less pronounced humps in the loss modulus G''(ω) at ~1/τ.
Statistically crosslinked polymer networks based on homopolymers are studied in the light of the tube confinement model for the topological interactions. Static experiments on KWS1 are described perfectly with the affine premise of chain deformation including a non-affine contribution due to the constraining potential. The tube model explains naturally the anisotropic scattering patterns as well as principal axes as a function of deformation ratio.
W. Pyckhout-Hintzen, A. Botti (Univ. Rome),
E. Straube (Univ. Halle-Saale)
Branched polymers behave rheologically different from linear chains. Here, time-dependent phenomena at several length scales can be decoupled by freezing-in states. Quenched melts of partially labeled branched copolymers can therefore be studied by insitu SANS and extensional rheology to locally investigate structural relaxations on different hierarchical levels. The topological tube is now intimately related to the relaxation of stress.
M. Heinrich, W. Pyckhout-Hintzen
Branched Polymers in Solution
The formfactor of polyamides (Nylon-6) of architectures from simple linear to random-branched structures are studied in several solvents covering up to the good solvent limit. A knowledge of the conformation allows the development of structure-property relationships and the investigation of the effect of polydispersity on processing. Due to their crystalline nature and the fact that reamidisation occurs above the melting temperature, these nylons can not be investigated in the melt state. The hydrogen-bonding capabilities of nylon-6 in solution on the other hand can be disrupted using a trifluoroacetylation modification. This makes nylon-6 soluble in many common solvents and a theta solvent can be found so that the conformation of the polymer in the melt can be predicted.
H. Hermes, W. Pyckhout-Hintzen
One of the fascinating properties of polymer melts is their rheological behaviour. The observation of slow molecular motions on a microscopic (molecular) scale can be realized by neutron-spin-echo (NSE)-spectroscopy, which measures directly the single-chain dynamic structure factor S(Q,t) or the segment mean square displacement. In the limit of short chains, S(Q,t) is successfully described by the Rouse model. For large molecular weights, topological constraints (entanglements) are dominant. The most successful model to take them into account is the reptation model of deGennes, where the topological constraints are represented by a virtual tube. This model was corroborated in the limit of long chain systems by NSE experiments (see figure). The ultimate goal in polymer science is to predict the optimal composition and molecular architecture of a polymer system for specific macroscopic properties or specific applications ('molecular design') by revealing all relevant relaxation mechanisms.
Polymers in confining geometries
Polymer melts in confining geometries exhibit novel and unique features, in particular with respect to applications. For example, if polymers and nanoparticles are mixed, then the rheological behavior of the mixture could be completely different in comparison to the single raw materials. At present a molecular theory which predicts the properties of the composite just based on the ingredients does not exist. Therefore, the ultimate vision is to derive the macroscopic function from the microscopic situation. In order to achieve this goal, we study the influence of confining geometries on the polymer by means of neutron scattering techniques. In particular SiO2 nanoparticles and nanoporous alumina or silicon are very well suited model systems. Confinement effects are introduced if their diameters are comparable with the typical size of the chain dimensions. Neutron scattering techniques offer the unique possibility to observe single polymer chains in these systems on the molecular and microscopic length scale. For example, the single chain structure factor can be observed by neutron small-angle scattering experiments. The dynamical features can be studied by the neutron spin echo technology, i.e. by the dynamical structure factor S(Q,t)/S(Q,0). By that means changes in comparison to the pure melt can be directly followed. The figure shows the influence of nanoparticles on S(Q,t)/S(Q,0).
G. J. Schneider