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NA4, Polymer Based Complex Systems
Introduction

The use of new polymeric systems can broaden a lot of applications. Mechanical properties of such system as pure materials or with solvents have many potential applications (elasticity, tackiness, …).
At the basis of these properties are polymer dynamics on a molecular scale. They determine linear and non-linear bulk properties of polymeric materials. In this network area we rationalise these molecular origins by an approach encompassing simulation, experimental techniques with molecular sensitivity (neutron, NMR, dielectric spectroscopy, …) combined with rheological methods and theoretical modelling. Simulation efforts will be directed towards systems like copolymers, where dynamics and thermodynamics are important.
Chemically realistic modelling and coarse graining in combination with SCF theories, the simulation of multicomponent systems emphasizing aspects such as the dynamic miscibility, the effect of external forces and boundaries are envisaged.




Figure : (a) Small-angle neutron scattering intensities for a stretched H-polymer sample (two-dimensional detector image). (b) Intensity cuts along the directions parallel to the deformation (open circles) and perpendicular to the deformation (open squares) compared with theoretical calculations for the sample strained to  = 2 after annealing times “0” and 6  10–2 s at 25C (the sample was quenched to –85C). The term d/d(q) represents the absolute macroscopic cross section. (c) H in polymer in its confining tube immediately after deformation (the arms are fully confined), and after 6  10–2s, when the arms have relaxed by 12%.

The SoftComp Scientific Activity

Scientifically SoftComp covers a broad range of scientific activities with more than 120 researchers actively involved. Therefore it is impossible to describe their important results. Instead we present a few highlights I an exemplary way.








Network Area 4 Highlights




Microscopic observation of structural relaxation in systems with tunable nanoconfinement and dynamic asymmetry

We have exploited the selectivity of neutron scattering combined with isotopic substitution to study the structure and dynamics of poly(n-alkyl methacrylates). Our diffraction results unambiguously prove the nanosegregated structure suggested from previous X-rays studies. Thereby, these findings support the scenario of self-confinement of alkyl nanodomains by the more rigid main chains (see the enclosed Figure). On the other hand, our neutron spin echo (NSE) investigation has revealed the following behaviour: On the one hand, the structural relaxation of the 'confining matrix' is standard, since (i) the results at different temperatures collapse into a single master curve when the timescale is scaled with the viscosity temperature dependence and (ii) the functional form can be described by a stretched exponential (Figure, left). The same functional form describes very nicely the dynamic structure factor at the peak revealing the correlations within the confined alkyl subsystem for PEMA. However, a qualitatively different relaxation pattern is revealed for these correlations in the higher order members PBMA and PHMA: their decay is nearly perfectly logarithmic (Figure, right, for PBMA). We attribute this behaviour to the high dynamic asymmetry present in PBMA and PHMA: the matrices relax almost two orders of magnitude more slowly than the confined alkyl side groups. The measurements were performed at the Institut Laue-Langevin (ILL, Grenoble) in collaboration between the groups of San Sebastian (Univ. Basque Country), Montpellier (CNRS) and Jülich (FZJ-Richter).









Figure: Center: diffraction patterns of PEMA (number of alkyl carbons n=2), PBMA (n=4) and PHMA (n=6). Left: Master curve built with the NSE data of PBMA at different temperatures obtained at the low-Q peak (confining matrix correlations); the times have been scaled with the mechanical temperature dependence. Solid line is a fit to a stretched exponential. Right: NSE results on PBMA at 1.3Å-1 (correlations within the alkyl nanodomains). Lines are fits to logarithmic decays.






Constraint release by contour length fluctuations
The tube concept by now dominates the approaches for an understanding of linear and nonlinear rheology of branched polymers. In this concept, contour length fluctuations of a polymer chain in the tube as well as constrained released processes are the important ingredients of these theories. The figure displays schematically both processes: chain end fluctuations lead to a shortening of the effective tube length while the dissolving of entanglements allows lateral chain motions beyond the initial tube constraints (FZ Jülich, Richter group/Univ. Leeds). Using neutron spin echo spectroscopy it has been shown on a molecular level that constrained release is also created by contour length fluctuations. A novel process which has not been considered so far by these tube theories (see Figure 1).












Figure 1: Schematic presentation of the CLF and CR mechanisms: chain and end fluctuations lead to a shortening of the effective tube length while the dissolving of entanglements allow chain motions beyond the initial tube constraints.






Snapshot of a polymer

Figure 3 presents a snapshot of a polymer blend, where the dynamics of the component with a low glass transition temperature (Tg) (red) is dynamically confined by the frozen matrix of a high Tg component. This schematic picture visualizes an experimental result, where for the first time such confinement effects were observed directly by neutron scattering and also by computer simulation. These results may be important for a design of new plastisizers which would serve their main purpose without weakening the material (San Sebastian/Jülich).
















Figure 3: Snapshot of the confinement effect within a miscible polymer blend of components with very different glass transition temperatures Tg (red: low Tg; blue: high Tg).