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).