Research

Soft Matter and Neutronscattering

We are trying to understand the beautiful world of soft matter by a variety of techniques. At present, we concentrate on polymer melts, polymers in solution that may form micelles or vesicles. In virtually all of these cases, internal interfaces determine the material behavior.

Our focus is on fundamental science with a mission to accomplish a broader societal impact, by trying to understand materials based on our results. We target mostly model materials synthesized in our laboratory or by our collaborators. We utilize fast field cycling NMR, dielectric spectroscopy and rheology to access the material behavior often called macroscopic properties. In many cases, understanding of these results requires the knowledge of the structure and dynamics at the nanoscale. Neutron and X ray scattering techniques are our most important tools to explore the structure in the region from 0.1 to 1000 nm, and the dynamics in a window from ps to several hundred ns. Typically we utilize small-angle neutron scattering and small-angle X ray scattering, neutron spin echo and quasi elastic neutron spectroscopy. Often, we complement scattering studies by transmission electron microscopy to advance development of models to analyze scattering data.

1 Synthesis

Figure 1. Vacuum synthesis line of our laboratory. We can reduce the pressure down to 10-7 mBar.

Often novel chemistry results in unique polymers. We are grateful to our collaborators and friends, currently sharing their materials with us: Dr. Allgaier, Prof. Newcomer, Prof. Nesterov, Prof. Nesterova, Prof. Sabliov, Prof. Watkins, Prof. Zhang.We utilize synthesis to obtain the best model polymers for our studies. Figure 1 illustrates our own vacuum line that can reach a pressure as low as 10^-⁷mbar. We often use anionic polymerization to synthesize polymers with a narrow molecular weight distribution, PDI = M_w/M_n < 1.1. We use mostly gel permeation chromatography to determine the PDI equipped with a multi-angle light scattering detector.

2 Local Characterization Techniques

Our most important techniques are dynamic light scattering, fast field cycling NMR, dielectric spectroscopy, rheology, transmission electron microscopy. Our group strongly contributes to our small-angle X ray scattering instrument at the synchtrotron of the Louisiana State University, the Center for Advanced Microstructures and Devices (CAMD).

2.1 Unified description of the viscoelastic and dielectric global chain motion…

… or chasing the Holy Grail of polymer science (Macromolecules 2011, 44, 7430–7437). Once knowing the microscopic dynamics of polymers chains, as revealed by neutrons the macroscopic properties can be calculated by one theory without any assumption. The following plots show the (a) mechanical and (b) the dielectrical properties of polymer chains. The full lines show the theoretical description, by one single theory without free parameters.

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Figure 2. (top) Rheology data and (bottom) dielectric spectra on polyisoprene with different molecular weights, represented by the number of entanglements Z. The lines illustrate the theoretical description.

Figure 2 a illustrates the loss modulus of entangled polymer melts obtained by oscillatory rheology and Figure 2b the dielectric loss of the same entangled polymer melts measured by broadband dielectric spectroscopy.

The lines depict a joint description of the rheology and dielectric data sets based on the tube model.

The result illustrates the insensitivity of dielectric spectroscopy to the so-called constraint release, which permits that reptation and contour length fluctuations can be well analyzed.

Constraint release has a substantial contribution to the rheology spectra. Using a combined analysis reduces the number of assumptions, and permit to identify the contribution of constraint release very precisely.

2.2 Power of Combining Dielectric Spectroscopy and Fast Field Cycling NMR

Figure 3. Susceptibility as a function of frequency from fast field cycling relaxometry at various temperatures.

Field cycling NMR ist sensitive to the nuclear spin. We can use isotopic labeling in concert with temperature variation to obtain information on the molecular relaxation processes, similarly to dielectric spectroscopy.

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Figure 4 Relaxation time as a function of the reciprocal temperature. Fast field cycling and dielectric spectroscopy can capture ~ 12 orders over magnitude in time.

The simplest approach ist to determine the relaxation time by dielectric spectroscopy and fast field cycling NMR using the same samples. Fig. 4 shows such a comparison capturing a time range of ~ 12 orders of magnitude

2.3 Microscopic origin of the terminal relaxation time of polymers in nanocomposites

Figure 5. (a) Dynamic correlation function S(Q,t), normalized to the static structure factor S(Q) as a function of time t, by neutron spin echo spectroscopy. (inset) Change of tube diameter due to adding nanoparticles. (b) Lossmodulus G" as a function of frequency by rheology.

(a) Coherent intermediate scattering function obtained by neutron spin echo spectroscopy. It represents a microscopic view of polymer motion in nanocomposites. The height of the plateau levels increases. It indicates the compression of chains by solid surfaces. More excitingly, the detailed analysis shows a disentanglement of chains, i.e. the disappearance of the polymer tube if entangled polymer chains are close to solid substrates.(Soft Matter, 2011, 7, 7988)(b) Loss modulus obtained by oscillatory rheology. Adding nanoparticles changes the peak position that represents the so-called terminal relaxation time. It is directly related to applications, e.g. it determines the friction of car tires on the road. The inset in (a) visualizes the relationship between the terminal relaxation time and the plateau levels. In other words, the inset in (a) demonstrates that changes of the polymer dynamics due to addition of nanoparticles determine the quality of the tires.

4. Characterization techniques at large-scale facilities

4.1 Neutron Spin Echo Spectroscopy to Understand the Dynamics in Nanocomposites

Figure 6. Neutron Spin Echo Spectroscopy on polymers adsorbed with backbone and chain ends.

Figure 6 illustrates differences in the dynamics if polymers are adsorbed compared to polymers moving freely. In addition, polymer can attach with their backbones and with their end groups. The difference in the decay of the correlation function evidences fundamentally different processes that determine the relaxation if free and adsorbed polymers.

4.2 Small-Angle Neutron Scattering and In Situ Sonochemistry

Figure 7. Recently we introduced sonochemistry at a small-angle neutron scattering instrument. Using this novel tools, unprecedented experiments can be conducted.

Recently, we invented a tool to conduct in-situ sonochemistry while determining the structure by SANS. The example shows that micelles respond to ultrasound, but in a very reversible way. In future it will help to understand exchange processes more in detail.