Dynamics of block co-polymer micelles and sponge phases probed by X-ray Photon Correlation Spectroscopy and Neutron Spin Echo
by
Peter Falus
→
Europe/Berlin
Bldg. 25b, 109
Bldg. 25b, 109
Description
X-ray photon correlation spectroscopy (XPCS) was used to characterize the wave-vector- and temperature-dependent dynamics of spontaneous thermal fluctuations in blends of a symmetric poly(styrene-ethylene/butylene-styrene) triblock copolymer (Mw=87k) with a polystyrene homopolymer (Mw=4k). XPCS is unique scattering technique which probes dynamics on molecular scales, while being sensitive to much slower motions than neutron spin echo. In the low concentration polymer blends small angle X-ray scattering and transmission electron microscopy measurements confirmed the existence of a vesicle (L4) phase. Measurements of the intermediate scattering function reveal stretched-exponential behavior versus time, with a stretching exponent slightly larger than 2/3. The corresponding relaxation rates show an approximate q3 dependence versus wave vector. Overall, the experimental measurements are well described by theories that treat the lamellar dynamics as movement of independent membrane plaquettes.
In the higher concentration blends a sponge (L-3) phase is present. Using XPCS the membrane dynamics was probed at much higher wavenumbers than the characteristic sponge wave number Measurements of the intermediate scattering function reveal an unexpected crossover from stretched- to compressed-exponential relaxations as the temperature is lowered from 180 to 120 degrees C. Additionally to the scientific results instrumental developments will be described, first the high speed detector which was developed to allow polymeric XPCS measurements. Finally the ongoing efforts to image polymeric fluids in flow cells will be presented. There are both developments at ILL to do neutron spin echo on polymer micelles under flow and at ESRF to do baseline XPCS measurements on colloidal solution under flow. The ultimate aim of these experiments is to characterize well the artifacts which arise in NSE and XPCS from the flow of simple fluids, and after that be able to study the dynamics of more complex polymeric systems under shear.
