Large scale motions in dense polymer systems

The single-chain and collective motions of polymer chains in melts are directly observable by neutron spin-echo spectroscopy (NSE). This method has both the necessary spatial resolution (10Å…500Å) as well as the adapted time domain from fractions of a ns to several 100ns. Neutrons as a probe offer also the unique possibility to label single chains or different parts of a composite melt by H/D-exchange. By recent instrumental developments the overall capacity to perform NSE experiments has been significantly increased by the Jülich NSE spectrometer and the IN15 at the ILL. Two actual examples for the NSE investigation of dynamics in polymer melts are presented: The direct observation of reptation in melts of higher molecular weight polyethylene that relies largely on the long times, up to 170ns uniquely accessible at the IN15. By the obtained results with different theoretical models a clear decision in favor of the reptation model of deGennes could be made. The other example comprising the investigation of collective concentration fluctuations relied on the efficient data collection capability of the Jülich NSE instrument. Here an extensive test of the dynamic RPA theory for multicomponent blockcopolymer melts was performed: Collective and single chain dynamics of diblockcopolymer melts have been made selectively observable by different labeling schemes of the A and B blocks and were investigated by neutron spin echo (NSE) spectroscopy. The results for the wavevector, q, dependence of the initial slopes of the intermediate scattering functions S(q,t) are compared to the predictions of the dynamic RPA. The dynamic structure factor reveals significant local concentration fluctuations on a length scale smaller than the radius of gyration even far above the order disorder transition. Such fluctuations are not predicted by RPA but have been suspected on the basis of spatially non resolving techniques.