NEUTRON SPIN-ECHO STUDIES ON THE SEGMENTAL DIFFUSION BEHAVIOR IN THE DIFFERENT CHAIN SECTIONS OF HIGH-MOLECULAR-WEIGHT POLY(DIMETHYLSILOXANE) MELTS

Using neutron spin echo spectroscopy, the dynamics of high molecular weight poly(dimethylsiloxane) in the melt (T = 473 K) was investigated. In order to decide whether the lateral constraints are uniform or nonuniform with respect to the contour of a test chain, chains were labeled either completely or partially by a corresponding proton-deuterium exchange. It was found that the central and the terminal chain sequences experience the same amount of lateral confinement, which in the framework of the reptation or tube model - can be characterized by a tube diameter d(t) of about 70 angstrom. Nearly the same d(t) value is obtained from the plateau modulus of rheological measurements, too. Thus, these experiments show unambiguously that the lateral constraints, responsible for the reptational chain dynamics, are quite homogeneous, even in the case of extremely large tube dimensions.