ENTANGLEMENT CONSTRAINTS IN POLYMER MELTS - A NEUTRON SPIN-ECHO STUDY

Using neutron spin echo spectroscopy, we have studied the melt dynamics of polyisoprene (PI), saturated polybutadiene (PEB-2), and poly(ethylene-propylene) alternating copolymer (PEP). The experiments on all three polymers show that beyond a characteristic length d and after a crossover time tau(e) the relaxational density fluctuations within a given chain are strongly impeded. The presence of an intermediate dynamic length scale in the chain dynamics establishes the microscopic existence of a well-defined entanglement distance confirming thereby one of the essential assumptions of the reptation concept. The microscopically determined lengths agree well with those obtained from the plateau moduli if interpreted in terms of the reptation theory. The detailed line shape of the dynamic structure factor supports the concept of local reptation.