Nuclear magnetic resonance investigations into long range chain fluctuations in molten poly(ethylene-oxide)

We studied by NMR the long range relaxation of poly-(ethylene oxide) chains , in the melt; the molecular weight was varied over the range 12-450 K. The transverse magnetic relaxation curves of protons attached to the polymer w ere thoroughly analyzed over the time interval 0-1 s and over the amplitude range 5x10(-4) to 1. The analysis revealed three ensembles of protons clea rly distinguished from one another from their different relaxation rates (a pproximate to 7, 10(2), and 10(3) s(-1), respectively), on the one hand, an d from the differences brought about by forming so-called pseudosolid spin- echoes specific to entangled polymers, on the other hand. In this work, it is shown that the main part of the relaxation curve (relative amplitude app roximate to 80%) can be interpreted in terms of exponential modes of isotro pic chain relaxation, corresponding to the partition of one chain into Gaus sian submolecules. The model provided relaxation functions in very good agr eement with experimental curves: the best fits showed that the submolecule molecular weight (11 K) and its associated correlation time (1 ms) are inde pendent of the chain molecular weight. Typical values of the terminal relax ation time, tau(R), of chain fluctuations were 0.002 and 0.17 s, for molecu lar weights equal to 55 and 450 K, respectively; correspondingly, the termi nal reptation times, estimated from self-diffusion data reported in the lit erature, were 0.002 and 2 s, respectively. On increasing the molecular weig ht, the isotropic chain relaxation observed from NMR was found to occur wit hin a time interval much shorter than viscoelastic relaxation times. (C) 19 99 American Institute of Physics. [S0021-9606(99)50139-6].