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

Abstract

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 were thoroughly analyzed over the time interval 0–1 s and over the amplitude range 5×10−4 to 1. The analysis revealed three ensembles of protons clearly distinguished from one another from their different relaxation rates (≈7, 102, and 103 s−1, respectively), on the one hand, and 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 ≈80%) can be interpreted in terms of exponential modes of isotropic chain relaxation, corresponding to the partition of one chain into Gaussian submolecules. The model provided relaxation functions in very good agreement with experimental curves: the best fits showed that the submolecule molecular weight (11 K) and its associated correlation time (1 ms) are independent of the chain molecular weight. Typical values of the terminal relaxation time, τR, of chain fluctuations were 0.002 and 0.17 s, for molecular weights equal to 55 and 450 K, respectively; correspondingly, the terminal reptation times, estimated from self-diffusion data reported in the literature, were 0.002 and 2 s, respectively. On increasing the molecular weight, the isotropic chain relaxation observed from NMR was found to occur within a time interval much shorter than viscoelastic relaxation times.