Pseudosolid nuclear magnetic resonance approach to poly(ethylene-oxide) chain dynamics in the melt

Residual spin-spin interactions of protons attached to highly entangled chains in molten polymers give rise to a time reversal effect detected from solidlike spin-echoes formed from the transverse magnetization. The quantitative analysis of such pseudosolid spin-echoes, observed on molten poly(ethylene-oxide), reveals that the transverse relaxation curve is the product of two contributions: M-x(R)(t), mainly sensitive to the existence of a temporary network and Phi(R)(t) arising from fast anisotropic segmental motions which give rise to residual spin-spin interactions. It is shown that the analysis provides a suitable method for distinguishing the two components from each other. The molecular weight was varied over the range 12-450 K. The description of M-x(R)(t) is based on the assumption that there exists two stochastically independent effects. In accordance with a previous study [J. P. Cohen Addad and A. Guillermo, J. Chem. Phys. 111, 7131 (1999)], the first process is interpreted in terms of exponential relaxation modes resulting from the partition of one chain into Gaussian submolecules. In addition to the effect of long-range fluctuations on the magnetization, an orientational memory effect is introduced along the chain. The proposed relaxation function accounts for the very specific shapes of both the experimental curves and of the ln(M-x(R)(t))/t plots; the minimum number of parameters required to describe such complex curves is 4. The analysis provides a coherent set of numerical values: the mean square spin-spin interaction and the correlation time tau(s), assigned to one submolecule are equal to 5x10(5) (rad s(-1))(2) and 0.002 s, respectively. Proton relaxation rates of end submolecules (approximate to 70 s(-1)) and of short free chains (12 K) in the melt (approximate to 20 s(-1)) have about the same order of magnitude. (C) 2000 American Institute of Physics. [S0021-9606(00)52135-7].