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

Residual spin-spin interactions of protons attached to highly entangled cha ins in molten polymers give rise to a time reversal effect detected from so lidlike spin-echoes formed from the transverse magnetization. The quantitat ive analysis of such pseudosolid spin-echoes, observed on molten poly(ethyl ene-oxide), reveals that the transverse relaxation curve is the product of two contributions: M-x(R)(t), mainly sensitive to the existence of a tempor ary network and Phi(R)(t) arising from fast anisotropic segmental motions w hich give rise to residual spin-spin interactions. It is shown that the ana lysis provides a suitable method for distinguishing the two components from each other. The molecular weight was varied over the range 12-450 K. The d escription of M-x(R)(t) is based on the assumption that there exists two st ochastically 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 orientationa l memory effect is introduced along the chain. The proposed relaxation func tion 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 correla tion 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 submolecul es (approximate to 70 s(-1)) and of short free chains (12 K) in the melt (a pproximate to 20 s(-1)) have about the same order of magnitude. (C) 2000 Am erican Institute of Physics. [S0021-9606(00)52135-7].