Slow dynamics of entangled polydimethylsiloxane chains observed by proton transverse magnetic relaxation

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Abstract
Specific proton magnetic relaxation properties of rotating methyl groups were applied to the observation of the diffusion of long polydimethylsiloxane chains in the molten state. NMR results were interpreted using a freely jointed chain model easily extended to PDMS chains. The magnetization relaxation function was calculated from a multiple step motional narrowing effect associated with normal modes of the polymer chain. Three main features characterize the present NMR study: (i) A NMR submolecule may be defined; it is similar to the average characteristic chain segment determined from viscoelastic measurements. (ii) The terminal chain relaxation time was found to be molecular weight dependent through the zero-shear viscosity only. This result disagrees with the well known Rouse formula; it is in accordance with recent expressions proposed by Doi, Edwards, and Graessley but discrepancies with numerical factors were found. (iii) Although the ’’free draining’’ model used by Rouse to calculate the absolute time scale factor is incorrect, the general shape of the relaxation spectrum associated with this model was found to be correct.