The intermediate coherent scattering function of entangled polymer melts: a Monte Carlo test of des Cloizeaux' theory

Using the bond fluctuation model for flexible polymer chains in a dense melt the intermediate coherent scattering function for chains containing $N = 200$ monomers is calculated and interpreted in terms of a recent theory of des Cloizeaux. The theory yields an explicit description for the crossover from the Rouse model to the regime where reptation prevails, for the limit $N\rightarrow \infty$. While the Monte Carlo data are qualitatively compatible with this description, an accurate estimation of the tube diameter is prevented due to the onset of a diffusive decay of the scattering function, not included in the theory. For a full quantitative analysis of the Monte Carlo data (as well as of experiments on chains with not extremely large molecular weight) an extension of the theory for finite N would be required.