Issue
EPL
Volume 77, Number 5, March 2007
Article Number 56003
Number of page(s) 5
Section Condensed Matter: Structural, Mechanical and Thermal Properties
DOI http://dx.doi.org/10.1209/0295-5075/77/56003
Published online 23 February 2007
EPL, 77 (2007) 56003
DOI: 10.1209/0295-5075/77/56003

Why polymer chains in a melt are not random walks

J. P. Wittmer1, P. Beckrich1, A. Johner1, A. N. Semenov1, S. P. Obukhov1, 2, H. Meyer1 and J. Baschnagel1

1  Institut Charles Sadron - 6 Rue Boussingault, 67083 Strasbourg Cedex, France
2  Department of Physics, University of Florida - Gainesville FL 32611, USA

jwittmer@ics.u-strasbg.fr

received 24 October 2006; accepted in final form 11 January 2007; published March 2007
published online 23 February 2007

Abstract
A cornerstone of modern polymer physics is the "Flory ideality hypothesis" which states that a chain in a polymer melt adopts "ideal" random-walk$\hbox{--} $like conformations. Here we revisit theoretically and numerically this pivotal assumption and demonstrate that there are noticeable deviations from ideality. The deviations come from the interplay of chain connectivity and the incompressibility of the melt, leading to an effective repulsion between chain segments of all sizes s. The amplitude of this repulsion increases with decreasing s where chain segments become more and more swollen. We illustrate this swelling by an analysis of the form factor F(q), i.e. the scattered intensity at wave vector q resulting from intramolecular interferences of a chain. A "Kratky plot" of q2F(q) vs. q does not exhibit the plateau for intermediate wave vectors characteristic of ideal chains. One rather finds a conspicuous depression of the plateau, $\delta (F^{-1}(q))=\vert q\vert^{3}/32\rho $, which increases with q and only depends on the monomer density $\rho $.

PACS
61.25.Hq - Macromolecular and polymer solutions; polymer melts; swelling.
64.60.Ak - Renormalization-group, fractal, and percolation studies of phase transitions.
05.10.Ln - Monte Carlo methods.

© Europhysics Letters Association 2007