  In systems such as block copolymer mesophases or physical gels formed by
associating copolymers, the dynamical properties are often controlled by the
extraction/association of a sticking group. We propose a description of the
extraction/association process of a single sticker. The statistical physics of
these associated systems is usually dominated by stretched brush-like regions.
A sticker has to overcome a potential barrier both to penetrate the stretched
structure or to escape a favorable region built by associated stickers. Our
main result is that these barriers are crossed by tension fluctuations and that
the corresponding processes are thus local with a friction independent of
molecular weight. When the potential barriers are high, the (very stretched)
equilibrium structures are not likely to develop on reasonable time scales.
Stretched model systems may also be grown in situ from nuclei bearing
initiating groups. These, irreversibly bound structures, are also briefly
discussed.
