Neutron Scattering

Finally, one may test whether only local reptation or also thé creep motion along thé tube is
important on this experimental time scale . Local reptation corresponds to Zd = ou and indeed
(Fig .15 .19) at low Q a différence between local reptation only and thé global reptation
mechanism appears to become distinguishable indicating thé présence of -rd. Here future
experimental work will have to set in . As it stands NSE spectroscopy accessing quantitatively
thé dynamic structure factor has by now seen clear and unambiguous signature of reptation in
a flexible linear polymer chain. The data cover a region ofthé time domain where reptation is
in principle applicable . Compared with other phenomenological approaches reptation is by
now thé only approach providing a consistent description of all NSE data. It implies that
reptation must emerge from any successful microscopie theory of polymer relaxation .
15.5 Summary
High resolution neutron spectroscopy permits to access thé molecular motions simultaneously
in space and time . Restricting itself to thé dynamics of homopolymers melts this lecture
attempted to transmit a flavour of what can be achieved in particular by NSE . Choosing
différent time and length scales, we covered thé range of molecular motions, commencing at
thé scale ofa few bonds to large scale motions reaching thé scale of thé entire chain.
In thé régime of thé `classical relaxations' of polymers neutron spectroscopy informs on thé
geometrical evolution ofthé motions in question . We have seen, that thé a-relaxation may be
understood as a sublinear diffusion process while thé fi-relaxation is in good agreement with a
local jump process of a few angstrom distances. Both processes may be considered with good
approximation as statistically independent. At scales where thé detailed chemical structure of
thé monomers ceases to be of importance, NSE measurements have by and large confirmed
thé prédictions of thé entropy govemed Rouse dynamics both for thé self and thé pair
conelation function . Recently, an in depth comparison of specially designed NSE
experiments with computer simulation also pointed out thé limits of this approach .
The dynamics of polymer melts under thé influence of topological interactions which result
from thé mutually interpenetrating chains poses high demands both conceptually and also
experimentally . NSE experiments on thé single chain dynamic structure factor of long chain
melts, established experimentally thé essential prédiction of local reptation namely thé tube
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