Neutron Scattering

15 . Polymer Dynamics
D. Richter
15.1 Introduction
In our every day life plastics or polymers play a very important role . Polymeric materials are
used, because they are durable, cheaply to produce, easily to process and because they exhibit
very versatile and favorable mechanical properties, e .g . depending on temperature or time the
same polymer may be viscose, rubber elastic, very tough with high impact strength or even
brittle . In the simplest case polymers are long linear chain molecules, build from one
repeating unit, the monomer ; such polymers are called linear homopolymers .
Since in gerreral rotational isomers may be easily formed at each bond of the chain backbone,
long chain polymers possess a very large number of irrternal degrees of freedora which
contribute importantly to the entropic part of the molecules free energy . At length scales
somewhat larger than the size of the monomer, the detailed chemical structure of the chain
building blocks ceases to be of importance and very general properties determined by the
statistical mechanics of the chains prevail, e .g . the conformational entropy follows from the
number of possible arrangements of a chain sequence in space . According to the central lirait
theorem the most probable arrangement is that of a Gaussian coil, e .g . the polymer chain
performs a rarrdom walk in space . If pieces of the chain are now stretched an errtropic force
arises and acts on these stretched segments endeavouring to restore them to the most probable
contorted state . Such forces are the basis ofrubber elasticity .
This lecture aims to identify general principles of chain motion on a molecular scale which
underlie the macroscopic mechanical properties, and presents concepts and experimental
results on these motional mechanisms in space and time . Thereby, we restrict ourselves to
melts ofhoraopolymers .
Neutron scattering with its space time sensitivity on a molecular and atomic scale unravels the
details of the molecular motions in question . Commencing at the scale of the single bond,
where movements take place at a pace as in normal liquids, quasielastic neutron scattering
(QENS) provides insight into local relaxation processes . At longer length scales first the
entropy driven Rouse motion and at even larger distances the effect of entanglement
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