Neutron Scattering

small but systematic deviations from the Rouse prediction . The origin of these discrepancies
are traced to intercharn interactions .
N
\J
N n
V
100
10
exp . S'(q,t), q = 0 .05 A'
exp . S'(q,t), q = 0 .08 A'
exp . S'(q,t), q = 0 .10 A'
+A exp . S'(q,t), q = 0 .065 X'
Figure 15 .16 :
time (ns)
Mean square center of mass displacement for PB chains in the melt obtained
from (r'(t)=-Q z Ën S(Q,t) . Solid line : simulation result ; dashed line
15 .4 Topological interactions - Reptation
The reptation model of de Gennes, Doi and Edwards proceeds from the intuitive concept that
the motions of a chain in a melt are heavily impeded in directions lateral to their own profile
by the other chains encircling them. The dominant diffusive motion proceeds along the chain
profile . A chain twists and turns through a melt lire a snalçe . The lateral restrictions are
modelled by a tube with a diameter d, parallel to the chain profile, whereby d relates to the
plateau modulus of the melt . The restrictions of the motion through other chains are not
effective on a monomer scale, but rather permits lateral excursions on intermediate length
scales (d - 50 . . . .Â) . The experimental observations for viscosity and diffusion can be made
directly comprehensible in this simple intuitive model .
As it concems the motion of an individual polymer, large scale lateral diffusion is quenched
during the life time rd of the tube constrains . Initially for short times the chain relaxes
according to the Rouse picture until the mean square displacement reaches about the tube
15- 22