Neutron Scattering

Figure 15.17 : Dynamic structure factor of a PEP melt for different Q-values . Solid lines :
Ronca model ; dashed line : Rouse model at the largest Q-value .
In Fig .15 .18 the same data are plotted versus the scaling variable of the Rouse model
(Eq .[15.20]) . In contrast to Fig .15 .12 the scaled data do not follow a coramon curve but are
rather split into Q dependent branches after an initial common course . This splitting is a
consequence of the existence of a dynamic length scale which invalidates the Rouse scaling
properties . We note, that this length is of purely dynamical character and cannot bc observed
in static experiments . In order to distinguish between différent models measurements up to
Fourier times 3 or 4 tiraes larger than ze are not enough . Here, the recent development of an
ultra high resolution NSE spectrometer (IN15 at the ILL in Grenoble opened new ground in
pushing the time lirait of NSE up to about 200ns) .
Fig .15 .19 displays recent experimental results on a polyethylene melt (MW = 36 .000) which
were carried over a time regime of 170ns. The data are compared with the dynamic structure
factors of the reptation model as well as the models of de Cloizeaux and Ronca . It is apparent
that these data clearly favour the reptation model which appears to bc the only so far existing
model yielding a dynamic structure factor which is in quantitative agreement with this NSE
data . The model of Ronca produces a plateau which is too flat . From Fig .15 .19 it is also
apparent that the Rubber like model of de Cloizeaux leads to an inconsistent Q dependence
which is most apparent at the larger Q values. We note that the fits were preformed vaiying
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