Neutron Scattering

The dashed line gives the Rouse prediction r cc W C4Q 4 . While at larger momentum transfers
the experimental results follow very well this prediction, towards lower Q, a systematic
relative increase of the relaxation rate is observed. Including the diffusion, we have
D+Q 2W f4
6
the solid lines in Fig .15 .13 represents the prediction of Eq .[15 .21 ] . Perfect agreement is
obtained.
15.3 .3 Computer simulation
In order to learn about the limits of the Rouse model, recently a detailed quantitative
comparison of molecular dynamics (MD) computer simulations on a 100 C-atom
polyethylene chain (PE) with NSE experiments on PE chains of similar molecular weight bas
been performed . Both, the experiment and the simulation were carried out at T = 509K .
Simulations were undertaken, both for an explicit (ea) as well as for an united (ua) atour
model . In the latter the H-atours are not explicitly taken into account but reinserted when
calculating the dynamic structure factor . The potential parameters for the MD-simulation
were either based on quantum chemical calculations or taken from literature . No adjusting
parameter was introduced. Fig .15.14 compares the results from the MD-simulation (solid and
broken lines) with the NSE-spectra . The time axis thereby is scaled with the centre of mass
diffusion coefficient, in order to correct for thé slightly différent overall time scales of
experiment and simulation . From Fig .15 .14 quantitative agreement between both results is
evident . Fig .15 .15 compares thé saine experimental data, which agreed quantitatively with thé
simulations with a best fit to thé Rouse model (Eq.[15 .17] . Here a good description is
observed for small Q-values (Q _< 0.14A-1 ), while at higher Q important deviations appear .
Similarly also thé simulations cannot be fit in detail with a Rouse structure factor .
15- 1 9