NSE Results on Chain Specific Effects Limiting the Rouse Dynamics

NSE Results on Chain Specific Effects Limiting the Rouse Dynamics 

Recently a very detailed study on the single chain dynamic structure factor of short chain PIB (M =3870) melts was undertaken with the aim to identify the leading effects limiting the applicability of the Rouse model toward short length scales [217]. This study was later followed by experiments on PDMS (M =6460), a polymer that has very low rotational barriers [219]. Finally, in order to access directly the intrachain relaxation mechanism experiments comparing PDMS and PIB in solution were also carried out [186]. The structural parameters for both chains were virtually identical, Rg=19.2 (21.3 A). Also their characteristic ratios C =6.73 (6.19) are very similar, i.e. the polymers have nearly equal contour length L and identical persistence lengths, thus their conformation are the same. The rotational barriers on the other hand are 3-3.5 kcal/mol for PIB and about 0.1 kcal/mol for PDMS. We first describe in some detail the study on the PIB melt compared with the PDMS melt and then discuss the results. 

Rouse rate at 470 K is 8100 AVns. The corresponding mode-dependent characteristic times are represented by the dashed-dotted line in Fig. 5.1. Taking into account the relationship between Dr and the corresponding diffusion coefficient Dr becomes Dr=1.22 0.17x10 mVs. 

While the NSE results show that, within the experimental accuracy, in the range Q 0.15 A the Rouse model gives a good account for the internal modes as well as for the diffusion of the chain centre of mass, it is also clear that for higher Q-values the experimental structure factors decay significantly more slowly than the Rouse model would require. These deviations are quantified in fitting the Rouse model to the different spectra separately. This procedure results in a strong dispersion of the elementary Rouse rate. The values determined for at Q 0.15 A follow a Q-dependence, which can be described by the power law  

Note that another origin of the slowing down of the chain relaxation compared to the Rouse prediction could be a reduction of the weights of the higher modes, which in the Rouse model are proportional top (see Eq. 3.19). 

---