Results and discussion on isotopically labeled chains

The orientation of the central block shows a plateau in an intermediate time scale. It can also be noted that at very short times, the orientation of the end part is slightly below that of the central block. This effect can be accounted for by the fact that, during the deformation process, no topological constraint is acting at the end of the chain to orient it. This behavioiu is similar to dangling chains in elastomeric networks [28]. 

In order to discriminate between subsequent relaxation processes, it is necessary to calculate the orientational relaxation as a fimction of the location of a segment along the chain and to compare the results of such predictions to experimental data. Theoretical expressions are given elsewhere [29] and reference should be made for further details. In this work, the more elaborate treatment of the chsiin fluctuation process has been used and equations used in the calculation of orientation have been extracted from reference [13]. The 

In order to avoid the use of adjustable parameters on rescaling the theoretical with the experimental orientation, and since we were interested in differences in the relaxation kinetics, we chose to study the orientation of the different blocks normalized by the average orientation, as a function of relaxation time. The relaxation times have been adimensionalized by the retraction time, Tg, which can be determined experimentally by applying the theoretical scaling law (see section 4.1). 

These results, as compared to a previous study [29], illustrate a better agreement between theory and experiments. This can be attributed to a better approximation of the mode distribution involved in the chain fluctuation process and to which the end of the chains are quite sensitive. These data also prove that under our experimental conditions, the relaxation of the chain is dominated by the chain retraction and chain-length fluctuation process. The 

The results are reported as a function of t/r since is independent of the molecular weight and nature (linear or star shaped) of the polymer. This avoids the arbitrary choice of a reference temperature 

---