Depth dependence sequence of events between 7 s and 12 s of shearing

The application of our depth sectioning strategy to matched experiments that differ only in the shearing time aims at revealing the structure development that occurs at each depth slice when shearing between ts = 7 s and ts = 12 s.

3.5.2.1 Saturation of precursor formation at high stress

At the highest stresses for the outermost depth section of each shearing time (both corresponding to highly oriented skin, from the wall to 35 µm), the development of the real-time observables is very similar. From the moment shear is started up to 7 s of shear, the rise of the retardance is the same for both experiments of ts = 7 s and 12 s, as expected (Figure 3-6). However, it is remarkable that for the next 5 s (that is, after cessation of flow for ts = 7 s but while the shear pulse is still imposed on the melt for ts = 12 s), the growth of birefringence is still very similar. In other words, the additional 5 s of shear in the case of ts = 12 s do not have an additional effect when compared with the case in which flow has stopped after 7 s. Thus, it appears that the structure formed during shear has already “saturated” by the time that flow has been imposed for 7 s.

After flow, both the initial rise in birefringence and the initial growth rate of the AP,110 in the outer 35 µm are very similar for ts = 7 s and 12 s. The impingement time is very sharp and occurs at similarly early times, denoting similar and small inter-shish spacing for these two shearing times. Consistently, there is no significant difference in the degree of orientation of the parent 110 peaks. In contrast, the only manifest difference is the higher P:D ratio for ts = 12 s when compared to ts = 7 s. These results suggest that all the thread-length that could form at this level of stress has already developed by 7 s of shearing, and that the additional 5 s of shearing for ts = 12 s contribute in a way that mainly affects the subsequent P:D ratio, but that does not create more thread-length. Perhaps the additional shearing contributes by either fattening the shish (adding oriented crystallites with chain in flow direction), or creating kebabs preferentially of the parent type (because the chain axis is aligned in the flow direction).

Saturation of the density of threads has been previously observed by Kornfield et al. [9]: In that study, increasing the shearing time from 4 s to 8 s did not enhance the density of threads in the oriented skin as observed by TEM. A number of possible explanations that could cause saturation in the number density can be put forward. Perhaps when the thread-length/volume is very high, the threads themselves hinder each other from elongating even more. Or perhaps a depletion of long chains (which are known to be crucial for the development of threads) has occurred in a section past a critical thread density: if there are many threads very close together, most long chains may already be involved in them and further propagation cannot occur.

It is very significant that depth sectioning is crucial to detect the advent of saturation for the outermost 35 µm under the current experimental conditions. If only the raw measurement of the whole experiment was investigated, the contribution of inner sections to the overall measured signal (in this case, the signal corresponding to the second depth slice of ts = 12 s) would mask the saturation behavior observed in the outer section.

3.5.2.2 Development of threads between 7 s and 12 s at intermediate stress

At intermediate stresses (second and third slices) there are large differences between ts = 7 s and ts = 12 s. The greater initial growth rates for the 12 s sections suggest that the amount of oriented nucleation surface created increases between 7 s and 12 s of shearing. The much more gradual character (or disappearance altogether) of the “knee” feature in AP,110 for ts = 7 s indicates much lower densities of threads (larger and irregular spacings). Consequently, the sections for ts = 7 s are also less well oriented than those of ts = 12 s.

The comparison of thread-length formation between different shearing times at stresses such that threads are formed (so we are above the threshold stress), but saturation has not been reached, point to a range of conditions that must be carefully selected in order to study the velocity of propagation of threads. Clearly real-time depth sectioning can only provide relative values; in order to deduce absolute velocities of propagation, TEM should be performed on some samples to provide a “calibration” for the real-time measurements. 3.5.2.3 No real-time changes at low stress

For the lower stresses that are too low (i.e., the innermost section of the sample), the real-time depth sectioning data does not display noteworthy differences between 7 s and 12 s. Minor dissimilarities for this region are found only in the ex-situ OPM (where slightly more sausages and smaller isotropic structures are observed for 12 s) and in the ex-situ microfocus WAXD (which reveals slightly more orientation on top of the isotropic contribution for the longer shearing times). These differences correspond to growth that occurred mostly after quenching the sample.