Decay of threads after cessation of flow

The observation of a range of Tshear (205˚C-215˚C) for which we see the signature for creation of oriented precursors, yet we do not observe any oriented growth upon cooling and ex-situ, indicates that the formed threads must have relaxed completely by the time the polymer was cooled down. It is well known that oriented precursors can survive at temperatures above the nominal melting point (~ 170˚C for iPP) for some time. At temperatures lower than that, shish do not decay. Above that temperature, the decay time quickly becomes faster at higher temperatures, reaching very small values near the equilibrium melting point (which for iPP is ~ 212˚C [10]). For Tshear ≥ 205˚C, the decay time of threads can be inferred to be < 180 s (which is the minimum amount of time that elapses from the imposition of shear to the moment at which the polymer melt has actually cooled down to temperatures at which the decay time diverges).

In contrast to the highest temperatures, the precursors formed between 190˚C-200˚C are annihilated significantly within 100 s, but a few threads are able to survive. We note that these few threads are not located near the wall where the shearing stress was highest, but at varying distances from the mold wall. This suggests that most of the highly oriented precursor population, which will be created in the regions of higher stress, decays within the annealing time at Tshear, but that a handful of shish have, by chance, greater stability. At temperatures 205˚C or above, even if particularly stable threads were formed, they relaxed completely. The existence of exceptional shish provides a cautionary note: In order to understand the formation of highly oriented structures and the concomitant change in material properties, methods are needed to discriminate between the prolifically formed shish (which correlate with the birefringence upturn) and the scant oriented structures that can be observed ex-situ.

It is a delicate matter to compare the decay times of the threadlike precursors observed here with those reported previously by Alfonso and Scardigli [5] and Eder et al. [4]. In Alfonso’s “fiber pull” experiments, the criterion for decay was the absence of any transcrystalline growth––meaning that not even one of the most stable shish survived. With this criterion remarkably long times were observed in iPP: ~ 1200 s at 190˚C and 60 s at 210˚C. In contrast, Janeschitz-Kriegl’s criterion for survival was the thickness of a highly oriented skin. His group reported a decay time of approximately 58 s at 190˚C, ~ 7 s at 200˚C, and < 1 s at 210˚C. The stark difference in decay timescales between the two studies is unlikely to be due to difference in iPP materials: Both groups used Ziegler-Natta iPP; Alfonso with Mw from 400 to 520 kg/mol and Mw/Mn from 6 to 25 and [mmmm] of 96% or 98%, while Janeschitz-Kriegl used a highly isotactic PP of Mw = 330kg/mol and Mw/Mn = 6. Our results are consistent with both studies: The high concentration of shish decays away in less than 300 s at T ≥ 190˚C and the most stable shish can survive for minutes at temperatures up to 200˚C. This similarity is interesting given that the present material is a bimodal blend of iPPs prepared using single-site catalysts.

At Tshear ≥ 205˚C, no threads and no significant population of point-like nuclei survive: The turbidity does not decay while cooling and holding at 140˚C, and the final morphology consists of relatively large spherulites that form after extracting the sample and cooling to ambient temperature. Their size and homogeneity suggests that they have started growing at temperatures not too far below 140˚C, where only small numbers of nuclei appear but where linear lamellar growth rates can rapidly approach large values, so that spherulites from those few nuclei fill all the space. We infer that no significant quantities of point-like nuclei survive between 190˚C-200˚C, given that the few threads that remain are able to grow “sausages” of large diameter (~ 50-60 µm) without encountering spherulites growing from point-nuclei in the neighborhood. We attribute the decay of turbidity while holding isothermally at 140˚C to the growth of these sausage-structures.

At 180˚C and 185˚C, negligible growth of kebabs occurs. The relaxation rates of precursors formed are slow enough that many of them survive the temperature jump, and a highly oriented skin forms upon cooling. These two temperatures would therefore be within the range adequate for the purpose of the T-jump experiments. Finally, for the two lowest temperatures (140˚C and 160˚C), the thread-like precursors do not decay at all: at these temperatures kebab growth can already occur.

4.6

Conclusion

In summary, the results presented in this chapter lay the ground for choosing the T- jump conditions under the heat transfer limitations inherent to our experimental apparatus. Shearing temperatures at and above 190˚C will not be suitable for our purpose of performing temperature jumps to determine the thread-length/volume formed during flow, since they will decay too rapidly and we will not have any oriented threads left to study, while temperatures of 160˚C and below do not allow dissection of the effect of growth of threads from the growth of kebabs. For the next chapter, we have thus chosen an intermediate temperature (170˚C), at which we apply a variety of shearing stresses for different durations of shearing times. In this way, we effectively interrupt flow at selected times after the onset of shish formation and then obtain quantitative measurements of the

total thread-length present by cooling to temperatures where growth of kebabs occurs and can be experimentally measured.

4.7

Figures

Figure 4-1. Experimental temperature and shear protocol for the “T-jump” method. The polymer melt is sheared at a temperature where negligible lamellar overgrowth occurs. Afterwards, it is cooled down to a temperature where the amount of kebab development is large enough to be detected. Before impingement has occurred, the signal arising from the kebab growth is proportional to the thread-length present.

T

T (˚C)

σ

(MPa)

T

T

time

0 t

t

Figure 4-2. Schematic showing the conditions under which the observable signal due to growth of kebabs on the threads is proportional to the total thread-length created during flow. Experiment 1 and 2 differ in the total thread-length present; before impingement, the kebab arising from the growth of kebabs serves to “read out” the relative amounts of shish present.

Experiment 1