Phases of flow-induced crystallization of i-PP: How remote pieces of the puzzle appear to fit

A synopsis of already published results of very different characters, as obtained at four independent institutes, yields a congruent picture of the processes occurring during flow-induced crystallization of polymers like i-PP. The research groups mentioned only had a common starting point: They adopted the protocol of short term mechanical loading. With this protocol, a nice separation of the flow-induced nucleation process from the succeeding growth process is usually obtained. For this achievement, the largely differing time scales of loading and subsequent waiting are responsible. In Linz, the required mechanical treatments were characterized by harsh conditions, as occurring during practical processing ( short loading times with high loads). Conclusions were drawn from the time-dependent optical retardation, as observed directly after cessation of flow, and from microphotographs obtained on cross sections of the finally solidified samples. The three other groups applied milder mechanical treatments, but used time-dependent small angle light scattering (SALS) and incipient depolarization. They could also follow the structure formation under the microscope. For this purpose they used transparent equipment. From a combination of the results of the four groups, one can draw the following conclusions: During flow a tremendously increasing number of nuclei of augmenting anisotropy are formed. Apparently, after cessation of flow, these nuclei act like pointlike nuclei, if their average lengths are still shorter than the average distances between their centers. In those cases, spherulites are formed, which fill the intermediate space. With light scattering, mainly the well-known invariant, reflecting density fluctuations, is of importance. It also appears that its time dependence can readily be predicted with the aid of the number density of nuclei and the growth speed of spherulites. With continued waiting, the internal structure of the just formed spherulites matures. As a consequence, orientation fluctuations of anisotropic entities, which grow larger, play a role. Along this route, the formation of "shishs", as well-known from oriented shish-kebab structures, may also be elucidated. The relaxation behavior of these threadlike precursors is discussed in connection with their stability at lower temperatures.