Crystallization of Polymers in Fast Extensional Flow

As is well-known qualitatively, crystallization in polymers is considerably enhanced by all kinds of flow. This means that one has to do with the consequences of this fact practically in all plastic parts made by injection moulding, extrusion, blow moulding and melt spinning. Mostly, the type of flow encountered is of a quite complex nature. So it seems wise to start with specific, well-defined types of flow as there are pure shear flow and uniaxial extensional (or stretching) flow. However, if the deformation rate is relatively low, as is the case with practically all commercial rheometers, one is confronted with crystallization phenomena which differ considerably from those experienced with high deformation rates, which are characteristic for polymer processing. This insight cause the research group at Linz University to concentrate on high deformation rates, and in particular, on interrupted creep flow. As has been proven in the past for shear flow, the advantage of creep flow is that practically steady state flow is obtained after an almost instantaneous compliance. As a consequence, one can neatly separate the influence of the deformation rate (shear rate in previous experiments) from that of the time of (shearing) flow. Whereas previous research at Linz University was concentrated on high shear rates, the present proposal is focused on high extensional rates. With low deformation rates one Just obtains a mere multiplication of the number of nuclei, which finally leads to a spherulitic structure with more or less deformed (little) spherulites. With sufficiently high flow rates, however, one obtains the so-called "shish-kebab" structures, each item containing a stretched, thread-like crystalline core (the shish) with a lamellar radial overgrowth (the kebabs) . With interrupted shear creep four stadia were found in the development of the shish-kebabs. This shows that the underlying mechanisms cannot not directly be called simple. For extensional flow an apparatus based on the principle of flow around a stagnation point was chosen first and adapted for creep experiments. Preliminary results are encouraging. With this apparatus very high flow rates and long flow times can be realised. The flow field, however, turns out to be quite inhomogeneous, so that one has to cope with this difficulty. Apparently, the said inhomogeneity is caused by the fact that the hyperbolic flow lines do not extend to infinity. Another difficulty is given by the fact that the pertinent crystallization experiments can be carried out only in a restricted temperature range, where normal crystallization is too slow for interference. An alternative apparatus, containing an expansion tube for the creation of the stretching force, is intended to replace the classical melt-spinning experiment which, despite all the perfection, which was introduced during the years, could never lead to the unravelling of the stadia of the development of crystallinity because of temperature and extensional rate varying strongly with the distance from the spinnert. According the gas-law the expansion tube creates a stretching force, which decreases like the cross-section of the stretched filament. As a consequence, one obtains a constant tensile stress in a typical short term experiment.