H. Janeschitz-Kriegl

The physics of athermal nuclei in polymer crystallization

Abstract The lower temperature limit for sporadic “thermal” nucleation, as estimated in a previous paper (Janeschitz-Kriegl (1997) Colloid Polym Sci), can be verified by a variety of earlier experiments which are compiled in this paper. Below the said limit only “athermal” nuclei are of importance in samples containing no large numbers of hetero-nuclei. The number of athermal nuclei increases tremendously (by many decades) with decreasing temperature. Why these nuclei are dormant and “awake” only at “their” temperature is explained in terms of their growth conditions, which depend on the length of the incorporated (helical) sequences. Also the sluggishness of molecular processes, occurring in the temperature range of metastable conditions, where sporadic nucleation can occur is demonstrated by some of the recalled experiments. Except for very fast industrial processes, where flow plays a dominant role, this sluggishness makes a noticeable influence of the sporadic nucleation on structure formation very improbable.

Linear elastico-viscous properties of molten standard polystyrenes

This paper contains an extensive presentation of dynamic mechanical data (complex moduli), as obtained on the melts of a series of standard polystyrenes of narrow molar mass distributions. It also shows the way of obtaining structural parameters (plateau modulus and friction factor) which are needed for an interpretation of these data in terms of simple theoretical models (Maxwell elements, Doi-Edwards model). A linear mixing rule is used for taking into account the finite width of the molar mass distributions.

On the development of oblong particles as precursors for polymer crystallization from shear flow : Origin of the so-called fine grained layers

Recent studies concerning shear-induced crystallization after isothermal short-term shearing enabled a theoretical explanation for the formation of highly oriented surface layers in isotactic polypropylene. Under these special conditions also an occurrence of so-called fine grained layers was observed but has not been understood, so far. For slightly supercooled melts of polyethylene the formation of oblong crystallites, which were oriented perpendicularly to the flow direction, was recently reported for very slow continuous shear flow. This phenomenon was explained by the rotational component of this flow. The present work describes the growth of similar structures in isotactic polypropylene melts after fast short-term shearing at mild degrees of supercooling. The pertinent crystallites, which are finally formed on transverse precursors, are observed in cross-sections perpendicular to the direction of previous flow. Their anisotropic growth was monitored with the aid of light scattering. The present work also emphasizes that these structures are formed as a consequence of shear rates that occur in industrial processes. In fact, in injection molded parts they are formed in the neighborhood of the highly oriented surface layers. It is suggested that the transverse thread-like particles act as activators for the formation of the precursors of the highly oriented surface layers, as soon as they switch into the flow direction.

Shear induced crystallization, a relaxation phenomenon in polymer melts : A re-collection

Thread‐like nuclei for the crystallization of polymers which are formed at high deformation rates in a temperature range close to the equilibrium melting point appear to be practically stable at temperatures where spherulites are melting. However, the fact that the great majority of experiments on flow induced crystallization have been carried out at temperatures below the melting temperature of the spherulites leads to the conclusion that the precursors for elongated structures, as formed under those conditions, are practically stable from the moment of their creation. In other words: their relaxation times are much longer than any deformation time applied. As a consequence, deformation times as independent parameters lose their importance in these experiments. Long lasting deformations under low stresses can yield the same precursors as short term deformations under high loads. Apparently the only condition is that the applied specific work is the same.

New apparatus for the simultaneous measurement of stresses and flow birefringence in biaxial extension of polymer melts

SummaryA new apparatus is described for the measurement of the viscosity of polymer melts in biaxial extension. Use is made of the method of two impinging fluid streams. The mentioned streams are guided by lubricated trumpet shaped metal walls. The force is measured which tends to separate the trumpets. At the same time, the flow birefringence is measured in the plane of symmetry, which contains the stagnation point. The linear stress-optical relation, which is well-known from shear experiments, turns out to be valid also in the new flow geometry, at least up to the deformation rates applied. Results of the steady state measurements carried out on a polystyrene melt are evaluated. Suggestions for a better performance of the mechanical measurements are given.ZusammenfassungEin neuer Apparat zur Messung der Dehnviskosität von polymeren Schmelzen unter biaxialer Dehnung wird beschrieben. Hierfür wird die Methode des Aufeinanderprallens entgegengesetzt gerichteter Ströme verwendet. Die genannten Ströme werden durch geschmierte trompetenförmige Bohrungen herangeführt. Es wird die Kraft gemessen, welche die beiden „Trompeten“ auseinanderzudrücken sucht. Gleichzeitig wird die Strömungsdoppelbrechung in der Symmetrieebene gemessen, die den Staupunkt enthält. Die lineare spannungsoptische Regel, die bei Scherexperimenten wohlbekannt ist, scheint auch für die neue Strömungsgeometrie zu gelten, zumindest im Bereich der angewandten Deformationsgeschwindigkeiten. Ergebnisse, wie sie im stationären Strömungszustand bei einer Polystyrol-Schmelze erhalten wurden, werden ausgewertet. Vorschläge zur besseren Ausführung der mechanischen Messungen werden gemacht.

Flow-induced crystallization in polymer melts: on the correlation between nucleation and specific work

A new counting technique for nuclei, as found after short-term shearing, has been developed with the aid of the plate-and-plate rheometer Linkam CSS450. For the purpose this apparatus was modified by the introduction of complementary polarizing optics (a polarizer and a λ-plate). With the aid of this technique the density of nuclei could be determined in a new way. When these number densities are plotted against the applied specific work, satisfactory agreement is found with results, which have been obtained previously with a sliding glass plate rheometer. Nevertheless, the limits of validity of the diagrams had to be discussed. A transition from a dynamic description with the aid of the specific work to a purely kinematic description with the aid of the frequency of successful encounters cannot be overlooked. In fact, with increasing specific work the influences of temperature and (non-Newtonian) viscosity appear to fade away.

Crystallization kinetics of two aliphatic polyketones

Abstract This is to describe the kinetics of crystallization from the quiescent melt of two aliphatic polyketones (Shell Carilon) and to document a new device called ‘Thin Slice Experiment II’ that allows to determine the number of nuclei and the growth speed of spherulites of fast crystallizing polymers in the entire temperature range from the glass point to the melting point.

Injection moulding of plastics

SummaryIn continuation of a previous investigation a simple analytical expression is derived in closed form for the thickness distribution of the “freeze-off” layer which is vitrified at the (flat) wall of an oblong rectangular cavity. As has been pointed out previously, this layer is marked for amorphous polymers by the molecular orientation (birefringence pattern) in the moulded sample. One can show that a more detailed study with the aid of the coupled equations of energy and of motion will not furnish essential improvements. Problems of polymer physics like glass transition or crystallization kinetics at extreme rates of cooling and shearing must be solved first.ZusammenfassungIn Fortsetzung einer früheren Untersuchung wurde ein einfacher analytischer Ausdruck in geschlossener Form für die Dickenverteilung der „eingefrorenen“ Schicht abgeleitet, die an der (flachen) Wand eines langgestreckten rechteckigen Formnestes während des Einspritzvorgangs glasig erstarrt. Wie früher auseinandergesetzt wurde, wird diese Schicht bei amorphen Polymeren durch die Molekülorientierung (Doppelbrechungsmuster) im gespritzten Formteil markiert. Man kann zeigen, daß eine eingehendere Studie mit Hilfe der gekoppelten Energie- und Impulsgleichungen keine essentiellen Verbesserungen bringt. Probleme der Polymerphysik, wie Glasübergang oder Kristallisationskinetik bei extremen Abkühlungs- und Schergeschwindigkeiten, müssen erst gelöst werden.

Reptation model in polymer melt rheology: On the validity of a useful basic assumption

A mixing rule as suggested by the Doi-Edwards theory is checked experimentally. For this purpose, the dynamic shear moduli as functions of circular frequency are measured first for the melts of a series of polystyrenes of narrow molecular mass distributions. After the careful preparation of mixtures the same moduli are also determined for these mixtures. A discussion is given of the remarkable but limited validity of the additivity of contributions by the single macromolecules to the mentioned moduli.

Quasi-linear rheological behaviour of polymer melts: Comparison between mechanical and improved flow birefringence measurements

SummaryThe latest improvement of the rotor unit (cone- and plate-type) for the measurement of the flow-birefringence of polymer melts is described. As a consequence of this improvement, the influence of parasitic birefringences, as produced in the corners near the windows, is minimized. It is shown that the limitation of the range of available rates of shear, as experienced with previous versions of the apparatus, was due to those parasitic birefringences which predominated at rates of shear where the main birefringence caused an optical path difference of one, two or more whole wavelengths (fringes) and was, as a consequence, virtually zero. This explains why great troubles were experienced in particular with polymers possessing a high back-bone anisotropy. The validity of the linear stress-optical rule up to shear rates, where melt fracture occurs in capillary flow, is well understood in terms of the “wriggling motion” of chain molecules. Comparison with direct mechanical measurements was very satisfactory.ZusammenfassungEine Beschreibung der jüngsten Verbesserung an der Rotorzelle (Kegel-Platte-Typ) für die Messung der Strömungsdoppelbrechung polymerer Schmelzen wird gegeben. Als Folge dieser Verbesserung wird der Einfluß der parasitären Doppelbrechung, die in den Ecken neben den Fenstern auftritt, minimalisiert. Es wird gezeigt, daß die Begrenzung des Bereiches der verfügbaren Schergeschwindigkeiten, wie sie in früheren Versionen des Apparates gefunden wurde, auf das Vorherrschen der parasitären Doppelbrechungen bei Schergeschwindigkeiten zurückzuführen ist, bei denen die Hauptdoppelbrechung optische Weglängen von einer, zwei oder mehreren ganzen Wellenlängen verursacht und daher effektiv null wird. Dies erklärt, warum große Schwierigkeiten gerade bei solchen Polymeren auftraten, die eine stärkere Kettenanisotropie aufweisen. Die Gültigkeit der linearen spannungsoptischen Regel bis zu Schergeschwindigkeiten, bei denen in der Kapillaren Schmelzbruch auftritt, kann man aufgrund der Ringelbewegung der Makromoleküle gut verstehen. Ein Vergleich mit direkten mechanischen Messungen ergab sehr befriedigende Ergebnisse.