Quantification of non-isothermal, multi-phase crystallization of isotactic polypropylene: The influence of shear and pressure

Abstract

Key issue in studying the crystallization process of semi-crystalline polymers, is the need for controlled (or known) boundary and initial conditions. Here dilatometry (PVT) is used to reveal the crystallization kinetics and the resulting morphology of isotactic polypropylene homopolymer as a consequence of the combination of non-isothermal cooling at elevated (isobaric) pressure and application of shear flow. Based on the flow strength the crystallization kinetics can be classified in three regimes; quiescent crystallization, flow enhanced point nucleation and flow-induced creation of oriented structures. Using ex-situ wide angle X-ray diffraction and small angle X-ray scattering we revealed the details of the (oriented) crystalline structures for the different regimes including the different phases (α-, β-, and γ-phase) that can occur in iPP. Some unique oriented structures of combined α- and γ-phase were found. The distribution of the crystal phase content is explained by the interplay of nucleation density and crystal phase dependent growth rate, which are functions of both temperature and pressure.

Introduction

With real processing conditions, polymers are subjected to high cooling rates, high shear rates and elevated pressures, causing the material's specific volume to change due to thermal shrinkage and crystallization, determining the product's final dimensions and dimensional stability. Proper characterization of polymers with respect to the specific volume is, however, not an easy task since it, especially of crystallizing polymers, depends on the complete thermal-mechanical history the polymer experiences during processing. To understand or even predict polymer solidification during processing an important requirement to be fulfilled is quantitatively mimicking such conditions, i.e. characterization with cooling rates of $\mathit{O}$ (10²) °C.s⁻¹ in combination with elevated pressures of $\mathit{O}$ (10³) bar and shear rates of $\mathit{O}$ (10²–10⁴) s⁻¹. A substantial amount of literature was devoted to polymer solidification, specifically the crystallization kinetics and structure development, in terms of specific volume using PVT [1], [2], [3], [4], [5], [6], [7], [8]. Unfortunately, most lack the ability to measure under realistic processing conditions; either the measuring range of a single processing parameter is insufficient, the control over processing history is inadequate or the combinations of processing conditions are limited.

Flow induced crystallization (FIC) strongly depends on the flow strength. Not only the number of nuclei can be increased dramatically, also the type of crystalline morphology can change from spherulitic to shish-kebab (fibre-like crystals with perpendicular lamellae attached). This can have a strong influence on the crystallization rate and, more important, on the final physical properties [9], [10], [11], [12]. Van Meerveld et al. gave a classification of FIC in terms of critical values for the Deborah number¹ for which specific crystalline structures were generated, i.e. point like nuclei from which spherulites grow or fibre like nuclei from which shish-kebab structure emerge [14]. This classification was extended by van der Beek et al. to classify the combinatorial effect of cooling rate, pressure and shear flow on the evolution of the specific volume and resulting crystalline morphologies of an isotactic polypropylene (iPP). Using a custom designed dilatometer, the time evolution of the specific volume of (semi-crystalline) polymers was investigated for an unusual wide range of realistic non-isothermal conditions, elevated pressures and strong shear flows [15], [16], [17].

Along with the different crystalline morphologies, a common phenomenon observed for iPP is polymorphism, that is the ability of the polymer to crystallize into different types of crystal phases. The processing conditions necessary to attain a given iPP polymorph was investigated in detail and well established [18], [19], [20]. However, although there is a vast amount of literature on iPP polymorphism, no consensus is reached on when and how which crystal phase occurs during melt processing as a function of the thermo-mechanical conditions. Consequently, predicting the type of structures and polymorphisms is still an open issue [21], [22], [23].

In this study the classification of van der Beek et al. [17] is followed and extended by incorporating, in a controlled way, the effects of pressure, cooling rate and shear flow. The existence of the three regimes with a) no influence of flow, b) flow generates extra point nuclei and c) flow generates oriented shish-kebab structures are confirmed. We reveal the details of the crystalline structures for the different regimes including the different phases (in this case α-, β-, and γ-phase) that occur. The results are explained using the present knowledge of flow induced and multi-phase crystallization and are related to the crystalline morphology, as investigated ex-situ using X-ray and visualized by transmission electron microscopy (TEM).