Quantification of non-isothermal, multi-phase crystallization of isotactic polypropylene: The influence of cooling rate and pressure
Graphical abstract
Introduction
In commonly used production processes like injection molding, film blowing and fiber spinning, polymers are processed at elevated pressures and/or high deformation rates and simultaneously cooled from the melt within tenths of seconds. The crystallization process of semi-crystalline polymers is strongly affected by these extreme processing conditions. Although flow has a pronounced effect on the crystallization kinetics and resulting morphology, the focus of this study is on the characterization of quiescent non-isothermal and isobaric crystallization of isotactic polypropylene (iPP) and model development. The influence of flow on the crystallization kinetics and crystal phase distribution is ongoing work and will be presented in the near future.
The most established physical picture of quiescent crystallization is nucleation and subsequent growth of spherulites; crystalline lamellae grow in three dimensions starting from point-like nuclei. The nucleation density and growth rates have been studied for a range of materials including iPP [1], and reference therein. The reported growth rates of iPP homopolymer are comparable for different grades, e.g. diverse molecular weights, while the nucleation density is always unique due to residuals and catalysts remaining from the industrial synthesis [2], [3]. The nucleation density and growth rate are usually measured by optical microscopy in conditions that typically promote the α-crystals. However, it is well known that iPP is a polymorphic material with several crystal modifications [4]. Most common is the monoclinic α-phase, a stable crystal form created under moderate conditions. Shear or nucleation agents results in formation of the hexagonal β-crystallite [5], [6], [7]. Available beta-nucleation agents contrast in efficiency and selectivity during quiescent crystallization; the beta-phase fraction is dependent on the nucleation agent concentration and its ability to solely induce growth of the beta-phase. Furthermore, orthorhombic γ-crystals are formed at elevated pressures or in copolymers [8], [9], [10], [11], [12]. The mesomorphic phase, with features intermediate to those of the crystalline and amorphous state, is obtained when a sample is cooled from the melt at high cooling rates [13], [14]. In general, the nucleation density or spherulitic growth rates of the different polymorphs in an iPP homopolymer are difficult to determine experimentally in a single experiment, especially when high cooling rates and/or elevated pressures are required to induce a specific crystal phase.
The crystallization kinetics of the different polymorphs is not well established as a function of temperature and pressure. An increase in pressure results in an increase of the nucleation density [15] and in the equilibrium melting temperature [9] and thus in a higher undercooling , which is the driving force for crystallization. However, the exact effect of pressure on the growth rate of a given crystal phase is not yet known. For example, it is speculated from modeling and numerical simulations that the growth rate of the α-phase shifts towards higher temperatures with pressure accompanied with a decrease in the maximum growth rate [16].
Many attempts are made to model the crystallization process in semi-crystalline polymers [17], [18], [19], [20], [21]. In the case of iPP, most models lack to incorporate the polymorphism behavior in a clear way or do not account for relevant processing conditions (especially the effect of pressure is often discarded). A counter example is a kinetic model [18], [19] that uses pressure dependent rate equations for the α- and mesomorphic phase. Unfortunately, the nucleation density and growth rate are indistinguishable in these rate equations and hence, the kinetic parameters lose their physical meaning. To our knowledge the simultaneous incorporation of all iPP polymorphs in a quiescent crystallization model is not done before.
The aim of this study is to develop a model that includes the effects of cooling rate and applied pressure, as experienced in industrial processing, on the formation of different crystal phases, i.e. the α-, β-, γ- and mesomorphic phase. Material functions for the quiescent crystallization process are determined; both the nucleation density and individual crystal growth rates are temperature and pressure dependent. These relations are incorporated in the Scheider rate equations [22] and combined with the Kolmogoroff-Avrami expression for space filling [2], [23], [24], [25]. A number of experimental setups are used in combination with in-situ and ex-situ X-ray collection to study the (time-resolved) structure formation of iPP and β-iPP.
Section snippets
Theory
Polymer crystallization from the melt is dominated by heterogeneous nucleation. The nuclei grow in time, depending on the temperature and pressure, forming spherulites that will impinge and stop growing when complete space filling is reached. Therefore, a model describing polymer crystallization should contain expressions for the nucleation density and the spherulitic growth rates. Herein, the effects of secondary crystallization are not considered and not included in the model. The proposed
Materials
Two isotactic polypropylene (iPP) homopolymer grades were used; iPP1 (Borealis HD234CF), weight averaged Mw = 310 kg mol−1 and polydispersity Mw/Mn = 3.4 [32] and iPP2 (Borealis HD601CF), Mw = 365 kg mol−1 and Mw/Mn = 5.4 [33], [34]. These two grades are selected due to their well known difference in crystallization kinetics [3]. To study the kinetics of the β-phase, β-iPP is produced by adding 40 ppm pure γ-quinadricone β-nucleation agent (Borealis) to iPP2 using an in-house mixer.
Fast cooling experiments
A quenching
Experimental approach
The crystallization process for iPP1, iPP2 and β-iPP is studied using multiple experimental setups allowing to probe the structure development in time for various cooling conditions at atmospheric and elevated pressures. The experimental data is used to determine the temperature and pressure dependent material functions in the model; the nucleation densities of both grades, the secondary nucleation density induced by the β-nucleating agent and the spherulitic growth rates of the different
Conclusion
The influence of pressure and cooling rate has been studied on the multi-phase structure development for two grades of iPP and one β-iPP by using various experimental setups in combination with in-situ and ex-situ WAXD collection. It is demonstrated that the proposed non-isothermal crystallization model accurately describes multi-phase crystalline evolution for the α-, β-, γ- and mesomorphic phase. Temperature and pressure dependent nucleation densities and growth rate functions for the α-, γ-
Acknowledgments
This work is part of the Dutch Technology Foundation (STW) research programme STW-EPC 7730. The authors are indebted to the personnel of beamline BM26/DUBBLE at the ESRF for their support during the X-ray measurements and to the people of IME technologies at Eindhoven University of Technology for their assistance with the Pirouette Dilatometer. Prof. Giovanni Carlo Alfonso and Dr. Dario Cavallo at the University of Genova are kindly acknowledged for supplying crystallization data of iPP.
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