Abstract
In this study, a non-nucleated homopolymer (HP) and random copolymer (RACO), as well as a nucleated HP and heterophasic copolymer (HECO) were investigated regarding their crystallization kinetics. Using pvT-measurements and fast scanning chip calorimetry (FSC), the crystallization behavior was analyzed as a function of pressure, cooling rate and temperature. It is shown that pressure and cooling rate have an opposite influence on the crystallization temperature of the materials. Furthermore, the addition of nucleating agents to the material has a significant effect on the maximum cooling rate at which the formation of α-crystals is still possible. The non-nucleated HP and RACO materials show significant differences that can be related to the sterically hindering effect of the comonomer units of RACO on crystallization, while the nucleated materials HP and HECO show similar crystallization kinetics despite their different structures. The pressure-dependent shift factor of the crystallization temperature is independent of the material. The results contribute to the description of the relationship between the crystallization kinetics of the material and the process parameters influencing the injection-molding induced morphology. This is required to realize process control in injection molding in order to produce pre-defined morphologies and to design material properties.
Highlights
Isotactic polypropylene is a polymorphic, semi-crystalline thermoplastic
It is shown that pressure and cooling rate have an opposite influence on the crystallization temperature of the materials
While the crystallization temperature increases linearly with increasing pressure, it decreases with increasing cooling rate
Summary
Isotactic polypropylene (iPP) is a polymorphic, semi-crystalline thermoplastic. It can crystallize into α-, β-, or γ-modification, as well as form a mesophase depending on the crystallization conditions. De Santis et al [23,24] investigated the cooling rate-related crystallization of a non-nucleated iPP homopolymer under cooling rates ranging from 15 to 1000 K/s They concluded that at cooling rates up to 90 K/s the investigated material forms the α-monoclinic crystalline phase. Studies [20,25] on the influence of presence of 1-alkene co-units in random copolymers on the cooling rate-dependent crystallization behavior indicated that the incorporation of 1-alkenes into the iPP chain results in a reduction of the crystallization rate and crystallinity. Investigations by Drongelen et al [29] on iPP homopolymers at pressures of 5 and 25 MPa and cooling rates of 0.1 to 2.0 K/s showed that the formation of α- and γ-phases always occurs combined with the γ-phase being formed preferentially at high pressures and decreasing cooling rates, as described elsewhere [31,35,36]. The results were evaluated according to the following criteria: non-isothermal crystallization temperature as function of pressure, cooling rate, and isothermal crystallization as a function of temperature/supercooling
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