Temperature dependency of cavitation in impact copolymer polypropylene during stretching
Graphical abstract
Introduction
Isotactic polypropylene (iPP) as one of the most widely employed plastic materials has been applied in the automotive industry, packaging, medical devices, and so on. The ultra-mechanical performances of iPP were revealed as high stiffness and fatigue resistance. However, at low temperatures it always shows poor impact strength [[1], [2], [3]]. Introducing the additives, such as soft rubbery phase into iPP, can effectively improve its impact property [[4], [5], [6], [7], [8]]. These modified iPP polymers are defined as the impact copolymer polypropylene (ICP).
The micromorphology of ICP and the compatibility between the iPP matrix and the dispersed particles have been massively studied during past years [[9], [10], [11], [12], [13], [14]]. It is well accepted that the dispersed particles uniformly distribute in the iPP matrix. It is also highly emphasized that the balance of high rigidity and toughness of ICP is sensitive to the morphology of dispersed phase [[15], [16], [17], [18], [19]]. For example, the presence of rubber particles as a multi-layered core-shell structure benefits for improving the toughness of ICP without sacrificing too much of its stiffness. For the so-called core-shell structure of rubber particles, the reason for increased toughness can be explained by using the model proposed by Zhang et al. [20]. The crystallizable polyethylene (PE) in ethylene-propylene segmented (EsP) copolymer dominates in the core layer, ensuring the stiffness. The shell region contains two layers. One is the inner layer composed of soft ethylene-propylene random (EPR) copolymer which is responsible for the toughness, and the other one is the outer layer constituted of ethylene-propylene block (EbP) copolymer in which a large proportion of PP segments is included. The co-crystallization can occur between the polypropylene chains in EbP fraction and iPP matrix, enhancing the compatibility between the iPP matrix and the dispersed phase.
Many semi-crystalline polymers show the stress-whitening phenomenon during tensile deformation process. This phenomenon is caused by the formation of heterogeneities (usually cavities) at a length scale similar to the wavelength of visible light, i.e., hundreds of nanometers [21,22]. Among the systematic studies of the cavitation in the semi-crystalline polymers, two distinct cavitation processes triggered at different deformation stages had been established. Galeski pointed out that there is a competition between the cavitation and shear yielding around the yield point during tensile deformation of semi-crystalline polymers [[23], [24], [25]]. The cavitation emerged around the yield point is named as the small strain cavitation and its occurrence is determined by the deformation temperature, strain rate, and also the microstructures such as crystallinity and lamellar thickness [[26], [27], [28], [29]]. The other cavitation occurred at large strain regimes has been noticed by Taraiya et al. [30] through stretching iPP during die-drawing process. This cavitation is initiated at the stage much beyond the yield region and usually ended up with a final breakage of polymers [[31], [32], [33], [34], [35]]. It was recently that the mechanism of this large strain cavitation was established based on results of stretching iPP and poly(4-methyl-1-pentene) (P4M1P) at elevated temperatures. The fragmentation of load-bearing inter-fibril/micro-fibril tie chains in the highly oriented amorphous network is suggested to be the main reason for inducing the large strain cavitation [31,33,34,36,37].
The appearance of cavitation in ICP is directly related to its toughness. The cavities, or microvoids, absorb deformation or fracture energy to facilitate shear yielding or crazing of the matrix in order to induce the brittle-ductile transition [8,38,39]. Thus, a better understanding of cavitation in ICP becomes more and more important. Different from the homo-isotactic polypropylene (homo-iPP), a more complicated cavitation could be expected in the ICP during stretching considering the multi-phase structure in the system [[40], [41], [42]]. According to the previous studies, when the ICP is under an external force, the stress concentrates around the dispersed particles in the plastic matrix [[43], [44], [45]]. The interfacial adhesion between the dispersed phase and the matrix is the key factor to determine the nucleation sites of cavities. If the interfacial adhesion is strong enough, the cavitation would take place inside the dispersed phase. Such case is usually observed in the low temperature deformation process. While the interfacial adhesion becomes weak, the interfacial debonding between the matrix and dispersed phase would occur. That case could be found in the system where the compatibility between the iPP matrix and the dispersed particles is poor. The high temperature might enhance the probability of the latter cavitation incident in ICP because the co-crystallization areas contributed by the EbP fraction and the iPP matrix display low stability against the high temperature [13,20]. Clearly, temperature plays a significant role in controlling the cavitation mechanism in ICP during stretching.
In this work, we employed the ultra-small angle X-ray scattering (USAXS) technique to explore the cavitation in ICP during tensile tests [29,[46], [47], [48]]. Only the small strain cavitation could be triggered in the ICP and the intensity of cavitation increased with increasing the drawing temperature. The latter phenomenon was a striking finding which was totally different from the dependency in iPP. Such peculiar performance in ICP hints that the stability of the dispersed rubber particles in the iPP matrix plays an important role in causing such an unusual phenomenon. Besides, the healing of cavities in the stretched ICP during step-cycle heating was also investigated in order to understand the initiation of cavitation better. The corresponding results further confirmed that the cavitation in ICP substantially depended on the stability of the rubber phase in the system.
Section snippets
Materials and sample preparation
One ICP and its corresponding homo-iPP used in the current work were kindly provided by ExxonMobil Asia Pacific Research and Development Company, Ltd. The ICP was synthesized through in-reactor two-stage polymerizations and the homo-iPP was obtained from the first step polymerization. The basic information of these two polymers are listed in Table 1. Liu et al. recently [49] analyzed the fractionated components using the same ICP through temperature rising elution fractionation (TREF) method
DSC results
Fig. 1 registers the DSC melting curves of two quenched samples. The ICP sample showed a lower crystallinity (38.3%) than the homo-iPP sample (47.5%). The introduction of rubber particles into iPP evidently reduced the weight crystallinity of ICP sample. However, on the other hand, the total proportion of rubber phase was around 28% in ICP and the decrease of ϕw in ICP was lower than 13.3% (47.5%*28%) manifesting that some components in the rubber phase could crystallize. Actually, the
Conclusions
In the present work, the formation and evolution of cavities in iPP and ICP samples during stretching at different temperatures were investigated using USAXS technique. Both the small strain cavitation and the large strain cavitation were traced in iPP at low Td. The former cavitation was suppressed at high Td while the latter one could be found in all deformation cases for iPP. However, the ICP showed different cavitation performances. Only the small strain cavitation could be recognized and
Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: None.
Acknowledgments
This work is supported by the National Key R&D Program of China (2018YFB0704200), the National Natural Science Foundation of China (51525305 and 21704102), and ExxonMobil. We thank Dr. Ran Chen for providing computer code to deal with the WAXD and USAXS data.
References (56)
- et al.
Reinforcement of PP with polymer fibers: effect of matrix characteristics, fiber type and interfacial adhesion
Polymer
(2020) - et al.
Study on synergistic toughening of polypropylene with high-density polyethylene and elastomer-olefin block copolymers under ultrasonic application
Compos. Sci. Technol.
(2018) - et al.
Design of high impact thermal plastic polymer composites with balanced toughness and rigidity: toughening with core-shell rubber modifier
Polymer
(2020) - et al.
Abnormal crystallization behavior of high impact polypropylene under shear
Polymer
(2018) - et al.
Influence on properties and phase structure of single gas-phase reactor made impact polypropylene copolymers
Eur. Polym. J.
(2018) - et al.
Relaxation of shear-enhanced crystallization in impact-resistant polypropylene copolymer: insight from morphological evolution upon thermal treatment
Polymer
(2010) - et al.
Characterization of the microstructure of impact polypropylene alloys by preparative temperature rising elution fractionation
Eur. Polym. J.
(2011) - et al.
Effect of in-situ formed core–shell inclusions on the mechanical properties and impact fracture of polypropylene
Eur. Polym. J.
(2015) - et al.
Correlation between impact properties and phase structure in impact polypropylene copolymer
Mater. Des.
(2015) - et al.
Morphology of polypropylene/poly(ethylene-co-propylene) in-reactor alloys prepared by multi-stage sequential polymerization and two-stage polymerization
Polymer
(2009)
Morphology, microstructure and compatibility of impact polypropylene copolymer
Polymer
Cavitation during deformation of semicrystalline polymers
Prog. Polym. Sci.
Deformation induced void formation and growth in β nucleated isotactic polypropylene
Polymer
Deformation mechanism of iPP under uniaxial stretching over a wide temperature range: an in-situ synchrotron radiation SAXS/WAXS study
Polymer
Inter-fibrillar tie chains determined critical stress of large strain cavitation in tensile stretched isotactic polypropylene
Polymer
Large strain cavitation induced stress whitening in propylene-butene-1 copolymer during stretching
Polymer
Brittle-ductile transition behavior of the polypropylene/ultra-high molecular weight polyethylene/olefin block copolymers ternary blends: dispersion and interface design
Polymer
Polypropylene-elastomer (TPO) nanocomposites: 3. Ductile-brittle transition temperature
Polymer
Effect of αc-relaxation on the large strain cavitation in polyethylene
Polymer
Chain microstructure of two highly impact polypropylene resins with good balance between stiffness and toughness
Polymer
Isotacticity effect on crystallization and melting in polypropylene fractions .1. Crystalline-Structures and Thermodynamic Property Changes
Polymer
Microscopy of isotactic polypropylene crazed and fractured in liquid-nitrogen
Macromolecules
Crack-resistance behavior of polypropylene copolymers
J. Appl. Polym. Sci.
Fatigue of polymers
Int. J. Fract.
Synergistic toughening of polypropylene with ultra-high molecular weight polyethylene and elastomer-olefin block copolymers
RSC Adv.
Influence of propylene-based elastomer on stress-whitening for impact copolymer
J. Appl. Polym. Sci.
Analysis of nanodomain composition in high-impact polypropylene by atomic force microscopy-infrared
Anal. Chem.
Morphology formation in polypropylene impact copolymers under static melt conditions: a simulation study
J. Appl. Polym. Sci.
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