Modeling of flow‐induced crystallization in blends of isotactic polypropylene and poly(ethylene‐ co ‐octene)

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Isothermal and non-isothermal crystallization behaviours of the blends of long chain branched polypropylene (LCB PP) and poly(ethylene-co-octene) (PEOc) with different weight ratios were studied under quiescent and shear flow using polarized optical microscopy (POM), differential scanning calorimetry (DSC), and rheological measurements. Experimental results showed that the crystallization of the LCB PP/PEOc blends were significantly accelerated due to the existence of the long chain branches (LCBs), the blends being able to rapidly crystallize even at 146 • C. The addition of PEOc that acts as a nucleating agent, could also increase the crystallization rate of LCB PP. However, the crystallization rate of LCB PP was reduced when the PEOc concentration was more than 60 wt %, showing a retarded crystallization growth mechanism. The morphology of the binary blend was changed from a sea-island structure to a co-continuous phase structure when the PEOc concentration was increased from 40 to 60 wt %. In comparison with linear isotactic iPP/PEOc, the interfacial tension between LCB PP and PEOc was increased. In addition, flow-induced crystallization of LCB PP/PEOc blends was observed. Possible crystallization mechanisms for both LCB PP/PEOc and iPP/PEOc blends were proposed.

Section: Resultssupporting

confidence: 68%

Section: Introductionmentioning

confidence: 99%

Isothermal and Non-Isothermal Crystallization Studies of Long Chain Branched Polypropylene Containing Poly(ethylene-co-octene) under Quiescent and Shear Conditions

Zhang

2017

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“…When the iPP sheet was stretched at a specific temperature, the free energy of crystallization decreased [17,39], and the molecular chains oriented along the stretching direction. Spherulites lamellae molecular chains did not relax in a short period of time due to the restricted movement of the molecular chains at the crystallization temperature.…”

Section: Resultsmentioning

confidence: 99%

“…Molecular chains of iPP become extended and slip under stress stretching [14,15]. Fengyuan Yu [16,17] studied the crystallization kinetics of iPP under shear. Their results indicated that shear treatments have a positive effect on the formation of nucleation, and the induction time for crystallization was more sensitive to the increase in the shear rate.…”

Section: Introductionmentioning

confidence: 99%

Effects of Solid-State Stretching on Microstructure Evolution and Physical Properties of Isotactic Polypropylene Sheets

Zhou

Chen

et al. 2019

The microstructure evolution of an isotactic polypropylene (iPP) sheet during solid-state stretching was studied. The transition of the spherulites—cylindrites was evaluated using in-situ two-dimensional wide-angle and small-angle X-ray scattering methods. The crystallinity of stretched iPP sheets was characterized by differential scanning calorimetry. The crystal morphology was observed by means of scanning electron microscopy. It was found that the differences of crystal microstructure of the iPP sheet depended on the stretching strain, which promoted the orientation of molecular chains. Amorphous molecular chains in the spherulites oriented and formed into a mesophase near the yield point, and the partially ordered mesophase was further stretched to form an oriented cylindrite structure after the yield point. The highest relative content of cylindrites appeared at 15% strain. Notably, as the amorphous phase embedded into the lamellae layer, the crystal size decreased with the increase of strain, which indicated that the crystallinity of the stretched iPP sheet was much higher than that of unstretched iPP sheet. The induced cylindrites structure played a more important role in improving the mechanical properties and heat resistance of iPP sheets. Compared with the unstretched iPP sheets, the tensile strength increased by 28%, the notch impact toughness significantly increased by 78%, and the vicat softening point increased from 104 to 112 °C.

“…This shows that annealing could not promote the structural rearrangement of the molecular chains of the β-modification lamellae, and the crystal phase transformation process did not occur. When the stretching temperature was below the crystallization temperature ( Figure 8 a,b), a β-bundle crystal structure was observed [ 39 , 40 , 41 ], indicating that β-modifications dominated in the system. Oriented lamellae were not observed along the stretching direction, implying that the amorphous molecular chains were oriented, but that the lamellae were not stretched.…”

Section: Resultsmentioning

confidence: 99%

Deformation-Induced Crystallization Behavior of Isotactic Polypropylene Sheets Containing a β-Nucleating Agent under Solid-State Stretching

Zhou

Chen

et al. 2020

The deformation-induced crystallization of an isotactic polypropylene (iPP) sheet containing a β-nucleating agent was evaluated. The phase transformation of the β-modifications was investigated and the crystal morphology was observed at room temperature after stretching at different temperatures. The results showed that the crystallinity increased after solid-state stretching. When the stretching temperature was below the initial crystallization temperature, stretching deformation promoted the orientation of amorphous molecular chains. When the deformation temperature exceeded the crystallization temperature, part of the β-modifications underwent a phase transformation process and was stretched into a shish-kebab structure. However, once the stretching temperature was close to the melting point, the β-modifications melted and recrystallized, and the shish-kebab structure underwent stress relaxation due to poor thermal stability, transforming into α-modifications. It was revealed that the crystal phase transformation mechanism of the β-modifications was based on the orientation of the molecular chains between the adjacent lamellae. In addition, the shish-kebab cylindrite structure played an important role in modifying the tensile and impact properties of the iPP sheet. The tensile and impact strengths increased by as much as 34% and 126%, respectively.