In situ studies on the temperature‐related deformation behavior of isotactic polypropylene spherulites with uniaxial stretching: The effect of crystallization conditions

Abstract

The deformation behavior of isotactic polypropylene (iPP) spherulites with uniaxial stretching was investigated at different drawing temperatures via in situ polarized optical microscope (POM) observation. The iPP spherulites were prepared by two procedures: cooled to the room temperature from melt and annealed at 135, 140, and 145°C for 3 h. It was found that the crystallization conditions dominate the crystalline morphology and even the tensile properties of iPP. For iPP which crystallized during cooling progress, the spherulites were imperfect and the boundaries of the spherulites were diffuse, displaying good toughness at various drawing temperatures. For iPP annealed at high temperatures displayed the brittle fracture‐modes and the crack happened between spherulites, which due to the large and perfective spherulites have thick lamellas and weak connection at interspherulitic boundary. The shape and size of the iPP spherulites formed at 140 and 145°C are affected with uniaxial stretching till to the fracture of the samples at different drawing temperatures. The spherulites obtained at 135°C are deformed along the drawing direction at 100°C but not affected at low drawing temperatures, indicating the toughness increased with the increase of the drawing temperatures. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers

The Extent of Interlayer Bond Strength during Fused Filament Fabrication of Nylon Copolymers: An Interplay between Thermal History and Crystalline Morphology

“…Maltese cross extinction patterns are clearly visible which indicate the formation of spherulites [ 69 ]. The spherulites present at the interface exhibit stronger birefringence compared to those in the bulk of the layer, indicating that more perfect spherulites are formed at the weld interface [ 70 ]. Yet, spherulite density appears to be greater in the bulk of the layer.…”

“…This observation could be explained by possible remelting of the top surface of a previously extruded layer, which might have already crystallized to some extent, upon deposition of a new layer, where nucleation at the interface will be facilitated by the presence of remaining nuclei. This can trigger crystallization to occur at higher temperatures, leading to the formation of more perfect spherulites with sharp boundaries [ 70 ]. The bulk of the layer will hardly be nucleated and will only start nucleating at lower temperatures at larger undercooling, resulting in less perfect spherulites with a higher level of defects and less intense birefringence, yet a higher spherulite density is achieved since primary nucleation will occur almost simultaneously across the bulk of the layer [ 13 , 70 ].…”

“…This can trigger crystallization to occur at higher temperatures, leading to the formation of more perfect spherulites with sharp boundaries [ 70 ]. The bulk of the layer will hardly be nucleated and will only start nucleating at lower temperatures at larger undercooling, resulting in less perfect spherulites with a higher level of defects and less intense birefringence, yet a higher spherulite density is achieved since primary nucleation will occur almost simultaneously across the bulk of the layer [ 13 , 70 ].…”

“…As a result, spherulite sizes will not grow much larger than those of the nuclei from which they originate [ 69 ]. Upon crystallization, partial disentanglement of polymer chains is necessary to embed the chains into growing crystals, while remaining entanglement points become excluded from the crystalline regions and become located in the interlamellar amorphous regions [ 13 , 70 ]. Under mechanical loading, these amorphous regions between crystallites are largely responsible for the final mechanical performance, as bridging entanglements and tie chains connecting lamellae are considered to be stress transmitters upon deformation, providing ductility [ 70 , 73 ].…”

“…Upon crystallization, partial disentanglement of polymer chains is necessary to embed the chains into growing crystals, while remaining entanglement points become excluded from the crystalline regions and become located in the interlamellar amorphous regions [ 13 , 70 ]. Under mechanical loading, these amorphous regions between crystallites are largely responsible for the final mechanical performance, as bridging entanglements and tie chains connecting lamellae are considered to be stress transmitters upon deformation, providing ductility [ 70 , 73 ]. The reduced primary nucleation and crystal growth rate at elevated temperatures, induced by a higher T build plate , will lead to larger spherulites of higher crystallinity and a higher level of chain disentanglement.…”