Tailoring microstructure and mechanical properties of injection molded isotactic–polypropylene via high temperature preshear

Abstract

In this work, isotactic-polypropylene (iPP) specimens were prepared by a modified injection molding machine, in which high temperature preshear (HTPS) can be imposed on the molten polymer during the plasticizing stage. The effect of HTPS on the microstructure and mechanical property of iPP was investigated. It was found that spherulite size in core region of iPP part decreased steadily with the increasing HTPS duration, indicating that HTPS could substantially enhance iPP nucleation. Moreover, b-iPP formation correlated strongly with HTPS duration. That is, in the absence of HTPS, b-iPP existed only in intermediate region; with moderate HTPS duration, b-iPP could be unexpectedly formed in core region; however, long HTPS duration inhibited b-iPP formation in both intermediate region and core region. Based on the relationship between b-iPP formation and HTPS duration, metastable nuclei, instead of a-row nuclei, were proposed to be responsible for the development of b-iPP. Notched Izod impact test showed that moderate HTPS duration enhance the impact strength of injection molded iPP by decreasing the thickness of shear region and elevating b-iPP crystallinity in core region. Dynamic mechanical test indicated that with the increase of HTPS duration, the storage modulus of injection-molded iPP improves drastically. POLYM. ENG. SCI., 55:2714-2721, 2015

Overview of the Experimental Trends in Water‐Assisted Injection Molding

“…It is well known that iPP is a polymorphic material with several crystal modifications, including monoclinic α‐phase, trigonal β‐phase, orthorhombic γ‐phase, and smectic mesophase . Thus, iPP is always used as a representative template to study the crystalline evolution during the injection molding process.…”

“…[92] It is well known that iPP is a polymorphic material with several crystal modifications, including monoclinic α-phase, trigonal β-phase, orthorhombic γ-phase, and smectic mesophase. [93][94][95][96][97][98] Thus, iPP is always used as a representative template to study the crystalline evolution during the injection molding process. Wang and Huang [99] suggested that lowering the melt temperature is favorable for the formation of β-form in the WAIM PP part (Figure 9a,b).…”

Phase transitions of the rapid‐compression‐induced mesomorphic isotactic polypropylene under high‐pressure annealing

“…Its rich polymorphism, which has attracted wide interest, affected greatly on the mechanical properties . Besides the three crystalline polymorphs of α‐monoclinic, β‐hexagonal, and γ‐orthorhombic forms, the mesomorphic form of iPP with ordering between the amorphous and the crystalline phase has been paid attention constantly . The molecular segments of mesomorphic iPP preserve the 3 1 helical conformation, but lose three‐dimensional long‐range order .…”

Competition effect of shear‐induced nuclei and multiwalled carbon nanotubes (MWCNT) on β‐isotactic polypropylene (iPP) formation in preshear injection‐molded iPP/MWCNT nanocomposites

“…By preshear injection molding, shear‐induced nuclei can be preserved even in the core region. As a result, β ‐ i PP could be induced in not only the shear region but also the core region of the injection molded sample . The relative content of β ‐ i PP ( ) in the core region is even equal to that in the shear region.…”

“…The relative content of β ‐ i PP ( ) in the core region is even equal to that in the shear region. What is more, it has been proved that the formation of β ‐ i PP depends strongly on preshear intensity : in the absence of preshear, β ‐ i PP exists only in the shear region of injection molded specimen; strong preshear intensity inhibits the β ‐ i PP formation in both shear and core region; only moderate preshear favors the formation of β ‐ i PP in the core region. More interestingly, decreases monotonously with an increase of relaxation time after shear cease .…”