High-pressure crystallization of isotactic polypropylene droplets

Area of Science:

• Polymer Science
• Materials Science
• Crystallization

Background:

• Isotactic polypropylene (PP) can crystallize in multiple polymorphic forms (α, β, γ, mesophase).
• Controlling PP crystallization under external stimuli like high pressure is crucial for tailoring material properties.
• Nanoscale confinement effects can significantly influence polymer crystallization behavior.

Purpose of the Study:

• To investigate the effect of droplet size and high pressure on the polymorphic crystallization of isotactic polypropylene (PP) within a polystyrene (PS) matrix.
• To determine the critical droplet size for the formation of the PP gamma (γ) crystal form under high pressure.
• To understand the role of nucleating heterogeneities in PP γ-phase formation under confinement and high pressure.

Main Methods:

• Fabrication of PP/PS multilayered systems using layer-multiplying coextrusion.
• Controlled formation of PP droplets in PS matrix via interfacially driven nanolayer breakup and melt blending.
• Annealing of PP/PS dispersions under high pressure (200 MPa) to induce crystallization.
• Analysis of PP crystal forms (α, γ, mesophase) using microscopy and diffraction techniques.

Main Results:

• PP droplet size was controlled by initial layer thickness (12–200 nm) and melt blending.
• Only the largest PP droplets (average size 170 μm) predominantly crystallized in the γ form under high pressure.
• Smaller droplets (42 μm from 200 nm layers) showed minor γ form content, while even smaller ones contained α form and/or mesophase.
• The γ phase formation was exclusively observed in droplets large enough to accommodate effective nucleating heterogeneities.

Conclusions:

• Droplet size is a critical factor governing PP polymorphic crystallization under high pressure.
• The presence of nucleating heterogeneities is necessary for the formation of the PP γ phase under high pressure.
• High pressure processing combined with controlled morphology offers a route to selectively form specific PP crystal structures.