A Criterion for Flow-Induced Oriented Crystals in Isotactic Polypropylene under Pressure.

Area of Science:

• Polymer Physics
• Materials Science
• Crystallography

Background:

• Flow-induced oriented crystals enhance polymer product stiffness and strength.
• Understanding the formation conditions of oriented crystals is crucial for industrial applications and polymer physics.
• The concept of specific work of flow provides a framework for analyzing crystal formation.

Purpose of the Study:

• To determine the critical specific work of flow required to induce oriented crystals in isotactic polypropylene.
• To investigate the influence of applied pressure on the formation of oriented crystals.
• To elucidate the relationship between processing conditions and the development of polymer microstructure.

Main Methods:

• Utilized the concept of specific work of flow (w_(T,P)) to quantify the energy input during polymer processing.
• Experimentally applied varying pressures (50, 100, 150 MPa) and a fixed undercooling (65 K) to isotactic polypropylene.
• Summarized and verified the expression for specific work of flow under different thermodynamic conditions.

Main Results:

• Identified a critical specific work of flow (w_c(T,P)) of (1.7 ± 0.7) × 10⁷ J m^-3 for inducing oriented crystals.
• Demonstrated that oriented crystals can be successfully induced in isotactic polypropylene under the specified flow conditions.
• Established that pressure influences the critical specific work of flow through its effects on melt viscosity (Barus law) and equilibrium melting temperature (Clapeyron equation).

Conclusions:

• The specific work of flow is a key parameter for controlling the formation of oriented crystals in polymers.
• The findings provide practical guidance for processing polymers to achieve enhanced mechanical properties.
• This research contributes to a fundamental understanding of flow-induced crystallization in polymers.