Growth kinetics of polymer crystals in bulk

Area of Science:

• Polymer science
• Materials science
• Thermodynamics

Background:

• Understanding polymer crystallization kinetics is essential for material properties.
• Previous models often simplify the complex transition from melt to crystal.
• The role of intermediate phases in crystallization is not fully elucidated.

Purpose of the Study:

• To investigate the temperature dependence of polymer spherulite growth rates.
• To identify and characterize the
• zero growth temperature
• (Tzg) in various polymers.
• To develop a thermodynamic framework explaining crystal growth mechanisms.

Main Methods:

• Temperature-dependent measurements of spherulite growth rates for i-polystyrene, poly(epsilon-caprolactone), and linear polyethylene.
• Application of a thermodynamic multiphase scheme.
• Analysis of data from small-angle X-ray scattering, calorimetry, and optical growth rate measurements.

Main Results:

• The activation barrier for spherulite growth diverges at a Tzg, 14K, 22K, and 12K below the equilibrium melting points for i-polystyrene, poly(epsilon-caprolactone), and polyethylene, respectively.
• Tzg is identified as the temperature of a transition to a mesomorphic phase mediating crystal growth.
• The rate-determining step involves chain sequence attachment to the mesomorphic layer, with straightening causing the activation barrier.

Conclusions:

• A novel thermodynamic multiphase scheme accurately describes polymer crystal growth.
• The Tzg is a critical parameter defining the stability of crystalline and mesomorphic phases.
• Polyethylene's facile crystallization is attributed to a low surface free energy of its mesomorphic layer.