A thermodynamic multiphase scheme treating polymer crystallization and melting

Area of Science:

• Polymer Science
• Materials Science
• Thermodynamics

Background:

• Polymer crystallization is a complex process.
• The role of intermediate phases, like mesomorphic phases, is debated.
• Understanding these phases is crucial for controlling polymer properties.

Purpose of the Study:

• To compare phase transition temperatures of n-alkanes with polyethylene lamellae.
• To develop a thermodynamic model for transitions between melt, mesomorphic, and crystalline polymer phases.
• To quantitatively evaluate experimental data for various polymers using this model.

Main Methods:

• Comparison of n-alkane transition temperatures with polyethylene lamellar thickness and melting points.
• Construction of a thermodynamic scheme for melt, mesomorphic, and lamellar crystallite transitions.
• Reconsideration and quantitative evaluation of Small-Angle X-ray Scattering (SAXS) and Differential Scanning Calorimetry (DSC) data.

Main Results:

• Polymer lamellar thickness variation correlates with n-alkane crystalline-mesomorphic phase transitions.
• Mesomorphic polyethylene layers are stable up to a thickness defined by this transition.
• The proposed thermodynamic scheme yields equilibrium transition temperatures, heats of transition, and surface free energies for PE, sPP, iPS, and P(epsilonCL).

Conclusions:

• Polymer crystallization likely proceeds via a mesomorphic phase.
• Mesomorphic phases exhibit thermodynamic properties intermediate between melt and crystals, not liquid-like.
• The study provides a quantitative thermodynamic framework for understanding polymer phase transitions.