PVT Relationships for Liquid and Glassy Poly(vinyl acetate)

Area of Science:

• Materials Science
• Polymer Physics
• Thermodynamics

Background:

• Poly(vinyl acetate) (PVAc) is a common polymer with complex behavior under pressure.
• Understanding its phase transitions and thermodynamic properties is crucial for material applications.

Purpose of the Study:

• To investigate the Pressure-Volume-Temperature (PVT) behavior of liquid and glassy poly(vinyl acetate).
• To analyze the influence of different thermodynamic histories on PVT measurements and resulting glass properties.
• To relate PVT data to kinetic and thermodynamic models.

Main Methods:

• PVT measurements were conducted on poly(vinyl acetate) from -30 to 100 °C and 0 to 800 bar.
• Three distinct thermodynamic histories were employed: variable formation pressure, constant formation pressure at 1 atm, and constant formation pressure at 800 bar.
• Isobaric cooling at 5 °C/h was used to form the glassy state in all histories.

Main Results:

• Variable formation pressure history yields glasses with structures dependent on pressure, complicating thermodynamic property determination.
• The liquid-glass intersection temperature (T_g(P)) is identified as a critical kinetic property, aligning with isoviscous state approximations.
• Constant formation pressure histories provide accurate thermodynamic properties but limited kinetic insights.
• Increased glass formation pressure significantly enhances density, while entropy remains largely unaffected.

Conclusions:

• Thermodynamic histories critically influence the interpretation of PVT data for polymers.
• The liquid-glass intersection temperature is a valuable parameter for understanding polymer relaxation dynamics under pressure.
• PVT data, when properly contextualized by formation history, can be reconciled with dynamic mechanical and dielectric measurements.