Coupling of Caged Molecule Dynamics to JG β-Relaxation II: Polymers

Area of Science:

• Condensed Matter Physics
• Materials Science
• Physical Chemistry

Background:

• At high frequencies (above 1 GHz), structural α-relaxation and Johari-Goldstein (JG) β-relaxation are too slow to be observed in glass-forming materials.
• The fast dynamics at these frequencies are attributed to molecular vibrations and caged molecule dynamics, manifesting as nearly constant loss (NCL).
• The secondary glass transition temperature (Tgβ) is a key parameter characterizing the JG β-relaxation.

Purpose of the Study:

• To investigate the high-frequency dynamics (100 MHz to THz) in the glassy state of various glass-formers, including polymers and small molecules.
• To identify and characterize the temperature at which the NCL's temperature dependence changes (THF).
• To establish the relationship between THF and the secondary glass transition temperature (Tgβ).

Main Methods:

• Analysis of existing and new experimental data from quasielastic light scattering, Brillouin scattering, quasielastic neutron scattering, and GHz-THz dielectric relaxation.
• Comparison of the temperature dependence of NCL, elastic scattering intensity (I(Q, T)), and mean-square-displacement (⟨u(2)(T)⟩) with Tgβ.
• Independent determination of Tgβ using positronium annihilation lifetime spectroscopy (PALS) and adiabatic calorimetry.

Main Results:

• A consistent change in the temperature dependence of NCL, I(Q, T), or ⟨u(2)(T)⟩ was observed at a temperature THF in the glassy state of diverse glass-formers.
• This temperature THF was found to be approximately equal to the independently determined secondary glass transition temperature (Tgβ) (THF ≈ Tgβ).
• This THF ≈ Tgβ relationship was not previously recognized in scattering studies of polymers and small molecular glass-formers.

Conclusions:

• The observed property (THF ≈ Tgβ) arises from the coupled nature of JG β-relaxation and caged dynamics, both influenced by density and entropy.
• The JG β-relaxation acts as a terminator for caged dynamics, making these processes inseparable.
• The universality of the THF ≈ Tgβ relationship provides crucial insights into the glass transition problem, necessitating its consideration in future research.