Scaling the dynamics of orientationally disordered mixed crystals

Area of Science:

• Materials Science
• Condensed Matter Physics
• Physical Chemistry

Background:

• Orientationally disordered (OD) mixed crystals exhibit complex dynamics near their glass transition temperature (T(g)).
• Understanding relaxation dynamics is crucial for characterizing the glass transition in these materials.

Purpose of the Study:

• To analyze the primary relaxation time evolution in OD mixed crystals [(CH(3))(2)C(CH(2)OH)(2)](1-X)[(CH(3))C(CH(2)OH)(3)](X) as they approach T(g).
• To compare the effectiveness of different models in describing these dynamics and explore potential universal behaviors.

Main Methods:

• Utilized a distortion-sensitive, derivative-based procedure to analyze relaxation times.
• Applied the Vogel-Fulcher-Tammann (VFT) parametrization and a critical-like description (tau proportional to (T - T(C))(-phi(' ))).
• Employed the dynamic scaling model (DSM) to interpret results and estimate dynamic heterogeneities.

Main Results:

• The critical-like description provided a superior fit to the relaxation dynamics compared to the VFT parametrization.
• A critical temperature (T(C)) approximately 10 K below T(g) was identified, with critical exponents phi(' ) ranging from 9 to 11.5.
• The dynamic scaling model (DSM) showed good agreement with the observed behavior, enabling estimation of cooperatively rearranging regions near T(g).

Conclusions:

• The findings suggest that a critical-like dynamic behavior may represent an optimal universal pattern for vitrifying OD crystals.
• The study supports the applicability of the dynamic scaling model to understand the dynamics of these plastic crystals near the glass transition.
• The results contribute to a deeper understanding of molecular dynamics and heterogeneity in disordered materials.