Fast surface diffusion of amorphous o-terphenyl and its competition with viscous flow in surface evolution

Area of Science:

• Materials Science
• Physical Chemistry
• Condensed Matter Physics

Background:

• Molecular glasses like o-terphenyl (OTP) exhibit complex behavior near their glass transition temperature (Tg).
• Understanding surface dynamics is crucial for predicting material properties and processing behavior.

Purpose of the Study:

• To quantify surface self-diffusion coefficients in o-terphenyl (OTP).
• To investigate the mechanism of capillarity-driven surface grating decay in molecular glasses.
• To compare surface diffusion in OTP with other materials and bulk diffusion.

Main Methods:

• Measurement of surface self-diffusion coefficients using surface-grating decay driven by capillarity.
• Analysis of the temperature-dependent transition from viscous flow to surface diffusion.
• Comparison of surface diffusion in OTP with bulk diffusion and other glass-forming materials.

Main Results:

• Surface diffusion in OTP is orders of magnitude faster than bulk diffusion, particularly below Tg.
• A transition in decay mechanism from viscous flow to surface diffusion occurs at Tg + 11 K for 1000 nm gratings.
• Surface diffusion rates in molecular glasses depend on intermolecular forces, unlike polystyrenes and silicates.

Conclusions:

• Surface diffusion is a dominant transport mechanism in molecular glasses at low temperatures.
• Intermolecular forces play a critical role in governing surface diffusion rates.
• Surface diffusivity is directly proportional to the velocity of surface crystal growth on molecular glasses below Tg.