Hydrogen Bonding Slows Down Surface Diffusion of Molecular Glasses.

Area of Science:

• Materials Science
• Physical Chemistry
• Polymer Science

Background:

• Surface-grating decay is a key phenomenon in understanding material properties.
• Organic glasses with extensive hydrogen bonding exhibit unique behaviors.
• Surface diffusion plays a crucial role in various material processes.

Purpose of the Study:

• To investigate the mechanism of surface-grating decay in organic glasses with extensive hydrogen bonding.
• To compare the decay mechanisms of hydrogen-bonded glasses with those lacking or having limited hydrogen bonding.
• To elucidate the influence of hydrogen bonding on surface diffusion dynamics.

Main Methods:

• Measurement of surface-grating decay for sorbitol, maltitol, and maltose using 1000 nm wavelength gratings.
• Studying decay across a temperature range corresponding to viscosities from 10^5 to 10^11 Pa s.
• Comparative analysis with organic glasses of similar molecular size but with limited or no hydrogen bonding.

Main Results:

• Surface-grating decay in hydrogen-bonded glasses (sorbitol, maltitol, maltose) occurs via viscous flow across the studied temperature and viscosity range.
• In contrast, similar non-hydrogen-bonded glasses transition from viscous flow to surface diffusion under identical conditions.
• Extensive hydrogen bonding significantly slows down surface diffusion in organic glasses.

Conclusions:

• Extensive hydrogen bonding hinders surface diffusion by maintaining molecular interactions near the surface.
• The preservation of hydrogen bonds increases the kinetic barrier for diffusion.
• This finding correlates liquid fragility with surface diffusion, suggesting resistance to dynamic excitation, and has implications for surface crystallization and vapor deposition processes.