Crystallization near glass transition: transition from diffusion-controlled to diffusionless crystal growth studied...

Area of Science:

• Materials Science
• Physical Chemistry
• Crystallography

Background:

• Glass-forming liquids can exhibit unusual crystal growth modes near their glass transition temperature (Tg).
• This fast growth, termed glass-crystal (GC) growth, is not limited by bulk molecular diffusion.

Purpose of the Study:

• To investigate the GC growth mode in the ROY system, known for its multiple polymorphs.
• To differentiate polymorph behavior regarding GC growth and its characteristics.

Main Methods:

• Studied GC growth by crystallizing seven ROY polymorphs from the liquid state.
• Analyzed polymorph growth rates, morphologies, density, and molecular packing.

Main Results:

• Some ROY polymorphs exhibited GC growth, characterized by higher density and isotropic packing.
• GC-growing polymorphs showed morphology changes (faceted, fiber-like, spherulites) and significantly faster growth rates (3-4 orders of magnitude) with lower activation energies compared to non-GC polymorphs.
• GC growth persisted above Tg and was linked to local molecular motions, distinct from the liquid's structural relaxation (alpha process).

Conclusions:

• GC growth is a solid-state transformation, not diffusion-limited, enabled by local molecular motions in the glassy state.
• Polymorphs with higher density and more isotropic packing are more prone to GC growth.
• The GC mode's characteristics differ significantly from conventional crystal growth from the melt.