Reports: AC10
43809-AC10 Nucleation of One Polymorph by Another
The grant period 2008-2009 is a time extension year. Since a narrative progress report is optional, I will briefly summarize the accomplishments in this period. We have continued to make progress toward understanding the cross-nucleation between crystal polymorphs, a newly discovered mechanism of crystal nucleation in polymorphic systems. Polymorphism is the ability of the same substance to crystallize in different structures. Polymorphism is important in developing drugs, pigments, explosives, and other materials because polymorphs have different properties. Controlling polymorphism remains an unsolved problem; for example, polymorphs may crystallize concomitantly and even suddenly disappear. Cross-nucleation is the nucleation of a new, faster-growing polymorph on the growing crystals of the initial polymorph. This phenomenon is important because seeding is the principal method for controlling polymorphs. It invalidates the common assumption that the polymorph of the initial nucleation is the polymorph of the final crystallization product. The phenomenon is also important to the theory of nucleation, a foundation of science and technology. It differs from other nucleation processes in that the new phase nucleates on an initial phase of the same substance that continues to grow.
Major accomplishments during this period:
(1) Jing Tao, a graduate student who worked on this project, received her Ph.D. in June, 2009.
(2) We have used polymorphs to study how an extraordinarily fast mode of crystal growth from organic glasses depends on the crystal structure being grown. A remarkable property of certain glass-forming liquids is that a fast mode of crystal growth is activated near the glass transition temperature Tg and continues in the glassy state. This growth mode, termed GC (glass-crystal), is so fast that it is not limited by molecular diffusion in the bulk liquid. We have studied the GC mode by growing seven polymorphs from the liquid of ROY, currently the top system for the number of coexisting polymorphs of known structures. Some polymorphs did not show GC growth, while others did, with the latter having higher density and more isotropic molecular packing. These observations enabled us to evaluate various explanations for GC growth and conclude that it is solid-state transformation enabled by local molecular motions native to glasses and disrupted by the liquids structural relaxation.
(3) This project has grown into a bigger project funded by the NSF titled Polymorphism of Organic Materials (DMR 0804786).
(4) In this year, five papers from this lab acknowledge the PRF support, as well as 17 presentations (all invited) at leading scientific societies, universities, and companies.
