Purpose: To mechanistically explain the origin of two distinct non-isothermal crystallization modes, single-peak (unimodal) and two-peak (bimodal), of organic glasses. Methods: Glasses of ten organic molecules were prepared by melt-quenching and cryogenic milling of crystals. Non-isothermal crystallization of glasses was monitored using differential scanning calorimetry and powder X-ray diffractometry. Results: The non-isothermal crystallization of glass, generated by milling, is either unimodal or bimodal, while that of melt-quenched glass without being milled is always unimodal. The mode of crystallization of amorphous phase depends on the relative position of the crystallization onset (T c) with respect to glass transition temperature (T g), and can be explained by a surface crystallization model. Bimodal crystallization event is observed when T c is below or near T g, due to the fast crystallization onset at milled glass surfaces. Unimodal crystallization is observed when T c is well above T g. We have verified this model by intentionally inducing flip between the two crystallization modes for several compounds through manipulating glass surface area and T c. Conclusions: The two modes of crystallization of organic glasses is a result of the combined effects of faster surface crystallization and variation in specific surface area by milling.
Bibliographical noteFunding Information:
We thank Boehringer-Ingelheim Pharmaceuticals Inc, Ridge-field, CT for financial support to this work. Some of the experiments were performed at the University of Minnesota I.T. Characterization Facility, which receives partial support from the NSF through the NNIN program.
Copyright 2012 Elsevier B.V., All rights reserved.
- Crystallization onset
- Glass transition
- Kinetic flip
- Surface crystallization