title: Origin of two modes of non-isothermal crystallization of glasses produced by milling.
abstract: OBJECTIVE
  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.
authors.affiliation: Pharmaceutical Materials Science & Engineering Laboratory, Department of Pharmaceutics, University of Minnesota, 9-127B Weaver-Densford Hall, 308 Harvard Street S.E., Minneapolis, Minneapolis 55455, USA.
authors.forename: Sayantan
authors.initials: S
authors.lastname: Chattoraj
authors.forename: Chandan
authors.initials: C
authors.lastname: Bhugra
authors.forename: Chitra
authors.initials: C
authors.lastname: Telang
authors.forename: Li
authors.initials: L
authors.lastname: Zhong
authors.forename: Zeren
authors.initials: Z
authors.lastname: Wang
authors.forename: Changquan Calvin
authors.initials: CC
authors.lastname: Sun
mesh_terms: D002152:Calorimetry, Differential Scanning / Q000379:methods; D003460:Crystallization / Q000379:methods*; D005898:Glass / Q000737:chemistry*; D007700:Kinetics; D044367:Phase Transition; D011108:Polymers / Q000737:chemistry; D011208:Powders / Q000737:chemistry; D044366:Transition Temperature; D014961:X-Ray Diffraction / Q000379:methods
chemical_list: D011108:Polymers; D011208:Powders