title: Predictions of onset of crystallization from experimental relaxation times I-correlation of molecular mobility from temperatures above the glass transition to temperatures below the glass transition.
abstract: OBJECTIVE
  Predicting onsets of crystallization at temperatures below T (g), from data above T (g), would require that the correlation between crystallization onset and mobility is same above and below T (g), and the techniques being used to measure mobility above and below T (g) are measuring essentially the same kind of mobility. The aim of this work is to determine if the relaxation times obtained using different techniques (DSC, TAM) below T (g) correlate with relaxation time obtained above T (g) using dielectric spectroscopy.
  
  
  METHODS
  Model compounds for this work were chosen based on their varied DeltaH (f), DeltaC (p)(T (g)) and H-bonding in crystalline state vs. amorphous state. Relaxation times above T (g) were determined by the simultaneous fit of real and imaginary permittivity to the Cole-Davidson model. Tau and beta below T (g) were determined using isothermal microcalorimetry (TAM) or MDSC. MDSC was used to calculate Kauzmann temperature and strength of the glass using established relationships.
  
  
  RESULTS
  Indomethacin, nifedipine and flopropione showed Arrhenius temperature dependence throughout the entire temperature range and extrapolation of tau ( beta ) measured above T (g) by dielectric relaxation agreed with tau ( beta ) measured below T (g) by TAM/MDSC. Ketoconazole, however, showed the expected VTF behavior. For at least two compounds compared (indomethacin and ketoconazole), relaxation times measured by TAM and MDSC did not agree, with TAM giving significantly lower values of tau ( beta ), but TAM and MDSC relaxation times appeared to extrapolate to a common value at T (g).
  
  
  CONCLUSIONS
  It was found that, for all cases studied, relaxation time constants determined above and below T (g) did appear to extrapolate to the same value around T (g) indicating that molecular mobility measured above and below T (g) using different techniques is highly correlated.
authors.affiliation: Department of Pharmaceutical Sciences, School of Pharmacy, U-3092, University of Connecticut, 69 North Eagleville Road, Storrs, Connecticut 06269, USA.
authors.forename: Chandan
authors.initials: C
authors.lastname: Bhugra
authors.forename: Rama
authors.initials: R
authors.lastname: Shmeis
authors.forename: Steven L
authors.initials: SL
authors.lastname: Krill
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Pikal
mesh_terms: D002151:Calorimetry; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003460:Crystallization; D004563:Electrochemistry; D005544:Forecasting; D006860:Hydrogen Bonding; D004364:Pharmaceutical Preparations / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared; D013059:Spectrum Analysis, Raman; D013696:Temperature; D013816:Thermodynamics; D014961:X-Ray Diffraction
chemical_list: D004364:Pharmaceutical Preparations