Dynamic Heat Capacity and Relaxation Time of Ultraviscous Melt and Glassy Acetaminophen

The real and imaginary components, C′p and C″p, of the complex heat capacity, Cp′=C′p−iC″p of supercooled, ultraviscous melt of acetaminophen have been measured at different temperatures during cooling through its vitrification range and during heating through its glass-softening range by using a mo...

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Bibliographic Details
Published in:Journal of pharmaceutical sciences 2006-05, Vol.95 (5), p.1006-1021
Main Authors: Tombari, E., Presto, S., Johari, G.P., Shanker, Ravi M.
Format: Article
Language:English
Subjects:
Acetaminophen - chemistry
Algorithms
Biological and medical sciences
Calorimetry
Chemical Phenomena
Chemistry, Physical
Entropy
General pharmacology
Medical sciences
Molecular Conformation
Pharmaceutical technology. Pharmaceutical industry
Pharmacology. Drug treatments
Temperature
Thermodynamics
Viscosity
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Summary:The real and imaginary components, C′p and C″p, of the complex heat capacity, Cp′=C′p−iC″p of supercooled, ultraviscous melt of acetaminophen have been measured at different temperatures during cooling through its vitrification range and during heating through its glass-softening range by using a modulation frequency of 3.3mHz. From these data, the distribution of relaxation time parameter, β, and a characteristic (calorimetric or configurational) relaxation time, τcal, have been determined. A constant value of 0.65 for β fits the data, and τcal varies with the temperature according to the Vogel–Fulcher–Tammann equation, τcal=10−12.95 exp[1813/(T−240.5)]. This relation differs significantly from the one deduced by others in which the configurational entropy theory was used to deduce τcal. The C′p and C″p values measured during the cooling of its ultraviscous melt and during the heating of its glassy state show a small hysteresis only at low temperatures. These investigations also provide a comparison of calorimetric and dielectric relaxation times in ultraviscous acetaminophen and highlight the role of faster modes of relaxation at low temperatures in organic, molecular glasses that can help in a better understanding of the crystal nucleation process in glasses at T below their Tg.
ISSN:0022-3549
1520-6017
DOI:10.1002/jps.20601