Thermal Analysis UserCom 16
Thermal Analysis UserCom 16; Table of Contents:
- Interpreting DMA curves, Part 2
New in our sales program
- HP DSC827e
- DSC and TGA crucible sets
- Characterization of drugs by DSC
- Determination of the glass temperature by DMA
- Investigation of the cold crystallization and melting of amorphous linear polyesters by ADSC
- Drying of the glass transition using IsoStep™
Characterization of drugs by DSC
It is well known that interactions between the active substance and excipients can influence the pharmacological properties and behavior of drugs in biological systems.
In this study, mixtures of excipients and piroxicam as the active substance were ground together and analyzed by DSC. The active substance, piroxicam (4-hydroxy-2-methyl-N-pyridinyl-2H-1,2-benzothiazine-3-carboxamide 1,1-dioxide) is a non-steroidal drug that is often used for the treatment of osteoarthritis and rheumatoid arthritis. It shows a therapeutic effect in very small doses and has few side effects. The drug’s mechanism has to do with the disruption of prostaglandin formation and inhibition of the enzyme cyclooxygenase.
Besides the active substance itself, piroxicam tablets contain the following excipients: anhydrous lactose, lactose monohydrate, microcrystalline cellulose type 102 and magnesium stearate.
Determination of the glass temperature by dynamic mechanical analysis
The glass transition temperature is a important criterion for the technological use of polymeric materials. The application range of elastomers is usually restricted to temperatures significantly above the glass transition temperature to ensure that deformation behavior is as far as possible entropy elastic.
The glass transition temperature of a polymer is, however, strongly dependent on frequency, and the deformation of elastomers is usually time dependent (e.g. seals, surface of tires, etc.). This means that a glass transition temperature measured under quasi static conditions (for example by DSC) is not a good criterion for the characterization of the low temperature behavior of dynamically stressed materials.
The temperature limits for dynamically stressed elastomeric materials can be determined by measuring the glass transition temperature using methods in which a periodic stress is applied. The measurement frequency should of course correspond to the frequency at which the component is in practice stressed.
If such a measurement is not possible, for example at frequencies that are significantly higher or lower than the measurement range of the instrument, then the glass transition temperature can be obtained by extrapolation using the time-temperature superposition principle. A quantitative relationship between the glass transition temperature and measuring frequency can be obtained through the semi-empirical WLF or analogous Vogel-Fulcher equation.
In this article, the experimental relationship between frequency and time are determined for a number of primary elastomers (NR, BR, SBR, NBR, IIR) from temperature-dependent and frequency-dependent shear modulus measurements. The quantitative description of the time-temperature superposition principle is performed using the semi-empirical WLF or analogous Vogel-Fulcher equation.
Investigation of the cold crystallization and melting of amorphous linear polyesters by ADSC
Ever since its introduction in the early sixties, differential scanning calorimetry (DSC) has been used as one of the main techniques to study the crystallization and melting of polymers .
The cold crystallization and melting of polyethylene terephthalate (PET) was often analyzed with non-isothermal DSC measurements. The degree of crystallinity was then determined from the enthalpies of crystallization and melting by peak area integration [2, 3].
The aim of the present work was to study the crystallization and melting of polyesters using temperature-modulated differential scanning calorimetry (ADSC) and to point out the additional information that can be obtained with this method compared with conventional DSC.
 B. Wunderlich, Thermal Analysis, Academic Press, New York, 1990.
 K. H. Illers, Colloid Polym Sci 258 (1980) 8.
 M. Schubnell, UserCom 13 (2001) 12.
Drying and glass transition using IsoStep™
If several thermal events occur simultaneously in a DSC measurement, the problem is then how to separate the different processes involved. Often, a change in heat capacity is overlapped by exothermic or endothermic peaks, e.g. through chemical reactions, crystallization or vaporization. One possible way to separate the different processes is to vary the measurement conditions in the conventional DSC. For example, heating and cooling measurements can be performed at different rates and in different temperature ranges using different types of crucible. This is of course relatively time-consuming.
IsoStep™ is a new technique that can be used to distinguish between such overlapping processes. It provides both the heat capacity curve and the non-reversing curve simultaneously.
In this article, the separation of different thermal events is demonstrated by measuring the glass transition of a spray-dried pharmaceutical substance. The measurement of a similar compound to determine the change of heat capacity during the vaporization of water has been described in a previous article [1, 2]. As an extension of this, the relationship between the water content and the glass transition temperature is analyzed quantitatively using IsoStep™. Knowledge of this relationship is important for processing the powder because it can become sticky if the glass transition temperature is below the processing temperature.
 M. Schubnell, J. Schawe, UserCom 14 (2001) 5.
 M. Schubnell, J.E.K. Schawe, Int. J. Pharm., 192 (2001) 173.