Thermal Analysis UserCom 8

Introduction

A substance is said to be polymorphic if it can exist in different crystalline forms, i.e. if the same chemical compound occurs in various modifications with different physical properties. The importance of polymorphism lies in the fact that the physical properties (melting point, color, solubility, refractive index, hardness, conductivity, etc) of a given compound vary from one polymorphic form to the other. The crystalline modifications, however, melt to the same liquid phase. Polymorphism (in the case of elements also known as allotropy) is exhibited by elements such as sulfur, carbon (graphite, diamond), phosphorus as well as by numerous minerals and organic substances. Plastics can also exist in polymorphic forms, e.g. isotactic polypropylene. The polymorphic forms of pharmaceutically active substances are of great practical importance [1]. Since the solubilities and dissolution rates of individual poly- morphic forms are very different, resorption and bioavailability in the body also differ [2]. The therapeutic efficiency therefore depends on the modification in question; a metastable form for example can be twice as active as the stable form. Polymorphism is not only important for pharmacological efficiency; it also plays an important role in production, processing and formulation.

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References

[1] J. L. Ford and P. Timmins, Pharma- ceutical Thermal Analysis, Ellis Horwood, 1989
[2] D. Giron, J. Pharmaceutical & Bio- medical Analysis, Vol. 4, n6, 755-770, 1986

Introduction

Pellets and tablets are often coated with aqueous dispersions of insoluble polymers such as polymethacrylates to provide a means for the controlled release of active ingredients. The mechanism of drug release can be described as diffusion of the drug through the swollen membrane or through the water filled pores [1]. The swelling behavior can be characterized by the determination of the increasing water content of the polymer films or by visual observation of the swelling process. In both cases, relative thick films are necessary in order to obtain measurable changes. In this study, a method using TMA (thermomechanical analysis) has been developed that enables the swelling behavior of thin polymer films to be measured directly. TMA allows the determination of extremely small changes in length of samples subjected to exactly defined forces and temperatures. In the past, TMA methods have been described for the swelling behavior of papyrus in water [2] and for elastomers in organic solvents [3].

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Literature

[1] Knop K. - Influence of buffer solution composition on drug release from pellets coated with neutral and quaternary acrylic polymers and on swelling of free polymer films. Eur. J. Pharm. Sci. 4, 293-300 (1996)
[2] Wiedemann H.G. and Bayer G. - Papyrus, the paper of ancient Egypt Anal. Chem. 55, 1220A-1230A (1983)
[3] Staub F. and Riesen R. - Quellungs messungen an Elastomeren. Mettler Applikation Nr. 3110

Introduction

During the isothermal cure of an epoxy resin the system changes from a viscous liquid to a highly crosslinked network. The rate of cure of the reaction in the liquid state is controlled by chemical kinetics. With increasing crosslinking, the glass transition temperature (Tg) of the system increases. When Tg equals the curing temperature T_c , the system changes to a glassy state and vitrifies. The mobility of the reactive centers becomes increasingly restricted and the reaction is controlled by diffusion. This results in the conversion remaining practically constant [1].

In the past, the curing time to vitrification (the so-called “vitifrication time”) has been determined by conventional DSC in a rather time-consuming manner. To do this, a number of samples are cured separately for different periods of time. The curing temperature must, of course, lie below the maximum attainable glass transition temperature of the formulation concerned (typically 10 ºC to 50 ºC degrees below). Afterwards, the glass transition temperatures of the partially cured samples are measured with DSC. At the same time it is possible to determine the degree of cure by measuring the post-curing peak. A plot is made of the glass transition temperatures against the corresponding curing time. The vitrification time is determined as the point of intersection of the best-fit curve of these data points with the curing temperature used.

The purpose of this article is to show that the vitrification time can be determined directly with the ADSC technique. The advantage of ADSC is that it is more accurate and faster than the conventional method described above.

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References

[1] S. Montserrat, J. Appl. Polym. Sci., 44 (1992) 545-554

TGA measurements are normally per- formed at atmospheric pressure. It can of course happen that a step in a TGA curve is the result of two overlapping processes. For instance, solvent or plasticizers can evaporate at the same time as the polymer has already begun to decompose. In some cases the overlapping processes can be separated by changing the heating rate, either manually or automatically (MaxRes). If this is not possible, there is another possibility that will be described in more detail below: measurements at reduced pressure. While the atmospheric pressure generally has little or no efffect on the course of a decomposition reaction, this is quite different for processes involving evaporation. In such cases, a reduction of pressure shifts the vaporization process to lower temperatures. The separation of the two overlapping effects (one of which is a vaporization process and the other a decomposition reaction) should therefore be possible by working at reduced pressure with the thermobalance. But how can we reduce the pressure in a thermobalance and yet still measure properly?

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Differential scanning calorimetry (DSC) is together with thermogravimetry (TG) the most important thermoanalytical method for the analysis of damage to plastic components or for the investigation of manufacturing problems. With the aid of this technique, possible causes of damage can be identified or eliminated. It helps the interpretation of results from other methods such as infrared spectrometry, viscosity determinations and mechanical testing. In many cases damage could have been avoided if DSC measurements had been routinely performed as part of the quality assurance of incoming goods. In some cases the results of DSC measurements have led to the integration of DSC in the goods-in control. In damage analysis, DSC is used primarily to investigate the following problems:

• is the plastic part made of the prescribed polymer?
• has the type of pellet been delivered that was ordered?
• is the material contaminated with another type of plastic?
• are any internal stresses “frozen” in the parts?
• has the plastic been thermally damaged during processing in use?
• is the material sufficiently stabilized?
• has the material been completely cured?

A number of relevant examples will now be discussed in order to illustrate the important role that DSC plays in this type of work.

The DSC20 measuring cell with standard sensor and TC10A controller were used for all measurements. Standard aluminum crucibles with pierced lids and nitrogen or air purging were used for sampling. The heating rate was normally 10 K/min. The sample weight ranged from 5 mg to 25 mg according to the problem concerned, whereby particular attention was paid to good contact with the bottom of the crucible [1]. The curves measured during this work were imported into a PC with STAR^e software.

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Literature

[1] J. Vogel „DDK - Einführung in die Messtechnik, Fehlervermeidung“, Thermische Analyse an Kunststoffen - Methoden und Anwendungen, LabTalk-Seminar der Fa. Mettler-Toledo 25.11.97