Thermal Analysis of Polymeric Materials

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At higher temperatures, monochromatic radiation pyrometers maintain the ITS 90. They can be calibrated at the freezing points of Ag, Au, or Cu. The evaluation is based on Planck’s radiation law. At lower temperatures, vapor-pressure-temperature relationships for 3He and 4He are used.

With the ITS 90, one achieves the ultimate in temperature precision. Thermal analysis is usually far from such precision. Root-mean-square deviations of ±0.1 K are the typical goals.

The krypton length standard. From the late 18th century until the middle of the 20th century, the reference meter was a particular bar of alloyed platinum and iridium, stored and protected in a site in Sèvres, outside Paris. Duplicates were kept by the industrialized nations and compared periodically. The meter bar had a cross section shaped like , to resist deformation, and it bore two marks at right angles to its length to indicate the meter. The distance was chosen to represent 1/10,000 of the pole-to-equator distance as closely as possible, to reproduce the 18th century definition of the meter. In 1960, the 11th General Conference of Weights and Measures chose the reddish-orange krypton-86 emission as the defined meter instead: “The meter is the length equal to 1,650,763.73 wavelengths in a vacuum, corresponding to the transition between the levels 2p10 and 5d5 of the krypton-86 atom.”

The precision of the Pt/Ir meter was about 1 part in 106. The krypton-86 meter can be reproduced in 1 part in 108. A remaining drawback of the krypton-86 meter is that the emitted light is not sufficiently coherent to produce measurable interference fringes past 80 cm. Ultimately, lasers with greater coherence should replace the krypton-86 meter.

820 Appendix 9

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Development of Classical DTA to DSC

The principle of heat-flux calorimetry is illustrated with a schematic of a classical DTA in Fig. A.9.1. The reference (Rfc) and sample (Spl) materials have immersed thermocouples for precise temperature measurement. An identical environment with a temperature that increases linearly in time is created by the DTA-furnace and a programmer. The furnace-control thermocouple checks the furnace (block) temperature, Tb, against the program temperature. Any difference is used to adjust the power to the heater. Additional cooling may be introduced by placing the DTA furnace in a cold bath and regulating the heating. The symmetrically placed Rfc and Spl should have the same heat flux for the same temperature difference from the furnace Tb Tr and Tb Ts, making the temperature difference, T = Tr Ts, proportional to the differential heat-flow rate, HF. The reference temperature for both thermocouples is provided by an ice bath (see Fig. 4.8). For higher precision one uses an automatic reference cell with Peltier cooling and heating that can establish the triple point of water to better than 0.01 K. Commercial instruments provide internal, electronic reference junctions.

Fig. A.9.1

The thermocouple voltage due to T is only 1% or less than the voltage due to the absolute temperatures and must, thus, be preamplified before recording. Both T (proportional to heat flow HF), and Ts are then recorded either directly, as illustrated in Fig. A.9.1, or both are given as a function of time, t. Whenever the DTA trace can be used for the quantitative evaluation of caloric variables, such as heat capacities and heats of transition, the experiment is called DSC, differential scanning calorimetry. Many traditional DTAs are capable of measuring heat, and thus can be called DSC.

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In contrast, one finds many DSCs which are used only for qualitative DTA work on transition temperatures. The often-posed question of the difference between DTA and DSC is therefore easily answered: DTA is the general term covering all differential thermal analysis techniques, while DSC must be reserved for scanning experiments that yield calorimetric information.

Eight schematics of classical DTA instruments of the last 50 years are illustrated in Figs. A.9.2 and 3 (A H). The sketches are more or less self-explanatory. Sample sizes range from as much as one gram (E) down to a few milligrams (G). The latter is a micro-DTA with the thermocouples serving as sample and reference cups. Equipment (D) and (F) is distinguished from all others by having direct contact between the metal block and the sample and reference holders; i.e., heat is directly

Fig. A.9.2

transferred from the metal of the furnace to the sample holder in form of a glass capillary. This design is often thought to be less quantitative, but this does not have to be so, as long as a reproducible thermocouple placement is achieved, or the temperature gradient within the sample is small. In fact, the sample temperature can be determined most precisely in such instruments. An intermediate design is (A), in which much of the conduction of heat goes through a metal bridge as a controlled thermal leak. All other designs rely largely on the surrounding atmosphere for the transfer of heat. The conduction of heat through the atmosphere is, however, difficult to control because of the ever-present convection currents. The compromise (A) of a well-proportioned heat leak has found the widest application in modern DSC design. Figure 4.54 displays an updated version of this DSC and Fig. 4.55 the present, modern heat-flux DSC based on the same heat-flux principle. A number of conditions for good quality thermal analyses have been developed over the years and apply as well for modern scanning calorimetry. A summary is given below.

822 Appendix 9–Development of Classical DTA to DSC

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Fig. A.9.3

Typical additional conditions for the design of early DTA instrumentation and its environment are:

1.Smooth and linear furnace temperature change.

2.Draft-free environment, closely regulated room temperature.

3.High-thermal-conductivity furnace (silver, gold, or high-purity Al).

4.Control of conduction, radiation, and eliminate convection.

5.Proper heating and cooling design (direction sample heater cooling).

For detailed references to the equipment and design criteria see: Wunderlich B (1971) Differential Thermal Analysis. In: Weissberger A, Rossiter BW (1971) Physical Methods of Chemistry, Vol I, Part V. Wiley-Interscience, New York.

Based on recommendations of the Committee on Standardization of ICTAC, Fig. 2.5, one should always give the following information which are excerpts with updates published by: Mackenzie RC (1969) Talanta 16: 1227; (1972) ibid. 19: 1079.

1.Identification of all substances (sample, reference, diluent) by their IUPAC name and formula, empirical name and formula, or equivalent compositional data (see Sect. 1.2).

2.A statement of the source of all substances, details of their histories, pretreatments and chemical purities, for polymers also molar masses and distributions.

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3.Measurement of the average rate of linear temperature change over the temperature range involving the phenomena of interest.

4.Identification of the sample atmosphere by pressure, composition, and purity.

5.A statement of the dimensions, geometry, and materials of the sample holder, and furthermore, the method of loading the sample holder and the sample, where applicable.

6.Identification of the abscissa scale in terms of time or of temperature at a specified location.

7.A statement of the methods used to identify intermediates or final products.

8.Faithful reproduction of all original records.

9.Wherever possible, each thermal effect should be identified and supplementary supporting evidence stated.

On DTA traces, the following specific details should be presented:

10.Sample weight and dilution of the sample.

11.Identification of the apparatus, including the geometry and materials of the thermocouples and the locations of the differential and temperaturemeasuring thermocouples, as well as their calibration.

12.The ordinate scale should indicate deflection in kelvins or heat-flow rate with the exothermic or endothermic direction clearly marked.

824 Appendix 10

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Examples of DTA and DSC under Extreme Conditions

For this summary, forms of thermal analyses under extreme conditions are described for the measurement of heat and temperature, as dealt within Sects. 4.1–4. The distinction between DTA and DSC seen in these methods is described in Appendix 9. In Appendix 10, DTA or DSC at very low and high temperatures and DTA at very high pressures are mentioned. This is followed by a discussion of high-speed thermal analysis which, in some cases, may simply be thermometry. Finally, microcalorimetry is treated. One might expect that these techniques will develop in this century [1]. The numbers in brackets link to references at the end of this appendix.

Low-temperature DTA and DSC needs special instrumentation [2]. In Fig. A.10.1 a list of coolants is given which may be used to start a measurement at a low temperature. From about 100 K, standard equipment can be used with liquid nitrogen as coolant. The next step down in temperature requires liquid helium as coolant. A differential, isoperibol, scanning calorimeter has been described for measurements on samples of 10 mg in the 3 to 300 K temperature range [3]. To reach even lower temperatures, especially below 1 K, one needs again another technique [4], but it is

Fig. A.10.1

possible to make thermal measurements even at these temperatures. Usually heat capacities and thermal conductivities are obtained in this temperature range by timedependent heat leak measurements.

High temperature and pressure DSC and DTA needs special materials for the construction of the calorimeter, as is shown in Figs. A.10.2 and A.10.3. At high temperature, the control of heat loss and gain by radiation becomes important, at high pressure the strength and high mass of the enclosure must be considered.

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Fig. A.10.2

Fig. 10.3

The diagram in Fig. A.10.2 displays an instrument based on the classical design of a heat flux DTA (see Appendix 9). With a related design, differential calorimetry and thermogravimetry can be carried out simultaneously. Figure A.10.3 illustrates a typical high-pressure DTA setup which is usable up to 500 MPa of pressure, 5,000 times atmospheric pressure. The pressure is transmitted by a gas, such as nitrogen,

826 Appendix 10–Extreme DTA and DSC

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or a liquid, such as silicon oil. Reference and sample are placed around their respective thermocouples inside the high-pressure container. The thermocouple output is recorded for the measurement of temperature and temperature difference. Special safety precautions must be observed when using high-pressure DTA. Particularly, special enclosures must be in place to contain the DTA in case of failure if the pressure-transmitting agent is gaseous or can easily evaporate.

High-speed thermal analysis is possible, as shown for example in Fig. 3.95 (see also Sect. 6.2). Many industrial processes are very fast. Spinning of fibers, for example, may be done at rates of 100 to 10,000 m min 1. A temperature change of 60 K over a length of 1 m in the thread-line, then, causes rates of temperature change of 6,000–600,000 K min 1 (100–10,000 K s 1). Such heating rates must be compared to times for large-amplitude molecular motion, such as rearrangements of polymer chains by conformational adjustments, described in Sects. 1.3.5–8 and 5.3.4. The conformational motion may have a picosecond timescale (10 12 s), i.e., in the time the fiber goes through the above temperature gradient, each backbone bond may rearrange billions of times, sufficient to cause intricate changes to various useful structures, particularly when, in addition, strains are imparted on the fiber.

Most DTA and DSC equipment can be adjusted to measure at rates from about 0.1 K min 1 to perhaps 100 K min 1. With some modification, changes of sample size and altering of heater size, etc., this can be brought to a range of 0.01 K min 1 to 1,000 K min 1. One can cover in this way five orders of magnitude in heating rates. Even faster DTA needs special considerations. Permitting a temperature gradient of ±0.5 K in a disc-like sample, the equation in Fig. A.10.1 can be used to calculate the maximum sample dimensions for given cooling or heating rates (see also Figs. 4.65 and 4.66). Obviously the limit of fast-heating DTA has not been reached. Just dipping samples in cooling baths or heating baths can produce rates up to 10,000 K min 1 with reasonable control [5].

A unique solution to fast DTA is the foil calorimeter, shown schematically in Fig. A.10.4. A copper-foil is folded in such a way that two sheets of the sample (also very thin, so that the mass remains small) can be placed between them. The copper foil is used as the carrier of electrical current for fast heating. Between the inner portion of the stack of copper foil and sample, a thin copper–constantan thermocouple is placed. Only three folds of the stack are shown. In reality, many more folds make up the stack so that there are no heat losses from the interior and measurements can be made under adiabatic conditions. Heating rates of up to 30,000 K min 1 (500 K s 1) have been accomplished. Measured is temperature, time, and the-rate-of- change of temperature for a given heat input. With such fast heating rates it becomes possible to study unstable compounds by measuring faster than the decomposition kinetics of the compound. This super-fast calorimeter has seen little application, likely because it requires a new calorimeter for each sample.

A more recent step to speed up DSC was taken in connection with a commercial power-compensation DSC (High Performance DSC) [6] and is now available as HyperDSC® from the Perkin-Elmer Inc. It is claimed to reach 500 K min 1. For a pan of a diameter of 5 mm, the heating rates calculated in Fig. A.10.1 corresponds to sample masses of 20, 2, and 0.2 mg, showing that it is the heating and cooling capacity of the DSC that limits fast calorimetry, not the properties of the sample.