Thermal Analysis of Polymers. Part 2: TGA, TMA and DMA of Thermoplastics

In Part 2, the focus is on the use of TGA, TMA and DMA techniques.

Effects such as decomposition, expansion, cold crystallization, glass transition, melting, β relaxation and recrystallization are discussed in detail.

TGA, TMA and DMA yield valuable complementary information to DSC measurements.

Thermogravimetric Analysis (TGA)

Thermogravimetric analysis is a technique that measures the mass of a sample while it is heated, cooled or held isothermally in a defined atmosphere. It is mainly used for the quantitative analysis of products.

A typical TGA curve shows the mass loss steps relating to the loss of volatile components (moisture, solvents, monomers), polymer decomposition, combustion of carbon black, and final residues (ash, filler, glass fibers). The method allows us to study the decomposition of products and materials and to draw conclusions about their individual constituents.

The first derivative of the TGA curve with respect to time is known as the DTG curve; it is proportional to the rate of decomposition of the sample. In a TGA/DSC measurement, DSC signals and weight information are recorded simultaneously. This allows endothermic or exothermic effects to be detected and evaluated. 

The DSC signal recorded in TGA/DSC measurement is, however, less sensitive than that obtained from a dedicated DSC instrument and the DSC curves are less well resolved.

The upper diagram of Figure 1 shows TGA and DTG curves of PET. The two lower diagrams are the corresponding DSC curves measured in a nitrogen atmosphere. The DSC curve on the right in the range up to 300 °C shows the glass transition, cold crystallization, and the melting process. The DSC signal can be corrected for the mass lost by the sample during the measurement (left); the blue curve is the uncorrected curve and the red curve is corrected for the loss of mass [2, 3].

Decomposition

In a decomposition process, chemical bonds break and complex organic compounds or polymers decompose to form gaseous products such as water, carbon dioxide or hydrocarbons.

Under non-oxidizing (inert) conditions, organic molecules may also degrade with the formation of carbon black. Volatile decomposition products can be identified by connecting the TGA to a Fourier transform infrared spectrometer (FTIR) or a mass spectrometer (MS).

Thermomechanical Analysis (TMA)

Thermomechanical analysis measures the dimensional changes of a sample as it is heated or cooled in a defined atmosphere. A typical TMA curve shows expansion below the glass transition temperature, the glass transition (seen as a change in the slope of the curve), expansion above the glass transition temperature and plastic deformation. Measurements can be performed in the dilatometry mode, the penetration mode, or the DLTMA (Dynamic Load TMA) mode.

Dilatometry

The aim of dilatometry is to measure the expansion or shrinkage of a sample. For this reason, the force used is very low and is just sufficient to ensure that the probe remains in contact with the sample. The result of the measurement is the coefficient of thermal expansion (CTE).

The dilatometry measurement shown in Figure 2 was performed using a sample about 0.5 mm thick sandwiched between two silica disks. It was first preheated in the instrument to 90 °C to eliminate its thermal history. After cooling, it was measured in the range 30 to 310 °C at a heating rate of 20 K/min using the ball-point probe and a very low force of 0.005 N.

The curve in the upper diagram of Figure 2 shows that the sample expands only slowly up to the glass transition. The expansion rate then increases significantly on further heating due to the increased mobility of the molecules in the liquid state. Afterward, cold crystallization and recrystallization processes occur and the sample shrinks. The sample expands again after crystallite formation above about 150 °C and finally melts. The melting is accompanied by a drastic decrease in viscosity and sample height.

Penetration

Penetration measurements mainly yield information about temperatures. The thickness of the sample is not usually important because the contact area of the probe with the sample changes during the experiment. The depth of penetration is influenced by the force used for the measurement and the sample geometry. 

Conclusions

Parts 1 and 2 of this article series illustrate the different possibilities that are available for characterizing a thermoplastic by thermal analysis. The techniques used were DSC, TGA, TMA, and DMA. The thermoplastic chosen for the measurements was PET. The results agree well with one another. The main effects investigated were the glass transition, cold crystallization, recrystallization, melting and decomposition. Topics such as OIT and the thermal history of samples were also covered. Similar effects to those described for PET occur with other polymers.

A particular effect can often be measured by different thermal analysis techniques. The results obtained from one technique are used to confirm those from another technique. For comprehensive materials characterization, samples are usually first investigated by TGA, then by DSC and TMA, and finally by DMA. 

_{Thermal Analysis of Polymers. Part 2: TGA, TMA and DMA of Thermoplastics | Thermal Analysis Application No. UC 321 | Application published in METTLER TOLEDO Thermal Analysis UserCom 32}