How to Determine the Optimum Experimental Parameters for DMA Measurements

Introduction

For successful DMA measurements, it is extremely important to choose the “right” experimental parameters, probably more so than in most other thermoanalytical techniques. In practice, the difficulty is that the deformation mode, sample geometry and mechanical measurement parameters (force and displacement amplitude) are interdependent. This article discusses important factors that need to be considered when planning a DMA experiment. 

Choosing the Deformation Mode

Samples can be mechanically stressed in a number of different ways in DMA. The mode you finally decide to use depends on the information required and the sample itself. For example, if you want to determine the shear modulus, you have to use the shear mode; and if you want to measure thin films, you cannot do this in the 3-point bending mode. Table 1 presents an overview of the different deformation modes and characterizes them with respect to their mechanical advantages and disadvantages as well as their most important applications.

Besides the mechanical properties of the sample holders, it is also important to consider their thermal properties and the frequency ranges accessible with the deformation modes.

Choosing the Heating Rate

DMA samples are usually large compared with those measured in other thermoanalytical techniques (e.g. DSC or TGA). As a result of this, the furnace is relatively large and has a corresponding large thermal mass and inertia. The sample clamping assemblies are also large and react slowly to temperature changes. DMA experiments are therefore generally performed at heating rates of less than 3 K/min. With isothermal experiments, the temperature should be allowed to stabilize for at least 30 minutes before starting the measurement.

Choosing the Sample Geometry

Three main points should be considered when choosing sample geometry:

• In the bending and the shear modes, there are additional contributions from the other deformation mode. These contributions can be neglected if the following guidelines concerning sample geometry are observed (ISO 6721):
  

Conclusions

A DMA experiment begins with the choice of the right deformation mode for the measurements. This depends on the information you require, i.e. Young’s modulus (bending, tension or compression) or shear modulus (shear), and on the type of sample. The next step is a trial experiment to determine the range in which deformation is linear and hence determine the maximum displacement amplitude. You can then use the “Modulus Calculator” to optimize the sample geometry and the force and displacement amplitudes so that the sample can be successfully measured in the expected modulus range with the DMA/SDTA861e . Based on the measurement parameters, the offset parameters can then be set for 3-point bending, tension and compression. Table 3 summarizes approximate values for the sample geometry and force and displacement amplitudes for typical samples.

_{How to Determine the Optimum Experimental Parameters for DMA Measurements | Thermal Analysis Application No. UC 231 | Application published in METTLER TOLEDO Thermal Analysis UserCom 23}