Dynamic Mechanical Analysis (DMA)
Dynamic Mechanical Analysis (DMA) has various applications, including determination of the following:
- Viscoelastic material properties, such as various moduli and the loss factor tan (δ)
- Temperatures that characterize the viscoelastic properties
- The glass transition temperature, in particular (for which DMA is the most sensitive method)
- The frequency-dependent mechanical properties of materials
In DMA (analysis of the tensile modulus/complex modulus of elasticity),a very small fluctuating mechanical sinusoidal stress is applied to the test specimen (which is under a constant preload, which may be dependent on the storage modulus), with variation of temperature over time. This results in deformation of the specimen in phase. The parameters measured are the force amplitude, the deformation amplitude and the phase displacement Δ φ between the force and deformation signals. The preload is used to keep the specimen sufficiently tensioned at the time of negative dynamic deformation amplitude. The DMA yields the complex modulus of elasticity of the specimen. A prerequisite here is that the specimen is not subject to stress outside the linear elastic region (Hooke’s region) – see Figure 1. The specimens may react in one of three ways:
- Perfectly elastic specimens react immediately to the applied force, the phase angle φ = 0. They oscillate without loss.
- Perfectly viscous specimens reach their deformation maximum at the crossover point of the force. Their phase angle φ = π/2 (90°). They convert the excitation energy entirely into heat.
- Viscoelastic materials, such as Desmopan, exhibit a certain lag in the deformation of the specimen after the application of force. Therefore, for the phase angle Δ φ, 0 < φ < π/2. The greater the phase angle, the more pronounced the damping of the oscillation.
For the determination of
- Softening behavior, heat resistance
- Glass transition temperatures
- Plasticizing effects
- Niveau of modules
- Phase characteristics
- Morphology, crystallization and melting
- Flow process
- Heating at 0,1 K/min up to 10 K/min; adjustable
- Temperature range: -150°C to +300°C
- Frequency: 0,01 to 20 Hz in 1-2-5-sequencies, adjustable
- Thickness of specimen: 0,1 to 2 mm
Figure 2 shows a possible arrangement of the measurement apparatus.
Figure 3 shows storage modulus curves for different polymers. If the storage modulus only falls slightly above the glass transition temperature and remains at this level up to the decomposition temperature, i.e., the material remains comparatively rigid over the entire temperature range in which it is used above the glass transition temperature, the material is an extremely strong one, crosslinked and with short chains.
If the storage modulus falls substantially in the region of the glass transition temperature and then remains at a more or less high level up to final softening, it is an elastomer, crosslinked to varying degrees, or a thermoplastic elastomer (e.g., thermoplastic polyurethane), depending on whether the final softening is a melting without decomposition or a decomposition process. If the storage modulus falls further with varying rapidity after passing through the glass transition temperature, it is a non-crosslinked material with a low or high molecular weight.
This is the standard method for the (mechanical/thermal) determination of glass transition temperature, plasticizing effects, phase behavior or multiphase systems and the corresponding morphological structures, the miscibility of polymers and the melting and crystallization properties of partially crystalline phases. In particular, it allows analysis of the different reasons for softening and attribution to molecular processes (the glass process) of an amorphous phase or melting (crystalline phases).
Figure 4 shows the modulus for a TPU that is available commercially. Measurements were carried out using forced oscillations at a constant measurement frequency of 1 Hz at different temperatures.
The glass transition at the maximum of the loss modulus E” is apparent; the glass transition temperature can be determined with a precision of 0.1 °C since it is here defined through a maximum. The two-stage softening of the TPU material is attributable to the melting of a partially crystalline soft phase, which follows a broad softening of an amorphous hard phase. The glass process and the melting of a material differ substantially in molecular terms. The glass process is a cooperative molecular relaxation process of main chain segments of the polymer, which has an activation energy based on the time-temperature-equivalence principle and therefore proceeds at different excitation frequencies at different temperatures. The temperature at which softening is observed upon melting of a material, however, is only dependent on its specific melting temperature, and not on the frequency of its dynamic excitation.
The modulus of elasticity of a Desmopan is compared to that of a cast PU and a hard thermoplast (ABS) in Figure 5.
The relationship between modulus of elasticity and Shore A hardness, as well as the limits of a linear plot, are shown in Figure 6. It is apparent that the modulus of elasticity (the “stiffness”) is a more accurate means of assessing the behavior of a material (in particular using the Finite Element Method) than the Shore A hardness, which is in widespread use because it is easier to determine.