D5 Dynamic-Mechanical Thermal Analysis (DMA)

Topic

This model measures, through dynamic-mechanical analysis, three related magnitudes of the compound: the storage modulus, the loss modulus and the loss factor. This analysis applies an oscillating load to the composite, and tracks how the material deforms in response to that load: the resulting deformation is not exactly in phase with the applied load, but rather lags behind it by a certain angle. That this deformation lags behind the applied load means that the response of the material can be separated into two different components.

The component of this response that remains in phase with the applied load is what defines the storage modulus, and represents the elastic energy that the compound stores and returns in each cycle; The component that is ninety degrees out of phase with respect to that same load is the one that defines the loss modulus, and represents the energy that the compound dissipates as heat in each cycle. The fact that these two components of the response are combined means that, together, they form the complex module, which completely describes the dynamic response of the compound against that oscillating load.

The fact that this complex module is formed by the storage module and the loss module is what allows us to calculate, from it, the loss factor: the relationship between how much energy the compound dissipates in each cycle and how much energy it stores and returns elastically. That the loss modulus is large in relation to the storage modulus causes the loss factor to be greater, and the compound behaves in a more viscous manner; The fact that the loss modulus is small in relation to the storage modulus means that the loss factor is lower, and the compound behaves more elastically.

The fact that this loss factor is measured at different temperatures is what allows, in turn, to determine the glass transition temperature of the polymer matrix within the composite: as the temperature of the composite increases, the loss factor reaches a peak at the temperature at which the polymer matrix stops behaving like a rigid solid and begins to behave like a soft material, because it is precisely in that transition where the matrix dissipates the most energy for each loading cycle. That this peak of the loss factor occurs at a certain temperature means that that same temperature is taken as the glass transition temperature of the matrix.

That is why this model, with the dynamic-mechanical analysis with which it was opened, makes the glass transition temperature that it determines to be, specifically, the effective glass transition temperature of the polymer matrix already described in two other models of this set, but measured within the already assembled compound and not in the resin alone: ​​that same glass transition temperature is, precisely, the one that another different model in this set shows is reduced when the matrix absorbs moisture and is plasticized by it.

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