Typical DMA thermogram of an amorphous thermoplastic (polycarbonate). Temperature-sweeping DMA is often used to characterize the glass transition temperature of a material.įigure 2. At the glass transition, the storage modulus decreases dramatically and the loss modulus reaches a maximum. Amorphous polymers have different glass transition temperatures, above which the material will have rubbery properties instead of glassy behavior and the stiffness of the material will drop dramatically along with a reduction in its viscosity. One important application of DMA is measurement of the glass transition temperature of polymers. Applications Measuring glass transition temperature When the strain is applied and the stress lags behind, the following equations hold: Viscoelastic polymers have the characteristics in between where some phase lag will occur during DMA tests.

For a purely viscous fluid, there will be a 90 degree phase lag of strain with respect to stress. For a perfectly elastic solid, the resulting strain and the stress will be perfectly in phase. The viscoelastic property of a polymer is studied by dynamic mechanical analysis where a sinusoidal force (stress σ) is applied to a material and the resulting displacement (strain) is measured. This solidlike and liquidlike behavior of polymers can be modeled mechanically with combinations of springs and dashpots.

The classical theory of hydrodynamics describes the properties of viscous fluid, for which the response of stress is dependent on strain rate.

Such response of stress is independent of strain rate. The classical theory of elasticity describes the mechanical properties of elastic solid where stress is proportional to strain in small deformations. Polymers composed of long molecular chains have unique viscoelastic properties, which combine the characteristics of elastic solids and Newtonian fluids. A sample is mounted on the grips and the environmental chamber can slide over to enclose the sample. A typical DMA tester with grips to hold sample and environmental chamber to provide different temperature conditions.

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