Dynamic mechanical spectroscopy
From Wikipedia, the free encyclopedia
 |
This article may be too technical for a general audience. Please help improve this article by providing more context and better explanations of technical details to make it more accessible, without removing technical details. See below for more information. |
In the technique of Dynamic Mechanical Spectroscopy a material (usually a slab of polymer) is exposed to a periodical deformation. The deformation can be in tensile, compression or bending mode but torsional deformations are the most practical ones because they tend to produce a linear response more readily. In other words the deformation (strain) can be described a linear function of the applied force (stress). The coefficient that links the two is called the modulus:
- Strain = modulus * stress
In DMS the modulus is measured as a function of the frequency of the deformation and/or the temperature of the experiment. Because the temperature is typically varied in a systematic way the technique is also known as dynamic mechanical analysis or dynamic mechanical thermal analysis. (DMTA) The modulus is generally a complex number, because when the applied stress is sinusoidal (i.e. a single frequency is applied) the strain can lag behind in time. The phase shift is due to viscous as opposed to elastic effects. When the material undergoes a glass transition these losses reach a maximum. The temperature at which this happens, however, is frequency dependent. The mechanical excitation does not have to be a single sine wave, in fact more than one frequency response can be measured simultaneously in a process called multiplexing. Often a square wave is used rather than a sine wave in an application of the Fourier transform principle. A requirement for its application is that the response is linear for all frequencies.
