Degradation wave, dynamics

Figure 6.15. Dynamics of the degradation wave for indicated values of parameter a. Solid lines - position of the wavefront z , dashed lines - wave velocity v .

In order to check these results, Lutz et al. degraded polymer samples which had been isolated shortly before macrogelation, by ultrasonic waves [213]. Figure 38A shows the decrease of Mw and of the hydrodynamic diameter dz, measured by static and dynamic light scattering respectively, on ultrasonic treatment of a polymer of Mw = 2.2 X10. Both Mw and dz decrease first abruptly but then... 

DMA measures the viscoelastic properties of a sample using either transient or dynamic oscillatory tests. Transient tests include creep and stress relaxation. In creep, a stress is applied to the sample and held constant, while deformation is measured versus time. After some time, the stress is removed and the recovery measured. In stress relaxation, a deformation is applied to the sample and held constant the degradation of the stress required to maintain the deformation is measured versus time. The most common test is the dynamic oscillatory test, where a sinusoidal stress (or strain) is applied to the material and the resultant sinusoidal strain (or stress) is measured. Also measured is the phase difference, 8, between the two sine waves. The phase lag will be 0° for a purely elastic material and 90° for a purely viscous material. Viscoelastic materials such as polymers will exhibit an intermediate phase difference. Since modulus equals stress divided by strain, the complex modulus, E, can be calculated. From E and 8, the storage modulus, E, the loss modulus, E", and tan 8 can be calculated ... 

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