Viscoelasticity torsion pendulum method

One of most popular techniques for dynamic mechanical analysis is the torsion pendulum method. In a modification of this method designed to follow curing processes, a torsion bar is manufactured from a braid of fibers impregnated with the composition to be studied this is the so-called torsional braid analysis (TBA) method.61 62,148 The forced harmonic oscillation method has been also used and has proven to be valuable. This method employs various types of rheogoniometers and vibroreometers,1 9,150 which measure the absolute values of the viscoelastic properties of the system under study these properties can be measured at any stage of the process. The use of computers further contributes to improvements in dynamic mechanical analysis methods for rheokinetic measurements. As will be seen below, new possibilities are opened up by applying computer methods to results of dynamic measurements. 

There are several other comparable rheological experimental methods involving linear viscoelastic behavior. Among them are creep tests (constant stress), dynamic mechanical fatigue tests (forced periodic oscillation), and torsion pendulum tests (free oscillation). Viscoelastic data obtained from any of these techniques must be consistent data from the others. 

This second group of tests is designed to measure the mechanical response of a substance to applied vibrational loads or strains. Both temperature and frequency can be varied, and thus contribute to the information that these tests can provide. There are a number of such tests, of which the major ones are probably the torsion pendulum and dynamic mechanical thermal analysis (DMTA). The underlying principles of these dynamic tests have been covered earlier. Such tests are used as relatively rapid methods of characterisation and evaluation of viscoelastic polymers, including the measurement of T, the study of the curing characteristics of thermosets, and the study of polymer blends and their compatibility. They can be used in essentially non-destructive modes and, unlike the majority of measurements made in non-dynamic tests, they yield data on continuous properties of polymeric materials, rather than discontinuous ones, as are any of the types of strength which are measured routinely. 

Dynamic mechanical measurements can mostly be divided into two groups. The reaction of a sample to a once applied light torque can be measured with the torsion pendulum. The sample oscillates freely, whereby the amplitude decreases steadily with each cycle for viscoelastic materials. The ratio of two successive amplitudes is constant for ideal viscoelastic materials. This procedure yields shear moduli. The torsion pendulum allows measurements to be relatively easily made the disadvantage is that the frequency is not an independent variable with this method. 

Because of the frequency dependence of Tg, the convention adopted for assignment of the glass transition temperature is an important consideration. Traditionally, a frequency of 1 Hz has been used as a standard value. This is based on the historic precedence, since the torsion pendulum was the most widely used DMA technique in the early days of viscoelastic property measurements. The torsion pendulum is a free vibration technique with a natural frequency of approximately 1 Hz. The 1 Hz value also is reasonably close (within 10 °C) to the Tg values determined by other widely used methods such as DSC, dilatometry, and TMA. The relation between DMA and DSC Tg values is considered further at the end of this chapter (Sircar and Drake 1990). Because of the ambiguity inherent in the kinetic nature of Tg, it is most important that the test frequency be reported along with any Tg value determined by a DMA technique. 

A torsion pendulum is a simple instrument to measure the dynamic viscoelastic properties of solid polymers. Using this method, the shear modulus G can be obtained as a function of temperature within the frequency range from 0.1 to 10 Hz. This method is actually separated into two methods, A and B, as shown in Fig. 5. In method A, the upper end of the specimen is rigidly clamped, and the lower end of the specimen is attached to an inertia member. In method B, the lower end of the specimen is fixed, and the inertia member, a rod, and an upper clamp are balanced by a counterweight. 

In the so-called torsional braid analysis,the liquid is absorbed in a long strip of soft braided thread or filter paper which has very slight torsional stiffness, and torsion pendulum measurements are made by the methods of Chapters 6 and 7. Both storage and loss components of the dynamic torsional response may be attributed primarily to the viscoelastic liquid, and examined on a relative basis, although absolute measurements are not possible. Care must be taken that a contribution from the braid does not lead to erroneous conclusions about the properties of the viscoelastic liquid.25a. a flexural braid method has also been utilized. 

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