Torsion oscillation method

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. 

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. 

Functions were calculated from the constants given above using the rigid rotator harmonic oscillator method. The entropy was increased by R tn 2 because two rotameric forms are implied by use of the torsional frequency. Small but arbitrary adjustments were made in the assignment of the bending mode frequencies in order to reproduce the vapor pressure data of Scott et al. (1 ) as closely as possible. Calculated values of S (298.15 K) = 57.03 and S (340 K) = 58.70 cal k" raol" may be compared with 56.99 and 58.69, respectively, derived from the data of Scott et al. Internal rotation calculations would require a complex potential function in order to fit the data. The barrier to inversion (990 cm 2.8 kcal mol ) is slightly less than the barrier to... 

Surface viscosity was determined by a torsion oscillation method in which the period of oscillation of a paraffin-coated mica ring was measured on a clean water surface and lipid-covered surface (6, 13). Since the period of oscillation is proportional to the surface viscosity, the difference in period (At) between lipid or protein film and a clean aqueous surface measures viscosity and relates to either molecular area, film pressure, or time of interaction (t). The period values were repro-... 

Joos data on distearoyl lecithin (DSL)-cholesterol mixed films (35) coincide with data from our DPL-cholesterol system in the sense that cholesterol reduced the viscosity of the lecithin film, and the surface viscosity decreased with increasing cholesterol concentrations. However, the comparison and interpretation of surface viscosity data require caution (2,6). For example, in Joos experiments the lipid was distearoyl lecithin (DSL), the subphase was distilled water, phospholipid and cholesterol were premixed, and viscosity was measured by the rotational surface Couette method. By the torsion oscillation method, at all film pressures... 

Free- Vibration Methods. Free-vibration instmments subject a specimen to a displacement and allow it to vibrate freely. The oscillations are monitored for frequency and damping characteristics as they disappear. The displacement is repeated again and again as the specimen is heated or cooled. The results are used to calculate storage and loss modulus data. The torsional pendulum and torsional braid analy2er (TBA) are examples of free-vibration instmments. 

Before considering particular test methods, it is useful to survey the principles and terms used in dynamic testing. There are basically two classes of dynamic motion, free vibration in which the test piece is set into oscillation and the amplitude allowed to decay due to damping in the system, and forced vibration in which the oscillation is maintained by external means. These are illustrated in Figure 9.1 together with a subdivision of forced vibration in which the test piece is subjected to a series of half-cycles. The two classes could be sub-divided in a number of ways, for example forced vibration machines may operate at resonance or away from resonance. Wave propagation (e.g. ultrasonics) is a form of forced vibration method and rebound resilience is a simple unforced method consisting of one half-cycle. The most common type of free vibration apparatus is the torsion pendulum. 

G Torsion pendulum method (Nielsen, 1962) (measurement of frequency of oscillations and rate of damping, at different temperatures) Measurement of speed of shear wave (Schuyer et al., 1954, Hartmann, 1980) G = pu2sh... 

A convenient method for determining transition times and transition temperatures of polymeric materials is dynamic mechanical analysis. One type of instrument which is particularly suitable for polymeric solids is the freely oscillating torsion pendulum (TP). Advantages of the TP include its simplicity, sensitivity, relatively low frequency ( 1 Hz) which permits direct correlation of transition temperatures with static nonmechanical methods (e.g., dilatometry and calorimetry), and its high resolution of transitions A major disadvantage of the conventional TP is that test temperatures are limited by the inability of materials to support their own weight near load-limiting transition temperatures. 

A torsion pendulum interfaced with a desktop computer form an automated instrument for dynamic mechanical characterization of polymeric materials. The computer controls the initiation of the oscillations, collects the digitized data and calculates the shear modulus and loss modulus from the damped oscillations, utilizing one of four methods of analysis ... 

Three techniques related to oscillating-body methods can be mentioned. One involves measuring the torsional oscillations of a piezoelectric crystal immersed in a fluid it can achieve accuracies of about 2% and has been used in a variety of conditions. A second involves measuring the drag on a rotating cylinder magnetically suspended in the fluid it can achieve uncertainties as low as... 

Another method of determining Aj is by studying the damped torsional oscillations of the nematic suspended in a static magnetic field. For D/x H V1, where D is the torsion constant of the wire and V the... 

Free vibration methods such as the torsion pendulum are covered by ISO 4663 and are limited to cry low strains and frequencies, and are in much less frequent use these days than the forced vibration nonresonant systems on which this chapter will focus. The early Du Pont DMA and German Myrenne used input energy to maintain the resonant oscillation amplitude, but the main limitations were variable frequency according to the sample size (which had to be glassy or plastic) or one frequency only (1 Hz) respectively. 

Of course, such dynamic mechanical test methods are only suitable for such probes as can support their own weight. In examining paints, lacquers, and non-self-supporting films, a glass fiber cord is impregnated with a solution of the test material and the solvent is evaporated. The impregnated cord is then subjected to periodic oscillations (torsional braid analysis). 

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. 

Rotational methods are particularly suitable for studying the flow of non-Newtonian liquids. An example is the concentric cylinder (or Couette) viscometer. The liquid is sheared between concentric cylinders, which are moving relative to one another. The outer cylinder can be rotated (or oscillated) at a constant rate and the shear measured in terms of the deflection of the inner cylinder, which is suspended by a torsion wire. Altema-... 

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