Analysis torsion pendulum

Glass-transition temperatures are commonly determined by differential scanning calorimetry or dynamic mechanical analysis. Many reported values have been measured by dilatometric methods however, methods based on the torsional pendulum, strain gauge, and refractivity also give results which are ia good agreement. Vicat temperature and britde poiat yield only approximate transition temperature values but are useful because of the simplicity of measurement. The reported T values for a large number of polymers may be found ia References 5, 6, 12, and 13. 

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. 

The preceding analysis provides a powerful method for determining the diffusivities of species that produce an anelastic relaxation, such as the split-dumbbell interstitial point defects. A torsional pendulum can be used to find the frequency, u>p, corresponding to the Debye peak. The relaxation time is then calculated using the relation r = 1/ojp, and the diffusivity is obtained from the known relationships among the relaxation time, the jump frequency, and the diffusivity. For the split-dumbbell interstitials, the relaxation time is related to the jump frequency by Eq. 8.63, and the expression for the diffusivity (i.e., D = ra2/12), is derived in Exercise 8.6. Therefore, D = a2/18r. This method has been used to determine the diffusivities of a wide variety of interstitial species, particularly at low temperatures, where the jump frequency is low but still measurable through use of a torsion pendulum. A particularly important example is the determination of the diffusivity of C in b.c.c. Fe, which is taken up in Exercise 8.22. 

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. 

It should be noted that a TL, l would not be expected in this polystyrene) braid sample because of its high molecular weight (approximately 100,000) (46). Also, this analysis of the composite structure of the braid supports Nielsen s (47) suggestion that rL, L arises solely from the residual elasticity of the glass. The torsional pendulum, braid, and TL, l will be discussed in more detail elsewhere (16,17). 

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. 

Topological limit of the reaction 24 Torsion pendulum (TP) 88 Torsional braid analysis (TBA) 88 Transesterification 48... 

ASTM standard specimens and procedures were used for flexure (D-690), compression (D 695), Izod impact and torsional pendulum analysis (TPA). For tension, D1822 tensile impact specimens were substituted for D638 specimens to conserve material. Test specimens were machined from the plates and cylinders using a water cooled dlamond wheel. All the specimens were dried in vacuo at 100°C for three weeks before testing or subsequent postcure treatment. Half the specimens were post-cured for 2 hours at 225°C in vacuo before testing. Selected specimens were Immersed in distilled water at 80°C for 6 weeks for moisture uptake determinations. 

Baer, Hiltner, and colleagues (see Hiltner, 1979, and references cited therein) have used dynamic mechanical analysis to examine the hydration of collagen, elastin, and several polypeptides. A torsional pendulum constructed of the sample was examined for low-frequency (i.e., IHz) mechanical loss as a function of hydration and temperature. A common feature is a dispersion that is absent in the dry protein and appears at... 

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 ... 

A variation of the torsion pendulum, torsional braid analysis (TBA), utilizes a supported specimen so that the dynamic mechanical properties of a sample can be monitored in the liquid as well as the solid states (1, 2). An inert multifilamented glass braid is impregnated with the sample (usually in its liquid state or in solution). The observed dynamic mechanical properties are relative due to the composite nature and complex geometry of the specimen. 

The purpose of this paper is to describe an automated torsion pendulum controlled by a desktop computer, to discuss four separate methods of data analysis, and to compare the results of a torsion pendulum experiment and a TBA experiment using the same epoxy resin. 

The automation of the torsion pendulum utilizing a desktop computer eliminates the tedious data analysis previously associated with that technique. Any one of four data reduction methods can be used the experimental conditions will determine which is the optimum one to employ. The torsion pendulum technique provides quantitative values of shear modulus and logarithmic decrement and in the torsion braid mode provides a qualitative analysis of materia i, especially in the liquid-to-solid transition region. 

The dispersion structure of the blends both in the melt and in the solid state was imaged partly by light microscopy (LM), and partly by scanning (SEM) and transmission electron microscopy (TEM). Wide-angle X-ray scattering (WAXS), Infrared (IR) measurements, and torsional pendulum analysis at IHz were performed too. 

Torsional pendulum analysis exhibits discrete relaxations at the glass transition temperatures of PVDF and PA-6 at -45°C and 50°C, respectively. This also indicates incompatibility of the blend components in the amorphous phase after solidification. 

The results of the characterization of cured phthalonitrile resins are presented. This includes their thermal properties from thermal gravimetric analysis and differential scanning calorimetry, spectroscopic properties from the infra-red and nuclear magnetic resonance techniques, and mechanical properties from torsional pendulum analysis and fracture mechanics evaluation. 

Torsional Pendulum Analysis (TPA). A freely oscillating torsional pendulum (7) operating at ca. 1 Hz was used for the determination of dynamic shear modulus of all cured samples as a function of temperature. The procedure recommended in ASTM-D-2236-70 was followed. 

Dynamic Mechanical Analysis (DMA) is a technique in which the elastic and viscous response of a sample under oscillating load, are monitored against temperature, time or frequency. This technique became well known by the impressive amount of information about the structure of polymers obtained with the torsion pendulum apparatus. The torsion pendulum DMA apparatus is a so-called resonant system i.e. the measuring frequency is not constant. The modern DMA systems are nearly always fixed frequency systems operating at frequencies between about 0.01 and 100 Hz. and in a temperature region ranging from about -150°C to 300°C. A survey of the DMA technique and the available commercial equipment was given by Wunderlich [1]. 

Dynamic mechanical analyzers can be divided into resonant and defined frequency instruments. The torsion pendulum just described is, for example, a resonant instrument. The schematic of a defined-frequency instrument is shown in Fig. 4.155. The basic elements are the force generator and the strain meter. Signals of both are collected by the module CPU, the central processing unit, and transmitted to the computer for data evaluation. The diagram is drawn after a commercial DMA which was produced by Seiko. At the bottom of Fig. 4.155, a typical sample behavior for a DMA experiment is sketched. An applied sinusoidal stress, o, is followed with a phase lag, 6, by the strain, e. The analysis of such data in terms of the dynamic moduli (stress-strain ratios, see Fig. 4.143) at different frequencies and temperature is the subject of DMA. 

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