Viscoelasticity dynamic mechanical analysis methods

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. 

Dynamic mechanical analysis (DMA) or dynamic mechanical thermal analysis (DMTA) provides a method for determining elastic and loss moduli of polymers as a function of temperature, frequency or time, or both [1-13]. Viscoelasticity describes the time-dependent mechanical properties of polymers, which in limiting cases can behave as either elastic solids or viscous liquids (Fig. 23.2). Knowledge of the viscoelastic behavior of polymers and its relation to molecular structure is essential in the understanding of both processing and end-use properties. 

In this section we are going to examine such viscoelastic properties in some detail and we will start by examining in turn three important mechanical methods of measurement creep, stress relaxation, and dynamic mechanical analysis. This will lead us to interesting things like time-temperature equivalence and a discussion of the molecular basis of what we have referred to as relaxation behavior. 

Pellicle and tea-immersed pellicle were analyzed using nanoDMA (dynamic mechanical analysis) to see if the tannins had an effect on the viscoelasticity of the pellicle. NanoDMA is a technique used to study and characterize mechanical properties in viscoelastic materials. The method is an extension of nanoindentation testing [58, 59], An analysis of the nanoindentation load-depth curve gives the hardness (H) and reduced elastic modulus (E ), provided the area of contact, A, between the indenter tip and the sample is known [ 13]. By... 

Dynamic-mechanical analysis (DMA) is a versatile method for measuring viscoelastic values over a wide frequency range commonly the modulus of elasticity and the damping values are determined. Moreover the testing method is used to investigate material behaviour as a function of temperature (e.g. for determine the glass transition temperature). 

Treny and Duperray [32] have pointed out that issues of human comfort relating to noise and vibration are one of the major priorities for materials structural research and development in the field of transportation. Dynamic mechanical analysis (DMA) testing provides a way to characterise in an accurate manner the viscoelastic properties of all the material used in vehicle interiors. Using a unique database software allowed easy material selection according to their viscoelastic properties. An approach for the optimisation of materials through the combination of selective database software and specific numerical calculation methods, to predict the final acoustic behaviour during the materials selection and systems development period are presented. 

Two manifestations of linear viscoelasticity are creep and stress relaxation-, the respective two testing methods are known as transient tests. One can also apply sinusoidal load, an increasingly more used method of study of viscoelasticity by dynamic mechanical analysis (qv) (DMA). We shall now briefly discuss each of these three approaches. 

Dynamic mechanical methods (typically oscillatory parallel plate rheometry) are commonly used to measure the dynamic mechanical properties from the liquid state to the solid state. By using small-amplitude oscillatory deformations (linear viscoelastic regime), the dynamic storage and loss moduli can be obtained. From these quantities, the viscosity and modulus can be calculated (71) (see Dynamic Mechanical Analysis). 

Creep obeys viscoelastic theory at small strains and it is possible to apply predictive methods using data from dynamic mechanical analysis to obtain creep data. Here, remarkable amounts of data can be obtained from... 

Polymers are viscoelastic materials, whose mechanical behavior exhibits characteristics of both solids and liquids. Thermal analysts are frequently called on to measure the mechanical properties of polymers for a number of purposes. Of the different methods for viscoelastic property characterization, dynamic mechanical techniques are the most popular, since they are readily adapted for studies of both polymeric solids and liquids. They are often referred to collectively as dynamic mechanical analysis (DMA). Thermal analysts often refer to the DMA measurements on liquids as rheology measurements. 

There are three fundamental test methods for characterization of the viscoelastic behavior of polymers creep, stress relaxation, and dynamic mechanical analysis. Although the primary focus for this chapter is DMA, it is useful first to discuss the fundamentals of creep and stress relaxation, not only because they are conceptually simpler but because most DMA instruments also are capable of operating in either a creep or stress relaxation mode. All three of the methods are related, and numerical techniques are available for calculating creep and stress relaxation data from dynamic mechanical data (Ferry 1980). 

The study of elastic and viscoelastic materials under conditions of cyclic stress-strain changes is called dynamic mechanical analysis (DMA). It is a large, separate field of study that will be briefly summarized in Sect. 6.6. Of key interest in this analysis method is the time-temperature correlation of the viscoelastic properties. The thermal antilysis versions of the instruments... 

The term viscoelasticity combines viscous and elastic stress-strain flow characteristics. If materials behavior is dominated by viscous flow it is generally referred to as a fluid, whereas if the elastic properties dominate the mechanical properties of a material it is considered to be solid. Most adhesives are applied in a liquid or pasty condition to allow wetting and promote spreading and then are required to phase change into a solid. In the liquid state, rheology provides the methods to differentiate between elastic and viscous flow characteristics while, for example, dynamic mechanical analysis of cured adhesive polymers uses similar principles to access elastic and viscous parameters of the stress-strain response. 

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 tests have been widely applied in the viscoelastic analysis of polymers and other materials. The reason for this has been the technical simplicity of the method and the low tensions and deformations used. The response of materials to dynamic perturbation fields provides information concerning the moduli and the compliances for storage and loss. Dynamic properties are of considerable interest when they are analyzed as a function of both frequency and temperature. They permit the evaluation of the energy dissipated per cycle and also provide information concerning the structure of the material, phase transitions, chemical reactions, and other technical properties, such as fatigue or the resistance to impact. Of particular relevance are the applications in the field of the isolation of vibrations in mechanical engineering. The dynamic measurements are a... 

DYNAMIC MECHANICAL METHODS FOR THE ANALYSIS OF THE VISCOELASTIC PROPERTIES OF PHARMACEUTICAL AND BIOMEDICAL MATERIALS... 

Oscillatory rheometry and dynamic mechanical thermal analysis, both termed dynamic oscillatory methods, allow for the convenient, accurate, and rapid quantification of the viscoelastic properties of pharmaceutical and biomedical systems. In light of this, considerations of the theory, practice, and applications of these methods will form the basis of this chapter. 

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