Viscoelastic properties, measurement apparatus

The paper discusses the application of dynamic indentation method and apparatus for the evaluation of viscoelastic properties of polymeric materials. The three-element model of viscoelastic material has been used to calculate the rigidity and the viscosity. Using a measurements of the indentation as a function of a current velocity change on impact with the material under test, the contact force and the displacement diagrams as a function of time are plotted. Experimental results of the testing of polyvinyl chloride cable coating by dynamic indentation method and data of the static tensile test are presented. 

With appropriate caUbration the complex characteristic impedance at each resonance frequency can be calculated and related to the complex shear modulus, G, of the solution. Extrapolations to 2ero concentration yield the intrinsic storage and loss moduH [G ] and [G"], respectively, which are molecular properties. In the viscosity range of 0.5-50 mPa-s, the instmment provides valuable experimental data on dilute solutions of random coil (291), branched (292), and rod-like (293) polymers. The upper limit for shearing frequency for the MLR is 800 H2. High frequency (20 to 500 K H2) viscoelastic properties can be measured with another instmment, the high frequency torsional rod apparatus (HFTRA) (294). 

Figure 7.11 Sketch of the range of frequencies for the different apparatuses measuring viscoelastic properties.

The dynamic mechanical properties of polymers are measured using various types of apparatus, as discussed in Chapter 2. By dynamic mechanical analysis, not only main chain motion but also the secondary relaxation can be detected. The mathematical structure of theories of dynamic viscoelastic properties has been presented [81,821 and application to polymers has been described [83]. 

Fig. 40 Experimental apparatus developed by Murayama and Silverman [44] for measuring viscoelastic properties of a polymer in a liquid medium. The letter A denotes a thermoregulator B, a temperature controller C, clamps and T, transducers.

Brief mention may be made of some methods in which apparatus designed for viscoelastic solids is applied to study of viscoelastic liquids compounded with solid matrices. Such measurements usually provide the loss tangent only and are not appropriate for absolute measurements of viscoelastic properties, though they can give a semiquantitative view of changes in properties over wide ranges of temperature. 

Reference has already been made in Chapter 5 to the exploitation of a resonance oscillation of a moving apparatus element in the measurement of dynamic viscoelastic properties, commonly used in characteristic impedance determinations. This principle is frequently employed for measurements on soft solids and will now be described in more detail. 

Numerous instruments (plastometers, penetrometers, extensiometers, etc.) and procedures have been devised for measuring the rheological behaviour of various viscoelastic materials. However, the results obtained from most of these instruments are of little fundamental significance, because the applied stress is not uniformly distributed throughout the sample, and the way in which the material behaves towards a particular apparatus is measured rather than a fundamental property of the material itself. Nevertheless, such empirical instruments are indispensable for control testing purposes in industry,... 

The design of effective sound and vibration damping materials assumes an understanding of the mechanisms controlling the dissipation process and knowledge of candidate material properties. The use of viscoelastic materials as sound and vibration absorbers is wide-spread and well-known. Accurate measurement of the complex dynamic moduli of these materials is therefore vital to the control of acoustic and vibrational energy. This chapter discusses and compares three apparatus used to measure the dynamic modulus of viscoelastic materials. 

A final comment seems to be pertinent. In most cases actual measurements are not made at the frequencies of interest. However, one can estimate the corresponding property at the desired frequency by using the time (fre-quency)-temperature superposition techniques of extrapolation. When different apparatuses are used to measure dynamic mechanical properties, we note that the final comparison depends not only on the instrument but also on how the data are analyzed. This implies that shifting procedures must be carried out in a consistent manner to avoid inaccuracies in the master curves. In particular, the shape of the adjacent curves at different frequencies must match exactly, and the shift factor must be the same for all the viscoelastic functions. Kramers-Kronig relationships provide a useful tool for checking the consistency of the results obtained. 

The study of fundamental adhesion has been hampered because standard Tests of adhesion provide a result that is a complicated combination of fundamental adhesion, the physical properties of the adherend and the viscoelastic/plastic character of the adhesive (see Adhesion - fundamental and practical, Peel tests). Our understanding of adhesion has been significantly improved with the advent of mechanical devices that are able to probe the forces of adhesion under conditions that minimize all of the confounding effects of adherend, viscoelasticity, and so on. The Surface Forces Apparatus (SFA) as developed by Israelachvili and Tabor is a mechanical device that has allowed adhesion scientists to directly measure the forces of adhesion under very low rate, light loading, almost equilibrium conditions. Attention is also drawn to Atomic force microscopy. 

In the field of rubber elasticity both experimentalists and theoreticians have mainly concentrated on the equilibrium stress-strain relation of these materials, i e on the stress as a function of strain at infinite time after the imposition of the strain > This approach is obviously impossible for polymer melts Another complication which has thwarted the comparison of stress-strain relations for networks and melts is that cross-linked networks can be stretched uniaxially more easily, because of their high elasticity, than polymer melts On the other hand, polymer melts can be subjected to large shear strains and networks cannot because of slippage at the shearing surface at relatively low strains These seem to be the main reasons why up to some time ago no experimental results were available to compare the nonlinear viscoelastic behaviour of these two types of material Yet, in the last decade, apparatuses have been built to measure the simple extension properties of polymer melts >. It has thus become possible to compare the stress-strain relation at large uniaxial extension of cross-linked rubbers and polymer melts ... 

An apparatus to measure the dynamic mechanical properties in a gas stream was developed by Murayama and Silverman [44], who then measured the dynamic viscoelasticity of polyamide 66 in an HCl gas stream. 

One of the problems faced with polymer systems is that of equilibrium. Not only may adsorbed polymers desorb during compression, but the polymer may not be able to achieve its equilibrium configuration for a given separation over the time-scale of the experiment. Several groups have thus built modified surface force apparatuses to perform viscoelastic measurements on confined films, not only to determine relaxation rates for grafted polymers, but also to determine the effect of confinement on the physical properties of the films and to study friction and lubrication (see Section 2.5 above). Storage and loss moduli can be extracted from the response to oscillations in the frequency range of approximately 10 to 10 Hz. Montfort and co-workers, for example, have used normal oscillations to study the viscoelasticity of polybutadiene films on metal surfaces in hydrocarbons (234, 235). At such sufficiently small separations that the polymers could interact with one another, the... 

Numerous factors militate against the widespread use of NMR microscopy the resolution is poor by optical standards, the apparatus is expensive, the technique requires a high level of scientific expertise and the arrangements for sample loading are inconvenient and restrictive. Set against these are the uniquely non-invasive character of the method, its sensitivity to fluid phases, its unique ability to measure specific molecular properties and the especially powerful insights it can provide regarding fluid dynamics. Studies in which NMR microscopy has been able to provide unrivalled information include those concerned with membrane filtration, flow and dispersion in porous media, non-Newtonian flow in viscoelastic fluids, nonequilibrium phase transitions, electrophoresis and... 

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