The Measurement of Viscoelastic Behaviour

For a satisfactory understanding of the viscoelastic behaviour of polymers data are required over a wide range of frequency (or time) and temperature. The number of experiments required ean sometimes be reduced by using either the equivalenee of creep, stress relaxation and dynamic mechanical data (described in Chapter 4) or the equivalence of time and temperature as variables (to be discussed in Chapter 6). Nevertheless a variety of techniques need to be combined to eover a wide range of both time and temperature. 

There are five main classes of experiment, which will be discussed in turn  

An Introduction to the Mechanical Properties of Solid Polymers I. M. Ward and J. Sweeney 2004 John Wiley Sons, Ltd ISBN 0471 49625 1 (HB) 0471 49626 X (PB) 

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Viscoelastic flow behaviour of dilute polymer solutions in porous media is described as a method for characterization of polymer-solvent-temperature systems. Porous media flow tests provide information on the solution state of polymer solutions and the molecular weight of the polymers used. Furthermore, flow-induced and thermally induced degradation effects - frequently observed in polymer solutions - can be characterized by the measurement of viscoelastic effects in flow through porous media. Decrease of molecular weight and changes of the conformation of macromolecules in solution are important parameters in these processes. 

More recently the treatment was extended to piezoelectric devices in contact with viscoelastic media (i.e., liquids and polymers). It was then realised that if the deposited mass was not rigidly coupled to the oscillating quartz crystal, separation of inertial mass and energy losses was not possible with the measurement of the resonant frequency alone. Quartz crystal impedance in the acoustic frequencies was introduced in order to study mass and viscoelastic changes and a full electrical characterization of the crystal behaviour near resonance was employed. 

Before the models are described, the two simple aspects of viscoelastic behaviour already referred to - creep and stress-relaxation - are considered. For the full characterisation of the viscoelastic behaviour of an isotropic solid, measurements of at least two moduli are required, e.g. Young s modulus and the rigidity modulus. A one-dimensional treatment of creep and stress-relaxation that will model the behaviour of measurements of either of these (or of other measurements that might involve combinations of them) is given here. Frequently compliances, rather than moduli, are measured. This means that a stress is applied and the strain produced per unit stress is measured, whereas for the determination of a modulus the stress required to produce unit strain is measured. When moduli and compliances are time-dependent they are not simply reciprocals of each other. 

A severe complication of the measurement of the viscoelastic behaviour of gellan gum gels is the occurrence of syneiesis. 

The mechanical and viscoelastic behaviours of natural rubber based blends and interpenetrating polymer networks (IPNs) are fimctions of their structures or morphologies. These properties of blended materials are generally not constant and depend on the chemical nature and type of the polymer blends, and also enviromnental faetors involved with any measurements. Preparations of natural rubber blends and IPNs are well known as effeetive modifieation methods used to improve the original meehanieal and viscoelastie properties of one or both of the eomponents, or to obtain new natural rubber blended materials that exhibit widely variable properties. The most common consideration for their mechanical properties include strength, duetility, hardness, impact resistance and fracture toughness, each of which can be deformed by tension, compression, shear, flexure, torsion and impaet methods, or a eombination of two or more methods. Moreover, the viseoelastieity theory is a way to predict the behaviours of deformation of natural rubber blends and IPNs. The time and... 

As was the case with the measurement of coating modulus (Section 2.1), viscoelastic behaviour and the influence of the substrate can affect hardness measurements. With organic coatings, it is common to distinguish between conventional quasi-static hardness and dynamic hardness, p, defined as the resistance to plastic indentation at very high strain rates, where it is assumed that both viscoelastic and elastic effects are negligible. 

Following the manner of presentation in the previous sections, the subject of viscoelasticity will be explained by reference to experiments familiar to the practitioner of rubber technology. This is a rather unorthodox approach and different from the usual one, which begins with an introduction of the theory. The experiment is tensile stress-strain measurement. In the rubber industry tensile measurements are routinely performed with crosslinked specimens. Here, we are concerned with gum-rubber behaviour. Therefore, we must perform the measurements with uncrosslinked specimens. First, compression-moulded specimens must be prepared they require special attention, which will be described next. 

Stress relaxation tests are alternative ways of measuring the same basic phenomenon in viscoelastic polymers as creep tests, Le. the time-dependent nature of their response to an applied stress. As such, they have also been of value in understanding the behaviour of these materials. The essence of stress relaxation tests is that strain increases with time for a given stress, so that if stress is decreased with time in a controlled manner ( relaxed ), a state... 

It is likely that most biomaterials possess non-linear elastic properties. However, in the absence of detailed measurements of the relevant properties it is not necessary to resort to complicated non-linear theories of viscoelasticity. A simple dashpot-and-spring Maxwell model of viscoelasticity will provide a good basis to consider the main features of the behaviour of the soft-solid walls of most biomaterials in the flow field of a typical bioprocess equipment. 

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