Polymers mechanical damping

An associated technique which links thermal properties with mechanical ones is dynamic mechanical analysis (DMA). In this, a bar of the sample is typically fixed into a frame by clamping at both ends. It is then oscillated by means of a ceramic shaft applied at the centre. The resonant frequency and the mechanical damping exhibited by the sample are sensitive measurements of the mechanical properties of a polymer which can be made over a wide range of temperatures. The effects of compositional changes and methods of preparation can be directly assessed. DMA is assuming a position of major importance in the study of the physico-chemical properties of polymers and composites. 

Dynamic mechanical experiments yield both the elastic modulus of the material and its mechanical damping, or energy dissipation, characteristics. These properties can be determined as a function of frequency (time) and temperature. Application of the time-temperature equivalence principle [1-3] yields master curves like those in Fig. 23.2. The five regions described in the curve are typical of polymer viscoelastic behavior. 

Dynamic mechanical tests measure the response of a material to a periodic force or its deformation by such a force. One obtains simultaneously an elastic modulus (shear, Young s, or bulk) and a mechanical damping. Polymeric materials are viscoelastic-i.e., they have some of the characteristics of both perfectly elastic solids and viscous liquids. When a polymer is deformed, some of the energy is stored as potential energy, and some is dissipated as heat. It is the latter which corresponds to mechanical damping. 

As shown in Chapter 10, molecular dynamics in polymers is characterized by localised and cooperative motions that are responsible for the existence of different relaxations (a, (3, y). These, in turn, are responsible for energy dissipation, mechanical damping, mechanical transitions and, more generally, of what is called a viscoelastic behavior - intermediary between an elastic solid and a viscous liquid (Ferry, 1961 McCrum et al., 1967). 

Below the glass-rubber transition temperature glassy polymers also show other, secondary transitions. Their effects are smaller and often less obvious, although they are important to the mechanical behaviour (to diminish brittleness). Secondary transitions can be detected by studies of mechanical damping, by NMR or by electric loss measurements over a range of temperatures. 

The resilience of a polymer will be high (i.e. tan <5 is small) in temperature regions where no mechanical damping peaks are found. This applies in particular to rubbery networks (T Tg), which therefore possess a high resilience. Various rubbers behave quite differently at room temperature the rebound resilience is for natural rubber, butadiene-styrene rubber and butyl rubber high, medium and low, respectively. In practice this means that tyres for cars must have medium rebound resilience high rebound resilience causes bumping on the road, whereas low rebound resilience causes a tyre to become very hot. 

Quantitative relationship of chemical structure to physical properties in network polymers has received considerably less attention and study than in the case of thermoplastics. However, in recent years, progress has been made towards elucidation of the quantitative relationships between structure and properties. We have chosen to illustrate the quantitative effect of structural factors on physical properties in four representative areas glass transition temperature, modulus of elasticity, mechanical damping, solvent resistance. 

Carbon itself has been successfully used as a biomaterial. Carbon based fibers used in composites are known to be inert in aqueous (even seawater) environments, however they do not have a track record in the biomaterials setting. In vitro studies by Kovacs [1993] disclose substantial electrochemical activity of carbon fiber composites in an aqueous environment. If such composites are placed near a metaUic implant, galvanic corrosion is a possibility. Composite materials with a polymer matrix absorb water when placed in a hydrated environment such as the body. Moisture acts as a plasticizer of the matrix and shifts the glass transition temperature towards lower values [Delasi and Whiteside, 1978], hence a reduction in stiffness and an increase in mechanical damping. Water immersion of a graphite epoxy... 

Figure 1.5 Change in modulus and mechanical damping in the region of the glass transition temperatures for (a) amorphous thermoplastic, (b) crosslinked thermoset, (c) a biend of two thermoplastic polymers. The 7g corresponds to the steepest slope in the modulus curves and (more approximately) to the peaks in the damping curves, assuming that the damping vibrations are of low frequency...

Hypotheses have been advanced to explain the increase of the impact strength of the epoxy resins by the integration of finely dispersed rubbers. There is an assumption [139] that the rubber particles in the epoxy polymer matrix absorb the impact energy in the manner of mechanical damping. In this case the dissipation of the excess energy occurs mainly inside the formed interphase layer with decreased density of the cohesion energy. In addition, to provide max-... 

Fatigue testing of polymers cannot be accelerated by simply increasing the loading frequency. The reason is the relatively high level of mechanical damping (internal friction) in common polymers which would produce an excessive heating of the specimen. 

The mechanical damping of a polymer is related to impact, vibration, and similar dynamic stresses. Although mechanical damping has been correlated with fatigue performance, neither it nor torsion modulus can be used to determine design data, because creep effects are not included in the test. 

Dynamic mechanical analysis techniques permit measurement of the ability of materials to store and dissipate mechanical energy during deformation. DMA is used to determine the modulus, glass transition, mechanical damping and impact resistance, etc., of thermoplastics, thermosets, elastomers and other polymer materials. Information regarding the phase separation of polymers is also available by DMA [2]. In DMA, viscoelastic materials are deformed in a sinusoidal, low strain displacement and their responses are measured. Elastic modulus and energy dissipation are the measured properties. 

The main advantages of foamed plastics are low consumption of raw materials, reduced weight, excellent heat and sound-insulation and mechanical damping. Important applications of polymer foams include packaging materials, insulation, sound absorption and upholstery. 

Viscoelasticity is a phenomenon observed in most of the polymers since they possess elastic and viscous characteristics when deformed. The properties such as creep, stress relaxation, mechanical damping, vibration absorption and hysteresis are included in viscoelasticity. If a material shows linear variation of strain upon the application of stress on it, its behavior is said to be linear viscoelastic. Elastomers and soft biological tissues undergo large deformations and exhibit time dependent stress strain behavior and are nonlinear viscoelastic materials. The non-linear viscoelastic properties of solid polymers are often based on creep and stress-... 

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