Viscoelasticity of Elastomers

Elastomer is a general term used for cross-linked polymeric materials such as rubber with elastic properties described above. However, elastomers are not purely elastic materials and they exhibit damping, that is, fractional energy loss per cycle in, say, an oscillatory experiment. The energy loss is due primarily to stress relaxation (viscous dissipation) of inelastic pendant chains and unentangled loops in the network. In the case of a prescribed stress sinusoidal deformation, the applied stress is expressed as 

The strain is then represented by two components one in phase and the other 90° out of phase with the stress. The compliance / of a viscoelastic material subjected to a small shear stress deformation is expressed as 

FIGURE 9.23 Loss tangent (777 ) of butyl rubber samples cross-linked to different extent by sulfur. The data for each sample were collected over a range of temperatures and shifted to a reference temperature of 25°C. Values indicated are the values of —log 7 (the equilibrium compliance). (Reprinted with permission from Sanders, J. R, and J. D. Ferry, Macromolecules, 7, 1974, 681. Copyright 1974 American Chemical Society.) 

In a shear stress relaxation experiment, an elastomer can be cured between a cone-and-plate or a parallel-plate fixture of a rheometer, which is subjected to a step strain y that is held constant for times t 0. The stress o(t) is monitored and reported in terms of a time-dependent modulus Gif) defined by 

Go is the initial modulus related to the initial value of the stress X is the relaxation time as exhibited by the single Maxwell element 

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Effect of Double Networking on Non-Linear Viscoelasticity of Elastomers... 

International Rubber Hardness. The International mbber hardness test (ASTM D1415) (2) for elastomers is similar to the Rockwell test ia that the measured property is the difference ia penetration of a standard steel ball between minor and major loads. The viscoelastic properties of elastomers require that a load appHcation time, usually 30 seconds, be a part of the test procedure. The hardness number is read directly on a scale of 0 to 100 upon return to the minor load. International mbber hardness numbers are often considered equivalent to Durometer hardness numbers but differences ia iadenters, loads, and test time preclude such a relationship. 

As mentioned earlier, the contact-mechanics-based experimental studies of interfacial adhesion primarily include (1) direct measurements of surface and interfacial energies of polymers and self-assembled monolayers (2) quantitative studies on the role of interfacial coupling agents in the adhesion of elastomers (3) adhesion of microparticles on surfaces and (4) adhesion of viscoelastic polymer particles. In these studies, a variety of experimental tools have been employed by different researchers. Each one of these tools offers certain advantages over the others. These experimental studies are reviewed in Section 4. 

The process of viscoelastic braking just described has certain parallels with the dynamic adhesion of elastomers. When, for example, a rubber strip is peeled from a rigid substrate, the effective, or apparent, work of adhesion, W, is usually much greater than the intrinsic, or reversible, energy of adhesion, Wq, given by the Dupre equation [15] ... 

BS 903 PA24 Guide to the determination of dynamic properties of rubbers DIN 53513 Determination of the viscoelastic properties of elastomers on exposure to forced vibration at nonresonant frequencies... 

The above theoretical considerations, the relationships between the mixing energy and the viscosity, and the tensile strain energy demonstrate in fact that mixing operations of elastomers are controlled by energy absorption processes which are dependent upon the viscoelastic properties of the polymer materials. 

Pawlowski, H.. and Dick. J.. Measurement of the viscoelastic properties of elastomers with a new dynamic mechanical rheological tester, pre.sented at the Rubber Div. ACS at Philadelphia, May 2 5, 1995, Paper 50. 

Conventional acrylic films, such as the VHB 4910 series of elastomers from 3M, possess excellent actuation strain, energy density, and coupling efficiency. However, in order to achieve these high performance values, the film must be prestrained. The addition of bulky support frames required to maintain the prestrain on the film significantly increases the mass of VHB acryUc based devices, reducing their effective energy densities to more pedestrian values. VHB acrylic films also suffer from viscoelastic effects, which limit their maximum response frequency to the 10-100 Hz range. The viscoelastic nature of these films also limits their overall efficiency and results in time dependent strain that can make their performance somewhat erratic. 

Measurements of linear and nonlinear viscoelastic behavior of elastomers have a long history. Instead of reviewing the works done in the past by various workers... 

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