Elastomers, viscoelastic behavior

Polymers are viscoelastic materials meaning they can act as liquids, the visco portion, and as solids, the elastic portion. Descriptions of the viscoelastic properties of materials generally falls within the area called rheology. Determination of the viscoelastic behavior of materials generally occurs through stress-strain and related measurements. Whether a material behaves as a viscous or elastic material depends on temperature, the particular polymer and its prior treatment, polymer structure, and the particular measurement or conditions applied to the material. The particular property demonstrated by a material under given conditions allows polymers to act as solid or viscous liquids, as plastics, elastomers, or fibers, etc. This chapter deals with the viscoelastic properties of polymers. 

The viscoelastic response of polymeric materials is a subject which has undergone extensive development over the past twenty years and still accounts for a major portion of the research effort expended. It is not difficult to understand the reason for this emphasis in view of the vast quantities of polymeric substances which find applications as engineering plastics and the still greater volume which are utilized as elastomers. The central importance of the time and temperature dependence of the mechanical properties of polymers lies in the large magnitudes of these dependencies when compared to other structural materials such as metals. Thus an understanding of viscoelastic behavior is fundamental for the proper utilization of polymers. 

Linear viscoelasticity is valid only imder conditions where structural changes in the material do not induce strain-dependent modulus. This condition is fulfilled by amorphous polymers. On the other hand, the structural changes associated with the orientation of crystalline polymers and elastomers produce anisotropic mechanical properties. Such polymers, therefore, exhibit nonlinear viscoelastic behavior. 

While mechanical conditioning is most commonly associated with elastomers, thermoplastics and composites are not excluded. Reference [36] for example looks at the influence of meehanical conditioning on the viscoelastic behavior of glass-liber-reinforccd epoxy resins. 

Plazek, 1986 Plazek and Choy, 1989 Plazek and Frund, 1990 Plazek and Chay, 1991) as well as some polybutadienes and fluorinated elastomers (Plazek et al, 1983 Plazek et al., 1988 Plazek and Rosner, 1998). Some nonlinear viscoelastic behavior is discussed. 

Three urethane-crosslinked polybutadiene elastomers (TB-1, TB-2, and TB-3) of varying crosslinking levels, along with a similarly crosslinked styrene-butadiene copolymer (HTSBR) and two polybutadiene polymers randomly crosslinked with dicumyl peroxide (PB-1 and PB-2), have been investigated to determine their viscoelastic behavior. Elsewhere, TB-1, TB-2, and TB-3 have been designated as HTPB-1, HTPB-2, and HTPB-3, respectively. 

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... 

The change of viscoelastic behavior of ciosslinked elastomers on swelling depends on polymer physical network. This, in turn, depends on the concentration and nature of the plasticizer in the material expressed by parameter m describing the ability of liquid to affect physical network of elastomer. 

VII. The Calculation of the Tear Energy of Elastomers from Their Viscoelastic Behavior... 

The viscoelastic properties of a series of fully cured epoxy resins with different crosslink densities and their trends have been discussed in some details in the previous section. Qualitatively these properties are shared by all crosslinked elastomers, although quantitatively they depend on molecular architecture and the chemical type of the network polymer and the crosslinking agent. Hence it is instructive to show the viscoelastic behavior of other model network systems and compare them. 

VII. THE CALCULATION OF THE TEAR ENERGY OF ELASTOMERS FROM THEIR VISCOELASTIC BEHAVIOR... 

One particular form of Eq. (47) wliich has been successfully used for representing the nonhnear viscoelastic behavior of elastomers is due to Bogue and coworkers [B22, B24, C9]. Both Middleman [M24] and Montes and White [M34] have used the form... 

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