Elastomer relaxation behavior

Figure 17 Calculated stress-relaxation behavior at 298 K for five uncross-linked elastomers of M = 200,000 EP, ethylene-propylene (56 44) styrene-butadiene (23.5 76.5), SB natural rubber, N butyl and dimethyl siloxane.

The measurement of stress relaxation at various temperatures allows for precise evaluation of elastomer aging behavior and offers detailed insight into the mechanisms of elastomer aging [229]. Multiple loads act on almost all elastomers seals and molded rubber technical parts [242] ... 

Natural rubber (NR) is a well studied elastomer. Of particular interest is the ability of NR to crystallize, specifically the strain-induced crystallization that takes place whilst the material is stretched. Moreover, in many elastomer applications, network chain dynamics under external stress/strain are critical for determining ultimate performance. Thus, a study on how the strain-induced crystallization affects the dynamics of a rubbery material is of outmost importance. Lee et al [1] reported their initial findings on the role of uniaxial extension on the relaxation behavior of cross-linked polyisoprene by means of dielectric spectroscopy. Nonetheless, to our best knowledge no in-depth study of the effects of strain induced crystallization on the molecular dynamics of NR has been undertaken, analyzing the relaxation spectra and correlating the molecular motion of chains with its structure. Broadband dielectric spectroscopy (BDS) has been chosen in order to study the dynamic features of segmental dynamics, because it is a comparatively simple technique for the analysis of the relaxation behaviour over a suitable frequency interval. This study is important from a basic and practical point of view, since an elongated crosslinked polymer at equilibrium may be considered as a new anisotropic material whose distribution of relaxation times could be affected by the orientation of the chains. 

They presented a theoretical approach to predict the behavior of silicone rubber under uniaxial stress. The model is based on the concept of the classical Maxwell treatment of viscoelasticity and stress relaxation behavior, and the Hookean spring component was replaced by an ideal elastomer component. From the test data, the substitution permits the new model estimation of the cross-link density of the silicone elastomer and allows a stress level to be predicted as a complex function of extension, cross-link density, absolute temperature, and relaxation time. Tock and co-workersh" ] found quite good agreementbetweenthe experimental behavior based on the new viscoelastic model. By using dynamic mechanical analysis (DMA), the authors would have been able to obtain similar information on the silicone elastomer. 

Elastomers are a versatile class of polymer materials with distinct advantages in sealing applications over other polymer classes such as thermoplastics, duromers, and thermoplastic elastomers. In particular, the superior relaxation behavior, together with the availability of low-hardness types, makes elastomers the most... 

For elastomers, factorizability holds out to large strains (57,58). For glassy and crystalline polymers the data confirm what would be expected from stress relaxation—beyond the linear range the creep depends on the stress level. In some cases, factorizability holds over only limited ranges of stress or time scale. One way of describing this nonlinear behavior in uniaxial tensile creep, especially for high modulus/low creep polymers, is by a power... 

One type of block polymer is known as thermoplastic elastomers. They consist of a number of rubber blocks tied together by hard crystalline or glassy blocks. These materials can be processed in injection molding and extrusion equipment since the crystalline blocks melt or the glassy ones soften at high temperatures. However, at lower temperatures, such as at room temperature, the hard blocks behave very much as cross-links to reduce creep and stress relaxation. Thermoplastic elastomers have creep behavior between that of very lightly cross-linked rubbers and highly cross-... 

It has been reported (4-6) that elastomers undergo very longterm relaxation processes in stress relaxation and creep experiments. The long time behavior of shear modulus can be represented by (18)... 

We will discuss some preliminary results, which have been performed recently l01). In Fig. 39a the results for polymer No. 2d of Table 10 are shown, which were obtained by torsional vibration experiments. At low temperatures the step in the G (T) curve and the maximum in the G"(T) curve indicate a p-relaxation process at about 120-130 K. Accordingly the glass transition is detected at about 260 K. At 277 K the nematic elastomer becomes isotropic. This phase transformation can be seen only by a very small step in G and G" in the tail of glass transition region, which is shown in more detail in Fig. 39 b. From these measurements we can conclude, that the visco-elastic properties are largely dominated by the properties of the polymer backbone the change of the mesogenic side chains from isotropic to liquid crystalline acts only as a small disturbance and in principle the visco-elastic behavior of the elastomer... 

While the ingress of liquids in elastomers is diffusion controlled and thus Case I, because the polymer chains are well above the glass transition temperature Tg, liquid transport into glassy polymers is more complex [112, 135]. The swelling behavior below Tg is characterized by a sharp boundary between the remaining polymer core and the swollen region which proceeds with constant velocity. This behavior is also called relaxation controlled because the diffusion is much faster than the segmental relaxation of the polymer. 

Stress relaxation experiments involve the measurement of the force required to maintain the deformation produced initially by an applied stress as a function of time. Stress relaxation tests are not performed as often as creep tests because many investigators believe they are less readily understood. The latter point is debatable, and it may only be that the practical aspects of creep measurements are simpler. As will be shown later, all the mechanical parameters are in theory interchangeable, and so all such measurements will contribute to the understanding of viscoelastic theory. Whereas stress relaxation measurements are useful in a general study of polymeric behavior, they are particularly useful in the evaluation of antioxidants in polymers, especially elastomers, because measurements on such systems are relatively easy to perform and are sensitive to bond rupture in the network. 

A new approach was developed to describe deformation behavior of crosslinked amorphous elastomers. - Stress relaxation arrd change of network properties during deformation of an elastomer in the whole range of strain up to the rapture of material is taken into accormt. 

During the 1940s and early 1950s, Leaderman [L4,L5],Tobolsky [A2,A3, Dll, T5], Ferry [F3], and others made extensive studies of the small-strain behavior of elastomers and related polymers. These studies involved creep (deformation under applied stress), stress relaxation following applied stresses, and imposed oscillatory strains. These and other experimental techniques used have been described in special detail in the monograph of Ferry [F3]. These studies showed that all of these deformations could be represented in terms of the superposition principle of Boltzmann [B26] [see Eq.(43)]. 

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