Rupture of Elastomers

Viscoelastic Rupture of Elastomers Failure in highly stretched elastomers is by no means instantaneous (27-29). A common home example... 

London, 19th-22nd Sept. 1994, p.72-5. 93 GAS DIFFUSION INDUCED RUPTURE OF ELASTOMERS... 

Minor morphological differences, as between MR and NATSYN show up by their thermomechanical responses. Work on polymer chain conformations in bulk, 1968-to date, especially on fluorinated acrylics with B. McGarvay and W. Lee, on elastomeric dienes with H. Mark and K. Sato, and of entanglements in GRS witn R. Meyers, followed. These experiences were reviewed, with emphasis on the structural factors in rubber-like behavior at all phases of extension, in a chapter coauthored by T. L. Smith, on the Rupture of Elastomers in Liebowitz s treatise "Fracture , while later analyses focused on the effect of fillers in elastomers, the role of interfaces in composites, failure mechanisms in plastics as a function of morphology, and on resulting lessons for polymer engineering. 

Summarizing, the two common denominators of the various aspects of Dr. Eirich s work are, on the other hand, emphasis on general concepts underlying ongoing diverse efforts in the physics and chemistry of macromolecules and colloids on the other, doing research basic to such diverse areas as adsorption of polymers and the hydration of peptides, or the rupture of elastomers and the calcification of bone and teeth, research which had been bypassed by mission oriented workers but now stands to enchance the development of new research and to help the reinterpretation and inter-gration of existing data. 

Molecular Mechanical Aspects of the Isothermal Rupture of Elastomers," Fracture, Vol. HI, H. Liebowitz, Ed., Academic Press, 1972 co-author T. L. Smith. 

Among elastomers, artificial rubbers have replaced natural rabber for many uses because of their high resistance to chemical attack by ozone, an atmospheric pollutant. When ozone reacts with polymer chains, it breaks CUCn bonds and introduces additional cross-linking. Breaking 7r bonds causes the rabber to sofien, and cross-linking makes it more brittle. Both changes eventually lead to rupture of the polymer structure. 

The large scale molecular motions which take place in the rubber plateau and terminal zones of an uncross-linked linear polymer give rise to stress relaxation and thereby energy dissipation. For narrow molecular weight distribution elastomers non-catastrophic rupture of the material is caused by the disentanglement processes which occur in the terminal zone, e.g., by the reptation process. In practical terms it means that the green strength of the elastomer is poor. 

The linearity limit of elastomers is large, because their deformation is of an entropic nature and does not involve bond rupture and reformation. 

Now let me try to get some insists into molecular mechanisms of the mechanic fracture in polymers 73). The l teilin model was originally proposed in ex]danation of the mechano-radical formatirm in the hi y stretched fibre. However, one can apply the Peterlin model to the fracture ptenomena in crystalline polymers, because large deformations proceed always in advance of a mechanical fracture. Thus, the tie molecules are assumed to be only parts vtdiich are broken in the case of destmction of bulk polymers. The fact that no mechano-radical is formed from the polymer having no tie mdecules even after the milling supports the interpretation mentioned ove. However, for amorjdious pd[ymers such as PMMA and PB, formation of the mechano-radkals is not attributed to the ruptures of the tie molecules, becau% neither the crystalline parts nor frie tie molecule exist in an amorphous polymer and no particular part of the polymer, on which the applied stress is concentrated, can be assumed in the amorphous polymer. It was found that the polymer chains are ruptured even in the case of an amorphous polymer, like PMMA, PB, and other elastomers, as mentioned in the Section III. The medianism other than the Peterlin model is needed to explain the bond scissions of polymer chains in the amorphous pdymers. 

While correlation of fracture behavior in glassy polymers with the time-temperature relationships is receiving increasing attention [see, for example, Broutman and Kobayashi (1972), Johnson and Radon (1972), Radon (1972)], the case of rupture in elastomers is rather well developed, and is discussed briefly below. 

TBP corrrpletely ruptures hard block domains of rrrethane urea re rdless of their initial concerrtration of elastomer. Orrly SPU chemical network resists swelling by TBP. The eqrrilibrirrm swelling of SPU in DBP decreases with increase in concerrtration of hard blocks. As a result tensile strength of a material is decreased to a lesser degree. The lowest decrease in terrsile strength is observed in SPU-1 swollen to equilibrium in DBP. SPU-1 has 37% hard blocks (Figirre 10.80). 

Abrasive wear consists of the rupture of small particles of elastomer under the action of frictional forces, when sliding takes place between the elastomer surface and a substrate. A suitable measure of the rate of wear is provided by the ratio A/p, where A is the volume of rubber abraded away per unit normal load and per unit sliding distance, and ju is the coefficient of friction. This ratio, termed abradability, represents the abraded volume per unit of energy dissipated in sliding. Master curves for the dependence of abradability on the speed of sliding, reduced to a convenient reference temperature by means of the... 

Filled rubbers form a complex network of cross-linked chains connected to surface-active particles such as carbon black or amorphous silica (see Carbon Black). Here we will only indicate the structural features of importance in unfilled cross-linked elastomers. Two breakdown mechanisms are conceivable the initiation and growth of a cavity in a moderately strained matrix and the accelerating, cooperative rupture of interconnected, highly loaded network chains. The second mechanism is more important imder conditions, which permit the largest breaking elongation Xbmax to be attained (29). In that case, the quantity >-bmax is expected to be proportional to the inverse square root of the cross-link density Vg in fact, an increase of A,bmax with to is found experimentally for a... 

The theory of rubber elasticity (Section 9.7) assumes a monodisperse distribution of chain lengths. Earher, the weakest link theory of elastomer rupture postulated that a typical elastomeric network with a broad distribution of chain lengths would have the shortest chains break first, the cause of failure. This was attributed to the limited extensibility presumably associated with such chains, causing breakage at relatively small deformations. The flaw in the weakest link theory involves the implicit assumption that all parts of the network deform affinely (24), whereas chain deformation is markedly nonaffine see Section 9.10.6. Also, it is commonly observed that stress-strain experiments are nearly reversible right up to the point of rupture. 

Elongation is the maximum extension of an elastomer at the moment of rupture. An elastomer with less than a 100% elongation will usually break if doubled over on itself. 

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