Crystallisation strain-induced

Raman and IR spectroscopic studies dealing with the qualitative and/or quantitative determination of rubber compounding ingredients, i.e., the elastomer itself [22, 26-31], fillers [32, 33], vulcanisation chemicals and other additives [34-37], are not included here. The same applies to studies dealing with the crosslinking of elastomers by means of chemicals other than sulfur or peroxide [38-41], self-crosslinking of elastomers blends [42-44], crystallisation (strain-induced) [45-48] and oxidation/ageing [49-53]. 

When a rubbery polymer, such as natural rubber, is stretched the molecules become aligned. This orientation leads to crystallisation. The effect of this so-called strain-induced crystallisation is to make the extended polymer stiffer than the unstrained polymer. Such crystallisation is not permanent but disappears when the sample is allowed to retract and regain its original dimensions. 

One effect of this strain-induced crystallisation is that there is a characteristic upswing in the plot of stress against strain for natural rubbers, as illustrated in Figure 7.11. 

Density is also found to increase in this region, thus providing additional evidence of crystallisation. Certain synthetic elastomers do not undergo this strain-induced crystallisation. Styrene-butadiene, for example, is a random copolymer and hence lacks the molecular regularity necessary to form crystallites on extension. For this material, the stress-strain curve has a different appearance, as seen in Figure 7.12. 

The stress-strain curve for unfilled NR exhibits a large increase in stress at higher deformations. NR displays, due to its uniform microstructure, a very unique important characteristic, that is, the ability to crystallise under strain, a phenomenon known as strain-induced crystallization. This phenomenon is responsible for the large and abrupt increase in the reduced stress observed at higher deformation corresponding, in fact, to a self-toughening of the elastomer because the crystallites act as additional cross-links in the network. This process can be better visualized by using a Mooney-Rivlin representation, based on the so-called Mooney-Rivlin equation ... 

No. 18, 1996,p.4009-24 FOURIER TRANSFORM RAMAN STUDY OF THE STRAIN-INDUCED CRYSTALLISATION AND COLD CRYSTALLISATION OF NATURAL RUBBER... 

Applications of Fourier transform Raman spectroscopy to the analysis of NR are described. Of particular interest is the observation of main chain modifications during vulcanisation and the ability to observe the conversion of insoluble to soluble sulphur under conditions appropriate to compounding and vulcanisation. The influence of crystallisation, both temperature and strain induced, on the Raman spectmm of NR is also demonstrated. 9 refs. EUROPEAN COMMUNITY UK USA WESTERN EUROPE... 

Titanium-polymerised rubber, CBll, is the most branched. However, the branches are relatively short. The rubber gives strain-softening and strain-induced crystallisation upon stretching [30]. 

Previonsly, it was noticed with the matrix rnbber [33] that in the presence of the larger amonnt of the crystalline particles there was an indication of more strain-induced crystallisation. Among the samples that we examined, the three rnbbers of the VCR series were the only ones, which gave an indication of strain-indnced crystallisation in milling. This means that in order for the gum rubber and the compound to have strain-induced crystallisation during milling the material must be under sufficient tension, i.e.. Region II. 

Another importance of linear viscoelasticity is that it provides a reference for non-linear behaviour that is, the latter is expressed as a deviation from the former with use of appropriate parameters. First is the universal parameter, the elongation ratio, a, which reduces the time-scale of nonlinear behaviour to that of linear behaviour. Next is the modulus shift factor, T(a), which indicates the degree of strain-hardening or strainsoftening. Finally, comparison of linearised elongation data with that of shear data indicates, if they disagree, the presence of strain-induced crystallisation or strain-induced association. All these deviations from linearity are related to the structure of rubber. 

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