Strained rubbers

A strainer equipped with a special die which extrudes the strained rubber as a tube, which is then slit at the head to produce a continuous sheet, which may then be cut into slabs of convenient length. 

Deuterium NMR is very sensitive to orientational behavior and order there are a number of papers dealing with constrained polymeric networks. For example, 2H NMR (in both, solid state and solution) is used in the study of the orientational order generated in uniaxially strained rubbers as a function of the crosslink density. Two sets of rubbers (model end-linked silicone rubbers and randomly crosslinked diene networks) were investigated directly (on perdeuterated silicone labelled chains) and indirectly, via C6D6 as an NMR probe for diene rubbers 45). 

Typical examples of stress corrosion are crack formation in strained rubber vulcanizates under the influence of ozone, hair cracks in PE under stress in the presence of a surface active agent (see also 7.4.2), crack formation in PC, when exposed to e.g. CCI4, within a few seconds after the application of a small stress. 

Figure 7.11 Double-quantum images of a strained rubber band with a cut (a) image, (b) 2H image of deuterated spy molecules incorporated into the rubber network by swelling, (c) finite element simulation of the stress distribution...

Sample deformations modify the number of accessible conformations of intercross-link chains (cf. Figure 7.7), so that they can be detected by analysis of relaxation and residual dipolar couplings. This is illustrated for strained rubber bands with a cut in Figures 7.6 and 7.11. 

In elastomer samples with macroscopic segmental orientation, the residual dipolar couplings are oriented as well, so that also the transverse relaxation decay depends on orientation. Therefore, the relaxation rate 1/T2 of a strained rubber band exhibits an orientation dependence, which is characteristic of the orientational distribution function of the residual dipolar interactions in the network. For perfect order the orientation dependence is determined by the square of the second Legendre polynomial [14]. Nearly perfect molecular order has been observed in porcine tendon by the orientation dependence of 1/T2 [77]. It can be concluded, that the NMR-MOUSE appears suitable to discriminate effects of macroscopic molecular order from effects of temperature and cross-link density by the orientation dependence of T2. 

The reliability and sensitivity of the probe method has been emphasised. It circumvents almost completely the perturbations inherent to some other probe techniques (electron spin resonance, fluorescence). In particular, free chains appear to be ideal, non-perturbative NMR probes for testing chain segment orientation in strained rubbers. The solvent probe method is easy to handle and unexpensive, since it does not require the synthesis of... 

For analyzing the fracture behavior of filler clusters in strained rubbers, it is necessary to estimate the strain of the clusters in dependence of the external strain of the samples. In the case of small strains, considered above, both strain amplitudes in spatial direction n are equal (t A F i). because the stress is transmitted directly between neighboring clusters of the filler network. For strain amplitudes larger than about 1%, this is no longer the case, since a gel-sol transition of the filler network takes place with increasing strain [57, 154] and the stress of the filler clusters is transmitted by the rubber matrix. At larger strains, the local strain eAtfl of a filler cluster in a strained rubber matrix can be determined with respect to the external strain if a stress equilibrium between the strained cluster and the rubber matrix is assumed ea GpX =6rm( u)) With Eq. (29) this implies... 

The first addend is the equilibrium energy density stored in the extensively strained rubber matrix, which includes hydrodynamic reinforcement by a... 

Fig. 8.4.4 [KM i double-quantum imaging of deuterated spy molecules in strained rubber bands with a cut on each side, (a) Pulse sequence. A 180 pulse in the preparation period refocusses the chemical shift and off-resonance interactions, (b) Spin-echo image, (c) Hi double-quantum image. The contrast is determined by the doubic-qiianium coherence uiiipliludc weighted by the spin density.

Where K, A, d, and y are the rubber bulk modulus, relative volume strain, rubber particle diameter and surface energy per unit area, respectively. 

Experimental Comparison of a Free Chain in a Strained Rubber. .. 98... 

LEONCIO GARRIDO, JEROME L, ACKERMAN AND JAMES E, MARK Chain Segment Ordering in Strained Rubbers 367... 

Figure 2 illustrates the length-temperature data for one of the elastomers, poly-cis-1, U-butadiene cured with 0.5 p.h.r. dicumyl peroxide, at a series of loads. From the slopes of the straight lines in this plot, the linear thermal expansion coefficients of the strained rubber can be readily obtained. In order to compute the relative energy contribution to rubber elasticity by eq. 6, we need the linear thermal expansion coefficients of the unstrained rubber and the temperature coefficient of the shear modulus. These can be determined by plotting a as a function of the quantity (X -l)/(X +2). If eq. 7 is indeed valid, then one would expect a linear relation. From the intercept and slope of such a plot, values of and dilnG/dT can be easily obtained. 

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