Styrene-butadiene rubbers relaxation

Cerveny, S. Bergman, R. Schwartz, G.A. Jacobsson, P. Dielectric alpha- and beta-relaxations in uncured styrene butadiene rubber. Macromole-cular 2002, 35, 337-342. 

Of particular importance for detection of chemical or physical change in polymer materials are mobility filters, which are sensitive to differences in the numbers of molecules within a given window of correlation times. Within reasonable approximation such filters are relaxation filters. Here, Tj filters are sensitive to differences in the fast motion regime while T2 and Tip filters are sensitive to the slow motion regime. Which time window is of importance can be seen from Fig. 5.7 [101]. It shows a double-logarithmic plot of the mechanical relaxation strengths Hi(t) for two carbon-black filled styrene-butadiene rubber (SBR) samples as a function of the mechanical relaxation time T. They have been measured by dynamic mechanical relaxation spectroscopy. In terms of NMR, the curves correspond to spectral densities of motion. But the spectral densities relevant to NMR are mainly those referring... 

Permanent or thermoreversible cross-links mediate the opposite effect on chain dynamics compared with dilution by a solvent instead of releasing topological constraints by dilution, additional hindrances to chain modes are established by the network. With respect to NMR measurands relatively large cross-link densities are needed to affect chain modes visible in the experimental time/frequency window, as demonstrated with proton spin-lattice relaxation dispersion of polyethylene cross-Unked by 10-Mrad irradiation with electron beams [123] and with styrene-butadiene rubbers [29]. However, there is a very strong effect on the dipolar correlation effect which probes much slower motions and can therefore be used favorably for the determination of the cross-Hnk density [29, 176, 177]. 

FIGURE 10.10 Stress relaxation of a cross-linked styrene-butadiene rubber at 1.7°C at elongations from 350% to 525%. Solid points indicate rupture. (Data from Smith, T. L.,... 

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 spin-lattice relaxation process is usually exponential. Theoretically, the effect of spin-diffusion, characterized by the coefficient D (order of 1(T12 cm2 s 1), has an influence on T, relaxation times when ix > L2/D, where Lis the diffusion path length. NMR studies of model systems f6r rubber networks, based on a styrene-butadiene-styrene block copolymer (SBSy, in which styrene blocks act as a crosslink for polybutadiene rubber segments of known and uniform length, indicate that spin diffusion operating between PS and PB phases causes a lowering of Tg for the PS component in SBS (as compared to the pure PS) and hindering of the motion of the PB component (as compared to the pure PB)51). 

Selective 1H T2 relaxation experiments were used to determine specific sites of styrene-butadiene (SBR) rubbery chains, which interact with the carbon black surface [84, 98, 99]. The T2of all the different kinds of protons in SBR chains was found to consist of at least two components attributable to a highly immobilised, tightly bound rubber around the filler particles and an outer region of loosely bound and free rubber. The relative... 

When simple crosslinked elastomers are stretched and held at. high elongation, two consecutive phenomena occur (1) a smooth relaxation period, and (2) sudden failure. This sequence is depicted in Figure 1.22 for poly-(styrene-co-butadiene) rubber (Rodriguez, 1970, Chapter 9 Scott, 1967 Smith, 1964 Smith and Stedry, 1960). 

Figure 1.22. Stress relaxation of a crosslinked poly(styrene-co-butadiene) rubber (at 1.7°C) at elongations from 350 to 525 %. Solid points indicate rupture, the dashed-line gives the ultimate stress. (Scott, 1967.)...

The identification of material heterogeneities based on differences in molecular motion is an important feature of NMR imaging. The importance of slow molecular motion for image contrast is demonstrated in Figure 31 with relaxation time parameter images through a partially aged sheet of carbon-black-filled styrene-co-butadiene rubber (SBR) (147). 

Figure 3.415. Curves of stress relaxation, at 30 °C of the vulcanised poly(butadiene-co-styrene) rubber reinforced with 30% black carbon [1202].

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