Polyisoprene crosslink density

In a second paper Brown and Tinker [101] examined the effects of a number of parameters, such as the accelerator used in the sulfur curing, peroxide versus sulfur curing, and swelling ratio at constant crosslink density. The results for c/s-polyisoprene show that the value of H% is independent of accelerator. However, lower values of H% were seen for peroxide-cured materials. The results for BR were independent of curant the reasons... 

The curing and dynamic properties of precipitated nano-silica on NR without and with the sulfur addition (NR with S), synthetic polyisoprene (IR), polybutadiene (BR) and SBR was investigated. Silica was treated with bis(3-triethoxysilylpropyl)tetrasulfane (TESPT) to form bonds at interfaces. Cure, Mooney viscosity, glass transition temperature, bound rubber, crosslink density and DMA were measured. The properties of silica-filled SBR and BR correlated with highest rolling resistance and SBR-silica correlated with best skid resistance. A Payne effect was observed in the loss modulus under some experimental conditions. In addition to possible filler de-agglomeration and network disruption, the nanoscale of the filler may have further contributed to the non-linear response typified by the Payne effect. ... 

Of course, the first manmade crosslinked polymer was Charles Goodyear s vulcanized rubber with low crosslink density. Since there were only a few sulfur crosslinks between the polyisoprene chains, Charles Goodyear s product, which he called vulcanite, was elastic but had a higher modulus than the original linear Hevea braziliensis. However, Nelson Goodyear s product, called ebonite, produced by the addition of larger amounts of sulfur (25-30%) to natural rubber, was a nonelastic intractable solid. 

The strongest residual dipolar couphngs were also edited in a nonrotating cross-linked polyisoprene series by exciting double- and triple-quantum coherences in the short time regime (68). From this, the dynamic order parameters of the methylene and methyl groups were estimated and correlated with the crosslink density. Essentially, the same behavior was foimd as for SBR. 

Fig. 2. Loss modulus and elongation at break as a function of temperature for a typical strain crystallizable rubber polyisoprene cured to 1.7x10 mol cm crosslink density. (Our data.)...

Some examples of the analysis of the apparent activation energy Ea of blends can be found in the literature. When evolution of the Ea of liquid cis-polyisoprene/ trans-polyisoprene (CPI/TPI) blends was analyzed using the VFTH model [41], the Ea for pure TPI was observed to be higher than that of TPI/CPI blends. Moreover, as the concentration of TPI decreased, then the Ea also decreased this was ascribed to the fact that in glass-rubber relaxation the motion of molecules is governed by the crosslink density. As the decrease in TPI content caused a decrease in crosslink density, this in turn enhanced the motion of chains, and consequently less Ea was required to promote segmental cooperative movements. 

Other studies on the properties of block copolymers with more complicated architectures exist, although not all of them consider tack. Some studies comparing properties of radial versus simple block architecture allow to compare the effect of chemical versus physical crosslinks in the case of structured systems. The data shows that there is a decrease of melt viscosity together with better adhesive properties for star copolymers [44]. This is related to a point that was discussed earlier the different architectures of the molecules lead to different physical crosslinking densities. In the same spirit, block copolymers with four different arms, two polyisoprene, and two polystyrene-fc-poly(ethylene/butylene) arms were studied, and led to a better combination of shear strength and melt viscosity for adhesive applications [45], compared to the conventional linear SIS and SEBS triblocks. 

The differences between torque values developed by the various synthetic polyisoprenes parallel differences in modulus values and reflect cross-linking efficiency. The higher torque development for natural rubber compared to Natsyn 2200 is believed to be due partly to a different viscoelastic behaviour since, for example, hardness is higher for Natsyn 2200 than for natural rubber and estimates of crosslink densities by swelling measurements show a close similarity. 

A polyisoprene rubber has 2.5% of its repeating units crosslinked by sulfur vulcanization. Estimate the modulus of the sample at low extensions. (Density of vulcanizate = 0.94 g/cm at 25°C.)... 

A sample of cross-linked natural rubber (polyisoprene) is found to have shear modulus G = 217 kPa at 20 C. Deduce TV, the number of sub-chains between crosslinks per m and hence the number average degree of polymerization between crosslinks. The chemical structure of polyisoprene is given in 3.1. You may take for relative atomic mass C = 12 and H = 1, and for density p = 909 kg/m. Assume... 

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