Styrene-butadiene, fractionation

Block copolymer chemistry and architecture is well described in polymer textbooks and monographs [40]. The block copolymers of PSA interest consist of anionically polymerized styrene-isoprene or styrene-butadiene diblocks usually terminating with a second styrene block to form an SIS or SBS triblock, or terminating at a central nucleus to form a radial or star polymer (SI) . Representative structures are shown in Fig. 5. For most PSA formulations the softer SIS is preferred over SBS. In many respects, SIS may be treated as a thermoplastic, thermoprocessible natural rubber with a somewhat higher modulus due to filler effect of the polystyrene fraction. Two longer reviews [41,42] of styrenic block copolymer PSAs have been published. 

Although PFE lacks a proven total concept for in-polymer analysis, as in the case of closed-vessel MAE (though limited to polyolefins), a framework for method development and optimisation is now available which is expected to be an excellent guide for a wide variety of applications, including non-polyolefinic matrices. Already, reported results refer to HDPE, LDPE, LLDPE, PP, PA6, PA6.6, PET, PBT, PMMA, PS, PVC, ABS, styrene-butadiene rubbers, while others may be added, such as the determination of oil in EPDM, the quantification of the water-insoluble fraction in nylon, as well as the determination of the isotacticity of polypropylene and of heptane insolubles. Thus PFE seems to cover a much broader polymer matrix range than MAE and appears to be quite suitable for R D samples. 

Figure 3.6 Variation of retention with the composition of the stationary phase in GLC. Stationary phase styrene-butadiene polymer blends and copolymers, the butadiene fraction is plotted on the horizontal axis, (a) Specific retention volumes for three n-alkanes and benzene. V is proportional to the capacity factor, (b) the retention index for benzene. The solid line is calculated from the straight lines in figure 3.6a. The circles (polymer blends) and triangles (copolymers) represent experimental data. Figure taken from ref. [310], Reprinted with permission.

Structure and Composition of Diene Copolymers. One finds that most of the reported copolymerization studies on butadiene or isoprene involve styrene as comonomer. In part this is due to the early interest in styrene-butadiene synthetic rubbers. The free radical produced copolymers (GRS, usually about 20—25% styrene units) contain about 20% of its butadiene fraction in the 1,2 form. The ratio of 1,2 to 1,4 units is little affected by polymerization variables such as temperature, conversion and styrene content (39). Butadiene and styrene copolymers contain 50 to 60% 1,2-diene units when prepared by sodium catalysts at 50° (39). This behaviour is once more significantly different when lithium is used in place of sodium as can be seen in Table 3. 

Mastral et al.23,24 have also investigated the effect of the main components present in tyres (carbon black, styrene-butadiene copolymer and polybutadiene) on the liquefaction of coal. Coprocessing of coal and carbon black confirmed the catalytic role of the latter, as it promotes hydrocracking reactions leading mainly to the formation of gaseous products. The addition of SBR to coal improves the yield of gases, oil and asphalt fractions, even at relatively low temperatures (350-375 °C). It is proposed that SBR favours the stabilization of the radicals involved in the process through alkylation reactions... 

Economical soy products including SPI, DSF, SPC, and SSF can be mixed and coagulated with polymer latex in the aqueous phase to form dry composites with significantly enhanced modulus. These dry soy products have a shear elastic modulus of 1-5 GPa within the temperature range of -40 to 140 C. The carboxylated styrene-butadiene composites filled with these soy products show a significant increase of shear modulus compared to that of the polymer matrix alone. The different compositions of these soy products generate a different reinforcement effect and approximately follow the order SPC > DSF > SSF SPI. The dehydration of these soy reinforcement fractions causes the... 

The most important synthetic elastomer is styrene-butadiene (SBR) which accounts for 41% of the world market in elastomers. It is used predominantly for vehicle tires when reinforced with carbon black. Nitrile rubber (NBR) is a random copolymer of acrylonitrile (mass fraction 0.2 to 0.4) and butadiene, and it is used when an elastomer that is resistant to swelling in organic solvents is required. The range of properties can be extended when styrene is also incorporated in the chain. 

As aggregate fractions, quartz sand (0-5 mm), styrene butadiene rubber (SBR) waste particles (0-12 mm) or polyurethane (PU) waste particles (0-12 mm) were used. Their bulk specific gravities were 2640 kg/m, 550 kg/m and 500 kg/m respectively and their gradations are shown in Fig. 1. 

Natural rubber, synthetic cw-1,4-poly(isoprene), butadiene rubbers, and styrene-butadiene rubbers are all sensitive to oxidation because of their high carbon-carbon double bond fractions. Attempts to reduce sensitivity to oxidation with maintenance of the vulcanizability have lead to the development of what are known as the butyl rubbers, IIR, which are copolymers of isobutylene with a little isoprene. But butyl rubbers only have a small rebound elasticity. However, since they also have poor gas permeability, they are mostly used for tire inner tubes. 

All diene rubbers discussed so far, natural rubber, styrene-butadiene rubbers, poly-butadienes), butyl rubbers, and ethylene-propylene rubbers, consist of aliphatic or aromatic monomeric units. They swell readily in aliphatics they have poor oil resistance. But the free radical copolymerization of acrylonitrile with butadiene leads to what is known as nitrile rubber, which has good oil resistance because of the many polar nitrile groups. However, the rebound elasticity and the low-temperature flexibility decrease with increasing nitrile fraction. Consequently, NBR is mainly used for fuel hoses, motor gaskets, transport belts, etc. 

Plots of the styrene-butadiene rubber blends give higher values of 2.45 to 2.65 for the exponent, as shown in Fig. 25. A possible explanation is that the styrene-butadiene rubber contains gels that do not participate in the dilution by the resin. Therefore, the resin concentration in the amorphous phase is higher than calculated which would reduce the modulus more than expected. This would result in an apparent higher power for the polymer volume fraction. 

For natural rubber, the volume fraction exponent is 2.25-2.40, close to the value of 2.25 predicted by DeGennes. For styrene-butadiene rubber, the exponent was found to be... 

MMD and CCD for styrene-butadiene block copolymers were determined by SEC followed by turbidimetric titration of the eluates [60]. The mobile phase was THF and methanol was added to the fractions in 7 minutes. The intensity of the scattered light was determined, and the point of inflection during titration indicated the precipitation of a new species of molecule. If the dependence of solubility on chemical composition and concentration is known for the particular retention volume, the chemical composition of each species may be evaluated from these inflection points. 

PE would have a low solubility even in hot toluene. In the case of styrene-butadiene copolymer, the uncrosslinked polymer is soluble in aromatic solvents, whilst the highly crosslinked (gel) fraction is completely insoluble and, indeed, this can be used as the basis of a method for separating gel from uncrosslinked polymer. Copolymers usually dissolve in a greater number of solvents than homopolymers. Thus, whilst PVC is only slightly soluble in acetone or methylene chloride, its copolymers with vinyl acetate or acrylates dissolve easily. 

Figure 4.2 ReIatioi ship between (a) T of PVC and mass fraction of plastizer (IX5P) (%) and (b) of styrene-butadiene random copolymer and ma.ss fraction of styrene...

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