Macrostructure, rubber

The information on physical properties of radiation cross-linking of polybutadiene rubber and butadiene copolymers was obtained in a fashion similar to that for NR, namely, by stress-strain measurements. From Table 5.6, it is evident that the dose required for a full cure of these elastomers is lower than that for natural rubber. The addition of prorads allows further reduction of the cure dose with the actual value depending on the microstructure and macrostructure of the polymer and also on the type and concentration of the compounding ingredients, such as oils, processing aids, and antioxidants in the compound. For example, solution-polymerized polybutadiene rubber usually requires lower doses than emulsion-polymerized rubber because it contains smaller amount of impurities than the latter. Since the yield of scission G(S) is relatively small, particularly when oxygen is excluded, tensile... 

The authors collected the test data shown in Table X. From these data it can be noted that the number-average molecular weight, or of a commercial emulsion SBR such as IISRP 1500 or 1712 is typically 90,000 to 175,000. The primary molecular weight of a solution-polymerized polymer produced with an anionic lithium catalyst can, in contrast, be increased toward 250,000 without gelation. In addition, emulsion-polymerized SBR contains only about 92% rubber hydrocarbon as a result of the presence of residues from the production process solution polymers tend to be near 100% hydrocarbon. As a consequence, the authors concluded that the number-average molecular weight can be considered the key parameter of polymer macrostructure, particularly with respect to the hysteretic characteristics of a tread formulation. Hence the differences in macrostructure between emulsion- and solution-polymerized polymers will dictate many of their properties in a tire tread compound. 

The main differences between the hot and cold emulsion SBRs is due to another macrostructural feature. The hot rubbers are more extensively branched and contain much microgel. The microgel ... 

The broad range of control of solution polymer structure and macrostructure of styrene-butadiene rubbers that is only possible using lithium catalysts was discussed in Section 2. We have seen how the microstructure of the butadiene units in the chain and comonomer sequence distribution can be controlled with the addition of polar modifiers and/or variations in pol5onerization process variables. Additionally, the unique control of macrostructure features and the new possibilities offered by reactive functional groups were discussed as part of the molecular engineering capabilities of solution anionic polymerizations. 

Cured vinyl ester resins are fragile after curing like standard UPRs and thus need flexibilization. The flexibilization of vinyl ester resins is made using the known reactive rubber vinyl-terminated liquid elastomeric butadiene-acrylonitrile copolymer (VTBN) [76]. The macrostructure of the VTBN flex-... 

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