Miscible elastomer blends mechanical properties

Where the solubility parameter rule is in error is for natural rubber and polybutadiene. The differential in solubility parameters is around 0.6 but the two polymers are immiscible. Polybutadiene grade IISRP 1207 and an oil extended polymer such as IISRP 1712 have a differential of less than 0.1 and in this case the two elastomers are nearly fully miscible between the lower and upper critical solution temperatures. The blended elastomers mechanical properties then become a function of the filler type, distribution, vulcanization system, and any processing aids present. 

The majority of elastomer blends are phase separated, but of interest, as crosslinking can achieve mechanical compatibilization due to crosslinking between the phases as noted in Chapter 3. This compatibilization method can lead to unique and useful blends with a compromise in properties, offering useful commercial products as well illustrated by the applications in tire construction. The unsaturated hydrocarbon elastomers without polar functional groups are rarely miscible with each other, because no specific interactions are present to achieve the necessary thermodynamic driving force for miscibility. The few miscible examples noted generally exhibit matched solubility parameters. 

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