SynopsisThe stress-strain properties of natural rubber and synthetic cis-polyisoprene vulcanizates have been investigated over a large range of extensions. The results are in qualitative agreement with those of Mullins, in that the upturn of a Mooney-Rivlin plot is ascribed to finite extensibility effects. It is further found that the stress-strain curves can be adequately described by a modification of existing non-Gaussian theory. Analysis of the curves shows significant finite extensibility effects even at extensions well below the upturn. Thus, division of stress-strain curves into Gaussian and non-Gaussian regions is to be avoided. Values for n, the number of statistical segments between crosslinks, were calculated using the proposed treatment. The values of n found were relatively unaffected by temperature. Further, the molecular weight of a statistical segment waa found to be about the same for synthetic cis-polyisoprene as for natural rubber despite differences in the crystallization behavior. Crystallization effects are therefore thought to bc of little importance in determining the position of the upturn. The equivalent statistical segment was found to be 4.3 isoprene units for natural rubber and cis-polyisoprene.
The effect of microstructure on crystallizability of polyoctenamers prepared by R3Al‐WCl6 catalyst was studied. The results indicate that polyoctenamers with a broad range of trans‐vinylene content do crystallize. The measured melting points are dependent on the trans‐vinylene content. From the dependence of melting temperature on copolymer composition, a value of 73 ± 2°C. for the melting point and a molar heat of fusion ΔHu of 3520 cal./mole are calculated for 100% trans‐polyoctenamer. From the melting point depression in the presence of diluent, a value for ΔHu of 4800 cal./mole is obtained.
Blends of polybutadiene with SBR show different crystallization behavior than blends with polyisoprene. One might suppose that the similarities in SBR and polybutadiene are sufficient to allow more intimate mixing than can be obtained by using entirely different polymers such as the polyisoprene polybutadiene pair. Possibly, bulk viscosities are important here, but no matter what the cause, crystallization properties have shown differences in the way the polybutadiene was situated. Also, large changes in the glass transition occurred for those materials which showed sizeable retardation in crystallization rate. An interpretation of the crystallization rate data based on particle size was given in this discussion because a precedent exists for this effect. However, molecular compatibility has not been ruled out. Further investigation of thermal properties should help clarify this point. It will be interesting to learn to what extent particle size alone affects the thermal behavior of blends. Perhaps more work along these lines coupled with improved optical and electron microscope techniques will provide some answers to our questions on how phase structure affects thermal and mechanical properties.
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